Correlation Between Whisker Initiation and Compressive Stress in Electrodeposited Tin–Copper Coating on Copper Leadframes

To evaluate the contribution of coating stress to whisker initiation from IC package leads, the stress distribution in the coating was investigated by finite-element analysis (FEA). Two different leadframe samples, which were composed of the same tin-copper coating on two different copper-leadframe materials, namely, copper-iron (hereafter, CUFE; corresponding to CDA number C19400) and copper-chromium (CUCR; CDA number C18045), were used to examine the whisker-initiation behavior on the coating surfaces. The two samples showed significantly different tendencies of whisker initiation from the coating. That is, after long-term storage at room temperature, no whisker initiation was observed on the coating on the CUCR sample, whereas long whiskers (with a maximum length of more than 200 μm) were formed from the coating on the CUFE sample. The FEA calculation on the leadframe samples revealed that the coatings had a two-directional stress gradient, namely, one gradient toward the surface and another toward the base leadframe material. It also indicated a difference between the stress distributions in the two samples. The gradient of normal stress on the coating´s grain boundaries (GBs), toward the surface of the CUFE sample, was found to be larger than that in the CUCR sample. This result implies that the tin-atom flux along a GB in the coating on the CUFE sample was larger than that on the CUCR sample because the atom flux along the GB was proportional to the stress gradient. It agrees with the above-mentioned whisker-initiation behaviors in the samples. We thus conclude that in the CUFE sample, a whisker initiates either from a surface grain immediately on top of a GB or from surface grains located on both sides of the same GB. To confirm this conclusion, the correlation between the tin-diffusion sites and whisker formation sites was investigated. Simulation of atom diffusion by molecular dynamics indicated that the dominant tin-diffusion site is a GB when com- - pressive stress is applied in the direction normal to the GB. Investigation of the correlation between the whisker roots and coating microstructures of the CUFE sample showed that the whisker roots were located on top of GB intersections in the coating. These results indicate that whisker-initiation sites are correlated with dominant tin-diffusion sites and that each whisker initiates either from a surface grain located immediately on top of a GB or from surface grains located on both sides of the same GB.