Intermetallic compound layer formation between Sn-3.5 mass %Ag BGA solder ball and (Cu, immersion Au/electroless Ni-P/Cu) substrate

The growth kinetics of intermetallic compound layers formed between eutectic Sn-3.5Ag BGA (ball grid array) solder and (Cu, immersion Au/electroless Ni-P/Cu) substrate by solid-state isothermal aging were examined at temperatures between 343 and 443 K for 0-100 days. In the solder joints between the Sn-Ag eutectic solder ball and Cu pads, the intermetallic compound layer was composed of two phases: Cu₆Sn₅ (η-phase) adjacent to the solder and Cu₃Sn (ε-phase) adjacent to the copper. The layer of intermetallic on the immersion Au/electroless Ni-P/Cu substrate was composed of Ni₃Sn₄. As a whole, because the values of the time exponent (n) are approximately 0.5, the layer growth of the intermetallic compound was mainly controlled by a diffusion-controlled mechanism over the temperature range studied. The growth rate of Ni₃Sn₄ intermetallic compound was slower than that of the total Cu-Sn(Cu₆Sn₅ + Cn₃Sn). The apparent activation energy for growth of total Cu-Sn(Cu₆Sn₅ + Cu₃Sn) and Ni₃Sn₄ intermetallic compound were 64.82 and 72.54 kJ mol^-1, respectively.