Is 5 the Magic Number?

摘要

It doesn't seem long ago I wrote a piece on Type 4.5 solder pastes and their printing advantages for miniaturized geometries. A quick search of my archives, however, revealed it's been nearly four years since that column appeared! In that time, much has changed in terms of manufacturers' movement toward the integration of devices like 01005s and 0.3mm CSPs, as well as the number of evaluations of metric 0201s and metric 03015s taking place. More and more, our printing applications experts are being asked about 150um deposit sizes and the process implications of such small material volumes. This, indeed, pushes the process limits and leads me back to the basics of what I refer to as the "five particle rule." In short, this rule I was taught when I began in SMT states that if the particle size of the solder paste is not such that five particles will fit shoulder to shoulder on the narrowest aperture, it's likely an indication of potential printing problems. And, these issues are regardless of speeds, squeegee pressures, stencil thickness, board quality and even the printer; they are solely dictated by particle count per aperture. While I've never come across any paper that explains the physics of this rule, in my experience it has proven to be absolutely accurate.