Semiconductor Die Attach with High Copper Solder Alloys
Authors: Keith Sweatman, Takatoshi Nishimura, and Tetsuro Nishimura Company: Nihon Superior Co., Ltd. Date Published: 2/11/2014
Pan Pacific Symposium
Abstract: With the expected expiration of the RoHS exemption for solders with more than 85% lead for die attach applications there is a need to identify lead-free solders that can replace the Sn-95Pb alloy that has been commonly used for power semiconductor die attach. While most of the research directed at finding such a replacement has focussed on alloys with a solidus temperature comparable with the 270°C of the Sn-95Pb alloy it been found that reliable performance at operating temperature up to the required 200°C can be achieved with a tin-copper alloy with a solidus of only 227°C when there is a high volume fraction of Cu6Sn5 intermetallic compound in the microstructure. That high volume fraction of intermetallic is possible because of additions to the tin-copper system that stabilize and greatly refine the grain size so that it provides a strong strengthening effect on the tin matrix of the solder that is effective even at temperature close to the solidus temperature. The fine grain size means that it is not necessary to fully dissolve the intermetallic during the soldering process so the high-copper alloy can be reflowed at a temperature of only 260°C rather than the temperatures above 300°C required with the Sn-95Pb alloy it would be replacing. In this paper the authors will describe the method used to achieve this fine, stable dispersion of intermetallic and report the result of extensive testing of the strength and reliability of joints to Si dies made with this alloy.