Effect Of PCB Surface Finishes On Lead-Free Solder Joint Reliability
Authors: Xu Zheng, Hongfeng Ran, Lei Wang, Hao Kou, Yexiang Ning, Bei Wang, Xian Lin, and Zuyao Liu Company: Shenzhen Kaifa Technology Co., Ltd. Date Published: 10/31/2010
Abstract: PCB surface finish technology has developed rapidly with the transition of soldering process in the past decade. Thus, a variety of surface finish processes have been widely used in lead-free electronic assembly. Different PCB surface finishes adopted in lead-free soldering could lead to the difference in solder joint interfacial reaction and evolution, which greatly affects the reliability of lead-free solder joints. In this paper, the reliability of Sn3.0Ag0.5Cu (SAC305) solder joints with four kinds of PCB surface finishes were studied under isothermal aging at 125°C, including Electroless Nickel and Immersion Gold (ENIG), Organic Solderability Preservative (OSP), Immerse Silver (ImAg) and Immerse Tin (ImSn). Their intermetallic compound (IMC) composition and morphology were investigated by SEM/EDX, and the crystallographic features of IMC were determined by XRD. Then the evolution of the microstructure of solder joints under isothermal aging was discussed. The results showed the IMC grains on ImSn pad were inequable, with larger length/diameter ratio, and had different crystal orientations from that on OSP and ImAg pads. The high content of phosphorus in ENIG pad resulted in the emergence of phosphorus-rich and Ni3SnP multi-layer after aging. Moreover, the mechanical properties of the solder joints with four PCB surface finishes were also studied via QFP pull test and high-speed solder ball shear test (HSSBS test). Combined with the fracture analysis and mechanical test results of solder joints, it was found that the differences of failure modes and reliability performance were related to solder joint microstructure. ENIG pad showed poor reliability performance in HSSBS test due to the presence of phosphorus-rich/Ni3SnP multi-layer structure after aging and interface micro-crack defect. Lastly, the electrical properties of different solder joints were evaluated by comparing their electrical resistivities. Owing to different composition and IMC crystal orientation, the resistivities of ImSn and ENIG pads after aging increased while that of OSP and ImAg pads decreased.