Effects of Multiple Reflows on IMC and Shear Strength of Individual Solder Joints

Solder alloys play a significant role in estimating the reliability solder joint. An intermetallic compound (IMC) layer between the solder and the substrate occurs during the soldering process, establishing a metallurgical connection. However, the growth of this IMC can drastically deteriorate the physical and mechanical characteristics of the solder joints. Reflowing electronic assemblies several times, as is frequent in real-world applications, can increase the IMC thickness, compromising the integrity of the solder joint. This study investigates the influence of reflow number on both IMC layer growth and shear strength of solder joints. Individual solder joints of SAC305 and SAC-3.3Bi were assembled on printed circuit boards (PCBs) with an organic solderability preservative (OSP) surface finish and reflowed up to six times. The microstructure and thickness of the IMC layer were examined using scanning electron microscopy (SEM). Shear strength at various shear rates was measured using a Nordson DAGE test system. The failure mode and failure surface morphology were identified using optical microscopy. The results revealed that the IMC thickness increased with the number of reflows for both solder alloys. The shear strength dropped slightly as the number of reflows increased, and all sample surfaces exhibited ductile fractures, indicating that the shear strength of these joints is insensitive to the IMC thickness and morphology. The shear strength and IMC thickness were mathematically fitted with the number of reflows.