Do Conformal Coatings Prevent Tin Whisker Failures?

Tin whiskers are small filaments that can grow spontaneously from tin and tin alloy surfaces.  The conditions necessary for tin whisker growth have been much debated.  Papers have been written, conferences have been held and PhDs have been granted, yet the exact mechanisms and growth conditions are still not completely determined.  The growth of tin whiskers is a stress relief mechanism with either internal plating stress or external mechanical stress driving the whisker growth mechanism.  For those who would like some additional background information, read G. Gaylon’s  A History of Tin Whisker Theory: 1946 to 2004.
Electronic suppliers have employed a variety of risk mitigation approaches.  An OEM has little control over the global supply chain and therefore the question arises as to what tin whisker mitigation methods are available to the OEM.  One risk mitigation idea is to use conformal coating as a tin whisker barrier.  On first appearances this makes sense since tin whiskers are thin and fragile and therefore it would seem that a layer of coating would easily prevent whisker growth.  However in 2006, Woodrow and Ledbury (1) demonstrated the surprising result that most conformal coatings could be penetrated by tin whiskers.  These include acrylic, silicones and urethanes.  Only the vacuum deposited parylene coatings showed good resistance to whiskers.  Their research was performed on brass coupons plated with bright tin which can easily grow tin whiskers.  In the July 2012 Journal of Electronic Materials, S. Han et al have reported (2) results of tin whisker experiments on real electronic modules.  These modules used a variety of component package styles and a variety of conformal coatings.  The results were the same as those by Woodrow.  That is, all conformal coatings (except parylene) will allow penetration by a tin whisker. This study also reported the very practical finding that conformal coating doesn’t consistently cover irregular surfaces.  For example, gull wing leads will not be covered continuously by conformal coatings.  Therefore the edges and the knee of the lead can (and will) be exposed.  For corrosion protection it is desirable that the component lead be entirely covered but in practice this cannot be achieved.  Studies have shown than even multiple coating passes will not provide complete coverage.
One area which has not been studied is the probability of a tin whisker to penetrate a conformal coating from the outside inwards to the lead. After all, in order for a whisker to short between two leads, it must penetrate the coating extending outwards and then penetrate a second time growing inwards. This second penetration seems unlikely although this must be corroborated with additional studies which will be experimentally difficult.
In conclusion, can it can stated that conformal coatings will greatly reduce the risk of shorts due to tin whiskers but this cannot be considered a foolproof risk mitigation strategy.  There are two reasons for this: 1) tin whiskers can grow through conformal coatings and 2) conformal coatings typically do not provide uniform coverage over uneven geometries such as the edges of electrical conductors. However having long tin whiskers grow through the coating or having a long whisker which grows from the small uncoated area of a leadframe is small. This low probability whisker must then contact an uncoated conductor which is also a low probability.  If these small possibilities are multiplied by the small possibility of having tin whisker failures on an uncoated module, then the resulting reliability is very high indeed. The chance of a tin whisker failure under these circumstances is virtually zero.

References:
1.    T. Woodrow and E. Ledbury, Evaluation of Conformal Coatings as a Tin Whisker Mitigation Strategy, SMTAI, September 2006.
2.    S. Han et al, Evaluation of Effectiveness of Conformal Coatings as Tin Whisker Mitigation”, Journal of Electronic Materials, July 2012.