Abstract
Thermoelectric elements and metallic conductors are connected by soldering reaction in thermoelectric cooling module. Owing to miniaturization of thermoelectric cooling modules, the size of thermoelectric elements is also decreasing. The contact resistance between thermoelectric elements and electrodes are expected to affect the cooling performance of thermoelectric modules. Lead-tin solder is widely used in microelectronic packaging industry. However, it will be replaced by lead-free solder soon due to WEEE and ROHS regulations. In this study, p-type and n-type thermoelectric elements were jointed by lead-tin and Sn95.5Ag4Cu0.5 solders, respectively, and contact resistivity and microstructure at the solder/thermoelectric junctions were compared. Besides, this study also discussed the changes of contact resistivity with different reaction time when lead-free solder and thermoelectric material are connected. It is observed that the contact resistivity of Sn95.5Ag4Cu0.5 solder/Cu junction is lower than that of lead-tin solder/Cu junction. Therefore, Sn95.5Ag4Cu0.5 solder is expected to have smaller influences on the cooling performance of thermoelectric device. But, the thickness of IMC at the interface of Sn95.5Ag4Cu0.5 solder/thermoelectric element is thicker than that at lead-tin solder/thermoelectric element interface, which may cause an implicit reliability problem. With the increase of soldering reaction time, the IMC thickness and contact resistivity between thermoelectric material and electrode also increased. The experimental results indicate that the IMC thickness is proportional to the reaction time, and a reaction-controlled kinetic process is suggested.