Effect of Differently Shaped Solder Joints of Chip Resistor on Fatigue Life

As surface mount components shrink in size, smaller apertures on stencils during solder paste printing can lead to uneven solder volume on a single resistor. This can result in the formation of irregular solder shapes, which are often deemed acceptable criterion depending on assembly house. However, such irregularities can potentially introduce variations in the fatigue life of the surface mount component. This study employs experimental and numerical approaches to investigate the reliability of differently shaped SAC305 solder joints on a single chip resistor. Four distinct solder shapes, concave, straight, tiny convex, and convex, are generated using varying volumes of solder paste controlled by uniquely designed apertures on a stencil. Eight pairs of differently shaped solder joints are assembled to hold a chip resistor in place. The test assembly involves R1005 and R0402 soldered with SAC305 and undergoes thermal shock cycling. As a single chip resistor assembly consists of two solder joints connecting termination and pads, the effect of solder volume difference between two solder joints are investigated. The fatigue life corresponds to decrease as the solder volume difference increase as 40%, 80%, and 120%. Conversely, total volume of two solder joints in a single chip resistor increases in scenarios with the same volume difference between two solder joints. Experimental data and finite element analysis lead to a new set of Darveaux's constants specific to this assembly. This study provides guideline to control the solder paste volumes in relation to fatigue life and enables numerical solution using a new Darveaux's constants.