Presentations by Wayne Tustin
Designing PCBs to Withstand Harsh Environments
PCB West 2002
Santa Clara, California
March 21, 2002
This paper was for a panel discussion at the PCB West meeting.
Figure 1
(courtesy Martin Testing Labs)
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
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
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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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
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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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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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.
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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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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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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.)
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.)
Alternately, your PCB might flex, with components at the center receiving greatest displacement, as in Animation 3.
Animation 4
(courtesy Cirvibe, Inc.)
Animation 4 shows us more complex resonances at frequencies, respectively 174, then at 258 and finally at 341 Hz.
Figure 5
(courtesy Cirvibe, Inc.)
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.
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.
