Stencil printing technology for wafer level bumping at sub-100 micron pitch using Pb-free alloys

In this paper solder paste printing is reported at sub 100 mu m pitch using Pb-free solder paste with IPC type-6 (15-5 mu m) particle size distributions. The results confirm that consistent sized paste deposits can be produced onto wafers at ultra fine pitch geometries using a stencil printing process. Furthermore, a stencil printing evaluation has determined the impact that the print parameters have on the reproducibility of the deposits. The investigation also reveals that the volume of solder paste deposit can be controlled by selecting different shapes of stencil apertures. Large volumes of paste are required during reflowing of the fine particle solder paste to produce sufficient stand-off between the flip chip device and substrate. Print consistency and uniformity of the bumps generated are also governed by the volume of solder paste for each deposit. Statistical examinations of printing defects from a large number of printing trials have been conducted for several bump geometries. Subsequently the best print parameters were then used to print onto wafers containing bond pads so the paste deposits could be reflowed to form solder spheres for analysis.