In the first two sections of this triad we discussed why Tin (Sn) whiskers are an issue in high reliability military electronics and what was known about their physical metallurgy (See Figure 1). Unfortunately, very little is known about Sn whisker growth which would allow for the development of a predictive failure model. Therefore, a collection of anecdotal information and empirically derived predictive criteria is now the best available methodology to manage the problem, based on the Kolmogorov - Johnson - Mehl - Avrami analysis (KJMA).

Much of this anecdotal information is misleading. Recent studies showing systems that are "whisker immune" according to the literature have been shown to produce whiskers, sometimes after as long as 30 months of observation (2).

The approach for DoD applications is to SOLVE the problem of tin whiskers, not simply mitigate it. Even the lowest level of occurrence of this phenomenon is sufficient to cause catastrophic failure, as in heart pacemakers or military systems (3, 4). Mitigation from some high level of occurrence to a lower level is therefore unacceptable for high reliability applications.

In order to eliminate this problem we have set as a goal discovery of a tin alloy finish for lead free electronics that would be equivalent to what the addition of at least 1% Pb to the Sn electroplated finish did to prevent tin whiskers for the tin-lead solder system. It has been proven over 60 years of experience that a minimum of 1% Pb essentially eliminates tin whiskers as a problem for any assembly system that uses SnPb solder and SnPb plating on the component contacts. Alternate lead finishes of Nickel over-plated with Palladium have been used as a whisker-immune component contact finish but its use is not widespread, primarily due to the cost of that system.

The EMPF will institute a two pronged strategy to accomplish this complete elimination of tin whisker growth risk. The first thrust will be to study the crystallography and phase transition behavior of Sn and SnPb alloys to develop the true mechanism of the growth of Sn whiskers and the crystallographic mechanism of the elimination of the of that growth through the addition of Pb to the Sn, either as solder or as the electroplated finish of the component contacts. The second thrust of this activity will be the experimental verification of the theoretical mechanism proposed in the first.

The second thrust will concentrate on the electroplating process to finish the components with the correct whisker-immune electroplated Sn alloy. This tin alloy can then be verified by use of XRF (X-Ray Fluorescence), just as the current SnPb alloy compositions are being verified by most large defense OEMs. Verification can initially be accomplished by electroplating tin or selected tin alloys onto a surface that initiates immediate whisker growth. The first of the experiments at the EMPF has been to duplicate the phenomenon of rapid (days rather than months or years) tin whisker growth so that this rapid whisker growth process can be used to quickly screen candidate mechanism theories. It is known that immersion Sn deposits on printed circuit boards (PCBs) can develop whiskers (5.).

Experiments have shown that immersion Sn plating operations which operate at 140oF, will cause whisker formation from the immersion plated tin on interior surfaces of Copper (Cu) plated through holes in PCBs (Printed Circuit Boards) in several days (apparently from the compressive stresses developed by cooling the printed circuit substrate from the plating bath temperature (140oF) to room temperature. However, such coatings plated at 100oF did not grow whiskers in the same 5-day period even when cooled to an equivalent differential temperature, 140oF- 75oF vs. 100oF- 32oF. These samples may now be investigated further to verify the theoretical crystallographic model that ACI is developing in the first thrust of this root cause determination. One can assume that the characteristics of the Sn plating developed in the 100oF vs the 140oF process are not equivalent, i.e. internal stress, grain size, thickness, contamination. This experiment indicates that whiskers can be eliminated by control of plating process in an equivalent way to controlling external stress. It should be noted that both of these observations have been reported in the literature, but without a coherent crystallographic explanation.

Elimination of compressive stress (including imposition of tensile stress) into the electroplated Sn finish layer will suppress whisker growth to a great extent (5). Although this elimination of compressive stress in the Sn plate has been successful, verification of the stress level in a plated deposit requires X-Ray diffraction on the deposit, an analysis that needs to be executed in a laboratory. The EMPF's intention is to select an alloying addition which (in the same manner as Pb) can be detected in the deposit by XRF (X-Ray Fluorescence), the method commonly used by the major defense OEMs to accept or reject components based on their inclusion of at least 3% of Pb in the Sn plating on the component contact areas. In the present case, the objective will be to select the proper tin alloy, verifiable by XRF, that eliminates the possibility of whisker growth in an analogous fashion to the way that Pb (at least >1%) in the Sn plating finish does today. This would allow verifiable elimination of whisker growth from Lead-Free components. Obviously, there are alloying additions to pure Sn that do not suppress whisker growth (such as Cu, Bi, or Ag), and the same crystallographic mechanism that will explain the non-effectiveness of those elements will be used to explain the effectiveness of the one identified by ACI.

References:

1) ACI papers Tin Whiskers I and II, this journal.

2) Douglas W. Romm, Donald C. Abbott, Stu Grenney, and Muhammad Kan, "Whisker Evaluation of Tin-Plated Logic Component Leads" Texas Instruments Application Report SZZA037A- February2003

3) ITG subject: Tin Whiskers - Problems, Causes, and Soloutions Date: 3/14/86 Number: 42 Inspector’s Technical Guide Dept of Health and Human Services.

4) Lessons Learned notice by Lockheed Martin Astronautics, Notice # LLN-98-06 pp1-4, July 1998, "Tin Whiskers formation in Electronic Components"

3) B.D. Dunn, "A Laboratory Study of Tin Whisker Growth, "European Space Research And Technology Centre, Noordwijk, The Netherlands ESA STR-223 September 1987.

4) Rich Parker Delphi Electronics Private Communication.

5) Chen Xu, Chongllun Fan, Anna Vysotskaya, Joseph A. Abys and Yun Zhang, Lucent Technologies, Electroplating Chemicals and Services, Leslie Hopkins, Bell Laboratories, and Fred Stevie Lucent Technologies Microelectronics, "Understanding whisker Phenomenon-Part II Competitive Mechanisms".

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