Microstructural evolution of low temperature solders in solder joints on a flexible substrate

Surface Mount Technology is widely practiced in present day electronic component assembly. Low temperature solders are finding increasing applications primarily when polymeric materials are involved in the assembly process. Although there is a significant amount of data on conventional Pb-Sn solders reflowed at temperatures above 200 °C, the information on the behavior of other solders is extremely limited. Hence, it is necessary to develop an understanding of the structure-property relationship, both after reflow and after long term use, in these alternate solders. The microstructure of the solder in an SMT joint and the reflow characteristics are influenced by a variety of factors including composition of the solder, reflow parameters such as temperatures and dwell times, other process variables, subsequent temperature exposure and so on. In the current study, two different solders, Pb-Sn-Bi solder and Pb-Sn-Ag solder, have been used to surface mount various components on a polyester substrate using various process conditions. Microstructures of the solder joints have been examined using cross-sectional scanning electron microscopy. The effect of some of the process variables on the solder microstructure will be outlined. Differences between the behavior of the solders will also be highlighted. The solder microstructure will be discussed on the basis of the reflow profiles and the flux used.