For solder joints subjected to current stressing, the formation of a pancake-type void in the solder and excessive consumption of the metallization layer are the two major degradation mechanisms reported in the literature. Nevertheless, the key factor determining which mechanism is dominant and responsible for failures remains unidentified. This paper describes a theoretical analysis and the ensuing experimental verification showing that temperature is the key factor determining the dominant mechanism. The formation of a pancake-type void requires the diffusion of Sn, which operates by the vacancy mechanism and exhibits a higher activation energy; the excessive consumption of metallization layer requires the diffusion of metallization metal (typically Cu or Ni) through Sn, which operates by the interstitial mechanism and exhibits a lower activation energy. At higher temperatures, both Sn and Cu electromigration flux are important, and consequently void formation and metallization consumption operate simultaneously. At lower temperatures, the Sn electromigration flux is relatively unimportant, and the metallization layer, accordingly, is excessively consumed before the void forms.
- Intermetallic compounds