Optimising process parameters for flip chip stencil printing using Taguchi's method

The solder paste printing process is an important process in the assembly of surface mount devices using the reflow soldering technique. There is wide agreement in industry that the paste printing process accounts for the majority of assembly defects, and most defects originate from poor understanding of the effect of printing process parameters on printing performance and the nature of their interactions. The key solder paste printing process parameters considered in this study is the squeegee pressure, squeegee speed, stencil-substrate separation speed and squeegee print direction. Our previous work shows that theses process parameters impact the printing process performance. As the current product miniaturisation trend continue for hand-held consumer products, area array type package solutions [such as chip scale packages and flip chip] are now being designed into,products. The assembly of these devices requires the printing of very small solder paste deposits consistently from pad to pad, and from board to board. This paper concerns the determination of the solder paste printing process window for flip chip assembly applications. Five different solder paste formulations [specially formulated for flip-chip assembly] were evaluated as part of a broader study on low cost solder bumped flip-chip assembly. The results have also been used for establishing guidelines for printing solder pastes for both solder bumping and the flip-chip assembly process. The experimental design for the study was based on the Taguchi method. A 2-level and 4 factors orthogonal array was used for investigating the main effects.