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Golden
Rules For Preventing Parts Headaches At PCB
Layout

Preventing parts headaches at printed
circuit board (PCB) layout involves a
number of key design considerations and
precautions. They include:

Carefully evaluating PCB
CAD design tools because each commercial
version has trade-offs. Some allow parts or
components to be easily designed on the PCB,
while others make it more difficult. One tool in
particular creates a component in a one-step
process, but provides limited access to
component pins. Another tool takes three to four
steps to create a component. However, it incurs
more time, yet is more flexible. Radial
components at the CAD level pose potential
issues due to the possibilities of coefficient
of thermal expansion mismatch. To avoid
problems, it’s best to select the proper CAD
tool for a given PCB layout. But regardless of
the tool used, PCB layout requires that IPC
design guidelines 7351 must be stringently
followed when designing through-hole components
on a PCB.

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Keeping in mind fabrication
tolerances. Sufficient margin on the pads
of a through-hole component must be maintained.
Otherwise, “breakout” can occur if requirements
aren’t maintained for the annular ring. Also,
correct coefficient of thermal expansion must be
carefully maintained for plastic- and
ceramic-based leadless chip carrier components,
especially Pb-free,
to avoid component thermal damage during high
temperature cycles.

Making sure component mechanical
specifications are carefully read and correctly
implemented. It’s vital that mechanical
footprints on the PCB’s physical surface are
viewed from top side. A few component
manufacturers do inadvertently specify their
components with a bottom side view without
noting them as such. By placing a component
specified this way on a PCB, the pin-out is
changed 180° and PCB functionality is adversely
affected. Checking out component height
requirements is also important. Heights of
components must comply with the PCB enclosure’s
mechanical dimensions.

Placing special attention to the PCB
application. For example, the end product may be
subjected to harsh environments with
considerable vibration. Special component
placement and robust PCB
design must be applied to guard against
these conditions.

Not overlooking component life cycles. A big
headache can occur when specifying an obsolete,
soon-to-be-obsolete, or non-existent
device.

Taking special pre-cautions when replacing
an obsolete component. Replacements or
substitutions must be carefully evaluated to
assure specs are correct and pin-to-pin
compatibility.

Following proper guidelines for component
placement. Components must be placed at least
1.5 millimeters or more from the PCB’s edge. A
pick and place line requires components to be as
much as three to five millimeters from the PCB’s
edge. A PCB undergoes different manufacturing
cycles, and each machine has its own
limitations. Hence, it is important to follow
set guidelines to avoid damaging expensive
components like a $3,000 ball
grid array (BGA).

Creating press fit components with very
tight tolerances, ensuring there’s no wiggle
room.

Assuring design-for-assembly
(DFA) engineers are highly trained and
experienced.

Checking that high voltage or current traces
are substantially distant from one another to
avoid thermal problems.

Providing thermal relief so that components
can be properly soldered on a PCB.

Applying heat sinks to dissipate heat from
thermal intensive
components.

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