Influence of Stencil Thickness and Surface Finishes of Soldering Pads on the Formation of the Voids Inside Solder Joints

This article focuses on the formation of the voids inside the solder joints during reflow soldering in surface mount technology (SMT). The stencil printing technology is commonly used during SMT for the solder paste deposition on the printed circuit boards (PCBs) respectively on the soldering pads. The stencil can have different thicknesses according to the requests. The thicknesses 100, 150 and $200 \mu \mathrm{~m}$ were chosen for the study of the stencil thickness influence on the voids formation inside the solder joints. The results showed that the number of macrovoids is lowest for the thickest stencil; on the other hand, the surface area of the macrovoids is highest for the thickest stencil. Additionally, we used three different surface finishes (organic surface preservative - OSP, hot air solder levelling HASL and electroless nickel immersion gold - ENIG) of soldering pads in the experiment to study the influence of surface finishes on the voids formation. When the lowest voids formation was observed in the case of HASL surface finish and highest in the OSP.