Analysis of Microcavity Morphology and Formation Mechanism in AuSn Solder Ring Seal

AuSn solder ring seal is a kind of typical high-reliability hermetic package. During the sealing process of components, Kovar alloy with Ni/Au-coated, AuSn solder ring and ceramic with Ni/Au-coated are the main components involved in the reaction. X-ray inspection is usually used to determine the sealing quality. The flow state of the area solder is judged to be unacceptable when the sealing cavity exceeds 25% of the design width. There are few studies on the micro-morphology of the sealed sealing area. The morphology and formation mechanism of the micro-cavity are not clear, which is not conducive to further control of the void rate and improve reliability. Based on the mainstream reflow process conditions, the cross section of the welded sample was scanned, the state and distribution of the interface compound were observed, the crystal precipitation process during the reflow process was deduced, the reaction phase of the base metal and its coating was further analyzed, and the AuSn solder ring was studied by scanning the interfacial compound element composition scanning. The results show that after the Ni-plated layer is exposed to AuSn
₂₀
, layered and dendritic Au-Sn-Ni ternary compounds are formed at the interface and grow into the weld. The sealing region forms the dominant segregation of Au
₅
Sn, and the size of the tiny cavity is concentrated below 10um, showing a diffuse part in the middle of the solder. The research indicates that the asymmetric diffusion of elements is the mechanism of forming tiny voids, which lays a theoretical foundation for further control of void ratio and reliability.