7.2 – Evaluation of wafer bumping stencils
In order to better characterize the performance of wafer bumping stencils, a novel solder transfer ratio has been defined that relates the amount of solder paste volume transferred to a wafer to the actual stencil aperture volume. This transfer ratio is a function of how fully an aperture fills with solder paste during the printing process, as well as how much paste is in turn released to the wafer. Knowing how the solder transfer ratio varies enables stencil designs that better achieve target bump heights with less bump height variation. Toward this end, an experimental technique has been developed that is used to measure solder transfer ratios. The technique is based on measuring the mass of transferred solder paste, and converting that mass to a transfer ratio based on the known aperture volume. Read the full article (PDF)...
This article was taken from Global SMT & Packaging Vol. 7 No. 2 - February 2007