Latest Technological Advancements in Stencil Printing Processes for Ultra-Fine-Pitch Flip Chip Bumping up to 60µm Pitch

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 brings up all the technological challenges and up to date advancements in all processing steps of 6wafer bumping of peripheral array structures at Ultra-fine-pitches (UFP) of 100µm pitch and especially 60µm pitch. Innovative electroformed stencils have been manufactured with a thickness up to 20µm. Both type 7 (2-11µm) and type 6 (5-15µm) pastes of eutectic composition Sn63/Pb37 have been successfully employed for wafer bumping at 100µm pitch. Bumping using 25 µm stencil thickness has yielded bump heights of 42.3±3.8µm and 43.6±3.5µm for type 7 and type 6 pastes, respectively. A newly developmental type 8 paste (2-8µm) has been for first time used for bumping of chips with peripheral contacts at 60µm pitch. A 20µm thick electroformed stencil was used with 35µmx80 µm oblong apertures. Printing at 60µm pitch has yielded very promising results and has proved the capability of electroformed technology to manufacture accurate and robust thin stencils. The bump height at 60µm pitch was measured to be 28 ±3 µm. The present study provides insights into the processing issues for further development of UFP technologies such as Ultra fine paste printing behaviour, its reflow and slump characteristics as well as discussion in stencil manufacturing and printing machine alignment issues for UFP bumping.