Designing PCBs to Withstand Harsh Environments

Figure 1

(courtesy Martin Testing Labs)

Figure 1

I asked the PCB (printed circuit
board) design and manufacturing audience to think with me
about MIL environments, where vibration and shock can be very
severe. And about the test lab HALT, ESS and HASS environment
of random vibration, used today by not only MIL but also commercial
suppliers.

I mentioned the military demand
for COTS, commercial off-the-shelf equipment that must be
slightly modified for MIL usage. In Figure 1, we are testing
the cushioning that has been placed under a “cocooned” (air
conditioned) equipment, to protect the equipment by lessening
the received shock and vibration.


Figure 2

Figure 2

Inside that equipment are
many printed circuit boards. We want to understand how they
behave when vibrated.

But first, let’s understand
how the ultra-simple one-spring, one-mass, one-dashpot system
of Figure 2 behaves when single-frequency sinusoidal vibration
excites it.


Figure 3

Figure 3

The family of transmissibility
graphs in Figure 3 describes how the system of Figure 2 responds.
Note the peak where forcing frequency Ff
= natural frequency Fn,
where Ff/Fn
= 1. Resonant behavior of this simple system is seen in Video
Clip 1.

The “region of isolation” (Figure
3) extends rightward from the black circle, where Ff
exceeds 1.414 Fn.


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video clip 1


Figure 4

Figure 4

The ultra-simple one-spring,
one-mass, one-dashpot system of Figures 2 and 3 does not describe
“real world” systems. “Real world” systems are much more complex.

Figure 4 suggests how a
more complex “real world” system behaves when single-frequency
sinusoidal vibration excites it. Note that we now have several
resonant peaks.


Animation 1

Animation 1

Consider,
for example, a simple cantilever beam, attached to a shaker
that has been tuned to excite the cantilever’s first resonance,
the “diving board” effect, Animation 1.


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Video Clip 2

At a higher forcing frequency,
a more complex response mode is excited. Note that this more
is characterized by one point of zero vertical motion, a node,
Video Clip 2.


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video clip 3

Let’s change the illumination
to a “strobe” light, adjusted to very near the shaker forcing
frequency. Now, in Figure 7, we can see rotation at the node.
This might harm a component attached there, if the component
was susceptible to rotation.

Between the shaker and the
node is an anti-node, a point of large dynamic forces. This
might harm a component attached there, if the component was
susceptible to tension and compression.


Click here to watch video clip 4

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video clip 4

Moving our forcing frequency upward, we can
miss observing resonances, because the beam displacements
are small. One technique involves sprinkling salt (video clip
4) onto the beam.


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video clip 5

In Video Clip 5, we see the salt migrating away
from the anti-nodes and collecting at the nodes (points of
minimal motion).


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video clip 6

In Video Clip 6 we see 8 nodes, indicating the
9th mode, the 9th natural frequency. Might there be more?
Yes, but the displacements are extremely small, and such resonances
are less likely to cause damage.


Animation 2

(courtesy Cirvibe, Inc.)

Animation 2

Now,
at long last, let’s consider one of your printed wiring boards.
Some vibration might cause your board to translate, to move
regularly from one location to another, as in Animation 2.


Animation 3

(courtesy Cirvibe, Inc.)

Animation 3

Alternately,
your PCB might flex, with components at the center receiving
greatest displacement, as in Animation 3.


Animation 4

(courtesy Cirvibe, Inc.)

Animation 4

Animation 4

Animation 4

Animation
4 shows us more complex resonances at frequencies, respectively
174, then at 258 and finally at 341 Hz.


Figure 5

(courtesy Cirvibe, Inc.)

Figure 5

The
exaggerated motions suggested by Animations 2, 3 and 4 can be
shown by single-frequency-at-a-time sinusoidal forcing. But
if your test lab employs broad-spectrum random vibration, as
in performing a “qual” test or during HALT (highly accelerated
life testing), ESS (environmental stress screening or HASS (highly
accelerated stress screening), all those motions can occur simultaneously.
“Real world” vibration is closer to random vibration than it
is to sinusoidal vibration. Vibratory forces at certain locations
can destroy components, as suggested by Figure 5.


The End

How
to avoid board failures? Use a relatively recent computer program
that tells you where NOT to place delicate components. It also
tells you the best locations if those components MUST be used.
That program can also tell you the effectiveness of a proposed
screen, component by component. Few screens are effective at
all locations on each board.

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