Solidification Condition Effects on Microstructures and Creep Resistance of Sn-3.8Ag-0.7Cu Lead-Free Solder
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THE transition of the electronic industry to the leadfree soldering process requires a clear understanding of the reliability of lead-free solder joints. There are many studies dedicated to the study of mechanical properties of lead-free solder materials, including tensile properties, creep, and fatigue, under different conditions.[1–7] These mechanical properties data are valuable to understanding short- and long-term solder joints reliability. However, most mechanical tests are conducted using bulk samples prepared by casting of solder alloys under different solidification conditions. Mechanical data generated from real solder joints produced with a realistic soldering process are better representative of the real microstructure in application. The drawback for mechanical properties based on small solder joints is that the testing samples often involve a number of variables related to complicated geometry and mechanical properties of PC boards and electronic components. It is obvious that most mechanical test samples, bulk or real solder joints, are manufactured under different solidification conditions, and thus have different mechanical properties. It is not surprising to have one or two orders of magnitude difference in comparison of fatigue life of solder joints J. LIANG, Consulting Scientist, and N. DARIAVACH, Principal Engineer, are with the EMC Corp., Hopkinton, MA 01748, USA. Contact e-mail: [email protected] D. SHANGGUAN, Vice President, is with Flextronics, San Jose, CA 95131, USA. J. LIANG (Specially appointed Guest Professor, Chongqing University, Chongqing, 400044, P.R. China) is currently on sabbatical leave from EMC Corp., Hopkinton, MA 01748, USA. This article is based on a presentation made in the symposium entitled ‘‘Solidification Modeling and Microstructure Formation: in Honor of Prof. John Hunt,’’ which occurred March 13–15, 2006 during the TMS Spring Meeting in San Antonio, Texas, under the auspices of the TMS Materials Processing and Manufacturing Division, Solidification Committee. Article published online July 3, 2007. 1530—VOLUME 38A, JULY 2007
under a similar testing condition. The most fundamental metallurgical reasons for such large scattering in mechanical property data for solders are due to the difference in solidification conditions, which are largely unmeasured and not reported in most cases. Nonetheless, there are few publications dedicated to the study of the effects of cooling rates on mechanical properties of lead-free alloys recently.[8,9,10] In the study of Ochoa et al.,[8,9] the effects of cooling rates on tensile behavior and creep of Sn-3.5Ag alloy were investigated with cooling rates of 24 C/s, 0.5 C/s, and 0.08 C/s. In general, the effects of solidification conditions on mechanical properties of the currently most promising lead-free solder candidate, SnAg-Cu eutectic alloy, are still not well known and require additional investigation. The SMT reflow process with application of lead-free solders is characterized with an increased maximum temperature around 250 C and up to
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