Technical challenges of stencil printing technology for ultra fine pitch flip chip bumping

Stencil printing remains the technology route of choice for flip chip bumping because of its economical advantages over traditionally costly evaporation and electroplating processes. This paper provides the first research results on stencil printing of 80 μm and 60 μm pitch peripheral array configurations with Type 7 Sn63/Pb37 solder paste. In specific, the paste particle size ranges from 2 μm to 11μm with an average particle size of 6.5 μm taken into account for aperture packing considerations. Furthermore, the present study unveils the determining role of stencil design and paste characteristics on the final bumping results. The limitations of stencil design are discussed and guidelines for printing improvement are given. Printing of Type 7 solder paste has yielded promising results. Solder bump deposits of 25 μm and 42 μm have been demonstrated on 80 μm pitch rectangular and round pads, respectively. Stencil printing challenges at 60 μm pitch peripheral arrays are also discussed.