The micro-impact fracture behavior of lead-free solder ball joints

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D.S. Liu Department of Mechanical Engineering, National Chung Cheng University, Chia-Yi, Taiwan 621, Republic of China (Received 30 August 2007; accepted 4 January 2008)

The present study investigated the micro-impact fracture behavior of various lead-free solder joints, including Sn–1Ag–0.1Cu–0.02Ni–0.05In, Sn–1.2Ag–0.5Cu–0.05Ni, and Sn–1Ag–0.5Cu. The fracture that occurs within the solder joint corresponds to a higher impact fracture energy (1.35 mJ), while the fracture at the interface between the solder joint and intermetallic compound acquires a smaller impact energy (0.82 mJ). Two types of fracture mechanisms were proposed based on observations of the fracture morphology and the impact curve for the solder ball joints. The longer deflection distance, referring to better elongation, exists for the mechanism corresponding to the higher fracture energy.

I. INTRODUCTION

Recently, due to environmental and health concerns, a variety of new lead-free solders have been developed. In 2000, the National Electronics Manufacturing Initiative (NEMI) recommended replacing Sn–Pb solders with Sn–Ag–Cu solders.1 Since then the lead-free solder alloys have been widely used in specific chip-scale package. Unlike Sn–Pb solders, research on lead-free solder joints lacks a mature manufacturing process and a boardlevel reliability test. Further, information is limited on their mechanical properties and applications over a long time period. The ever-increasing changes in package density and the number of input/output (I/O) pads have rendered solder joint reliability a rather critical issue. The drop test is one of the most common assessments for high-quality products. But the adoption of the Joint Electronic Devices Engineering Council (JEDEC) drop test condition may not be cost effective and efficient.2 An electronic package is generally subjected to different thermal and mechanical loads during manufacturing and application. The solder joint provides the electrical and mechanical connections for a chip–substrate or substrate–board combination. The strength of the solder joint is a key factor that affects the reliability of the integrateda)

Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2008.0129 J. Mater. Res., Vol. 23, No. 4, Apr 2008

circuit package. Over the past decades, a considerable number of investigations of the correlation between joint strength and fracture behavior have been reported.3–6 For the joint-strength evaluation, the methods of investigation that have been adopted include the shear test, the pull test, and the impact test.7,8 Most of the studies were focused on the strength and reported the fracture morphology of the constituents of the solder joint, such as intermetallic compounds (IMCs) and bulk solder.9 A number of studies10,11 have investigated the shear strength of solder joints; yet, the shear mechanism is still controversial. The investigations revealed that the intermetallic layer does not exhibit a significant effect on the shear strength of the sold

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