Solder Paste Printability Tester

Solder paste printability is one of the most critical properties in SMT (surface mount technology). Investigation has shown that poor solder paste printing results in various SMA (surface mount assembly) defectives. In this letter, a novel solder paste printability tester has been constructed with a characteristic "squeegee-roller" print motion, in which the roller served as an "annular stencil". This motion can perform a continuous scraping action and ensure an accurate control of printing distance, as well as printing pressure, angle, and speed. As a result, this tester enable both online and offline method to evaluate the solder paste printing performance. Offline method such as soldering ball test and wetting test reveals that, with the printing distance increasing, flux residues became darker; solder beading number and wetting time increased, while wetting area reduced. These behaviors represented the decay of printability, wettability, solderability, etc. Based on the rolling phenomenon upon solder paste printing, a tooth-like stainless wire were immersed in and rotated with the paste sample, and thus the rolling rate can be detected. It is found that at first this rate decreased gradually, and finally dropped down suddenly to zero. The corresponding critical printing distance was suggested to act as a quantitative criterion to evaluate the printability of solder pastes. The results of the project illustrate that combination of offline and online method is a complete approach for solder paste printability evaluation and improvement