A study of SnAgCu solder paste transfer efficiency and effects of optimal reflow profile on solder deposits

The reliability of solder joints in electronic products are greatly enhanced by good stencil printing and quality reflow soldering. The stencil printing process is widely used in Surface Mount Technology (SMT) to deposit solder paste on the substrate and is a critical step in SMT assembly as it has been widely claimed that up to 50% of the defects found in the assembly of printed circuit boards (PCBs) are attributed to stencil printing. Solder paste release from stencil during printing is a key factor which affects the quality of solder prints. Thus, the efficient transfer of paste from stencil through aperture to pad is a fundamental concern in SMT production process. The recent trends on further miniaturisation of electronic products have introduced more process challenges in the SMT assembly. The assembly of surface mount packages such as flip chips, chip scale packages and fine pitch ball grid arrays are challenging the current stencil printing and reflow profiling capabilities. The effective mounting of these miniaturised packages requires good transfer efficiency of solder paste through small stencil aperture. As further electronic product miniaturisation culminates in progressive use of decreasing stencil aperture sizes, the in-depth understanding of the dependency of transfer efficiency of solder paste on diminishing stencil aperture areas has become vital to improving solder joint eliability. This study investigates the transfer efficiency of type 3, 96.5Sn3.0Ag0.5Cu solder paste through linearly decreasing rectangular stencil aperture sizes typically used in PCB assembly. In addition, the research examines the effects of optimal reflow profile (ORP) on the solder deposit volumes. The results from the study show a power law relationship between actual solder deposit volume (SDV) and aperture cavity. The observed effect of ORP on actual SDVs was a 46% volume change which was fairly constant across the pad geometries. Our study shows that the transfer efficiency of solder paste decreases with decreasing stencil aperture size. A novel mathematical model gT ¼ 100e�jVA�VC j�jVA�VCþDmþe10�5 D�1 m j for VC P 0 is proposed for computing the transfer efficiency of solder paste through rectangular stencil aperture sizes