Effects of peak temperature reflow on the solder joint strength of lead-free solder ball on semiconductor units (PBGA) and correlation of the adhesion strength to intermetallic thickness and type

摘要

This research paper focused on the effects of peak reflow temperature on the solder joint strength and interfacial reaction of 95.5%Sn-3.8%Ag-0.7%Cu solder ball on PBGA packages with Cu/Ni/Au pad metallization. PBGA units were reflowed at peak temperatures of 235°C,240°C,245°C,250°C,255°C,and 260°C. Lead free solder requires a ramp to peak profile unlike the commonly used ramp-soak-peak profile for leaded solder. The resulting reflowed samples were tested with the ball shear machine to evaluate the solder joint strength. Solder ball shear strength gradually increases from peak reflow temperature of 235°C until peak temperature of 250°C. At above peak reflow temperature of 250°C,the solder ball shear strength was significantly reduced. Fracture mode of the ball shear test was found to be a mix of ductile and interfacial fracture. Observations of the interfacial fracture increases as the peak reflow temperature gets higher. It was found that the higher occurrence of interfacial fracture could contribute to the lower solder ball shear strength. During reflow,Au in the pad metallization diffuses into the bulk solder while Ni reacts with Sn in the bulk solder to form intermetallic compound (IMC). IMC thickness increases gradually from peak reflow temperature of 235°C to peak reflow temperature of 260°C. At peak reflow temperature of 255°C where the solder ball shear strength reduced significantly,the mean IMC thickness reach 0.945μm. The IMC layer continues to grow at peak reflow temperature of 260°C where the mean IMC thickness measured was 0.963μm. From the EDX analysis,the composition of the IMC layer was found to consists of Cu,Ni,and Sn. Morphology of the IMC layer changes from a needle-like structure at peak reflow temperature of 235°C to a more planar structure at peak reflow temperature of 260°C. The morphological change could lead to lower shear strength as the surface of the IMC layer is smoother and easier to slip on the application of the shear force. Key Words: Peak Reflow Temperature,Solder Joint Strength,Intermetallic.