Joint properties of solder capped copper pillars for 3D packaging

Demand for high speed, high density, small size, and multifunctional electronic devices has driven the development of 3D integration. Cu pillar bumps have received great attentions in ultra fine pitch 3D packaging. In this study, Sn-3.5Ag and Sn-58Bi cap bumps were fabricated on Cu pillar for fine pitch bonding of 3D packaging. Sn-Ag and Sn-Bi cap bumps with eutectic composition were fabricated with electroplating. The capped Cu pillars were bonded on the PCB substrate by flip chip bonding with the optimized experimental condition. The bonding interface of Cu-SnAg-Cu was composed of only Cu
₆
Sn
₅
IMC layers after flip chip bonding. The bonding strength of Cu-SnAg-Cu increased as the cap bump height increased. Bonding strength of Cu-SnBi-Cu bonding suddenly decreased after 200 h aging because of continuous Kirkendall void formation at the interface between Cu
₃
Sn and Cu.