Effect of Electromigration on Mechanical Behavior of Solder Joints
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Effect of Electromigration on Mechanical Behavior of Solder Joints Fei Ren 1, Jae-Woong Nah1, Hua Gan 1, Jong-ook Suh1, King-Ning Tu1 Bingshou Xiong2, Luhua Xu 2, John Pang2 1. Dept. of Materials Science and Engineering, UCLA. Los Angeles, CA 90095-1595; 2. Dept. of Mechanical and Production Engineering, Nanyang Technological University, Singapore
ABSTRACT Electromigration in solder joints causes a void formation between intermetallic compounds (IMC) and solder at the cathode. The effect of electromigration in mechanical test of Cu wires joined by solder was performed. The current density of electromigration was 1~5×103 A/cm 2. The working temperature was 100~150ºC. Tensile stress and shear stress were applied either before or after electromigration. The tensile strain rate was 3 µm/min. We observed that, without electromigration, tensile stress caused a ductile break at the middle of solders because the solder was softer. On the other hand, if combined with electromigration, a brittle failure always occurred at the cathodes interface during tensile test. The ultimate tensile strength decreased with longer electromigration time or higher current density. In shear test, the daisy chain of solders failed alternatively at the cathodes after electromigration.
INTRODUCTION Solder joints are widely used to bond chips to their substrates for electrical connection and packaging.[1] The electronic industry is actively searching for Pb-free solders due to environmental concern over Pb-containing solders. Due to the unique geometry of a flip chip solder joint, composition of solder alloys, and thin film metallization used at the interfaces of a joint, electromigration in flip chip solder joints is very different from that in Al and Cu interconnect.[2-5] Electromigration of solder joints becomes a serious reliability problem in electronic packaging. It is a phenomenon of coerced mass flow driven by a high current density.[6] Mechanical property of solder joint is important in evaluating the quality of electronic packaging. Solder joints exhibits a different mechanical behavior with or without electromigrtaion. In this paper, the tensile test and shear test were taken to show a different failure mode of solder joints after electromigration.
EXPERIMENTAL RESULTS AND DISCUSSION One dimensional copper-solder-copper samples were developed to study the mechanical property with electromigration. Fig.1 shows the sample preparation for this study. As shown in Fig. 1(a), a V-groove with 300µm opening was created on a silicon wafer. Two copper wire electrodes were aligned by the V-groove. The eutectic SnCuAg
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solder was reflowed between the two electrodes at 250ºC. The number of the solder balls could be changed to adjust the length of the solder between the electrodes. After reflow, the sample was taken out from the V-groove as shown in Fig.1(b). The advantage of this test structure is that mechanical stress and electric current can be applied simultaneously. And unlike bulk solder samples; this structure involves i
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