Flip-chip on FR-4 integrated circuit packaging

Technology advances have made it possible to extend the IBM C4 (Controlled Collapse Chip Connection) IC attachment method, also known as flip-chip, to printed circuit boards with low-cost dielectrics, such as FR-4. This new technology is referred to as Flip Chip Attach (FCA). In the past, C4 attachment had been limited to ceramic substrates due to the high temperature (360/spl deg/C) required to renew the low tin solder on the chips, and the need for the substrate to have its thermal expansion relatively closely matched to that of the silicon chip. The development of methods to deposit uniform, small (approximately 1/spl times/10/sup -3/ mm/sup 3/) volumes of eutectic tin/lead solder on printed circuit cards has allowed C4 bumped chips to be reflow soldered to the cards using standard SMT joining time/temperature profiles. The high melt (3-5% tin, balance lead) solder bump on the chip is used simply as a solder wettable device lead in this case. The resulting solder joint is not a true controlled collapse joint, but the process still maintains the self-aligning characteristic of C4, due to the surface tension of the molten eutectic solder and the low mass of the chip. The other enabling technology development for FCA is the use of a controlled expansion epoxy encapsulant between the chip and the substrate to minimize the cyclic strain on the solder joints induced by thermal expansion mismatch between the silicon chip and the FR-4 card. Without the use of such an encapsulant, the thermal cycle fatigue life of the FCA joints would be totally unacceptable, and the technology would be useless. In this paper we will explain how the enabling technologies work to make FCA a viable packaging method, and present reliability data for several different chips using FCA packaging. >