Investigation of ENEPIG/Sn-10Sb solder joint

Abstract

ENEPIG (electroless nickel electroless palladium immersion gold) surface finishing is widely used on IC (integrated circuit) packaging substrate for wire-bonding and soldering. Also, Sn-5Sb solder is a widely used high melting point Pb-free solder in some special area.In this study, the microstructure of the interface of ENEPIG/Sn-5Pb is studied with different Pd thickness in the ENEPIG surface finish. For ENEPIG/Sn10Sb solder with Pd thickness of 0.04μm, the IMC is flat and thin. With the increasing of the Pd thickness 0.1μm and 0.19μm, the IMC show increasing trends and with needle-like shape.The metal can shear force also was tested by ball shear equipment. The shearing force of the metal lid test results shows that the shearing force of the solder joint with Pd thickness of 0.04 is the largest among all the test samples. The shearing force shows decreasing trends with the increasing of the Pd thickness.After the metal lid shear test, the SEM and EDX analysis on interface of the ENEPIG side shows that lots of Ni-P area exposed for the solder joint with Pd thickness of 0.1μm and 0.19μm, while little Ni-P area can be detected for the solder joint with Pd thickness of 0.04μm.The schematic of ENEPIG/Sn10Sn solder joint interface is proposed to explain the phenomenon that be detected in this study. For the ENEPIG coating, it is composed by Ni-P coating (5~7wt% P contained), Pd coating (2~3wt% P contained) and Au coating. During the soldering, the Au and Pd will be dissolved into the Sn phase and form a small amount of PdSn4 and AuSn4. The AuSn4 and PdSn4 diffuse to the melting Sn phase quickly during the soldering. For the P element in the Pd-P phase will be remained at the interface between the Ni-P and Ni3Sn4. For the ENEPIG/Sn10Sb solder joint with Pd thickness of 0.04 μm, the P element contains at the interface is very little and cannot separate the Ni3Sn4 and the IMC shows flat shape. With the increasing of Pd thickness, more and thick P phase form P grains and distribute randomly. The P grains are strong enough to block the reaction between some Ni and Sn, so the Ni3Sn4 IMCs grow at the boundary between the P grains and the IMC shows needle-like shape.The Ni-P area is detected for the solder joint with Pd thickness of 0.1μm and 0.19 μm can be explained by the higher melting point of Pd metal compare with Au metal, and thick Pd thickness with poor solder-ability.