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Basic Skills in Electricity and Electronics Sixth Edition Charles A. Schuler Soldering and Desoldering Including SMT Rework ©2003 Glencoe/McGraw-Hill Acknowledgement: Many of the photos and all of the video clips used in this presentation are courtesy of The Electronic Industries Association 2001 Pennsylvania Avenue Washington, DC 20006 Soldering safety tips • Wear eye protection. • Work in a well-ventilated area. • Do not put wires or solder in your mouth. • Wash your hands when you are finished. Basic requirements • Clean metal surfaces • Clean, tinned soldering iron • Correct solder and flux • Correct temperature and time The actual methods used depend on the tools and materials at hand, the physical nature of the parts and boards, and the skill of the worker. Cleaning with wire brush Tinning the iron Cleaning by wiping tip on a damp sponge A diameter of 0.025 inches is common for most electronic work. Solder Eutectic alloy (lowest melting point) 63% tin, 37% lead 62% tin, 36% lead, 2% silver is preferred for some work Electrostatic discharge (ESD) Potentials as low as 200 volts can cause damage. Wrist strap Conductive work surface A workstation equipped to prevent ESD damage Wrist strap connection point When ESD measures are not available Touch a known earth ground before touching circuits and components. Tools and materials Temperature controlled iron with grounded tip. A magnifier is helpful for inspection. Circuit board holder Solder bridge Desoldering methods Desoldering braid Vacuum pump Vacuum desoldering station Using desoldering braid Click the center of the photo to start the video. Using a vacuum pump Click the center of the photo to start the video. Using a vacuum desoldering station Click the center of the photo to start the video. After device removal Clean the pads with desoldering braid Soldering Apply the iron, then the solder. Remove the solder, then the iron. (About 3 seconds per lead) Click the center of the photo to start the video. Removing a solder bridge with braid Clean the area Spray with flux cleaner and then brush to remove flux residue. Click the center of the photo to start the video. Surface-mount technology (SMT) • Is driven by economics. • Has largely replaced through-hole technology. • Is repaired when cost-effective. • Is repaired often enough that technicians should know about the general procedures. • Is evolving. This dual-inline package (DIP) served well, but …. Board stuffing is the term applied to loading circuit boards with their component parts. Today, board stuffing is automated. It’s done by pick and place robots. The DIP and other through-hole mounted components do not work well with today’s automated assembly techniques. The leads go through holes in the printed circuit board. Surface-mount technology (SMT) • • • • • • Does not require holes in the circuit board. Works well with robotic parts placement. Allows higher component density. Lowers the cost of manufacturing. Improves the reliability of products. Allows smaller and less expensive products. These leads do not need holes and are soldered on the component side of the circuit board. Discrete devices with leads do not lend themselves to robotic placement. These leads can be preformed (bent) to go through the appropriate holes in a printed circuit board but automated board stuffing is a problem. These devices are readily placed with robots. Unleaded discrete devices (also known as chip components) No leads (tinned ends) The end terminals are soldered on the component side of the circuit board. Automated parts placement works well! Used for resistors, capacitors, inductors, diodes, fuses, etc. Small-outline transistor (SOT) Gull-wing leads Used for BJTs, FETs and diodes Small-outline integrated circuit (SOIC) Gull-wing leads Used for ICs … analog, digital and digital memory The preferred removal method for chip components, SOTs and some SOICs is thermal tweezers. (A.P.E. South) Quad flatpack (QFP) Gull-wing leads Used for ICs … mixed signal, digital and microprocessors This QFP is less than 1 inch by 1 inch and has 144 pins! Plastic leaded chip carrier (PLCC) J-leads Can be socketed or directly soldered to PC board The preferred removal method for most SMT ICs is hot air. Click the center of the photo to start the video. Hot air rework station 1000° F 750° F Nozzle 800° F Accepts various nozzles Nozzle shape, size and distance are important factors. Portable hot air devices Electric gun Butane pen Custom tip for quad flat pack removal with suction and hot air Click the center of the photo to start the video. Use braid to clean pads after device removal. Apply liquid flux to the pads Tin the pads with a clean, freshly tinned iron. Apply flux again. Tin a corner pad with solder. Position the new device. Click the center of the photo to start the video. Clean the board with flux remover and a brush. Solder paste consists of powdered solder, solvent and flux. Solder paste dispenser How do you solder a device when the spacing between the pins is only 0.013 to 0.009 inches? 0.007 to 0.011 inches (pin width) Pitch = 0.02 inches (center to center spacing) The space between pins can be as small as 9/1000 of an inch! After soldering two corner pins, solder paste is applied and then hot air is used to melt and flow the solder. Click the center of the photo to start the video. The pitch keeps getting finer! 196 pins! Pitch = center to center lead spacing Ball Grid Array (BGA) Bottom view Side view The connections are underneath the chip. These devices are removed with hot air. The new part is installed with hot air. Accurate positioning is required. Fine pitch devices and BGA devices require a rework station. Rework stations offer hot air, vacuum pickup and precision positioning of the replacement IC. Chipmaster SMT rework station (A.P.E. South) Hot air is used for device removal and reflow device replacement. Vacuum pickup (A.P.E. South) Using desoldering braid to level the pads After removal of a BGA, the pads must be prepared