A Study of Impact Reliability of Lead-free BGA Balls on Au/Electrolytic Ni/Cu Bond Pad
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A Study of Impact Reliability of Lead-free BGA Balls on Au/Electrolytic Ni/Cu Bond Pad Shengquan Ou, Yuhuan Xu and K. N. Tu Department of Materials Science and Engineering, UCLA, Los Angeles, CA, 90095-1595 M. O. Alam, and Y. C. Chan Department of Electrical Engineering, City University of Hong Kong, Kowloon, Hong Kong ABSTRACT The most frequent failure of wireless, handheld, and movable consumer electronic products is an accidental drop to the ground. The impact may cause interfacial fracture of wire-bonds or solder joints between a Si chip and its packaging module. Existing metrologies, such as ball shear, and pull test cannot well represent the shock reliability of the package. In this study, a micro-impact machine is utilized to test the impact reliability of three kinds of lead-free solders: 99Sn1Ag, 98.5Sn1Ag0.5Cu and 97.5Sn1Ag0.5Cu1In (hereafter called Sn1Ag, Sn1Ag0.5Cu, and Sn1Ag0.5Cu1In). The effect of thermal aging on the impact toughness is also evaluated in this study. We find a ductile-to-brittle transition in SnAg (Cu) solder joints after thermal aging. The impact toughness is enhanced by the thermal aging. This is a combination effect of the growth of intermetallic compound (IMC) at the interface provided strong bonding, and the softening of the solder bulk during the thermal aging absorbed more energy during plastic deformation. INTRODUCTION The drop impact induced solder joint fracture has become one of the critical system failure modes of interest in the electronics industry due to the migration of market focus to portable applications. For instance, cellular phones might be broken due to impact by dropping. Especially, in recent years, while the weight of the personal digital assistant (PDA) devices is reduced greatly, ball grid arrays (BGAs) and chip size packages (CSPs) are adopted to effectively reduce mounting areas. The major damage of the BGA/CSP solder joints is not from external temperature changes but short-time stresses like drop impacts [1]. To evaluate the joint reliability between BGA ball and under bump metallization (UBM) pad, industry widely uses shear test and pull test [2]. Unfortunately, those tests can not evaluate the impact reliability of solder joints, because the testing speeds are typically lower than 1 mm/s, well below the velocity of impact applied to solder joints by dropping. Some previous study by Chiu et. al. [3] proved that there is a very strong correlation between drop reliability and voiding at the UBM/solder interface. However, ball shear testing does not correlate to drop test performance, and ball pull strength is not a good indicator of shock reliability either. Therefore, how to characterize the impact reliability induced by dropping becomes very crucial. Recently, the research group from Hitachi Metals, Ltd., Japan [4,5] proposed a miniature impact test for solder bumps by adopting the principle of the classic Charpy impact test [6]. Based on their work, we have built a micro-impact testing machine to quantitatively study the bonding strength betwe
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