An Evaluation of Wafer Bumping Stencils Based on Solder Transfer Ratios and Predicted Bump Height

摘要

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. Ratios have then been measured for different stencil designs (flip chip versus CSP) and stencil types (laser cut versus nickel plate-up), as well as for different solder alloys (leaded versus lead free) and solder deposition techniques (squeegee blade versus enclosed print head). These transfer ratios, along with the corresponding stencil dimensions, are then fed into a solder bump height model. As the correlation of predicted to measured bump height is very strong, so to is the concept of the solder transfer ratio. Further, a correlation of measured solder paste mass to measured bump height is generated to show how target bump height and bump height variation can be controlled and monitored prior to reflowing a printed wafer.