Stencil technology for wafer level bumping

Wafer level stencil printing of a Type-6 Pb-free SAC solder paste was statistically evaluated at 200 mu m and 150 mu m pitch using three different stencil manufacturing technologies: 1) laser-cut, 2) DC electroformed & 3) microengineered electroformed stencil. Factors evaluated in this study include differences between stencils in terms of printability, pitch resolution, maximum bump height achievable, print coplanarity, paste release efficiency and stencil frequency cleaning. Box plots were used to graphically view print performance over a range of aperture sizes for the three stencil types. Stencil technology impacts print performance, with the microengineered electroformed stencil producing the highest bump deposits and the lowest bump height deviation followed by the conventional electroformed and finally the laser cut stencil. Comparisons between the 1st and 5th print demonstrate consecutive printing without stencil cleaning is possible for all but the smallest spacings between apertures. High paste transfer efficiencies above 85% were achieved with the microengieered stencil using low aperture area ratios of 0.5.