Microstructure and failure mode of Sn–37Pb soldered in laser diode packages

The influence of solid-state aging on the microstructure development and failure mode of Sn–37Pb soldered laser diode (LD) package is presented. The metallization at the LD electrode and Cu heatsink consists of Ti/Pt/Au and Ni–P/Au, respectively. During reflow, Pt–Sn intermetallic compound (IMC) was observed at the LD/solder interface whereas Ni3Sn4 IMC was formed at the solder/heatsink interface. Complete dissolution of Au into the solder formed needle-like AuSn4 IMCs and diffusion of Ni into the AuSn4 IMCs formed a coarse anisotropic (Au,Ni)Sn4 IMC structure across the solder joint. Mechanical shear testing exhibited good bonding integrity with brittle die fracture. During aging, the IMCs settled to the interfaces and grew with time. The failure mode switched to ductile solder fracture for thermal aging of up to 9 days and then brittle interfacial fracture at the solder/IMC interface with further aging. The rapid growth rate of (Au,Ni)Sn4 IMCs and the poor adhesion strength of the IMC layers resulted in the change of mechanical integrity in the LD package.