Heat Resistant Cu-Sn based Joint Paste for less than 30μm joint thickness

Abstract

We have developed a heat resistant joint paste by Cu-Sn IMCC (Intermetallic Compound Composite) for thin joint thickness. The developed IMCC paste contains only IMCC fine particles as a metal composite. Re-melting point of the paste can be above the sintering temperature. For example, 320C remelting point was observed by 280C sintering. The paste is adoptable for heat resistant joint. The paste also indicated there is no whisker in joint area after TCT 1,000cycles (-55~+175C). When the joint thickness to be thin, voids in the joint region becomes conspicuous. There are two root causes to form voids. The first is non-uniform interaction between joint material and objects (joint targets). The second is gas void. For the paste, we chose a simple composition using only IMC source (fine particles which already contains IMC micro-colonies in them) as a metal component to provide moderate and uniform interaction with joint targets. To eliminate gas voids, two-stage sintering process was adopted. The first sintering (pre-sintering) is to evaporate all volatiles from the paste and to make appropriate interconnection with the substrate keeping the paste shape convex and providing enough joint strength for subsequent cleaning treatments. The second sintering (main-sintering) is for the device joint to the paste without voids. For making thin joint thickness around 30μm, weak pressure onto the device (~0.7MPa) is effective. Test sample with Si die (700μm thick, 6mm width) with Ni/Au finished backside metal was mounted on to Cu substrate. The joint region is almost filled by grown IMC layers. The thickness of the joint region is 30μm for entire joint region without voids.