Flip chip interconnect systems using wire stud bumps and lead free solder

Abstract

This research focuses on flip chip interconnect systems consisting of wire stud bumps and solder alloy. Conventional gold (Au) wire stud bumps and new copper (Cu) wire stud bumps were formed on the chip by wire bumping. Cu wire studs were bumped by controlling the ramp of ultrasonic power to eliminate the occurrence of under-pad chip cracks which tend to occur with high strength bonding wire. Lead free 96Sn3.5Ag0.5Cu alloy was used to interconnect the wire studs and printed circuit board. A comparison is made with conventional eutectic 63Sn37Pb alloy and 60In40Pb alloy. A more stable solder connection was created when Cu wire stud bumps were used, compared with Au wire stud bumps. The improved stability is due to reduced intermetallic compound formation with the solder alloy. Test vehicles were assembled with two different Direct Chip Attachment (DCA) processes. When a conventional flip chip assembly and reflow was used, the lead free test vehicles exhibited process failure. On the other hand, when solder reflow and underfill cure were performed at the same time by using a high accuracy flip chip bonder, the reliability of lead free test vehicles in thermal shock improved.