Amity School of Engineering & Technology Amity University, Uttar Pradesh Instruction Manual of PCB Laboratory Subject Code: BTE 322 DEPARTMENT OF ELECTRONICS & COMMUNICATION List of Experiments. EXPERIMENT NO. 1 “STUDY OF TRANSFORMER” AIM: Study of transformer. THEORY: Transformer is an ac machine that (i) transfers electrical energy from one electric circuit to another (ii) does so without a change of frequency (iii) does so by the principle of electro-magnetic induction and (iv) has electric circuits that are linked by a common magnetic circuit. The energy transfer usually takes place with a change of voltage though' this is not always necessary. When the transformer raises the voltage i.e. when the output voltage of a transformer is higher than its input voltage, it is called the step up transformer and when it lowers the voltage it is called the step-down transformer. Construction & working Principle of Transformer: Since its basic construction requires no moving parts, it is often called the step up transformer' and it is very rugged machine requiring the minimum amount of repair maintenance. Owing to the lack of rotating parts there are no friction or windage losses Further, the other losses are relatively low, so that the efficiency of a transformer is high Typical transformer efficiencies at full load lie between 96 % and 97 % and with extremely large capacity transformers the efficiencies are as high as 99 %. Construction & Working Principle Of Transformer: An elementary transformer consists of a soft iron or silicon steel core and two winding placed on it. The windings are insulated from both the core and each other. The core is built up of thin soft iron or silicon steel laminations to provide a path of low reluctance to the magnetic flux. The winding connected to the supply main is called the primary and the winding connected to the load circuit is called the secondary. The winding connected to higher voltage circuit is called the high-voltage (hv) winding while that connected to lower voltage circuit is called the low-voltage (lv) winding. In case of a step-up transformer lowvoltage winding is the primary and high voltage winding is the secondary while in case" of a step-down transformer the high-voltage winding is the primary and low-voltage winding" is the secondary. The action of a transformer is based on the principle that energy may be efficiently transferred by induction from one set of coils to another by means of a varying magnetic flux, provided that both the sets of coils are on a common magnetic circuit. In a tranceformer, the coils and magnetic circuit are all " nary with respect to one another. The emfs are induced by the variation in the magnitude of flux with time, as illustrated in fig. Although in the actual construction the two windings are usually wound one over the other, for the sake of simplicity, the figures for analyzing transformer theory show the windings on opposite sides of the core as in fig. When the primary winding is connected to an a c supply mains, a current flow through it,since this winding links with an iron core, current flowing through this winding produces an alternating flux in the core. Since. this flux is alternating and links with the secondary winding also, so induces an emf in the secondary winding. The frequency of induced emf in secondary winding is the same as that of the flux or that of the supply voltage.. The included emf secondary winding enables it to deliver current-to an external load connected across it. Thus the energy is transformed" from primary winding to the secondary winding by means of electro-magnetic induction without any change in frequency, The flux of the iron core links not only with the secondary winding but also with -the primary winding, so produces self induced emf in the primary winding. This induced emf in the primary winding opposes the applied voltage and, therefore, sometimes it is known as back, emf of the primary. In fact the induced emf in the primary winding limits the primary current in much the same way that the back emf in a dc motor limits-the armature current. RMS value of emf induced in primary E1 = 4.44 F N1 Фmax volts RMS value of emf induced in secondary E2 = 4.44 F N1 Фmax x N2volts Voltage and current transformer ratio 1 2 = 21 = 21 = 21 = K Where K is transformation ratio voltage CLASSIFICATION OF TRANSFORMER: Types Of Transformers: Depending upon the type of construction used, the transformer are classified into two categories viz (i) Core Type and (ii) Shell Type Transformer. Depending upon the type of service, in the field of power system, they are classified as (iii) Power Transformer (iv) Distribution Transformer (i) Core Type Transformer: In core type construction as shown in fig the coils are wound around the two limbs of a rectangular magnetic core. Each limbs carries one half of the primary winding and one half of the secondary winding so as to reduce the leakage reactance to the minimum possible. The lv winding is wound on the inside nearer to the4 core while the hv winding is wound over the lv winding away from the core in order to reduce the amount of insulation materials required. Small transformers may have cores of rectangular or square x -section with rectangular or circular coil but it is wasteful in case of large capacity transformers. In case of large sized transformers stepped cruciform core with circular cylindrical coils is employed as, illustrated in fig. such a core employs laminations of different sizes. Though the cost of manufacturing of such a cruciform core is much greater, but the circular coils that are used are easier to wind and provide more mechanical strength, especially when short-circuit occurs. Other advantages of using cruciform core are high space factor and reduced mean le4ngth of turn resulting in reduced copper loss. (ii) Shell Type Transformer: As shown in fig, the coils are wound on the central limb of a three limb core. The entire flux passes through the central limb and divides into parts going to side limbs. Consequently; the crosssectional area (and hence width) of the central limb is twice of that of each of the side limbs. Sandwich type winding is used in such a construction. If the core type and shell type transformers are compared, then we conclude that the core type transformer has a longer mean length of iron core and a shorter mean length of coil turn. The core type transformer has smaller x-section of iron and, therefore, a greater number of turns. Core type construction provides more space for insulation making it best suited for EHV requirements. In shell type transformers advantage is gained through the core being used to protect the windings from mechanical damage. The shell type construction gives better support against electro-magnetic forces between current carrying conductors. These forces are of considerable magnitude under short-circuit conditions. The shell type construction is commonly used for small transformers where a square or rectangular core cross section is suitable for economic consideration. The shell type construction needs more specialized fabrication facilities than core type, while the latter offers an additional advantage permitting visual inspection of coils in the case of a fault and ease of repair at substation site For these reasons, the present practice is to use core type transformers in large high voltage installations. (iii) Power Transformer: The term is used to include all transformers of large size (kVA and above) used in generating stations and substations for transforming the voltage each end of a power transmission line. They may be single phase or 3-phase; 3-wire de delta or delta/ star connected; and of voltage rating 220/11 kV or in high voltage range. are put in operation during load hours and disconnected during light load hours i.e they usually operated on approximately full load. This is possible because they are arranged banks and can be thrown in parallel with other units or disconnected at will. So power tr: formers are designed to have maximum efficiency at or near full load (i.e. with iron lo-: full load copper loss ratio of 1: 1). Power transformers are designed to have considerable gre: leakage reactance than is permissible in distribution transformers because in power tr: formers inherent voltage regulation is not as much important as current limiting effect of higher leakage reactance. Power transformers usually make use of flux density of 1.5 to T, have percentage impedance ranging from 6-18 % and regulation of 6-10 percent. They ~~ be self oil cooled, forced air cooled or forced water cooled. (iv) Distribution Transformer: Transformers of rating upto 200 kVA, used to step down distribution voltage to a standard service voltage are known as distribution transformer They are kept in operation all the 24 hours a day whether they are carrying any load or, In such transformers iron loss occurs for all the time where copper loss occurs only when ~ are loaded. Therefore, distribution transformers should be designed with iron loss small comparison to full-load copper loss (say with iron loss to full load copper loss ratio of 1: 3) other words, they are designed for good' all-day efficiency' and not for highest efficiency one are of the self cooling type and are almost invariably oil-immersed. The distribution transformers are usually 3-phase, 4-wire 11 kV/400 V delta/star connected. Such transformer are designed to have an inherently good voltage regulation which is possible by arranging the coils in such a way as to have minimum leakage reactance. Such transformers percentage impedance of 4-5 % and voltage regulation of 4-8 %. They make use of cold teel with flux density of 1. 7 T TRANSFORMER LOSSES: 1. Iron or Core Losses: Iron loss is caused by the alternating flux in the core and consists of hysteresis and eddy current losses. (a) Hysteresis Loss: The core of a transformer is subjected to an alternating magnetizing force and for each cycle of emf a hysteresis loop is traced out. The hysteresis loss per second is given by the equation. Hysteresis loss, Ph = '(B m a x ) x f v joules per second or watts whe.re f is the supply frequency in Hz, v is the volume of core in cubic metres, / ' is the hysteresis coefficient, Bmax is peak value of flux density in the core and x lies between 1.5 and 2.5 depending upon the material and is often taken as 1.6. (b) Eddy Current Loss: Eddy current loss arises from the fact that the core itself is composed of conducting material, so that voltages induced in it by the varying flux produces circu lating currents in the iron. These are called eddy currents and are accompanied by i2r loss in core the core called the eddy currents loss. The eddy current loss is given by equation (6.28) i.e. Pe = Ke (Bmax )2 f 2 t2 v watts For expressing the core loss in terms of voltage, frequency etc., substitute from transformer emf equation max= 4.44 = ∅ = 4.444 in expressions for hysteresis loss and eddy current loss respectively. Thus we get P h = ( 4.44 ) = ′ ( 1 4.44 ) 1− P h 1 − for a given transformer v (volume of core), N and a are constant It means that hysteresis loss depends on both the applied voltage and its frequency. The eddy current loss is given as Pe=K e (4.44 ) or P e V2 2 22 for a given transformer v,t, N and a are constant. The eddy current loss, is, therefore, proportional to the square of applied voltage and is independent of frequency. According to equations (6.27) or (10.1), (10.2) and (10.3), for any given voltage ( Bmax f), if f decreases B max increases correspondingly and if f increases Bmax decreases correspondingly. Hence eddy current loss, at any given voltage, is independent of frequency. In above discussion, applied voltage V is taken almost equal to the induced emf given by the referre. equations. 2. Copper or Ohmic Losses: These losses occur due to ohmic resistance of the transformer windings. If 1, and 12 are the primary and secondary currents respectively and R1 and R2 are the respective resistances of primary and secondary windings then copper losses occuring in primary and secondary windings will be I12 Rl and 122 R2 respectively. So total copper lo will be (I12 R1 + I22 Rd. These losses vary as the square of the load current or kVA. For example if the copper losses at full load are PC then copper losses at one-halforone-third of full load will be respectively.Copper losses are determined on the basis of constant equivalent resistance R~Q de mined from the short-circuit test (refer Art 10.18) and then corrected to 75 °C (since the st and operating temperature of electrical machines is taken 75 °C). RESULT: Study of transformer is completed. EXPERIMENT NO. 2 “CONSTRUCTION OF TRANSFORMER” AIM: Winding shop : step down transformer winding of less than 5VA. APPARATUS REQUIRED: 1. Transformer winding machine 2. Copper wire of (two different thickness). 3. Die. 4. Bobin. 5. Insulation material strips. 6. Cutter, plier. 7. E- section, Isection. 8. Soldering kit. 9. Clamp. 10. Varnishing material. 11. Tray. 12. Multimeter. THEORY: Transformer:- A transformer is a device that: 1. Transfers electrical energy from one electrical circuit to another. 2. Does so without a change in frequency. 3. Has electric circuit that one linked by a magnetic circuit. Since the basic construction requires no moving part it is often called as “static transformer” CLASSIFICATION OF TRANSFORMER: On the basis of the value of transformation can be classified as under : 1. Step Up Transformer 2. Step Down Transformer When the transformer raises the voltage when the output voltage of a transformer is less than its input voltage , it is called the step down transformer. Thus in case of step down transformer V2>V1, so In case step up transformer V2<V1 and thus K<1. Constructionally, the transformer are two type namely 1. Core type transformer 2. Shell type transformer These from next the two are distinguish from merely the manner in which the primary and secondary coil are placed around the laminated core. In the so called core transformer, the winding surrounds a considerable part of core whereas in the shell type transformer, the core surround considerable part the winding. In the shell type transformer, the iron core surround the copper winding the entire flux passes through the iron core, but outside this central core it divides into half going in each direction. The leakage flux reduced to very by sandwiching the primary winding between the section of secondary winding. In these type of transformer, the coils are better braced mechanically so that they are less easily displaced by high electromechanically so they are easily displaced by high electronically force that frequently developed during short circuit. PROCEDURE: 1. Take the bobbin and fit it on to die. 2. Tight the die in the bobbin with the help some paper (butter paper) such that it gets completely fit in it. 3. Fix the frame (bobbin) on the transformer winding machine. 4. The machine at the proper scale as per the setting of frame and wire used. 5. Set the counter at zero. 6. As first primary is to wound so placed the thin wire roll on the holders. 7. Rotate the handle manually, in such a manner that the copper get wound up the frame tightly. 8. Take the desired number of turns. 9. Take care that initial and final ends of the copper wire does not get mixed up. 10. Place a strip of insulation material round the frame over the primary winding in order to isolate primary from secondary. 11. Calculate the secondary no of turns by N2= N1*(V2/V1) N1= 2300, N2= 90 V1=230, V2= ? N!/N2=V1/V2 V2=9V And wound them over the insulating material over the frame as per before. 12. Again wrap the secondary winding with a strap of insulating material. 13. Apply soldering process to cover the terminals. 14. Now insert the E-section in the frame followed by the inserting of I-section. 15. Now the frame is placed in a tray and dipped in varnish. 16. Transformer is ready and is now placed in a clamp. 17. Test the transformer for 5VA (=9v*500mA) with a multimeter. RESULT : A step down transformer of less than 5VA has been winded. PRECAUTIONS: 1. The winding should be tight and uniform. 2. Secondary and primary winding should be properly isolated from each other. 3. The I-section should be placed in between the E-section at proper position. EXPERIMENT NO. 3 “STUDY OF CRYSTAL DIODE RECTIFIER” AIM : Study of crystal diode rectifier. RECTIFIER : Semiconductor Diode A pn junction is known as a semi-conductor or **crystal diode. The outstanding property of a crystal diode to conduct current in one direction only permits it to be used as a rectifier. A crystal diode is usually represented by the schematic symbol shown in Fig. The arrow in the symbol indicates the direction of easier conventional current flow. A crystal diode has two terminals. When it is connected in a circuit, one 'thing to decide is whether the diode is forward or reverse biased. There is easy rule to ascertain it. If the external circuit is trying to push the conventional current in the direction of arrow, the diode is forward biased. On the other hand, if the conventional current is trying to flow opposite the arrowhead, the diode is reverse biased. Putting in simple words : (i) If arrowhead of diode symbol is positive w.r.t. bar of the symbol, the diode is forward biased. (ii) If the arrowhead of diode symbol is negative w.r.t. bar, the diode is reverse biased. CLASSIFICATION OF RECTIFIER: 1. Half-Wave Rectifier: In half-wave rectification, the rectifier conducts current only during the of input a.c. supply. The negative half-cycles of positive half-cycles a.c. supply are suppressed i.e. during negative half-cycles, no current is conducted and hence no voltage appears across the load. Therefore, current always flows in one direction (i.e. d.c.) through the load though after every half-cycle. Circuit Details: Shows the circuit where a single crystal diode acts as a half-wave rectifier. The a.c. supply to be rectified is applied in series with the diode and load resistance RL. Generally, a.c. supply is given through a transformer. The use of transformer permits two advantages. Firstly, it allows us to step up or step down the a.c. input voltage as the situation demands. Secondly, the transformer isolates the rectifier circuit from power line and thus reduces the risk of electric shock. Operation: The a.c. voltage across the secondary winding AB changes polarities after every half-cycle. During the positive half-cycle of input a.c. voltage, end A becomes positive w.r.t. end B. This makes the diode forward biased and hence it conducts current. During the negative halfcycle, end A is negative w.r.t. end B. Under this condition, the diode is reverse biased and it conducts no current,Therefore, current flows through the diode during positive halfcycles of input a.c. voltage only it is blocked during the negative half-cycles (see Fig). In this way, current flows through load always in the same direction Hence d.c. output is obtained across It may be noted that output across the load is pulsating d.c. These pulsations in the output are further smoothened with the help of filter circuits discussed later. Disadvantages : The main disadvantages of a half-wave rectifier are : (i) The pulsating current in the load contains alternating component whose basic frequency is equal to the supply frequency. Therefore, an elaborate filtering is required to produce steady direct current. (ii) The a.c. supply delivers power only half the time. Therefore, the output is low. Efficiency Of Half-Wave Rectifier: The ratio of d. c. power output to the applied input a.c. power is known as rectifier efficiency i.e. Rectifier efficiency, = .. .. Consider a half-wave rectifier shown in . Let be the alternating voltage that appears across the secondary winding. Let rf and RL be the diode resistance and load resistance respectively. The diode conducts during positive half-cycles of a.c. supply while no current conduction takes place during negative half-cycles. D.C. Power: The output current is pulsating direct current. Therefore, in order to find d.c. power, average current has to be found out. I= I= I= ∫ 2 ( + 0 2 ( + ) ×2 = = 2 ( + ) ( + ) [− ]0 × d.c. power. P dc = 2 2 × = ( ) × ……… (i) a.c. power input : The a.c. power input is given by : 2 Pac = ( + ) For a half-wave rectified wave. I rms = 2 Pac = 2 …… (ii) ( ) × ( + ) 2 *Avergae value = ℎ = ∫0 2 ** It may be remembered that the area of one-half cycle of a sinusoidal wave is twice the peak value. Thus in this case, peak value is and, therefore, area of one-half cycle is *Iav = Idc= 22 = I *Rectifier efficiency = .. = .. ( )2 × 2 ( ) ( + ) 2 = + = 1 1+ The efficiency will be maximum if rf is negligible as compared to RL Max. rectifier efficiency = 40.6% This shows that in half-wave rectification, a maximum of 40.6% of a.c. power is converted into d.c. power. 2. Full-Wave Rectifier: In full-wave rectification, current flows through the load in the same direction for both halfcycles of input a.c- voltage. This can be achieved with two diodes working alternately. For the positive half-cycle of input voltage, one diode supplies current to the load and for the negative half-cycle, the other diode does so current being always in the same direction through the load. Therefore, a full-wave rectifier utilises both half-cycles of input a.c. voltage to produce the d.c. output. The following two circuits are commonly used for full-wave rectification ( i ) Centre-tap full-wave rectifier (ii) Full-wave bridge Rectifier (i.) Centre-Tap Full-Wave Rectifier: The circuit employs two diodes and as shown in Fig. A centre tapped secondary winding is used with two diodes connected so that each uses one half-cycle of input a.c. voltage.In other words, diode utilises the a.c. voltage appearing across the upper half (OA) of secondary winding for rectification while diode , uses the lower half winding B. Fig. Operation: During the positive half-cycle of secondary voltage, the end A of the secondary winding becomes positive and end B negative. This makes the diode , forward biased and diode D2 reverse biased. Therefore diode D1 conducts while diode D2 does not. The conventional current flow is through diode D1 load resistor RL and the upper half of secondary winding as shown by the dotted arrows. During the negative half-cycle, end A of the secondary winding becomes negative and end B positive. Therefore, diode D2 conducts while diode D1 does not. The conventional current flow is through diode D2 load and lower half winding as shown by solid arrows. Referring to Fig. it may be seen that current in the load RL is in the same direction for both half-cycles of input a.c. voltage. Therefore, d.c. is obtained across the load . Also polarities of the d.c. output across the load should be noted. Peak Inverse Voltage: Suppose Vm is the maximum voltage across the half secondary winding. shows the circuit at the instant secondary voltage reaches its maximum value in the positive direction. At this instant, diodeD1 is conducting while diode D1 is non-conducting . Disadvantages: (i) It is difficult to locate the center tap on secondary winding. (ii) The output is small as each diode utilises only one-half of the transformer secondary Voltage. (iii) Diodes used must have high peak inverse voltage. (ii.) Full-Wave Bridge Rectifier: The need a centre tapped power transformer eliminated in the bridge rectifier. It contain four diodes D1, D2, D3, and D4, connected to, form bridge as shown in Fig. The a.c. supply to be rectified is applied to the diagonally opposite ends of the bridge through the transformer. Between other two ends of the bridge, the load resistance RL is connected. Operation: During the positive half-cycle of secondary voltage, the end P of the secondary winding becomes positive and end Q negative. This makes diodes D1 and D3 forward biased while diodes D2 and D4 are reverse biased. Therefore, only diodes D1 and D3 conduct. These two diodes will be in series through the load RL as shown in Fig. (i) The conventional current flow is shown by dotted arrows. It may be seen that current flows from A to B through the load RL During the negative half-cycle of secondary voltage, end P becomes negative and end Q positive. This makes diodes D2 and D4 forward biased whereas diodes D1and D3 are reverse biased. Therefore, only diodes D2 and D4 conduct These two diodes will he in series through the load RL as shown in Fig. (ii). The current flow is shown by the solid arrows. It may be seen that again current flows from A to B through the load i.e. in the same direction as for the positive half-cycle. Therefore, d.c output is obtained across load RL. Peak inverse voltage. The peak inverse voltage (PTV) of each diode is equal to the maximum secondary voltage of transformer. Suppose during half cycle of input a.c, end P of secondary is positive end Q negative. Under such condition, diode D1 and D3 are forward biased while diode D2 and D4 are reversed biased. Since the diodes are considered ideal D1 and D3 can be replaced by wire as shown in Fig. (i). This circuit is the same as shown in Fig. (ii)Referring to it is clear that two reserve based diodes (D2 and D4 ) and the secondary of transformer are in parallel. Hence PIV of each diode (D2 and D4 ) is equal is to the maximum voltage (Vm ) across the secondary. Similarly, during the next half cycle, (D2 and D4 ) are forward biased while D1 and D3 will be reserve biased. It is very easy to see that reserve voltage across D1 and D3 is equal to Vm. Advantages: (i) The need for center-tapped transformer is eliminated. (ii) The output is twice that of the center-tap for the same secondary voltage. (iii) The PIV is one-half that of the centre-tap circuit. Disadvantages: (i) It requires four diodes. (ii) As during each half-cycle of a.c. input two diodes that conduct are in series, therefore, voltage drop in the internal resistance of the rectifying unit will be twice as great as in the centre tap circuit. This is objectionable when secondary voltage is small. Efficiency Of Full-Wave Rectifier: Fig. show the process of all wave rectification. Let v= Vm sin be the a.c. voltage to be rectified. Let rf and RL be the diode resistance and load resistance respectively. Obviously, the rectifier will current through the load in the same direction for both half-cycle of input a.c. voltage. The instantaneous current I is given by : i= + = + d.c. output power. The output current is pulsating direct current. Therefore, in order to find the d.c. power, average current has to be found out. From the elementary knowledge of electrical engineering, = 2 d.c. power output = I2dc x = 2 x a.c. input power. The a.c. input power is given by : = I2rms + For a full wave rectifier wave, Irms = Im √2 ∴ = ( + ) √2 full –wave rectification efficiency, Η = = = 82 × (2 /)2 √2 + ( + ) = 0.812 + = 0.812 1+ The efficiency will be maximum is negligible as compared to Maximum efficiency = 81.2% This is double the efficiency due to half-wave rectifier. Therefore, a full – wave rectifier is twice as effective as a half – wave rectifier. Comparision Of Rectifier: HALF WAVE FULLWAVE FULLWAVE 1. No. of diode 1 2 4 2. Transformer necessary no Yes no 3. Max. Efficiency 40.6% 81.2% 81.2% 4. Ripple factor 1.21 0.48 0.48 5. Output frequency fn 2fn 2fn 6. PIV Vm 2 Vm 2 Vm RESULT: Study of crystal Diode Rectifier is completed. EXPERIMENT NO. 4 “STUDY OF DESIGN RULES A PCB” AIM: Study of printed board (PCB). THEORY: Designing of printed circuit board. The design of a PCB can be considered as the last step in electronic circuit design as a first major step in production of PCB’s. The designing of PCB of the designing of the layout followed by the generation or preparation of the artwork. The layout therefore should include all relevant aspects and details of the PCB design, the artwork preparation brings it to the from required for the production process. We can say that layout design is the stage where engineering capacity with creating are the governing input while practical technical common have to be predominant in artwork preparation. Layout planning: The layout of a PCB as to incorporate all the information on the board before one can artwork preparation. This means that a concept, the clearly define all the details of the circuit & partly also the final equipment, is a prerequisite before the actual layout can start. Layout Scale: Depending on the accuracy required artwork should be produced at 1:1 or 2:1 or even 4:1 scale. Layout Approaches: Simple approach with sketching of components:-The method work with the simplest of mean by a sheet of paper with a gride printed on it, a pencil,& an eraser are the material required. Grid sheet is placed under tracing which makes all the information drawn on the grid sheet & tracing sheet simultaneously available to the designer. PROCEDURE & GENERAL RULES : 1) Prepare each & every layout as viewed from component side, stopside to avoid to confusion. 2) Design layout only when clear circuit diagram is available. 3) Among components, larger ones are placed first & space in between is filled with smaller. 4) The layout can first roughly sketched Layout Sketch: Layout sketch contains all contains relevant information for preparation of artwork besides that it should also contains information about component holes, conductor width & minimum spacing to be provided. Standard hole Standard width 1).8mm 2).1mm 3).1mm 4).3.2mm 1).5mm 2).1mm 3).2mm 4) mm Layout Check : After layout design is completed a thorough check is carried out to ensure that all requirement have been considered suitable. ARTWORK: Basic approach: There are five approaches for generation of PCB artwork. 1) Ink drawing on white cardboard sheet. 2) Black taping on one transparent base foil. 3) Red / blue taping on one transparent base foil. 4) Black taping on diazo films. 5) Cut-and –slip artwork. Among five black taping approach is the most versatile method. Black Taping On Transparent Base Foils: With the availability of self – adhesive or transfer type pads & adhesive precision tapes to the artwork designer, the pads are two types – 1) Self Adhesive Pads: These pads are supplied sticking on a backing paper. To fix artwork base sheet we have to peel it off from the backing. 2) Transfer Pads: The pads are printed on a thin adhesive film of a typically of a 10µm thickness it is mounted on transparent carrier striped & then transferred on the network base of with a stick on the carrier strip while the pads is exactly pointed. Taps: Self adhesive precision taps are available in a wide range of width They are supplied on the rolls & have a width tolerance of as little as depending on the manufacture. Artwork base foil:- The artwork base foil mainly used today are polyester film which provided an excellent dimensional stability 100µmm polyester thickness is considered as minimum for artwork. PRACTICAL HINTS: Order of network generation :- After fixing of artwork base foil table PCB outline are identified by suitable corner making with 1mm thickness. The taps are fixed outline the PCB area the inner tape edge represented the cutting edge. The next step fixing of all pads. The pads placed are always exactly properly central grid. Tap Handing: As a generally rule call the tape rolls & artwork pads should always be kept in their original package. This is necessary to protect from dirt & dust. The pads center nodes are kept free. Reduction scale of artwork to actual PCB size. Dimensional stability of the film master. There are again factor influencing the dimensional stability of a film master such as temperature variation changing in relative humidity. Process Effect: ageing. Round Bent Or Cut Corner: As they easily be accomplished with block steps have the advantages that no tape joining. Artwork Check : it is very essential where the work is comprehend a through check, is carried out. Master File Preparation: the film negative is used direct exposure of the light screen is called film master. Desired Feature Of Film Used For PCB Film Master:The film used should be high contrast capability only fully black & fully transparent portion should be produced. The density of the film is to be 2.5 The emulation should be scratch from to a high degree. Since a high resolution power maintain it make a fine emulsion necessary. Dimensional Stability Of Master Film: The absolute dimensional accuracy of a film master is determine byAccuracy of origin artwork Dimensional stability of the network RESULT : Study of PCB is completed. EXPERIMENT NO. 5 “DESIGN OF PCB” AIM: Design of PCB. PROCEDURE: STEP 1: Print the layout on a transparent & mark the front side of it. STEP 2: Take one-one glass of water in separate three trey's. STEP 3: Add powder A (two tea spoon) & powder B (two tea spoon) [i.e. film developer powder" ] in first try & mixed it gently in water . STEP 4: Add “lith film fixer" (2 teaspoon) with Water III third trey. STEP 5: Take "lith film paper" (black in colour) sheet & cut it as per the size of the transparent sheet . STEP 6: Put the transparent sheet (that is positive) font size up, in “film maker machine” i.e. is negative machine) & put this sheet file lith film paper from the green side on above the transparent sheet & white side upwords. STEP 7: "ON" the film maker machine for about 5 sec and take out die lith film paper nut of the machine. STEP 8 : Dip this, lith film paper in solution (i.e. trey 1) for about I to 1.5 nun & shake it. STEP 9 : now wash it in water (in trey 2) for about 1 min STEP 10: put the lith film paper in fixer soluti0il (in trey 3) for about 1 min only. . STEP 11: Now wash this with water. STEP 12: Now put this lith film paper in "prot-cure machine" (i.e. oven) for about 5 min. (at the temp (4O degree centigrade) to draw it. STEP 13: Now cut the piece of "copper plate" with tile help of "shearing machine" according tile size of circuit . STEP 14: Rub this copper plate with "steel woul" In remove the dust from copper. STEP 15: Now dip this copper plate in "photo-resist" in "dip coating machine" once. STEP 16: Now draw this copper plate in "},r0to~-cure machine" for about 5 min. to draw it STEP 17: Now put this copper plate and above it lith film paper in “U.V exposure machine” & on the light for about 2.5 min. STEP 18: Take "PCB developer solution" (white) in jar 1& "photo dye" (blue dry developer) in jar 2. STEP .19: Dip the copperplate in the PCB developer solution in jar 1 for about 1.5 min. STEP 20: Now wash this copper plate with waler. STEP 21: Now again dip the copper plate in blue dye (in-jar 2) Once for a sec. & again wash it with water. STEP 22: Now put this copper plate in "Etching" machine (having solution ofFcC13 at a temp. of 40 degree centigrade) for about 5 min. STEP 23: Take out this copper plate from the "Etching machine" again wash it with water & draw it with some cloth. STEP 24: Rub this copper plate with the "steel woul" so that the CKT start becomes of "copper colour". STEP 25: Now drill this copper plate with "drilling machine". STEP 26: Now put the "flux" on copper plate &,- pass this copper plate through [lie "R'I'M machine" so that (lie solder Will be pasted on the ckt sort. STEP 27: Cut the extra portion of this Copper plate with tile help of the "shearing machine". STEP 28: Now the P('13 is prepared ('M- soldering the different components. RESULT : Design of PCB is completed. PRECAUTIONS: 1. Always open the lith film paper in a dark room. 2. Don't touch the film paper by hand unless & until the image of the circut is being fixed. 3. Always open the dip coating machine in dark. 4. Don't touch the copper clad by hand before the etching is being done. 5. Don't raise the tcmp. of RTM machine above to 206 degree centigrade. EXPERIMENT NO. 6 “ETCHING AND DRILLING OF PCB” AIM: Etching and drilling of PCB. THEORY: In the all subtractive PCB processes, etching is one of the important steps: The final copper pattern is formed by removel of all the unwanted copper, which is not protected by an etch resist. This looks very simple at first glance but in practice there are factor like underetching and overhang which complicate the matter especially in the production of final and precise PCBs. Underetching: During the etching process it is expected that the etching progress vertically. However, in practice there is also an etching action side away which attacks the pattern below the etch resist. If the etching action is not stopped immediately after all the unwanted copper has been removed, underetching, especially when screen- or photoresists are used, is the etch factor, defined as the ratio of etching depth (copper foil thickness) to the width of the side attack (Fig.) Overhang: The exact control over conductor widths is further complicated where metal etch resists are used i.e., in pattern plating processesThe metal plating built up shows growt sideways also, resulting in overhang (Fig.). The difficulties with overhang are much reduced with the use of dry-film resists. Dry-film resists are available in thicknesses of as much as 70 µm and can therefore act as an effective barrier against sideway growth of plating layers. Another effective remedy against overhang, if solder-metal etch resist is applied, is the fusing operation, in which the solder plating is melted after etching and covers thereafter the side flanks of the copper conductors. In critical cases, the influences of overhang and underetching are compensated for by modifying the conductor widths already on the film master. Etching Machines: Etching of PCBs, as required in modern electronic equipment production, is usually done in spray-type etching machines. Tank or bubble etching, in which the boards, kept in a rack, were lowered and fully immersed into the agitated etchant, has almost disappeared. Tank etching could not provide the fast, precise and uniform etching which is desired in today's electronic equipment production. Another disappearing method is the etching with splash-type etching machines. In these machines, the etchant is thrown by centrifugal forces via a rotating device in the centre of the etching machine onto the surface of the surrounding boards. Spray Etching: In spray etching, the etchant is pumped under pressure from the pump via a pipe network to the nozzles and from there gets -splashed onto the boards. Spray etching machines can offer very high etching uniformity and a fast etching rate. The high etching uniformity is achieved by spraying through a full number of equally distributed nozzles in ring supply. A high etching rate is the result of the continuous flow of fresh etchant over the boards and the air oxygen absorbed in the etchant through spraying. From the design point of view, spray etching machines can be classified into the laboratory type and the conveyorised etching machines. Laboratory-type spray etchers: They are preferably used in a batch-type production setup. The boards are kept in a frame for the complete etching process and the following water rinse. The number of boards going for etching with each loading operation depends on the size of the frame and the size of the boards. This is the limiting factor in the productivity rate of laboratory-type etchers. Such etchers are available for single-sided etching and for double sided etching which also permits back-to-back etching of twice the quantity of single-sided boards. There are designs available with a fixed frame to keep the boards but with rotating frames also (swash plates) for highest etching uniformity. Conveyorised etching machines: Machines of this type are employed today in all major PCB industries because of their high productivity (up to several square meters of laminates per hour), their excellent etching uniformity (designs with adjustable nozzles for regularity down to ± 1%) and the short etching time required (faster conveyor speed). There are designs with fixed nozzles but for high etching uniformity, wiping or oscillating nozzles are used. In PCB production transfer-lines, where the etching machine is the speed limiting factor, there are often two or even three etching machines (modules) cascaded to utilise the full capacity of all the other equipment in the line. The typical material used for spray-type etching machines is PVC, a plastic which withstands almost all the different etchants currently used. PVC can easily be machined, bent, welded and adhesively joined. PVC is available in transparent or in different colors and is reasonably priced. It can withstand continuous etchant temperatures up to 60° C until it starts 'softening. However, for safety, heating elements are usually connected via a safety relay which switches off at 5758oC.The etching chambers are covered with transparent PVC. This permits control of the `etching process. Furthermore, removable top covers are secured with an automatic pump-off switch to disable an accidental removal of the cover while spraying is on. Metal parts in contact with the etchant have to be made in titanium. Heaters are encapsulated either in titanium or in quartz glass. Sophisticated designs also include etchant cooling to have a perfect control on the etchant temperature. Etchant pump inlets are usually provided with a filter to protect the pump from crystallised salts or undissolved pieces. If wiping or oscillating nozzles are used, the movement should be synchronised with the conveyor speed to provide also the high etching uniformity when thin copper layers are etched at a faster conveyor speed. Comparison Of Echants: FACTOR Corrosiveness ETCHANT Neutrali- Toxicity sation Required Operational ventilation Cost incl. disposal Disposal Pproblem Ferric High Medium Low Low Medium chloride High Low Medium Medium Low Cupric High High High High High chloride High Medium Medium High High Chromic acid Alkaline Ferric Chloride: Ferric chloride shall be explained here first because it is an etchant very simple to use especially in small-scale PCB production. In high-volume production, ferric chloride is not of much importance because it can hardly be regenerated and it attacks the common metal etch-resists. Chemistry Free acid to attack the copper is formed by the hydrolysis reaction. FeC13 + 3H2O — Fe (OH)3 + 3HCL The copper is oxidised by the ferric ions, forming cuprous chloride (CuCI) and ferrous chloride (FeCl2). FeCl3 + Cu ~ FeCl2+ CuCI (2) Cuprous chloride (CuCI) oxidises further in the etching solution to cupric chloride (CuCl2). FeC13 + CuCI – FeCl2 + CuCl2 The built-up cupric chloride (CuCl2) itself reacts also with copper and forms cuprous chloride (CuCI). Advantages: The high corrosive power of ferric chloride leads to short etching times and little underetching. Ferric chloride matches well to photo- and screen-printed resists, both for solvent and alkali soluble ones. Also, shelf-life is very long, a feature important for small set-ups where only occasional etching is done. Ferric chloride can dissolve copper up to approximately, 120 g/ l(practical limit) if the prolonged etching time is acceptable. Proprietary ferric chloride formulations include anti-foaming agents and other additives. They give the advantage of less odour and fuming, more even etching due to better surface wetting, longer life and less slime formation. Disadvantages: A major drawback of ferric chloride etching is the impossibility to regenerate it economically. This practically rules out its use in bigger set-ups where a constant etching speed is a must. The dependence of etching time versus copper content in the etchant is shown in Fig. 16>6. Metallic etch resists are attacked by ferric chloride which rules it out as an etchant for pattern plating processes. The copper reclamation of the spent etchant is not possible economically. OPERATION: Ferric chloride is available as lump Fe C13&6H20 as aqueous solution with or without additives. The working concentration can vary within certain limits I Low etching Optimum High etching Power power Fe Cl3 [g/1] 365 542 350 608 Fe Cl3 [% weight ] 28 34 38 42 Specific gravity [ kg/ dm3] 1.275 1.353 1.402 1.450 All the values are related to room temperature (20-25°C) In practical etching with ferric chloride, it is essential to know about the conditions and state of the etchant. Control is easiest done by observing the etching time while other parameters like etchant temperature, copper foil thickness and etching pressure are kept constant. Absolute values for the etching time range cannot be given since this depends on the equipment and solution strength used for etching. But practical saturation is somewhere around, when etching time is two to three times what it was with fresh mix-up. To know the exact copper content in the etchant, a chemical analysis has to be done. Less accurate but useful in practice is the etchant colour comparison (colometric comparison) with standard solutions of a known copper content. Best results are obtained when sample and standard solutions are in a 2 : 1 dilution. Standard solutions for colometric comparison can easily be self-prepared; however, the initial FeCl3 content must for comparable results, be the same both for the sample as well as the standard solution. In order to increase the copper solving capacity and to bring the etching time slightly down, often Hydrochloric acid (HCI) is added, after the copper content has reached about 80g/l. Muriatic acid acts simultaneously against excessive sludge formation. The concentrated muriatic acid can be added up to a maximum of 10 % of the etchant volume. To etch 1 kg of copper, 5.1 kg of feric chloride will be consumed. Etching temperatures should be in the range of 20-45'C. Drilling: Drilling of component mounting holes into PCBs is by far the most important mechanical machining operation in PCB production processes. Holes are made by drilling wherever a superior hole finish for plated-through hole processes is required and where the tooling costs for a punching tool cannot be justified. Therefore drilling is applied by all the professionalgrade PCB manufacturers and generally in all smaller PCB production plants and laboratories. The importance of hole drilling into PCBs has further gone up with electronic component miniaturisation and its need for smaller hole diameters (diameter less than half the board thickness) and higher package density where hole punching is practically ruled out. RESULT: Study of Etching and Drilling process is completed. EXPERIMENT NO. 7 “PRINCIPLES OF SOLDER & SOLDERING TECHNIQUES” AIM: Principles of solder & soldering techniques. APPARATUS USED: (1). Soldering iron flux. (2). Plier. (3). Diode. (4). Resistance. (5). Nutbolt. (6). General purpose pcb board. (7). Multimeter. (8). Stand step down transformer. PROCEDURE: 1. Mount on pcb as per the lay out. 2. Soldering of all the components as per diagram of pcb. 3. Diode polarity should be same while connecting. THEORY: Soldering: ‘Soldering is a process for the joining of metal parts with the aid of a molten metal (solder), where the melting temperature is situated below that of the material joined, and whereby the surface of the parts are wetted, without them becoming molten'. (The definition is from the international Institute of Welding, 1955.) Sofi soldering or soldering generally implies that the joining process occurs at temperatures below 450° C: The filler metal (solder) the base metals and gets drawn, by capillary action, into the gap between them. This process forms a metallurgical bond between the parts of the joint. Therefore, solder acts by wetting of the base metal surfaces forming the joint flowing between these surfaces which result in a completely filled space between them Metallurgical bonding to these surfaces when solidified. If the basic constituents in making a soldered joint are represented in a diagram, it will look as follows: On the face of this diagram, soldering is a simple operation.It consists of the relative positioning of the surfaces to be joined, wetting of these surfaces with molten solder and allowing the solder to cool down until it has solidified. It is essential that the surfaces to be joined are sufficiently wetted in order to ensure adequate adhesion and a sound electrical contact (Fig. 20-i). Wetting, however, is possible only if the solder can come into direct contact with the metal surfaces to be solderedThe atoms of the solder must be able to come within atomic distances from the component parts to be soldered to ensure sufficient attraction. Any contamination firmly adhering to the surfaces to be soldered (e.g., oxides) will act as a barrier to metallic continuity and thus prevent wetting. A drop of molten solder on a contaminated surface will behave like an isolated drop of mercury . After solidifying, this drop can easily loosen when subjected to mechanical shocks. The quality of the electrical contact in such a case will be very poor The current has to pass through more or less insulating oxide film. If the surfaces are clean (with their metal atoms thus lying immediately at the interface), wetting takes place and the solder will flow across the surface . The solder atoms are now able to come very close to the atoms of the base metal; hence, they are attracted. The solder thus gradually diffuses into the-solid metal and provides a good adhesion. The direct metallic contact with the atoms of the base metal, furthermore, ensures a good electrical contact. But strength and reliability of solder joints are not only dependent on the wetting: Design/ geometry of the joint as well as presence or absence of corrosive substances are the influencing factors. The amount of wetting and spreading of solder depends on the free energies of the various surfaces involved in the soldering system. This situation could be represented as shown in Fig. When a molten drop of solder is placed on a hot metallic surface in the presence flux, a configuration is formed which ideally consists of 3 forces (A, B, C) resulting from the surface energies or rather interfacial surface energies. Every interface (metal-air, liquid solder-air, metalliquid solder) has a quantity of energy which is proportional to the size of the surface. As every system tends towards a minimum of free energy, these interfaces tend to become as small as possible. While doing so, however, they counteract one another: Reduction of one interface leads to the enlargement of another. The boundary lines between the 3 phases (G I, G2, G3) adjust themselves equally and in such a way that the whole configuration has a minimum of interfacial surface energy. The three imaginary forces on the boundary lines are as follows: Force A-- for the interface between the part to be soldered and the air (also referred to as the surface tension of the metal) Force B- for the interface between the solder and the air (also referred to as the surface tension ( the solder) Force C- for the interface between the solder and the component to be soldered (also referred to as the solder metal interfacial tension). Starting from the dihedral angle B, force B can be resolved into - a force B X, acting along the surface, and - a force B X , perpendicular to the surface. This component force is compensated for by the rigidity of the base metal. If the forces acting on the border of the solder drop and along the surface of the component to be soldered keep each other in equilibrium, neither further wetting nor dewetting will occur. In this case, A=B x cos + C and thus cos = − Cos is a measure of the wetting. If cos is large, then small and wetting is good. Conversely, if cos B is small, then B is large and wetting is poor. Consequently, wetting is influenced by changes in the forces A, B, C. Such changes could occur as a result of chemical changes on the surface of either the solder or the component to be soldered. Such chemical changes can, for instance, be caused by the diffusion of tin from the solder into the base metal, thereby forming alloys. These alloys are solid solutions, occasionally with chemical compound of the base metal (e.g. Cu,Sn on copper or Fe,Sn on iron base metal). due to the replacement of the air by another gas or by vapour (e.g., flux). It is generally accepted that a good flow is obtained when the dihedral angle B is less than 10° Solder Alloys: All the allows have a low melting with the liquidus below the melting point of pure lead (327C). The bulk of the solders even melt below 250'C. In any solder system, lead (Pb) is mainly used as a dilutant only to lower the costs. The wetting phenomenon is dependent on tin only The higher the tin content, the better would be the wetting. There are various systems of solders in practical use the most important ones are mentioned below. Tin-Lead The binary mixture of tin and lead constitutes a simple and classical eutectic system with the eutectic point at 61.9% tin and the eutectic temperature at 183°C. Point C in Fig. is the eutectic point. Compositions which vary from the eutectic one have rather a melting range than a melting or solidification point The solder which corresponds to the point D, as an example, has a melting range of 183-273'C and a tin content of 30%. If a solder has a melting range, it is only fully molten at the highest temperature of this range. Similarly, cooling off must come down to the lowest temperature of the range for a complete solidification of the alloy above this temperature, the mechanical strength is zero. If a soldered joint is disturbed during solidification, it becomes unreliable. Therefore, one generally chooses a solder with a narrow melting range for the soldering of electronic components since wide-range solders increase the soldering time. 100% tin Specific gravity [ kg / dm2] Latent heat of melting [cal /g] Specific heat [cal / oC] Final melting point [oC] Required energy from 20 oC [cal / g] Required energy from 20 oC [cal / cm3] 7.3 14 0.06 232 30 219 63% tin 37% lead 8.4 13 0.05 183 24 292 37% tin 63% lead 9.5 10.5 0.04 257 21 200 100% lead 11.4 5.5 0.03 327 17 191 Other Solder Systems: Tin-Antimony: From the metallurgical equilibrium diagram for the binary tin-antimony alloy system, it can be observed that only a composition with 5% Sb by weight forms a single-phase solid solution in tin. Tin-Lead-Antimony: For the most commonly used `antimonial' tin-lead solder with a tin content of 30-50%, the liquidus temperature is slightly raised compared to binary system. The presence of Sb generally restricts the flow of solder. Tin-Silver: In this system, the eutectic point is reached with 3.5% Ag/96.5% Tin sb giving an eutectic temperature of 221'C. A higher silver content results in very steep rise in liquid us temperature. Tin-Lead-Silver: For solder joints which must possess some strength at elevated temperatures, an alloy of 97% Pb/ 2.0% Ag/ 1.0% Sn is used. Tin is added because of the poor wetting qualities of lead. Tin-Zinc: These solders are specifically used for soldering of aluminium. Tin-zinc represents a simple eutectic system with the eutectic point at 8.9% Zn/91.1 % Sn resulting in a melting temperature of 198°C Low-Melting-Point (Fusible) Alloys: The most commonly used alloy of this category, featuring in several specifications, is the ternary eutectic composition 50% Sn/ 32% Pb/ 18% Cd which melts at a temperature of 145'C. The use of cadmium in solders, however, poses certain health hazards and will be dealt with separately.Another useful but quarternary system, melting at only 70'C, is 12.5% Sn/ 25% Pb/ 50% Bi/ 12.5% Cd. This alloy is also called wood metal. Special Solders: Near-eutectic Sn-Pb solder dissolves such metals like copper and silver, especially if these are present in thin films. Therefore, in order to reduce the solubility of these metals, the neareutectic Sn-Pb system is loaded with a small percentage of these metals, i.e., 1.5% Cu or 2% Ag (% weight). A gold-tin eutectic system, containing 20% Sn and melting at 280°C, has a special application in soldering of semiconductors. Pure tin, tin-cadmium, cadmium-silver or cadmiumzinc-silver systems are some of the other systems used mainly for obtaining a higher strength of soldered joints. Influence of Impurities on Tin-Lead Solder: Copper: The final melting point of solder is very much raised even by only a very small addition of copper. The molten solder becomes easily dirty and sticks to all nonmetallic materials. Consequently, such solder has much less capillary action which is a disadvantage. The appearance of copper-containing solder is also less fine because of the tin-copper crystals. The copper solubility of solder with 60% Sn/40% Pb is around 0.4% at the temperature of 2500C. If the copper content is above 0.29%, the solder will show grittiness. It is therefore suggested to maintain the copper content in eutectic Sn-Pb solder at less than 0.25%. Iron: The iron content should be kept below 0.1%. Antimonv: Above 2.5% (% weight), flow properties are reduced but wetting does not decrease significantly. Aluminium: It is an oxide promoting element and its presence above 0.001% results in surface deterioration of molten solder (grittiness). Zinc: Zinc also reduces the flow of solder and its content should be kept below 0.02%. Mechanical Properties of Solder Alloys: The mechanical strength of solder is comparatively small, amounting to only a few percents of that of normal construction materials. The mechanical strength is particularly small at elevated temperatures. In this connection, room temperature must be considered as already a relatively high temperature for these alloys. Shear strength Composition 100% Sn 62% Sn / 38% Pb 40% Sn / 60% Pb 10% Sn / 90% Pb Shear strength [N / mm2] 130oC 100 90 70 50 20oC 20 30 30 20 150oC 7 8 9 10 Tensile strength: Results of tensile strength are dependent on the rate of loading the test specimen. As an example, the increase of the loading rate by a factor 400 at 20'C, results in an increase of the tensile strength by a factor 4. Moreover, the permissible stress in soldered joints depends to a large degree on the geometry of the gap between the parts joined by the solder and on the quantity of solder in the joint. Note that a large quantity ofsolder does not necessarily result in stronger joints. Creep strength: At long-term loading, the mechanical strength decreases further. This is 10 hrs loading Mm² 1,000 hrs loading 100,00hrs By alloying the solder with antimony, the strength can be increased to some extent but it does not change essentially. Note: Tin-lead solders are generally not suitable for mechanical joining if the load at 20°C lies permanently above 1 N/mm2. In case of higher temperature, the mechanical strength of solder gets reduced to a fraction of the strength at room temperature (e.g., a strength of N,/ mm2 at 20ºC will become only 0. 1 N/ mm2 at 10° C). Where mechanically stronger connections are required some other mechanical fixation devices have to be used along with soldering. Soldering Fluxes: During the soldering operation, an auxiliary medium is mostly used to increase the. properties of molten solder or to improve the degree of wetting. Such a medium is calledflux. Following characteristics are required in a flux: It should provide a liquid cover over the materials and exclude air up to the soldering temperature. It should dissolve any oxide on the metal surface or on the solder and carry such unwanted elements away. It should be readily displaced from the metal by the molten soldering operation. Residues should be removable after completion of the solder. Fluxes are usually divided into 3 groups according to the nature of their residues: corrosive, intermediate and non-corrosive fluxes. The corrosive or acid fluxes consist mainly of inorganic acids and salts. They are generally used to solder `difficult' materials, e.g., where rapid wetting by molten solder is required of where the surface conditions are critical. Corrosive fluxes may also be needed when using lead based solders. ' Note: Because of the corrosive nature of such fluxes, they should only be used on components and in conditions where the residues can be completely removed after soldering. The most basic ingredient in corrosive fluxes is zinc chloride since it melts at solderin temperatures and forms a convenient source to perform fluxing action. Ammonium' chloride, sodium chloride and tin chloride may also be present to increase the activity or lower the fusion point of the mixture. Composition (% weight) EXAMPLE: Melting temp. 87% ZnCl2 / 13% NH,CI 232°C 73% ZnC12 / 27% NH4CI 180°C 82% ZnC12 / 18% NaCI 262°C 23% ZnC12 / 77% SnC12 171°C As solvent, mostly water is used but also bromides or fluorides are applied. Phosphoric acid is particularly useful for soldering of stainless steel. Two typical general-purpose, corrosive flux compositions shall be given: 1) ZnCl2 100 g/ l 2) ZnCl2 33 g/ l NH4CI 10 g/1 NaCI 8 g/1 HCI 25 ml/l NH4Cl 4 g/l HCI 10 ml/l The rapid heating of aqueous fluxes produce spattering due to violent evolution of steam. This can be reduced by using polyethylene glycol as a solvent, perhaps mixed with isopropanol. A suitable mixture is 70% polyethylene glycol and 30% iso-propanol (% volume). Such solvents boil gently over a wide range of temperature. Residues of these halogen base corrosive fluxes, however, have often caused serious corrosion problems: These residues, being hygroscopic in nature, absorb water and then get strongly dissociated thus liberating halogen radicals which corrode the base metal. Since the danger of corrosion of the soldered joint with corrosive fluxes is not immediately noticeable, corrosive fluxes should be used only where adequate facilities for water washing are available. It is also possible to reduce the corrosivity to some extent by using petroleum jelly, lanolin, tallon, etc. along with zinc chloride. The proportion used may be around one-third zinc chloride and two-thirds carrier medium (by weight). Note: Because of their corrosive nature, corrosive fluxes are hardly applied in electronics industry. Intermediate Fluxes: They are weaker in nature if compared to inorganic fluxes. They consist of organic acids and certain of their derivatives such as hydrohalides. These fluxes are active at soldering temperature but, with some of them, the activity period is short because they volatile or oxidise. Typical intermediate fluxes are the following: Lactic, oleic, glutamic, stearic and succinic acids. Some of them are active also below soldering temperature. Hydrohalides or amines or amino acids from which halogen acids are liberated during heating, e.g., diethyl ammonium chloride, cetyl pyridinium bromide, anilin hydrochloride or glutamic acid hydrochloride. Amines or amides such as triethanolamine, ethylene diamine and urea, etc. Non-Corrosive Fluxes: For electrical components, electronic assemblies, delicate instruments and all other parts where it is difficult to wash off flux residues after soldering, a non-corrosive flux is required. Pure waterwhite rosin, 20-25% by weight respectively volume, dissolved in a solvent such as iso-propanol, is the closest approach to a non-corrosive flux solution. Colophony or natural rosin (obtained from a particular type of pine wood) contains chiefly abeitic acid and its isomers which become mildly active at soldering temperatures, being a weak acid. It is then in a form which will attack not too strong impurities such as the natural oxide layers of rolled copper. Impurities which will not be removed are the oxides of iron, chromium, aluminium, etc. The principal difference between rosin (colophony) and most other fluxes is due to the following properties: Rosin is not dissociated at room temperature. Rosin is practically insoluble in water; thus rosin cannot form an electrolyte. Non-corrosive fluxes would be the ideal solution if they would show a better activity and a better degree of flow. To overcome these limitations, activators are added to the rosin flux to improve both the degree of flow as well as the activity without significantly increasing the corrosivity or lessening the electrical insulation resistance. Suitable activators are glycol acid, urea and its derivatives, ammonium chloride and halogen containing amines. Such liquid fluxes containing organic halides should not contain more than 0.5% halide, as chloride, based on the solid contents of the flux. Solderability is generally increased in proportion to the halide content in the flux. The most important criteria for the selection of a flux are: -flow promoting properties of the flux solder composition composition of materials to be soldered surrounding in which soldering has to take place soldering technique form in which the flux has to be applied soldering temperature properties of the flux residues possible health hazards. Testing of Fluxes: For measuring flux as well as corrosion characteristics of a flux, a number of methods have been specified. Generally, a known weight of solder and flux are placed on a freshly (chemically) cleaned ,copper sheet and left floating on molten solder for a specified time. The area of solder spread determines the flow-property of the flux. Resistance to corrosion is then assessed by placing the specimen into the humidity chamber at 40°C and 98% RH for a duration of 100 resp. 500 hrs. The presence of green-blue copper corrosion products indicates the corrosion behaviour of the flu x. RESULT : Study of solders &soldering Techniques is completed. EXPERIMENT NO. 8 “WIRING AND FITTING OF POWER SUPPLY IN A CABINET” AIM: Wiring and fitting of power supply along with a meter in cabinet. Apparatus used: (1) Cabinet. (2) Nut, bolts. (3) Wires. (4) Soldering iron. (5) Meters. (6) Multimeter. (7) Bananna switch (S)Transformer. (9) Transformer wiring knobs. (10) Tool kit. THEORY: Transformer prepared (step down 0-9) prepared in the be fitted along with the full wave rectifier (bridge rectifier) is to be fitted in a cabinet along with the meters. PROCEDURE: (1) Transformer is tested and fitted in the cabinet. (2) The power supply PCB prepared is fitted along with the transformer. (3) The connection between the secondary of transformer and power supply PCB is soldered. (4) Finally an ammeter and a voltmeter is fitted in the cabinet for monitoring the output and currents. PRECAUTION: (1) AI1 the connection should be made properly. (2) Voltmeter should be connected parallel with the output voltage. (3) Ammeter should be connected in series with the Output voltage power supply. EXPERIMENT NO. 9 “TESTING OF POWER SUPPLY FABRICATED” AIM: Testing of power supply fabricated. APPARATUS REQUIRED: (1). Test Zig. (2). Power supply. (3). Multimeter. (4). Connecting wires. (5). Single strand wires. THEORY : The power supply prepared is tested on the test zig. The output of the power supply should be approximately 9 volt D.C. and output current should be 500mA approx. power supply tested on load and no load condition with include as well as resistive loads inbuilt in the test zigs. PROCEDURE: (1) Make the connection of power supply with test zig. (2) Then switch on the power supply. (3) Note the voltage reading. PRECAUTION: (1). All the connection should be tight. (2). Switch on the power supply carefully. (3). Note the reading on observation table accurately. RESULT: The output voltage of the power supply will be approximately 9 Volt D.C. and output current will be 500mA approx.