Reliability of BGA Solder Joints on the Au/Ni/Cu Bond Pad-Effect of Thicknesses of Au and Ni Layer

A systematic experimental work has been carried out to investigate the effect of thicknesses of the Au and Ni layers in the bond pad metallization on the reliability of ball grid array (BGA) solder joints as well as to understand the mechanism of Au diffusion to the solder interface. BGA solder balls of Sn-37 wt.% Pb have been bonded on BGA solder bond pads of Au/electrolytic Ni/Cu by reflowing at 225 degC for 0.5 min. The thickness of the Ni layer was varied from 0.35 to 2.8 mum, while the thickness of Au layer was varied from 0.1 to 1.3 mum. Solid state aging at 150 degC up to 1000 h has been carried out to simulate the ultimate interfacial reactions during the operational life of electronic devices. Cross-sectional studies of the solder-to-bond pad interfaces have been conducted by scanning electron microscopy equipped with an energy-dispersive X-ray analyzer to investigate the interfacial reaction phenomena. Ball shear tests have been carried out to obtain the mechanical strength of the solder joints and to correlate shear behavior with the interfacial reaction products. After the shear tests, fracture surfaces have also been investigated to understand the fracture modes. It has been found that Au embrittlement in the BGA Sn-Pb solder joints on Au/Ni/Cu BGA bond pads can be reduced by using either a thin Au layer or a thin Ni layer in the trilayer metallization of Au/Ni/Cu. It is easy to understand why Au embrittlement is reduced when a thin Au layer in the BGA bond pad is used; however, contrary to the usual expectation, it has been realized in this paper that a very thin Ni metallization is an effective means to maintain the mechanical integrity of the BGA SnPb solder joint. It is revealed that the out-diffusion of the Cu atoms during the aging period changes the chemistry and morphology of the intermetallic compounds (IMCs) at the solder interface, contributing to a significant reduction in brittleness. During aging at 150 degC, the thin Ni layer facilitates - - Cu diffusion from the bond pad toward IMCs. As a result, the brittle continuous layers of (Au,Ni)Sn₄ and Ni ₃Sn₄ are transformed into a nodular-shaped discontinuous layer of AuSn₄ and a layer of (Au,Cu,Ni)₆ Sn₅, respectively. New interfacial reaction products with the modified morphology change the typical scenario of Au embrittlement at the BGA solder interface and thus improve the interfacial joint strength significantly