Thermal Fatigue Behavior of Sn-Rich (Pb-Free) Solders
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INTRODUCTION
INCREASING concerns over the environmental and health hazards of Pb-Sn solders used in electronic packaging have prompted the need for Pb-free solder alternatives.[1–3] The design and development of Pb-free solders for electronic packaging requires a thorough understanding of microstructure and its effect on properties.[1,4] The thermal fatigue resistance of Sn-rich solder joints is dependent on the initial microstructure as well as on the damage evolution during thermal cycling. Extensive work has been done on characterizing the life of solder-joint specimens by testing entire electronic packages.[2–8] This approach enables the determination of the overall reliability of the component, from the macroscopic mechanical response of the solder joints. A better way of elucidating the fundamental microscopic mechanisms for fatigue damage is through singlejoint experiments. Several studies have been conducted on the thermal fatigue behavior of individual solder joints.[9–16] Subramanian and co-workers[9–13] examined the effect of dwell times and ramp rates on the damage accumulation in lap-shear solder/Cu joints. Microstructural analysis showed that faster heating rates resulted in more localized damage, particularly crack formation at Sn grain boundaries. The Sn grain orientation may also influence the damage evolution in Sn-rich solder joints during thermal fatigue, due to the elastic and thermal anisotropy of Sn.[17–24] This anisotropy may induce local stress mismatch at Sn-grain boundaries, resulting in sliding or separation at the grain boundaries.[19,22] R.S. SIDHU, formerly Graduate Research Associate, School of Materials, Fulton School of Engineering, Arizona State University, is with Assembly Technology Development, Intel Corporation, Chandler, AZ 85226. N. CHAWLA, Professor is with the School of Materials and Department of Aerospace Engineering, Arizona State University, Tempe, AZ 85287-8706. Contact e-mail: [email protected] Manuscript submitted June 26, 2007. Article published onlined February 16, 2008 METALLURGICAL AND MATERIALS TRANSACTIONS A
In lap-shear joints, it has been shown that a stress concentration at the corner of the joint is the likely site for crack initiation. Thus, it is necessary to determine whether Sn grain anisotropy or solder-joint geometry effects are responsible for crack initiation. Recently, Matin and co-workers[22–24] studied the damage evolution and grain orientation in solder balls and solder/Cu joints. They found very different damage mechanisms occurring for the different specimens. In the individual solder balls, damage was localized along Sn grain boundaries (confirmed by elastic anisotropy analysis). In the joints, where the solder was constrained by copper, damage was also observed, in the form of persistent slip bands and crack formation near the Cu/solder interface, due the coefficient of thermal expansion (CTE) mismatch between the solder and Cu. In general, the failure of solder interconnects during thermal fatigue has often been correlated with the region of largest
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