Measurement of impact toughness of eutectic SnPb and SnAgCu solder joints in ball grid array by mini-impact tester
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Kejun Zenga) and Rajiv Dunne Texas Instruments, Inc., Dallas, Texas 75265 (Received 5 July 2007; accepted 25 February 2008)
The most frequent cause of failure for wireless, handheld, and portable consumer electronic products is an accidental drop to the ground. The impact may cause interfacial fracture of ball-grid-array solder joints. Existing metrology, such as ball shear and ball pull tests, cannot characterize the impact-induced high speed fracture failure. In this study, a mini-impact tester was utilized to measure the impact toughness and to characterize the impact reliability of both eutectic SnPb and SnAgCu solder joints. The annealing effect at 150 °C on the impact toughness was investigated, and the fractured surfaces were examined. The impact toughness of SnAgCu solder joints with the plating of electroless Ni/immersion Au (ENIG) became worse after annealing, decreasing from 10 or 11 mJ to 7 mJ. On the other hand, an improvement of the impact toughness of eutectic SnPb solder joints with ENIG was recorded after annealing, increasing from 6 or 10 to 15 mJ. Annealing has softened the bulk SnPb solder so that more plastic deformation can occur to absorb the impact energy.
I. INTRODUCTION
Recently, drop-impact-induced solder joint fracture has become one of the critical system failure modes of interest in the electronics industry due to the increasing use of Pb-free solder joints in wireless, handheld, and portable applications. For example, cellular phones might be broken due to impact by dropping. While the weight of personal digital assistant (PDA) devices has been reduced greatly, ball grid arrays (BGAs) of solder joints and chip size packages (CSPs) have been adopted to reduce mounting areas effectively. Often, the major damage of the BGA/CSP solder joints is not from thermal stress fatigue but from high speed stresses in drop impacts.1 Solder joints using Pb-free materials have been found to be more brittle than those using SnPb. To evaluate the joint reliability between BGA ball and the bond-pad, the shear test and pull test are widely used in industry.2 These tests usually cannot evaluate the impact reliability of solder joints because the testing speeds are typically lower than 1 mm/s, which is two to three orders of magnitude below the velocity of impact applied to solder joints by dropping. A previous study by Chiu et al.3,4 proved that there was a very strong correlation between drop reliability and voiding at the solder/Cu pad interface. However, ball shear testing did not correlate it a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2008.0186 1482 J. Mater. Res., Vol. 23, No. 5, May 2008 http://journals.cambridge.org Downloaded: 31 Mar 2015
to drop test performance, and the ball pull strength was not a good indicator of drop/impact reliability, either. Therefore, how to characterize the impact reliability induced by dropping becomes very crucial, especially for the Pb-free solder joints. Recently, a research group of Hitachi Metals, Ltd., Japan5,6 propo
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