Role of Bi in microstructure formation of Sn-Cu-Ni based BGAs on Cu metallizations

There is ongoing research seeking solders with improved performance under harsh environmental conditions, elevated operation temperatures, and higher mechanical loads while, at the same time, meeting new emerging requirements for the continuous miniaturization of electronics. Bi has been identified as one of the advantageous alloying elements that significantly improve solder performance. The present investigation explores the influence of Bi on microstructure formation in Sn-Cu-Ni-Bi/Cu joints containing 0-14 wt% Bi. It is shown that Bi additions have no catalytic effect on βSn nucleation, however, they alter βSn growth textures in Sn-Cu-Ni-Bi/Cu joints causing a transition from a columnar grain growth to a virtually single grain structures as the Bi content increases above 8wt%. Bi additions are demonstrated to reduce the thickness of the Cu
₆
Sn
₅
IMC layer and to cause formation of non-equilibrium grain boundary βSn + (Cu, Ni)
₆
Sn
₅
+ (Bi) eutectic in joints with Bi contents ≥2wt% Bi.