Effects of Sn grain structure on the electromigration of Sn−Ag solder joints

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Kuan H. Lu Intel Corp., Chandler, Arizona 85226

Vikas Gupta, Leon Stiborek, Dwayne Shirley, and Seung-Hyun Chae Texas Instruments Inc., Dallas, Texas 75243

Jay Im and Paul S. Hoa) Microelectronics Research Center, University of Texas at Austin, Austin, Texas 78758 (Received 17 November 2011; accepted 9 January 2012)

In this article, we investigated the effect of Sn grain structure on the electromigration (EM) reliability of Sn–2.5Ag (wt%) solder joints used in flip-chip packages. The electron backscattering diffraction technique was applied to characterize the Sn grain size and orientation of the solder joints. Failure analyses on Sn–2.5Ag solder joints after EM tests showed that the Sn grain structure was important in controlling the kinetics of the intermetallic compound growth and void formation under EM. Further microstructural analysis revealed that the grain sizes and orientations of the solder joints after multiple solder reflows were statistically different from those with a single solder reflow and resulted in an improved EM reliability. Thermal annealing effect was also investigated to separate the thermal effect from the EM-induced effect. Results obtained in this study demonstrated that EM reliability of Pb-free solder joints could be improved by optimization of the Sn grain structure.

I. INTRODUCTION

With continuing device scaling, electromigration (EM) has become a critical reliability concern for flip chip solder joints due to the demand in higher input/output density, higher power density, and finer interconnect pitch. The problem is further complicated by the recent European Union Directive for Reduction of Hazardous Substances, which mandated the replacement of the Pb-based solders by Pb-free solders in electronics products. This has generated extensive interests in the industry to study the EM reliability for Pb-free solders, particularly for applications in highdensity and high-performance flip-chip packages. The Pb-free solder joints that have been widely implemented by the industry are Sn-based alloys with minor alloying elements.1 One important characteristic in addressing EM reliability of Sn-based Pb-free solder joints is the formation and growth of intermetallic compound (IMC) induced by material reaction between Sn and the contact metallization. A uniform IMC layer promotes adhesion between the solder and the substrate. Excessive formation of IMC, on the other hand, can lead to complete consumption of the under-bump metallization (UBM) and a)

Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2012.10 J. Mater. Res., Vol. 27, No. 8, Apr 28, 2012

cause EM failure. Unfortunately, this is usually a more serious concern for Pb-free solder joints as compared with Pb-based solders, since the former has an ample supply of Sn that is available to react and consume the finite source of UBM material at the contact. Moreover, the IMC growth rate is enhanced under current stressing. Consequently, the kinetics of EM-enhanced IMC formation becomes a key factor for i