Characterization of printed solder paste excess and bridge related defects

Surface Mount Technology (SMT) involves the printing of solder paste on to printed circuit board (PCB) interconnection pads prior to component placement and reflow soldering. This paper focuses on the solder paste deposition process. With an approximated cause ratio of 50 – 70% of post assembly defects, solder paste deposition represents the most significant cause initiator of the three sub-processes. Paradigmatic cause models, and associated design rules and effects data are extrapolated from academic and industrial literature and formulated into physical models that identify and integrate the process into three discrete solder paste deposition events - i.e. (i) stencil / PCB alignment, (ii) print stroke / aperture filling and (iii) stencil separation / paste transfer. The project’s industrial partners are producers of safety-critical products and have recognised the in-service reliability benefits of electro-mechanical interface elimination when multiple smaller circuit designs are assimilated into one larger Printed Circuit Assembly (PCA). However, increased solder paste deposition related defect rates have been reported with larger PCAs and therefore, print process physical models need to account for size related phenomena.