Effect of Sb addition in Sn-Ag-Cu solder balls on the drop test reliability of BGA packages with electroless nickel immersion gold (ENIG) surface finish

Recently, Sn-Ag-Cu solders have been widely used as lead-free candidates for the Ball-Grid-Array (BGA) interconnection in the microelectronic packaging industry. However, widely used Sn-Ag-Cu solders such as with 3.0-4.0 wt% Ag in microelectronics exhibit significantly poorer drop test reliability than SnPb solder due to the low ductility of Sn-Ag-Cu solder bulk. The brittle failure of solder joints occurs at intermetallic compound (IMC) layer after drop test. Because the brittle nature of IMC or defects around IMC transfers a stress to the interfaces as a result of the low ductility of solder bulk. For the improvement of the drop test reliability by solder alloys, the low ductility of solder bulk and the IMC control at the interface are needed. In this paper, the bulk property of solder alloys and interfacial reactions with ENIG of Sb-added Sn-Ag-Cu solder were studied and finally, drop test was performed. Low Ag solder such as Sn1.0Ag0.5Cu and Sn1.2Ag0.5Cu0.5Sb showed higher ductility than high Ag solder such as Sn3.0Ag0.5Cu. In the interfacial reaction, all of the solders had (Cu,Ni)₆Sn₅ IMCs and P-rich Ni layer, however, Sn1.2Ag0.5Cu0.5Sb solder showed the lowest P-rich Ni layer thickness, because less Ni participated in the formation of (Cu,Ni)₆Sn₅ IMCs. In the drop test, the longer lifetime was in order of Sn1.2Ag0.5Cu0.5Sb, Sn1.0Ag0.5Cu, and Sn3.0Ag0.5Cu. Sn1.2Ag0.5Cu0.5Sb solder showed the best drop test reliability compared with other two solders due to the thinnest P-rich Ni layer. The failures of all packages occurred along P-rich Ni layer which is the most brittle phase at the solder/ENIG interface.