Kozo Shimizu

Solder joint reliability of indium-alloy interconnection

Recent high-density very large scale integrated (VLSI) interconnections in multichip modules require high-reliability solder interconnection to enable us to achieve small interconnect size andlarge number of input/output terminals, and to minimize soft errors in VLSIs induced by α-particle emission from solder. Lead-free solders such as indium (In)-alloy solders are a possible alternative to conventional lead-tin (Pb-Sn) solders. To realize reliable interconnections using In-alloy solders, fatigue behavior, finite element method (FEM) simulations, and dissolution and reaction between solder and metallization were studied with flip-chip interconnection models. We measured the fatigue life of solder joints and the mechanical properties of solders, and compared the results with a computer simulation based on the FEM. Indium-alloy solders have better mechanical properties for solder joints, and their flip-chip interconnection models showed a longer fatigue life than that of Pb-Sn solder in thermal shock tests between liquid nitrogen and room temperatures. The fatigue characteristics obtained by experiment agree with that given by FEM analysis. Dissolution tests show that Pt film is resistant to dissolution into In solder, indicating that Pt is an adequate barrier layer material for In solder. This test also shows that Au dissolution into the In-Sn solder raises its melting point; however, Ag addition to In-Sn solder prevents melting point rise. Experimental results show that In-alloy solders are suitable for fabricating reliable interconnections.

Development of second-level connection method for large-size CPU package

This paper reports on second-level interconnection development for a large-scale Ball Grid Array (BGA) package. Generally, control of warpage becomes a problem as BGA packages become larger. To solve this problem, the following two measures were executed. The first was adoption of a low-temperature solder, and the second was warpage control using a heat spreader as a fixture. We were able to decrease the reflow temperature to 200°C by applying the low-temperature solder, and the effect was a warp reduction of 200 μm. Moreover, the shape of the heat spreader was optimized through a thermal-stress simulation, obtaining a warp reduction of 100 μm. Verification with a test vehicle was executed, no short/opening was observed, and the results of a thermal cycle test and simulation confirmed there was no problem in reliability.