For additional consideration you might look at some of the material written on Conductive Anodic Filament (CAF) to see if it applies to this situation. Dr Turbini has documented some excellent work as have others. CAF has the ability to form internally in the board given the presence of contamination, moisture and electrical bias, especially considering the occurrence of measles or voids in the laminate. The failure will not be evident initially in test, but will occur after sustained circuit operation.
Best regards,
Mel Parrish
-----Original Message-----
From: TechNet [mailto:[log in to unmask]]On Behalf Of - Bogert
Sent: Wednesday, November 24, 2004 2:43 PM
To: [log in to unmask]
Subject: [TN] Tin whiskers
November 24, 2004
We have experienced problem with power planes (laminated bus assemblies) that have embedded copper planes it them with electrical boss connections brazed thereto. The copper planes are laminated together in an operation similar to a PWB. Epoxy is then added to the assembly via the edges. The process produced voids. These voids, in conjunction with the tin whiskers that grew from the tin-plated copper, caused short-circuit. Although the tin whiskers vaporized from the short-circuit energy available from large capacitors, an arc was sustained that ultimately cleared some fuses.
Our equipment initially passed 3500 VDC dielectric strength test and functional test of the equipment. However, after about a year in storage (no voltage applied), the equipment failed when 700 VDC was initially applied. We also did dielectric test on some power planes that were not assembled yet. The product failed the first time. However, when the dielectric test was repeated, it did not fail second time.
Bottom line is that we believe tin whiskers grew in the internal epoxy voids and ultimately caused short circuit.
Our solution to justify continued use of previously built product was to retest at 1,000 VDC for all product that was not energized for at least a six month period. We also redesigned the power plane for new product.
My question is, what happens to tin whiskers once they are vaporized by short-circuit current? What is the residue that results (some form of tin)? I assume it is still conductive. If we do the dielectric test at 1000 VDC, will it destroy all tin whiskers after six months, or will more grow?
Will the vaporized tin whiskers form a conductive film within the epoxy void such that we could still get a short-circuit?
I recognize that one does not need voltage to cause tin whiskers. However, if tin whickers exist with no voltage applied, will they grow larger if we apply a voltage (700 VDC)?
I have read papers regarding how to preclude tin whiskers. Some papers indicate that a coating of conformal coating or epoxy that is 0.006 inch thick will prevent whiskers. Other papers indicate that this only slows whisker growth but does not prevent it.
Some papers indicate that nickel plate under the tin, or annealing after plating at 200C will stop whiskers.
Some papers indicate that doping the tin with 3 % lead will prevent whiskers. However, other papers indicate one can get whiskers at higher lead doping rates.
What is the best way to prevent whiskers from forming in tin plated
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copper bus bars?
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