Ph.D. Proposal Presentation by Min Pei
Wednesday, April 27, 2005

(Dr. Jianmin Qu, Chair)

"Sn/Ag Based Lead Free Solder Under Fatigue Load"


Solders are used extensively as electrical interconnects in microelectronics packaging. Lead-based alloys have been the de facto solder materials due to their low melting temperature, high ductility and good wetting to other metals. Because of environmental concerns, lead-based solders are being replaced by Sn/Ag and Sn/Ag/Cu based solder materials. Since the thermomechanical reliability of modern electronic devices depends on, to a large extent, the fatigue and creep behavior of the solder joints, it is imperative to understand the fatigue and creep behavior of these new lead-free solders, and to formulate new alloy compositions that will have equal or better performance than the traditional lead-based solders.

To this end, this study will focus on two aspects. First, extensive thermomechanical testing will be conducted on several commercial lead-free solder alloys. The purpose is to develop constitutive laws that can be used to describe the fatigue and creep behavior of these materials. Specifically, we will investigate, based on the test data, the suitability of exiting constitutive models developed originally for lead-based solder alloys. If necessary, new or modified constitutive models better suited for lead-free solders will be developed. Second, modifications to commercial lead-free solders will be made by introducing additives. Specifically, nano-size oxide particles will be added to improve fatigue and creep resistance, and appropriate rare earth elements will be added to improve the wetting performance.

Major results expected from this research include (1) accurate constitutive laws for lead-free solders used in electronic packaging, and (2) optimized lead-free solder alloys with high fatigue and creep resistance and superior wetting property. The constitutive laws will enable more efficient design, fabrication and operation of microelectronic packaging using lead-free solders; and the new lead-free solder alloys developed here will improve packaging assembly yield and reliability.