Enabling Cu Pillar Flip Chip Assembly with Water Soluble Lead-free Solder Paste

This paper reports the evaluation of printability along with attachment of Copper (Cu) Pillar Flip Chip (FC) and passive component, such as 008004, for the solder paste AP5112 T7. The solder paste can sustain continuous printing up to 8hrs using $70\mu \mathrm{m}$ stencil opening and $50\mu \mathrm{m}$ line spacing. The pot-life and shelf-life of the paste have been evaluated to be 72hrs and 4 months, respectively. The solder paste has been designed for use in Systems in Package (SIP) such as components including 008004 and Cu Pillar FC. The advantage of using this paste would be the single step stencil printing process for attachment of both passive(s) and Cu pillar FC, without involving flux dipping/printing. The results indicate the paste is suitable for Cu pillar FC attachment, with <1% voids, enable consistent stand-off heights, with stencil opening, down to 90%. Results using 008004 components show <1% voids, no solder beads and no tombstones.