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Tin Whisker Growth Facility

The Situation

The surfaces of Lead-free Tin surface finishes (including solders) can – suddenly and unexpectedly – form Tin “whiskers.” Whiskers are long, thin protrusions that can be several hundred to several thousand microns in length and that have the potential to cause short circuits either by contact with the surroundings or by detachment and settling on another location.

Another mechanism of circuit failure or malfunction is through the formation of tin vapor or plasma – to which whiskers are susceptible due to geometry – and which can provide conductive, short-circuiting paths. Extremely long whiskers have been observed periodically – e.g. 18 mm long whiskers were found in 2006 on the Card Retainers of high-criticality (Criticality of 1/1 and 1/R) Space Shuttle Flight Control Boxes.1

The phenomenon of Tin (Sn) whiskers has been known for over 65 years and was not a serious problem because it was found that the addition of Lead (Pb) suppresses or eliminates whisker formation.

However, the RoHS2 legislation enacted by the European Union on 1 July 2006 restricts the use of Pb in electronic products sold in the EU. This has led many electronic circuit board manufacturers to remove Pb from Pb-Sn plating and solders, leaving essentially pure Sn. While this approach is the most convenient and the least costly Pb-elimination strategy for the majority of board manufacturers,3 it also makes the resulting components susceptible to sudden failures due to Tin whiskers.4,5

The dimensions of the problem (of Tin whiskers) are such that they have also started to affect high reliability (Hi-Rel) circuits. In theory, the manufacturers and users Hi-Rel components have asked for – and received – exemptions from RoHS restrictions on Lead-free solders.

In practice however, the Hi-Rel market is miniscule in comparison with the commercial and thus does not offer a particularly strong incentive to the circuit board manufacturers for maintaining multiple production lines (pure Sn and Sn-Pb, for example).

As a result, a majority of the board manufacturers have not only switched to Pb-free solders, but in many cases have done so without changing the part numbers of the circuit boards. This in itself presents a unique problem for Hi-Rel circuits and adds urgency to resolving and managing the destabilizing influence of Tin whiskers.


The Problem

There have been numerous attempts at managing Tin whisker formation. Major laboratories around the world are involved in solving this problem, but so far with only very limited success. One of the important factors for the lack of success is the unavailability of an appropriate facility and methodology that will form Tin whiskers in a controlled, quantified, and predictable manner.

The Solution

Develop a facility that will provide highly controlled growth and growth-tracking environments to rigorously test all proposed mitigation strategies for the suppression of Tin whisker growth. The proposed facility will include the following key features:

  • Innovative coupon design and the use of advanced materials to produce a desired stress level and stress gradient (both compressive and tensile) while maintaining identical growth conditions (other than stress) at all locations on the coupon surface.

  • Ability to maintain the starting stress level during thermal cycling.

  • Converse ability to change the stress level and stress gradient during thermal cycling.

  • Ability to provide thermal cycles of any desired amplitude at any desired mean temperature (within the context of electronic assemblies).

  • Ability to precisely control the growth atmosphere with respect to %RH, %O2, %N2, and convection (natural or forced).

  • Ability to scan the entire coupon surface for whiskers and to track whisker growth through extended periods.

  • Ability to measure whisker growth characteristics in 3-d (angle, diameter, kinks, length, etc.).

  • Ability to accommodate multiple coupons while providing identical growth conditions to each.

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1Space Shuttle Program Tin Whisker Mitigation, K. Nishimi - United Space Alliance, LLC, International Symposium on Tin Whiskers, University of Maryland - CALCE April 24-25, 2007

2The Restriction of the use of certain Hazardous Substances in electrical and electronic equipment.

3Lead-Free Solders: Reliability and Other Issues a Year after the Legislation, S. Chada, Journal of Metals, p. 19, July 2007.

4DoD Executive Lead Free IPT – www.leadfreedod.com

5NASA Goddard Tin Whisker Homepage – nepp.nasa.gov/whisker/


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