Effect of Cu-containing solders on the critical IMC thickness for the shear strength of BGA solder joints

Shear strengths of BGA solder joints on Cu bond pads were studied for Sn-Cu solders with various compositions of 0, 1.5 and 2.5wt.% Cu. Focuses were placed on the effects of the intermetallic compound (IMC) layer thickness and the interface roughness between IMC layers and solders. Ball shear tests were performed both for as-soldered solder joints with soldering reaction times of 7, 15, 30, 60, and 120 sec at 270°C and for aged solder joints at 150°C for up to 16 days. During the soldering reactions, the Cu-containing solders enhanced considerably both the IMC layer growth and the solder/IMC interface roughness. Upon aging for 1 day, however, the effect of the Cu content of solders on the IMC layer growth and the interface roughness nearly disappeared. The critical thickness of the IMC layer for the maximum shear strength was about 1.2 μm for all three solders. At the critical IMC layer thickness, the shearing fracture occurred inside solder balls. As the IMC layer thickness deviated from the critical thickness, the fracture tended to occur near the solder/IMC interface and thus the solder/IMC interface roughness had an influence on the shear strength.