WE'RE CONTINUING OUR summary of a year-long experiment aimed at identifying lead-free solder alloys that mitigate tin whisker growth. This study cut right to the chase, beginning with an elimination round. The first phase knocked out two-thirds of the nine original candidate alloys based on their whisker production.
The results of the tests were both intriguing and enlightening, and forced us to rethink what we thought we knew about tin whisker propagation. The investigation focused on the conventional wisdom that tin whiskers are the result of compressive stresses caused by electroplating, bending, intermetallic growth, and thermal expansion mismatches.
It assessed the influence of these most commonly cited potential causes by creating test specimens with wire dipped in molten alloy and bent after cooling (FIGURE 1), and then subjecting them to thermal cycling and environmental storage conditions.
[FIGURE 1 OMITTED]
The dipped specimens grew whiskers, thereby eliminating the residual stresses of electroplating as the sole cause. Whiskers grew all over the test specimens' bent area, including the outside--or tension side (this is still under debate)--of the bend, challenging the notion that they are a form of relief from compressive stresses.
[FIGURE 2 OMITTED]
Whiskers are becoming better understood to grow in locations where the grains of the tin are less mobile. When tin confronts a non-moving section, it tends to squeeze out at that location. This mechanism can occur many levels away from the source of the original driving force.
Thermal excursions did not produce whiskers, either. What caused the most whisker growth? High heat and humidity storage conditions. The effect of the environmental storage on whisker growth was unmistakable. All the lead-free solders grew whiskers under the most adverse storage conditions, but some grew more than others. More details on that first phase of the study can be found in our May column (circuitsassembly.com/cms/ component/content/article/159-currentissue-articles/15973-tech-tips).
The next phase of the study tested seven alloys. It took the top three performers from Phase 1, added a few tweaks to those compositions, and the SAC 305 control, and put them through a variety of tests, including wetting, spread, differential scanning calorimetry (DSC) and mechanical properties.
[FIGURE 3A OMITTED]
Wetting balance tests offer both numerical and visual results. Numerically, they demonstrate the alloys'...