The role of soldering process variables on properties of lead-free solder joints

This paper reports some new experimental findings and analysis on the tensile fracture strength of solder joints made with the lead-free solder alloys 96.5Sn3.5Ag, 95.5Sn4.0Ag0.5Cu and 96.5Sn3.0Ag0.5Cu using convection reflow soldering techniques. The employed specimens were all sandwiched solder joint assemblies subjected to various reflow soldering process variables: reflow temperature, reflow time, cooling rate and joint thickness. It was found that reflow time was the most important process variable in determining joint strength. The joint strength decreases rapidly with increasing reflow time. Cooling rates alter the reflow time, thus altering the joint strength. Faster cooling rates increased joint strength. Typically, a 20°C band in which peak reflow is to occur is used when processing electronic assemblies. Strength variations were found to be broad over such a band. Peak temperature needs to be precise to optimize strength. Joint strength in correlation with joint thickness was found to be random. Thin joints produced results better than thick joints and vice versa under similar testing conditions. Joint fracture strengths over 150% greater than recorded bulk solder tensile strengths were found. The method by which a joint was processed aided in the morphology of the intermetallic compound (IMC) formed in the joint. The shape and size of the intermetallic whiskers and scallops truly dictate joint strength. Porosity is inherent to joints made with a solder paste. Porosity can be controlled by the process conditions. Minimizing the fraction of the surface area covered by pores will result in stronger joints. The pore size, shape and movement were found to be controlled by IMC formation and subsequent dissolution from around the pore.