Effect of Stand-Off Height on Microstructure and Tensile Strength of Cu/Sn9Zn/Cu Solder Joint

To investigate the effect of the stand-off height (SOH) on the microstructure and mechanical behavior of certain solder joints, Cu/Sn9Zn/Cu solder joints with SOH of 100¿m, 50¿m 20¿m and 10¿ were prepared and studied. It was found that as the SOH is reduced, Zn content in solder bulk has a rapid decrease due to consumption by the reflow reaction, hence, the microstructure of solder bulk is changed significantly from a Sn-Zn eutectic structure to a hypoeutectic structure. By contrast, Cu content in solder bulk has a rapid increase with the reduced SOH, and this leads to form more dissociative IMC in solder bulk. These composition and microstructure changes of solder bulk induced by the reduced SOH correlate closely with the mechanical properties of solder joints. In our study, it´s found that as SOH is reduced, tensile strength of solder joints decreases, fracture path of solder joint transfers from the solder bulk into the IMC/solder interface, and the fracture mode tends to change from ductile mode into brittle mode. These findings would point out a new way to enhance the mechanical properties of miniaturized solder joints by controlling the composition and dissociative IMCs in solder bulk.