Effect of La on the Growth of Cu 6 Sn 5 Intermetallic Compound for Improved Sn-Pb Solder Joints
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Effect of La on the Growth of Cu6Sn5 Intermetallic Compound for Improved Sn-Pb Solder Joints Xin Ma1, Hongyuan Fang, Yiyu Qian and Fusahito Yoshida2 National Key Laboratory of Welding, Harbin Institute of Technology, Harbin, 150001, P.R.CHINA 1 National Key Laboratory of Reliability Physics of Electronic Product, P.O.Box 1501-05, Guangzhou, 510610, P.R.CHINA 2 Department of Mechanical Engineering, Hiroshima University, Higashi-Hiroshima, 739-8527, JAPAN ABSTRACT Small amount of rare earth element La were added into the traditional Sn60-Pb40 solder alloy in order to improve the reliability of solder joints. The results of aging tests of solder joints showed that, the growth of interface Cu6Sn5 intermetallic compound was depressed by La addition. Furthermore, the thermal fatigue life of solder joints was improved by 3 times. Thermodynamic analysis indicated that La had higher affinity with Sn in the Sn-Pb-La system, and then the driving force for Cu6Sn5 formation was reduced since the activity of Sn was lowered. INTRODUCTION When molten Sn-Pb solder alloy wets copper pads on the printed circuit board (PCB), Cu6Sn5 intermetallic compound (IMC) will form at the solder/copper interface [1-4]. Recently, it is reported that the interface Cu6Sn5 IMC layer has played an important role on the reliability of solder joints for microelectronic packaging at PCB level. Experimental results showed that, with the thickness of Cu6Sn5 IMC layer increasing, the thermal fatigue life [5], isothermal shear fatigue life [6] and fracture toughness [7] of solder joints will decrease. Therefore, depressing the growth of Cu6Sn5 IMC should be an effective method to improve the reliability of solder joints. In this work, small amount of rare earth element La (only 0.05wt.%) had been added into the traditional Sn60-Pb40 solder alloy which is widely used in electronic packaging. Cross section observation of aged solder joints was made to compare the thickness of interface Cu6Sn5 IMC layer. Furthermore, accelerated thermal fatigue tests were done to evaluate the reliability of solder joints. Finally, thermodynamic models were used to explain the effect of La on the growth of Cu6Sn5 IMC. EXPERIMENTAL RESULTS Lap solder joints were used for aging tests, the base metal is copper and the solder alloy is Sn60-Pb40 and Sn60-Pb-0.05La, respectively. The aging temperature is 125 ± 5oC and aging EE5.18.1
time is 120 hours. SEM observations of the cross-sectioned samples were shown in figure 1. It can be seen that, after aging tests, the mean thickness of Cu6Sn5 IMC layer is 7.7µm for Sn60-Pb40 alloy while 4.2µm for Sn60-Pb-0.05La alloy, i.e., the growth of Cu6Sn5 IMC was depressed. (a)
(b)
Sn60-Pb40 Sn60-Pb-0.05La IMC
IMC Cu
Cu
Figure 1. Comparison of the thickness of interface Cu6Sn5 IMC layer of solder joints after 120 hours aging (a) Sn60-Pb40 solder alloy; (b) Sn60-Pb-0.05La solder alloy.
Simulated surface mount assembly was used for accelerated thermal fatigue tests [8]. Al2O3 ceramic substrate (Ni coated) is joined with FR-4 PCB (Cu pads on it
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