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Investigation of Overprinting BGA Pads
The added benefit of using overprinting stencil design strategy is that this should compel further improvement in paste transfer efficiency and uniformity by raising its area ratio. In this study the circle shape overprint apertures were designed for 0.5mm pitch BGA land patterns that measured to an area ratio value of 0.70.
Technical Paper
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Authored By:
Jeff Schake
ASMPT SMT Solutions
GA, USA
Summary
Stencil apertures for BGA components are traditionally round shaped to match the land pad geometry, and when spacing permits, it is common to enlarge the aperture size relative to the pad to achieve an overprint condition. The added benefit of using overprinting stencil design strategy is that this should compel further improvement in paste transfer efficiency and uniformity by raising its area ratio. In this study the circle shape overprint apertures were designed for 0.5mm pitch BGA land patterns that measured to an area ratio value of 0.70.
While such sized apertures should produce quite acceptable print quality, the resulting print volumes for a customer application were unexpectedly non-uniform and lower in solder paste transfer efficiency despite using a well setup stencil printing machine and process. Deeper investigation identified pad design factors interacting with stencil aperture gasketing thought to be the main factors contributing to inhibited paste transfer performance. This body of work ultimately serves to improve stencil design logic applied to BGA printing applications.
Conclusions
Based on the series of simple print tests conducted here, an aperture designed at 0.7 area ratio has been demonstrated to achieve an average print volume transfer efficiency level of 85% at a standard deviation level of 3% on a bare featureless substrate. This test condition offers the greatest opportunity to achieve the best possible print results. When including proper board topography using the same stencil apertures to overprint 0.5mm pitch BGA non-solder mask defined pads, a similar level of print volume transfer efficiency performance was achieved, but with more scatter in the volume distribution (i.e. larger standard deviations).
The largest misregistration of solder mask openings was found to raise the printed average volume transfer as expected, but without widening the print scatter. It is recognized that making a general conclusion on stability of print scatter under this severe level of solder mask misalignment is ignorant without due consideration to influences of outer layer copper thickness and solder mask thickness variables that were not specifically measured and tested in this work. Although the influence of a stencil nano coating in this test was not strongly detected to affect print quality, the overwhelming positive track record attached to using stencils with a nano coating warrant recommending its application to all solder paste printing stencils.
Initially Published in the SMTA Proceedings
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