Quality and yield of ultra fine pitch stencil printing for flip chip assembly
Stencil printing for SMT and fine pitch BGA structures is established as a low cost standard process. Using the same equipment with modified printing parameters and materials, a low cost bumping process has been transferred to serial production. This paper presents the results of ultra fine pitch stencil printing of solder paste on wafers (down to 200 mu m and 150 mu m pitch) with regard to quality and yield. A software tool for design of the stencil layout was developed and the predicted bump heights were compared to the experimental results. In the first part of the paper, a low cost bumping method for flip chip technology is described in detail. This technology is based on chemical Ni/Au deposition on wafers. For solder bumping on wafers, the key aspects of solder paste printing with optimized printable apertures are described and the printing results are presented. The second part of this paper is a comparison of measured standard deviations of bump heights and the quality demands for ultra fine pitch flip chip assembly.