Analysis of Solder Paste Release in Fine Pitch Stencil Printing Processes

Advanced electronics packaging technologies such as chip scale packages, fine pitch ball grid arrays, and flip chip are pushing solder paste stencil printing to the limit. In order to achieve solder print deposits of the sizes required for emerging electronic packaging technology, a rigorous understanding of the process is required. This paper seeks to expand our understanding of the physical characteristics of stencil printing specifically focusing on the solder paste release process based on experimental and analytical approaches. First, designed experiments were conducted to identify the main process variables affecting final print quality. An in-situ measurement system using a high speed imaging system monitored the solder paste release process. Based on experimental observations, different modes of solder paste release and their corresponding mechanisms were identified. A model was developed to predict print quality for fine pitch applications. The proposed model was experimentally verified showing good agreement with measured values for fine pitch and very fine pitch printing. It was found that the cohesive and adhesive forces acting on the paste tend to govern the release process rather than the viscous and inertial forces.