Aerospace & Defense Technology - May 2022 - 24

PCB Design
tary applications should meet the
performance specifications described in
the MIL-PRF-19500 standard.
A design compliant with the MILSTD-461
standard will not suffer from
EMI and also will not interrupt other surrounding
devices.
Military and aerospace PCB designs have to meet strict requirements from a variety of regulatory organizations
including the IPC trade association, US Department of Defense (DoD), NASA, and the Federal
Aviation Administration (FAA).
Based on function and design, different
types of PCBs can be used in military
and aerospace applications. Rigid PCBs
are used in aircraft cockpit instrumentation,
axillary power units, control tower
instrumentation, etc. Flexible and rigid-flexible
PCBs are ideal for high shock
and extended vibration environments.
RF PCBs are used in communication circuits.
Metalcore PCBs are preferred in
both military and aerospace applications
as they can enhance heat dissipation.
HDI PCBs can reduce the size and
weight of the device. They are mainly
used in emergency response devices as
they can provide quick signal transmission
with reduced signal loss.
Design Guidelines for Military and
Aerospace PCBs
1. Only military-grade components
should be used in the design, having
a tolerance range of 1-2%.
2. In the stack-up design, the dielectric
thickness between two planes should
be 3.5 mil minimum.
One of the destructive forces impacting deployed aerospace and military electronic systems is radiation.
for these applications are electroless
nickel with immersion gold coating
(ENIG) or hot air solid levelling (HASL).
2. Shock and Vibration
In these applications, the operating
environment can shake the PCB to a great
extent. Avoiding counterfeit parts and
mounting military-grade components is a
standard followed to reduce the vibration
effect on the PCB. Also, using the throughhole
assembly technique can ensure
strong adherence of the components and
improve signal transmission.
To reduce the vibration effect, the circuit
board design should avoid geometrical
deformities like warping, twist, or
bows. Designing a symmetrical stack up
with uniform copper distribution is highly
recommended in military and aerospace
PCBs.
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The military equipment designed
should meet the MIL-STD-810 standard
to be certified as field-ready.
3. Radiation Impact
One of the destructive forces impacting
deployed aerospace and military electronic
systems is radiation. This occurs
naturally in space at higher altitudes and
calls for radiation-hardened components
to be used in the PCB design. Antifuse
technology is used in aerospace PCB
assembly to enhance the reliability and
security of programmable devices. Military
standards for rad-hard components
are outlined in MIL-PRF-38535.
4. Radiofrequency Operation
Radiofrequency is used for communication
in military and aerospace applications.
The RF components used in milimobilityengineeringtech.com
ADT
Feature 5 - PCB Design 0522_1.indd 24
Intro
Cov
ToC
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3. The layout design should emphasize
handling the maximum current load
of the circuit. Using shorter traces,
heavy copper (trace thickness between
3-4 ounces per square foot) and heat
dissipation techniques are necessary
for high-current circuit design.
4. The annular ring is the copper pad
area around the drilled hole and is
used to provide a good connection
with the copper trace and via. For military-grade
PCB design, the defined
width of the ring for the hole is 6-7
mils minimum for the external layer
and internal layers. Recommended
drill to copper clearance for military
PCB is 7-8 mil.
5. The aspect ratio is the board thickness
to the diameter of drilled via. The recommended
through-hole aspect ratio
for military PCB is 10:1 and the maximum
board thickness can be 100 mil.
6. Guarding the clock signal is essential
to generate a clean clock pulse. Using
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Aerospace & Defense Technology - May 2022

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