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TOC
Section 2
Layout Principles
IPC Designer Certification Study Guide
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GLOSSARY QUIT
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Section 2.1
Printed Board and Assembly
Viewing Principles
Layout Principles
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GLOSSARY QUIT
When only through-hole assembly
existed, the viewing principles were
fairly straightforward. One viewed the
board from the component side and
the opposite side was the solder side.
With the advent of surface mounting,
the definition was no longer as clear.
New terms were needed as were new
rules for identifying each side.
Printed Board and Assembly Viewing
Principles - 2.1
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The term primary side replaces
component side, and the term secondary
side replaces solder side. The primary
side is the side of the packaging and
interconnecting structure (printed board)
that is so defined on the master drawing.
It is usually the side that contains the
most complex or the largest number of
components.
Printed Board and Assembly Viewing
Principles - 2.1
TOC
2221
11.2.1
Fig
11-2
D325
4.2.6
Fig
4-2
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In any case, it is the side the designer
determined to be the most critical. The
secondary side is opposite the primary
side, and when viewed from the primary
side, will appear mirrored, as will the
secondary legend and secondary solder
mask.
Printed Board and Assembly Viewing
Principles - 2.1
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The design dictates how each layer should
be named. The designer develops the
assembly drawing so in many cases the
reasons for selecting one side over the
other is related to the intensity of the
assembly operation. It is conceivable that
the primary side could be the side opposite
the through-hole components even though
that side has, for example, only one
component, a microprocessor.
Printed Board and Assembly Viewing
Principles - 2.1
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D325
Fig
4-2
Table
4-1
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That is the side the designer wanted to
view, as the layout proceeded. Primary
and secondary sides should not be
confused with primary and secondary
datums. The datums are used for
dimensioning purposes. The primary
datum plane is that side which is
opposite the primary side.
Printed Board and Assembly Viewing
Principles - 2.1
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Datum features are used to position the
printed board in relation to a set of three
mutually perpendicular planes.
Typically, printed board drawings are
oriented with layer one facing up. This
orientation establishes the backside of
the printed board as the first (primary)
of the three required datum planes
according to ASME Y14.5M.
Printed Board and Assembly Viewing
Principles - 2.1
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The other two datum planes (secondary
and tertiary) are typically established
using holes or etched features of the
board.
These are then the datum features that
set up the planes from which all
dimensions are determined.
Printed Board and Assembly Viewing
Principles - 2.1
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2615
3.3.2
Fig
3-2
2221
5.4.3
Fig
5-5A
thru
5-5E
Fig
5-6
Fig
5-7
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All of these conditions are used to
convey intent to the manufacturer of
the board or the assembler. They take
the information and establish the
panels for board fabrication and board
assembly. Nevertheless, the master
drawing descriptions form the final
accept or reject criterion for the final
product.
Printed Board and Assembly Viewing
Principles - 2.1
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GLOSSARY QUIT
Understanding how layers are viewed and
numbered is important to maintain
consistency in communication between
design and manufacturing.
Usually conductive layers are numbered
sequentially starting with the primary side
as layer one. If there are no conductors or
lands on the primary side, then the next
conductive layer becomes layer one.
Printed Board and Assembly Viewing
Principles - 2.1
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D350
4.5
Fig
4-2
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Nonconductive layers are numbered after
all the conductive layers have been
identified, starting over at the primary
side. Nonconductive layers are those for
which there is an image or dimensional
configuration in the secondary/tertiary
datum planes. Legend, solder mask,
cover coat, or any layer that remains with
the board is numbered according to these
principles.
Printed Board and Assembly Viewing
Principles - 2.1
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Table
4-1
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For example, if a twelve layer board has
solder mask on both sides, the solder
mask near the primary side becomes
layer thirteen. Then the secondary
mask becomes fourteen. The primary
silkscreen becomes fifteen, and the
secondary silkscreen will be sixteen.
Printed Board and Assembly Viewing
Principles - 2.1
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D350
4.5
Fig
4-2
Table
4-1
Extra
1.7f
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Sequential layer conventions only
apply to things that stay with the
board.
Data layers for temporary masking,
solder paste stencils, hole drilling
templates can be numbered in any
fashion because, as of this date, there
is no industry-accepted consensus
standard.
Printed Board and Assembly Viewing
Principles - 2.1
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Section 2.2
Schematic/Logic Transformation
Layout Principles
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There are many characteristics of electronics
that must be understood in order to properly
convert the electrical engineering description
into a working interconnected arrangement of
components.
The Institute of Electrical and Electronic
Engineers (IEEE) has a standard coordinating
committee (number 11) that specifies the
graphic symbols for both discrete electronic
components and integrated circuits.
Schematic/Logic
Transformation - 2.2
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GLOSSARY QUIT
Graphic symbols of discrete components
include schematic symbols for resistors,
capacitors, diodes, connectors, etc.
Graphic symbols for integrated circuits
(ICs) describe each circuit using logic
functions or Boolean equations.
2221
3.4
D325
8.5
Schematic/Logic
Transformation - 2.2
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The documentation for the symbols is published by the
American National Standards Institute (ANSI). The two
primary standards that establish the part to symbol
relationship are:
• ANSI Y32.2 Graphic Symbols for Electrical
and Electronic Diagrams (IEEE STD 315)
Extra
1.8
• ANSI Y32.14 Graphic Symbols for Logic
Diagrams (IEEE STD 91)
Extra
1.8b
Schematic/Logic
Transformation - 2.2
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When converting the symbols to a
physical arrangement, it is important to
use the maximum material condition
(MMC), which is the maximum
component outline of the parts, so that
they do not overlap or interfere with each
other at time of assembly.
Extra
1.8c
Schematic/Logic
Transformation - 2.2
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The component locations on the final board
are important so that the circuit functions as
intended and the arrangement meets all the
physical requirements of the assembly.
One of the primary placement concerns is
that components should not be located in
the guide area or in any area that has been
defined as keepout by the engineering
electromechanical conditions.
Schematic/Logic
Transformation - 2.2
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2221
3.2.1
3.4
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Another major consideration is that of
component locations which have been predefined, for adjustment, heat dissipation or
electrical performance (being near
companion components and similar
components for electronic functionality).
The designer should also be aware if
he/she is responsible for adding pull down
resistors to unused logic pins for signal
noise reduction, or decoupling capacitors
for power management purposes.
Schematic/Logic
Transformation - 2.2
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GLOSSARY QUIT
The ground and power distribution is an
important consideration in the design of
the printed board assembly. If different
grounds are used, the conductors or
planes should be well separated. Digital
and analog circuits have different rules
for positioning the components and the
ground and voltage distribution.
Schematic/Logic
Transformation - 2.2
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GLOSSARY QUIT
Digital circuits are composed of
components that can provide state
information (1 or 0) as a function of the
overall circuit.
Analog circuits are usually made up of
discrete devices, and provide the
waveform characteristics necessary to
describe a circuit.
Schematic/Logic
Transformation - 2.2
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2221
6.1.2
Fig
6-1
Fig
6-2
Fig
6-3
6.1.3
6.1.3.1
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Resistors, capacitors, diodes, transistors,
power transformers, coils, chokes, etc.,
are usually the type of components that
make up an analog circuit.
As opposed to digital circuits, analog
design should have their signal
conductors considered first, and ground
planes or ground conductor connection
considered last.
6.1.3.2
Schematic/Logic
Transformation - 2.2
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When determining the component
arrangement, the shortest possible
conductor length should be used between
devices.
Following the initial placement and
preliminary routing, many CAD systems
provide an image of the high conductor
density areas and show which component
lands have not been connected.
Schematic/Logic
Transformation - 2.2
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Extra
1.9
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This is called the rats-nest view and
permits the designer to move
components to different locations in
order to improve the interconnection
capability. This tool is useful in helping
the designer meet the goal of having the
shortest length of interconnection.
Schematic/Logic
Transformation - 2.2
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Typically, performance areas within a
design are identified by electronic
function.
Fig
6-3
Another good practice is to separate the
board into areas of high, medium, and
low frequency circuits. It is a usual
practice to maintain the high frequency
circuits near the connector so that the
length of the conductor is minimized.
Schematic/Logic
Transformation - 2.2
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2221
6.1.2
Fig
6-2
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But, one should recognize that the
signals to the low frequency circuits
(away from the connector) must have
the conductors route through the high
frequency area. Care must be taken
so that these low level signals do not
degrade the performance of the
critical circuits near the connector.
Schematic/Logic
Transformation - 2.2
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Section 2.3
Schematic and Logic Symbology
Layout Principles
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2221
3.3
Extra
1.10a
D325
10.24
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GLOSSARY QUIT
Logic and schematic diagrams are used to
convey the electrical description of analog and
digital circuit functions.
A logic diagram is the drawing that depicts a
multi-state device implementation of logic
functions with logic symbols and supplementary
notations that show the details of signal flow and
control, but not necessarily the point to point
wiring.
A schematic diagram shows, by means of
graphic symbols, the electrical connections,
components, and functions of a specific circuit.
Schematic and Logic
Symbology - 2.3
TOC
Extra
1.10b
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Both logic and schematic diagrams
use reference designators to indicate
each. The reference designator
consists of one or two letters which
are followed by a specific number that
identifies each unique component in
the circuit.
Schematic and Logic
Symbology - 2.3
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D325
10.16
10.19
10.24
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GLOSSARY QUIT
Examples of component designators are:
resistor (R), capacitor (C), diode (CR or
D), plug (P), socket (J), and delay line
(DL). The letter U is usually used to
signify an IC. The logic reference
designator is also followed by a number,
however, since the U represents an IC
assembly there may be more than one of
a particular function in a single IC
package.
Schematic and Logic
Symbology - 2.3
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GLOSSARY QUIT
As an example, a two input NAND gate
consists of two signals as inputs with one
signal as the output. The logic symbol for
this gate type can be drawn as a graphic
element that describes the logic function.
Extra
1.11
Examples of common logic symbols are
AND, NAND, NOR, OR, INVERTER, and
FLIP FLOP. Each has their own unique
graphic symbol.
Schematic and Logic
Symbology - 2.3
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There is a truth table associated with each
logic symbol to indicate the state of the inputs
necessary to achieve an output.
The standard Dual-Inline Package (DIP), for
example, contains four two input gates. Each
gate has the same characteristics and they
share the same voltage and ground in the IC
package. This characteristic allows the
designer to swap gates within the package in
order to be more efficient interconnecting the
circuit.
Schematic and Logic
Symbology - 2.3
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Extra
1.12
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Gate swapping is permitted when the logic
functions are identical and no other
performance criteria must be maintained.
The original logic reference designator, as
paired by the circuit design engineer, is
then modified during the component
interconnection process. This requires
that the reference designators be
redistributed in order to properly pair the
logic that is in a particular IC package with
its companions.
Schematic and Logic
Symbology - 2.3
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GLOSSARY QUIT
This practice is referred to as back-annotation
and is accomplished once the layout has been
completed. The final logic diagram must
reflect the board component positions. For
example, R12 would designate a specific part
in the parts list, just as U12 would designate a
specific IC package.
It is important to maintain the correct gate
segmentation within an IC package to the way
the gates are interconnected on the final
assembly.
Schematic and Logic
Symbology - 2.3
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The assembly drawing shows the
position of each part by its reference
designator and the bill of materials
indicates which components relate to
which designator. Usually, schematic
and logic designators are assigned on
the schematic/logic diagram in a left to
right, top to bottom sequence.
Schematic and Logic
Symbology - 2.3
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Sometimes the board assembly controls the
number assignment in a similar manner or in
a coordinate matrix fashion where parts
within a coordinate zone of the board
assembly are assigned the number of that
zone. In this process, letters will be used
along the board edge in one axis, while
numbers will be used along the board edge
in the other axis to establish a coordinate
system, much like that which is used on
maps.
Schematic and Logic
Symbology - 2.3
TOC
D325
8.3
8.5
10.8
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In either case, the intent is to make
finding the correct gate or part easier
during trouble shooting or maintenance.
Sometimes it is necessary to show
components of a separate subassembly.
This is for reference only and these
components are typically enclosed by
phantom lines.
Schematic and Logic
Symbology - 2.3
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Section 2.4
Functional Electrical Characteristics
Layout Principles
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GLOSSARY QUIT
In electronic systems the flow of electricity
is called current.
The current-carrying capacity is the
maximum current that can be carried
continuously by a conductor under
specific conditions without causing
degradation of electrical and mechanical
properties of the product.
Functional Electrical
Characteristics - 2.4
TOC
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Current is measured in amperes.
The resistance to the flow of electrons is
called resistance and is measured in
ohms.
In order to move electrons through
conductors, a pressure is required much
like water pressure in a water pipe. The
electrical equivalent to this pressure is
voltage which is measured in volts.
Functional Electrical
Characteristics - 2.4
TOC
REF
GLOSSARY QUIT
The conductive materials used to
interconnect components are usually
selected because of their low resistance
to the flow of electricity.
Copper continues to be the material of
choice for printed boards because it has
one of the least resistance properties to
current flow.
Functional Electrical
Characteristics - 2.4
TOC
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GLOSSARY QUIT
In an electronic system where things tend
to be uniform, the voltage, current, and
resistance are related by a rule called
Ohm’s Law.
Ohm’s Law states that the voltage (V) is
equal to the current (I) multiplied by the
resistance, or V = I x R.
Functional Electrical
Characteristics - 2.4
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REF
GLOSSARY QUIT
An example of Ohm’s Law:
If one were trying to determine the
current flowing through a resistor of 100
ohms, where the voltage was 12 volts,
the equation would be inverted to solve
for (I) by dividing the resistance into the
voltage, I =V/R. Which is 12/100, 0.12
amps, or 120 milli-amps.
Functional Electrical
Characteristics - 2.4
TOC
REF
GLOSSARY QUIT
Ohm’s Law is very useful in determining the
heat rise of conductors that are required to
carry heavy current. The cross-sectional
2221
6.2 current carrying capability for copper wire
has long been known and has been
Fig translated into equivalent copper conductor
6-4
widths and thicknesses. The circuit
requirements can therefore be analyzed to
Extra make sure that the board does not degrade
1.12b
due to insufficient copper to carry the
required current.
Functional Electrical
Characteristics - 2.4
TOC
REF
GLOSSARY QUIT
There are many parameters that relate
to the manner in which a circuit is
designed. Some of these deal with
performance requirements, others deal
with the ability to test the circuit.
Functional Electrical
Characteristics - 2.4
TOC
2221
6.1.2
Fig
6-1
Fig
6-2
Fig
6-3
REF
GLOSSARY QUIT
Proper shielding is important in order to
maintain circuit integrity. Electromagnetic
Interference (EMI) is the unwanted
radiated electromagnetic energy that
couples into electrical conductors. Proper
layout of the circuitry can minimize this
condition. The use of ground or voltage
planes as a shield can also reduce the
interaction of electro-magnetic interference
upon devices, circuits, or portions of
circuits.
Functional Electrical
Characteristics - 2.4
TOC
REF
GLOSSARY QUIT
Some other rules that are useful in circuit design include:
• Make ground conductors large enough to avoid noise
problems.
2221 • Group signal lines of similar logic families together.
6.1.2 • Avoid termination of logic outputs directly into
transistor bases.
• Terminate unused logic pins with a resistive pull-down.
• Do not tie signal outputs together.
• Where possible, divide complex logic functions into
smaller combinational logic sections.
Appendix A
Functional Electrical
Characteristics - 2.4
TOC
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GLOSSARY QUIT
Section 2.5
Characteristics of Grid Systems
Layout Principles
TOC
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GLOSSARY QUIT
A grid is an orthogonal network of two
sets of intersecting perpendicular
parallel equidistant lines.
Grid systems are used for locating
points, components, plated-through
holes, conductor patterns, and other
features on a printed board or printed
board assembly.
Characteristics of
Grid Systems - 2.5
TOC
2221
5.4.2
5.4.3
Fig
5-5A
thru
5-5E
Fig
5-6
Fig
5-7
REF
GLOSSARY QUIT
They are useful in defining the
requirements for manufacturing,
assembly, and testing because the
locations do not need to be individually
dimensioned as they are referenced to
the nearest grid intersection. When a
grid is specified and parts or board
features are off the selected grid, they
must be individually dimensioned and
toleranced.
Characteristics of
Grid Systems - 2.5
TOC
REF
GLOSSARY QUIT
Grid systems are always basicthe
intersection of the lines have no
tolerance. They are referred to as
being the true position of the desired
location of a part or feature. Tolerances
are therefore specified in relation to the
grid.
Characteristics of
Grid Systems - 2.5
TOC
2221
5.4.2
REF
GLOSSARY QUIT
The tolerance zone may be square,
which is defined by a plus or minus
variation from the true position; or
round, which is defined as a diameter
or radius of true position.
The center of the part, feature, or hole
must be located within the tolerance
zone as specified.
Characteristics of
Grid Systems - 2.5
TOC
REF
GLOSSARY QUIT
Grid systems are specified on the
master drawing. The master drawing is
the document that defines the board
fabrication requirements. Additionally,
grids may be specified on the assembly
drawing especially when the grid
describes the true position of test points
to be contacted during in-circuit testing.
Characteristics of
Grid Systems - 2.5
TOC
D325
4.2
Table
4-2
REF
GLOSSARY QUIT
Grids are located with respect to a minimum
of two datums. Datum features are used to
define the datums. The choice of features
are dependent on the design elements and
may be holes, lands, fiducials, or some other
symbol. These may be on the board or
assembly, or off, as might be required for a
board that is delivered in a panel format.
Where the two datums cross is usually
referred to as the point of origin or 0/0.
Characteristics of
Grid Systems - 2.5
TOC
REF
GLOSSARY QUIT
The choice of grid increment specified
on the master drawing is based on the
component terminal, or land location for
through-hole components, and on the
component centers for surface mount
components.
Characteristics of
Grid Systems - 2.5
TOC
REF
GLOSSARY QUIT
Typical increments for through-hole
components are in multiples of 0.13mm
[.005"], and 0.05mm [.002"] for surface
mount components.
The accepted grid for many years in both
the United States and internationally was
based on the imperial system, or inchbased. Spacings were 0.100", 0.050",
0.025", or increments of 0.005".
Characteristics of
Grid Systems - 2.5
TOC
D325
4.2
Table
4-2
SM782
3.6.1.4
REF
GLOSSARY QUIT
In the early 1980s the international
standard for all new designs (IEC 97)
adopted the international grid of 0.5mm
or increments of 0.05mm. The IPC and
EIA JEDEC (EIAJ) have accepted this
decision. All new component designs or
board grid routing schemes follow these
concepts.
Characteristics of
Grid Systems - 2.5
TOC
REF
GLOSSARY QUIT
Two problems created by random
component placement are:
• the loss of a uniform grid based test
node accessibility
• the loss of logical, predictable
conductor routing channels
Characteristics of
Grid Systems - 2.5
TOC
SM782
3.6.1.4
REF
GLOSSARY QUIT
Although some CAD systems have zero
grid routers, in reality they use a very fine
grid to complete the routing analysis. This
feature makes the final placement or test
point grid difficult to manage. In addition,
the algorithms sometimes do not take into
account the conductor width and clearance
allowances. Thus, CAM systems find
design rule violations at the time of board
panelization and must re-engineer the
defective design.
Characteristics of
Grid Systems - 2.5
TOC
REF
GLOSSARY QUIT
Section 2.6
Features Formed in Copper
Layout Principles
TOC
REF
GLOSSARY QUIT
The primary current carrying metal for
printed boards is copper. Copper clad
laminate is a metal-clad base material that
has copper as the conductive material, and
is used as the starting base for all rigid
printed boards.
When the copper cladding is imaged and
unwanted metal is removed through etching,
the remaining metal pattern forms the printed
wiring or printed circuit.
Features Formed in
Copper - 2.6
TOC
2221
6.2
Fig
6-4
REF
GLOSSARY QUIT
Copper, in the form of plating, is used to
make connections between the various
layers of copper cladding, or copper foil,
thus the copper pattern or conductive
pattern is the foundation carrying the
current in the printed board.
Features Formed in
Copper - 2.6
TOC
REF
GLOSSARY QUIT
Copper is also used as a method to
remove heat from the components.
Heat sinking planes are extended to
the edge of the board to make contact
with other metal to continue the heat
dissipation path.
Features Formed in
Copper - 2.6
TOC
2221
6.2
Fig 6-4
2222
10.1.1
REF
GLOSSARY QUIT
However, under most circumstances, it is
desirable to keep conductors slightly
away from board edges with a
recommended spacing of 0.4mm [.016"]
added to the electrical spacing required
for conductors.
Note that this recommendation does not
apply to card-edge connector contacts
which are brought to the edge of the
board so as to mate fully with pins in a
connector cavity.
Features Formed in
Copper - 2.6
TOC
REF
GLOSSARY QUIT
Lands used on printed boards for throughholes and vias usually fully circumscribe
the hole. In the design, the relationship
between land size, hole size, and the
desired amount of material left around the
hole (annular ring), are all considered as
the manufacturing allowance that must be
taken into consideration to accommodate
variations in registration of the different
conductive layers or hole location.
Features Formed in
Copper - 2.6
TOC
REF
GLOSSARY QUIT
Lands are connected by a conductor
that is usually less in width than the
diameter or dimension of the land. The
conductor provides a path for the
current from the component lead,
through the plated-through hole to the
next component.
Features Formed in
Copper - 2.6
TOC
2221
3.5.6.5
9.1.2
Table
9-2
REF
GLOSSARY QUIT
When a land is used simply to help
balance the construction, and has no
connection to a conductor, the land is
considered non-functional to the
performance of the circuit.
Fig
9-2
10.1.1
Table
10-1
Fig
9-3
Table
10-2
Table
10-3
Features Formed in
Copper - 2.6
TOC
REF
GLOSSARY QUIT
Component leads placed into platedthrough holes must consider all the
tolerances to allow easy insertion of the
lead into the hole. But, the hole must
not be overly large so that the solder
intended to connect the lead to the
barrel of the hole, or the land on a
single sided board, allows the lead to
fall out.
Features Formed in
Copper - 2.6
TOC
2222
9.2.1.1
REF
GLOSSARY QUIT
Specific precision recommendations exist
that describe the conditions of annular ring,
conductor width, and the relationship of
lead size to hole diameter for both platedthrough holes or unsupported holes. For
unsupported holes the requirements are
based on a single range (0.15-0.5mm
[.006-.020"]) to permit a proper solder fillet
to the lead, whereas plated-through holes
have a broader range with three levels of
requirements.
Features Formed in
Copper - 2.6
TOC
REF
GLOSSARY QUIT
When a plated through-hole connects
to a large copper plane it is important
to relieve the connection by leaving
space around the land to prevent the
solder cooling before it has completed
the path through the hole. The thermal
relief is intended to remove sufficient
copper without impairing the electrical
function of the hole and land.
Extra
1.6
Features Formed in
Copper - 2.6
TOC
2222
9.1.2
9.1.3
9.1.4
10.3
Fig
10-2
REF
GLOSSARY QUIT
Equations have been developed that
indicate how much copper can be
removed based on land/hole size. As
the conductive path, lands and holes
intended to be a connection for
component leads should be kept
clean and away from contamination
to permit good solder bonding of the
lead to the land or wall of the plated
through-hole.
Features Formed in
Copper - 2.6
TOC
REF
GLOSSARY QUIT
Section 2.7
Legend and Polarity Markings
Layout Principles
TOC
REF
GLOSSARY QUIT
Electronic diagrams , schematic or
logic, determine the polarity of polarized
components, and their identification.
These documents designate the
electrical functions and the
interconnectivity to be provided by the
printed board and printed board
assembly.
Legend and Polarity
Markings - 2.7
TOC
2221
4.6
D325
8.11
10.20
Fig
8-2
REF
GLOSSARY QUIT
Electronic diagrams use reference
designations made up of letters and
numbers to identify components. Symbols
and lines are used to define the electrical
function and interconnectivity. The letters
of reference designations indicate the type
of device (i.e., R=resistor, C= capacitor, CR
or D =diode, U=IC or assembly). The
numbers are sequentially assigned starting
with 1. Thus, C9 is the ninth capacitor in
the circuit.
Legend and Polarity
Markings - 2.7
TOC
REF
GLOSSARY QUIT
The legend is defined as marking on the
printed board. It may be in the form of
letters, numbers, symbols, and patterns
that are used primarily to identify
component locations and orientation for
convenience of the assembly and
maintenance operations. These
designations also serve to aid when
trouble shooting a board that is in the
field.
Legend and Polarity
Markings - 2.7
TOC
2221
4.6
REF
GLOSSARY QUIT
For manual component placement
techniques legend serves the function of
assisting the assembly operator in
identifying the exact component
insertion location. Having legend on the
board is not quite as necessary for
automatic operations, however, most
companies require the markings to
assist their field representatives in
maintaining the equipment.
Legend and Polarity
Markings - 2.7
TOC
REF
GLOSSARY QUIT
There are other techniques to aid
assembly and maintenance. Special seethrough overlays, handbook drawings, and
coordinate board assignments of
reference designations rather than
sequential numbering have been
successful in avoiding the extra
manufacturing steps incorporating ink
markings (legend) on the printed board.
Legend and Polarity
Markings - 2.7
TOC
REF
GLOSSARY QUIT
The specifications that require legend
allow for alternate techniques for defining
the component locations. What is critical
is that the reference designations and
polarity indicators match those that are
documented on the schematic or logic
diagram.
Legend and Polarity
Markings - 2.7
TOC
2221
4.6
REF
GLOSSARY QUIT
Often many designs are started before
the final designations are assigned. It is
important that back-annotation techniques
are accomplished before any final
decision is made on the incorporation of
legend on the board or any of the other
alternatives.
Legend and Polarity
Markings - 2.7
TOC
REF
GLOSSARY QUIT
As designs become more complex the
need for having legend increases and the
room for incorporating the legend on the
board decreases. One of the most
important things to be added to a board is
the polarity marking of various
components. ICs need pin 1 identified for
proper placement, this can be identified
with a dot or a different size or shape land.
Legend and Polarity
Markings - 2.7
TOC
2221
4.6
REF
GLOSSARY QUIT
Also, polarized capacitors and diodes need
polarity identification. The positive side of
the capacitor, or cathode of the diode, are
many times selected to have the land
shape identify the polarity. However, to
avoid confusion, the technique and
meaning should be well documented on
the assembly drawing and maintenance
handbooks.
Legend and Polarity
Markings - 2.7
TOC
REF
GLOSSARY QUIT
Once the decision has been made to
incorporate a legend into the design a
clear strategy should be developed as to
exactly what should and should not be
included. Company name and logo, part
numbers, serial number blank, and
assembly revision level are all
candidates for legend incorporation.
Legend and Polarity
Markings - 2.7
TOC
REF
GLOSSARY QUIT
Legend can be added using marking ink
and can be incorporated into the copper
circuitry of the board. The decision is
based on need to include legend, the
amount of available room, and bare board
surface topology.
Legend and Polarity
Markings - 2.7
TOC
2221
4.6
REF
GLOSSARY QUIT
Incorporation adds cost to the
manufacture of the board, so the decision
should be based on the value added in
assembly and maintenance. Many times
the decision is based on what was
appropriate in the past, instead of
addressing current specific needs.
Legend and Polarity
Markings - 2.7
TOC
REF
GLOSSARY QUIT
Section 2.8
Legend Marking Location
Layout Principles
TOC
REF
GLOSSARY QUIT
Legend marking is usually used to identify
the location of a particular component. As
such, the reference designation of the
component is the major form of
identification.
There are many components located on the
printed board assembly and open space is
usually at a premium; thus, a good
approach is to determine, in advance,
exactly what will and what will not be
incorporated into the marking.
Legend Marking Location - 2.8
TOC
2221
4.6
REF
GLOSSARY QUIT
There are specific requirements for the
location of these markings in order for
them to be useful. Usefulness applies to
the purpose of the markings, however, in
general, most requirements dictate that
the marking is visible after the assembly
is finished.
Legend Marking Location - 2.8
TOC
REF
GLOSSARY QUIT
With every square millimeter being
precious, the designer is challenged to
find the appropriate room by the side of
a component body that already has
other components very close. Putting a
partial designator on the board is not
the answer.
Legend Marking Location - 2.8
TOC
REF
GLOSSARY QUIT
Board manufacturers consider legends
a nuisance and frequently have
rejections of their product because the
legend is not clear or of a sufficient
size, yet these conditions were
dictated by the design.
Legend Marking Location - 2.8
TOC
REF
GLOSSARY QUIT
Some designers have even forgotten
the location of some of the holes in the
board and have had the reference
designations end up in the hole.
Elimination of the need for legend
would not make the manufacturer
unhappy, however, the benefits in
assembly and maintenance can be a
strong reason for their necessity.
Legend Marking Location - 2.8
TOC
2221
4.6
REF
GLOSSARY QUIT
The most important criteria for use is
that the marking must serve a
purpose.
Reference designators should be
placed as close as possible to the
device, without being under it, and still
be visible.
Legend Marking Location - 2.8
TOC
REF
GLOSSARY QUIT
The designer has the responsibility to
assure that the marking is of sufficient
size, clarity, and location to allow for
legibility during processing, inspection,
storage, installation, and field repair. If
this can not be accomplished it is better
to leave the marking off.
Legend Marking Location - 2.8
TOC
REF
GLOSSARY QUIT
In general, the minimum character height
for the numbers and letters is 1.5mm
[.060"], with a line width of 0.2 - 0.3 mm
[.008" - .012"]. Using text size less than
acceptable minimums will create nonlegible legends. A minimum line width
must be established to eliminate skips
and voids in lines and text.
Legend Marking Location - 2.8
TOC
2221
4.6
REF
GLOSSARY QUIT
Other rules that should be adhered to
include:
• avoid close proximity to surfaces that
must be solderable.
• avoid putting on surfaces covered with
melting metals.
• avoid placing reference designators under
components or in hidden locations.
• avoid placing reference designators on
any conductive surface.
Legend Marking Location - 2.8
TOC
REF
GLOSSARY QUIT
Solder mask, originally intended to
prevent certain board surfaces from
being soldered, is now useful in making
the marking legible and clear. Certain
solder masks have evened the topology
of the board and thus improved the clarity
of the marking. Contrast between the
color of the solder mask and the marking
ink is also appealing to many customers.
Legend Marking Location - 2.8
TOC
2221
4.5.1
4.5.1.2
REF
GLOSSARY QUIT
In general, it should be recognized that
both the mask and the marking ink
have little to do with the electrical
performance of the board.
They certainly help in the process of
building the assembly, and because of
this the benefit should justify the added
cost to the bare board.
Legend Marking Location - 2.8
TOC
REF
GLOSSARY QUIT
If copper is used to perform the marking
function, it should be incorporated into
the phototool used to prepare the outer
circuit patterns. Since this process is
done anyway there is no added cost,
however, the minimum letter height and
line width might need to be increased
slightly to allow for etching or plating of
the copper conductors.
Legend Marking Location - 2.8
TOC
REF
GLOSSARY QUIT
Electrical clearance must be maintained
from circuitry although the marking could
be attached to one circuit provided there
is no bridging to adjacent circuits with
different electrical characteristics.
These same rules apply when using
conductive marking inks; they must be
permanent and properly isolated.
Legend Marking Location - 2.8
TOC
2221
4.6
REF
GLOSSARY QUIT
Legend is also used to identify critical
circuits or groups of components that
relate to a particular part of the system
function. The ink marking is usually in
the form of an outline to contain the
group. Warning symbols are also
incorporated into the legend to identify
electrostatic sensitive devices (ESD)
status.
Legend Marking Location - 2.8
TOC
REF
GLOSSARY QUIT
• Why are grid systems used for locating
components, PTH’s conductor patterns and
other features on printed boards and printed
board assemblies?
–
–
–
–
because CAD systems require it
because design standards require it
because it helps in the manufacturing operations
because it avoids having to dimension individual
locations
Answer: because it avoids having to dimension
individual locations
Quiz 2
TOC
REF
GLOSSARY QUIT
• What is the total capacitance of three 18mf
capacitors connected in series?
–
–
–
–
6 microfarads
23 microfarads
54 microfarads
180 microfarads
Answer: 6 microfarads
Quiz 2
TOC
REF
GLOSSARY QUIT
• An internal plated hole has an LMC size of 1.0 mm
diameter. What is the mininum land size assuming a
fabrication allowance of 0.25 mm?
–
–
–
–
–
1.28 mm
1.31 mm
1.35 mm
1.40 mm
1.55 mm
Answer: 1.31 mm
Quiz 2
TOC
REF
GLOSSARY QUIT
• What is a transmission line called that has a
configuration consisting of a conductor over a
parallel ground plane, separated by a
dielectric material?
–
–
–
–
a strip line
a microstrip line
a capacitive coupled line
a characteristic impedance line
Answer: a microstrip line
Quiz 2
TOC
REF
GLOSSARY QUIT
• What is the total resistance of three 100 ohm
resistors connected in series?
–
–
–
–
30 ohms
300 ohms
3000 ohms
30000 ohms
Answer: 300 ohms
Quiz 2
TOC
REF
GLOSSARY QUIT
• In general, where should field adjustable
components be placed?
–
–
–
–
near the ground bus
in the center of the board
adjacent to the high speed circuits
at the edge opposite the connector
Answer: at the edge opposite the connector
Quiz 2
TOC
REF
GLOSSARY QUIT
• If analog circuits are restricted to the center of
the board, what is the effect of placing a
power supply in the connector zone, below
the analog circuits?
– the analog circuit performs better
– the heat from the power supply causes instability
– the power supply, influenced by the analog circuit,
is erratic
– the analog circuit requires additional protection to
overcome spikes
Answer: the heat from the power supply causes instability
Quiz 2
TOC
REF
GLOSSARY QUIT
• The maximum permissible operating temperature for
PCB is 45oC. For an external trace the current may
reach 10 A. Assuming a laminate board made of 1 oz
copper, what would be the minimum conductor
width?
–
–
–
–
–
70 mil
100 mil
150 mil
250 mil
300 mil
Answer: 150 mil
Quiz 2
TOC
REF
GLOSSARY QUIT
• The maximum permissible operating temperature for
PCB is 45oC. For an internal trace the current may
reach 10 A. Assuming a laminate board made of 1 oz
copper, what would be the minimum conductor
width?
–
–
–
–
–
70 mil
100 mil
150 mil
250 mil
300 mil
Answer: 300 mil
Quiz 2
TOC
REF
GLOSSARY QUIT
• What are the characteristics of a connector
with a “P” type reference designator?
–
–
–
–
fixed
movable
male pinned
female pinned
Answer: movable
Quiz 2