Rolf Ehrat said:

>Dear Colleagues,
>
>I am writing with respect to a 4 layer (mixed technology) PCB with the
>following layering:
>
>Component side traces
>Ground plane
>Vcc plane
>Solder side traces.
>
>We have, on three occasion, had boards burn out after the boards were in
>service for some time. The burns are between the 12V battery traces and
>other traces/planes. The 12V is fused with a 1.5V quick fuse. The post
>mortem on one occasion indicated that the short occurred between a component
>pin and the ground plane. The other cases are not so clear as the short
>occurred on an inside layer along an edge of the board. The spacing
>generally is 0.015".
>
>I recall to have read in the past about wisker growth. Can anyone point me
>to such literature, preferably in EDN, Electronic Design. Any help would be
>appreciated.

Sorry, I can't give you any citations (this ain't no li-berry), but:

If the PWB is finished with pure tin , plated finishes of this type are
known to have high residual stress, and to have tendencies to form
"whiskers" through a process known as dendritic corrosion.

Silver is known to form filaments by what is called electromigration.

I'd suggest that if the fuse holder (or fuse) has any silver finish, look
there, and change the finish. I've seen lots of fuses with Ag finished end
caps...

Likewise, bright tin (without lead alloy) either as the PWB finish or the
finish on the metal fuse holder could be a culprit.

You said that some zapping appeared to be on an internal layer, and the
above suggestions do not fit well with that scenario. It could also be
simply a matter of a very adverse (condensing, high temp, etc) environment
combined with some type of contamination.

It's amazing what a 12V lead-acid battery can do given a low impedance
path. The current flow, even from a momentary short, will give you the
effect of an arc welder. It may be hard to distinguish the secondary effect
of that "arcing" from the original source of the unwanted current path, or
from some temporarily caused impedance drop elsewhere in the circuit.

If the card cage or enclosure is finished with zinc-chromate, you might
consider the possibility that the "whiskers" formed on the enclosure, and
fell (or were blown) to a place on the PWB where the EMF and supply current
were large enough to make the teensy stray piece of metal go POW. At a
former employer, we stipulated that sheet metal card enclosure elements
should not be Zn-Cr plated. Nickel makes a nice electroplate alternative
for such sheet metal.

If the enclosure has Zn-Cr finish, look at some pieces under a bright
light, espcially near corners and formed edges with a 4X pocket loupe. You
may see some little hair-like pieces of metal up to several mm in length!

I hope the above is sufficiently cryptic to keep you entertained. At least
it didn't cost you much.


regards,


Jerry Cupples
Interphase Corporation
Dallas, TX USA
http://www.iphase.com


***************************************************************************
* TechNet mail list is provided as a service by IPC using SmartList v3.05 *
***************************************************************************
* To unsubscribe from this list at any time, send a message to:           *
* [log in to unmask] with <subject: unsubscribe> and no text.        *
***************************************************************************