Critical variables of solder paste stencil printing for micro-BGA and fine pitch QFP

Stencil printing continues to be the dominant method of solder deposition in high volume surface mount assembly. Control of the amount of solder paste deposited is critical for fine pitch and ultra-fine pitch SMT assembly. The process is still not well understood as indicated by the fact that industry reports 52-71% of SMT defects are related to the solder paste stencil printing process. The purpose of this paper is to identify the critical variables that influence the deposited solder paste volume, area, and height. An experiment was conducted to investigate the effects of relevant process parameters on the amount of solder paste deposited for BGAs and QFPs of 5 different pitches, ranging from 0.76 mm (30 mil) to 0.3 mm (12 mil). The effects of aperture size and shape, board finish, stencil thickness, solder type, and print speed were examined. The deposited solder paste was measured by an in-line fully automatic laser-based 3D triangulation solder paste inspection system. Analysis of variance (ANOVA) shows that aperture size and stencil thickness are the two most critical variables. A linear relationship between transfer ratio (defined as the ratio of deposited paste volume to stencil aperture volume) and area ratio (defined as the ratio of the area of the aperture opening to the area of the aperture wall) is proposed. Analysis indicates that proper stencil thickness selection is the key to controlling the amount of solder paste deposited and that the selection of maximum stencil thickness should be based on the area ratio. The experimental results are shown to be consistent with a theoretical model, which is also described.