Understanding and mitigating tin whiskers: Page 8 of 9

November 21, 2011 // By John O’Boyle
Understanding and mitigating tin whiskers
John O’Boyle, Maxim Integrated Products examnes the problems caused by the removal of lead from electronics and describes some techniques to mitigate tin whiskers.

Schlumberger, Lockheed, Raytheon, The National Physical Laboratory (UK), CALCE, and NASA, among others. A summary of the studies shows that no conformal coating meets all the criteria outlined above (see table 2). 7 Ultimately, no coating is 100% effective and whiskers still grow. The Arathane coating seems promising when applied sufficiently thick, however, and the conformal coating does prevent shorts from debris. Thermal effects need to be considered if a conformal coating is used on parts which will need to dissipate heat when operating. If necessary, the device may need to be derated. 

Table 2: Whiskers and different conformal coatings

The shift away from lead solder presents risks for high-reliability applications, particularly in the form of tin whiskers on tin-containing finishes. NiPdAu presents one alternative since it has also proven resistant to whisker formation, but the suitability of NiPdAu for a high-vibration environment is still under evaluation. It is a higher temperature solder and may, indeed, be less ductile than traditional SnPb solder.  

When a tin-bearing finish is used, conformal coatings have been somewhat effective and may also be suitable. Although the whiskers are contained, the conformal coating adds processing steps, possible thermal issues, and cannot totally prevent whisker formation.

Either solution above adds cost. When considering finish material for electronics, the SnPb solution is still the best because industry has more experience working with the material. Unlike NiPdAu, which must be plated to the entire lead frame before die bond and encapsulation, SnPb is electroplated to the lead frame after plastic encapsulation. More important, SnPb has been shown not to have a whisker problem and to be very resilient in high-vibration environments.

Both the RoHS directive and the tin-whisker issue present challenges, but they are not insurmountable. With proper choice of solder, manufacturers of high-reliability devices can achieve RoHS compliance while still maintaining reliability, performance, and cost objectives.

References
1. NASA Tin Whisker (and Other Metal Whisker) Homepage,

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