Evaluation of Bismuth/Tin Solder Intermetallic Layers Based on Heating Factor

The intermetallic compounds growth at the interface between the eutectic solder Bi58Sn42 and the copper soldering pad was examined in dependence on the temperature profile selection that was expressed by a heating factor (combined influence of time and temperature above the melting point of the solder alloy). Eight temperature profiles were used for reflow soldering. In contrast to other studies, almost the identical value of the heating factor was achieved by the different shapes of the profiles in order to evaluate the effect of the whole profile shape. Consequently, the metallographic cross-sections of the solder joints were analyzed by scanning electron microscopy. Additionally, the shear strength test of solder joints and the transition resistance measurement were conducted. As expected, the intermetallic layer thickness increased with an increasing heating factor, but a different thickness was observed for the identical heating factor when the whole shape of temperature profile including the preheating phase was changed. On the other hand, the prediction of IMC growth based on heating factor can be used in situations, where only the melting phase varies and the preheating remains constant - but the influence of the heating factor on intermetallic layer thickness cannot be generally formulated for a specific solder alloy due to a strong dependence on the flux.