Reliability Evaluation of Sintered Silver and Sn–3Ag–0.5Cu Solder Joints Using a Thermal Cycling Test Bin Li1,2, Zhenqing Zhao1*, Mingyu Li2*, Tao Wang1, and Hui Li1
1 Delta Power Electronics Center, Delta Electronics (Shanghai) Co., Ltd., Shanghai 201209, China 2 Department of Materials Science and Engineering, Shenzhen Graduate School, Harbin Institute of Technology, Shenzhen 518055, China * Corresponding authors. E-mail addresses:
[email protected] (Z. Zhao),
[email protected] (M. Li). Abstract—Sintered silver has received a great deal of recent attention as a high-temperature die-attach material. A detailed study of the sintered silver for bonding insulated gate bipolar transistor dies is reported. The purpose was to compare the reliability between sintered silver and Sn–3Ag–0.5Cu (SAC305) solder joints. It was found that the sintered silver had a porous and crack-free structure. And compared to the SAC305 solder, the sintered silver possessed an obviously better thermal conductivity. Moreover, the sintered silver exhibited stable thermal and electrical properties after 600 thermal cycles. Keywords—sintered silver; high-temperature packaging; power electronics
I. INTRODUCTION Recently, high-temperature power devices have received a great deal of attention [1–5]. This class of devices allows the use of power modules at high temperatures. In order to ensure the reliability of modules for high-temperature applications, all packaging materials with specific requirements must be correctly selected. Among them, die-attach materials have received an increasing interest because they play an important role in the system. High lead solder has been the main hightemperature die-attach material over several decades. Currently, a great deal of effort has been devoted to the alternative dieattach materials which can with-stand high temperatures, for example, the reported work on sintered silver [6–11], gold-tin solder [12,13], gold-germanium solder [14] and bismuth-based solder [15]. Among those materials, sintered silver has been most actively researched because of its high melting point, high thermal conductivity and lead-free composition.
Heraeus, and the IGBT dies (IGC16T65U8Q, 4.02×4.01 mm2) were supplied by Infineon. The process flow for sintering of silver paste is illustrated in Fig. 1. The silver paste was firstly printed onto a substrate, and subsequently dried at 120 °C for 10 min. Then, an IGBT die was placed on the dried paste, followed by sintering at 230 °C for 5 min with a pressure of 5 MPa. The temperature and pressure profiles for sintering are presented in Fig. 2, and the photograph of hot press (Vigor VLP-60) used in this work is shown in Fig. 3. In order to compare with the sintered silver sample obtained above, the Sn–3Ag–0.5Cu (SAC305) solder sample was synthesized on a Cu substrate without silver-plated layer.
Fig. 1. Process flow for sintering of silver paste.
In this study, samples with two kinds of die-attach materials (sintered silver and lead-free solder) were synthesized using the respective process profiles. The aim was to compare the thermal cycling reliability of two different joining techniques. II.
EXPERIMENTAL
A silver sintered sample was fabricated by bonding an insulated gate bipolar transistor (IGBT) die on a silver-plated Cu substrate. The silver paste (ASP 043-04) was obtained from
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Fig. 2. Temperature and pressure profiles for sintering.
The thickness distribution analysis was carried out by a solder paste inspection system (REAL SPI6000). A HITACHI S-3400N microscope equipped with an energy dispersive spectroscopy (EDS) detector was used for the characterization
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structure with uniform thickness (~42 μm). The porosity of the sintered silver joints strongly depends on the sintering pressure. The porosity will become lower with the sintering pressure increases. The EDS map in Fig. 6c shows that the sintered silver only contains Ag element, which is in accordance with the chemical composition of pure silver.
Fig. 5. Thickness distribution image of the silver paste after printing and drying processes. Fig. 3. Photograph of hot press for sintering.
Fig. 4. Photograph of the silver sintered device for thermal and electrical property tests.
of scanning electron microscopy (SEM) image and elemental mapping analyses. A thermal shock chamber (KSON KSRA415T-RBS) was used for the thermal cycling test. The thermal resistances were measured by a MicReD 1500A power tester. And the electrical properties were observed by an Agilent B1505A power device analyzer/curve tracer at room temperature. Fig. 4 exhibits the photograph of silver sintered device for thermal and electrical property tests in this study. The scanning acoustic microscopy (SAM) images were investigated with a Sonoscan GEN5 instrument. III. RESULTS AND DISCUSSION The printing process of silver paste has an important influence on the sinter result. Fig. 5 exhibits the thickness distribution of dried silver paste printed on a silver-plated Cu substrate. We can see that the dried paste possesses a quite smooth surface, which indicates that the sample has a good printing quality. The cross-sectional morphology image and corresponding EDS maps of the sintered silver are shown in Fig. 6. It can be seen that it has a porous and crack-free
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Fig. 6. (a) SEM image of the sintered silver cross-section. Corresponding EDS maps for (b) Si, and (c) Ag.
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acoustic analysis could provide useful information in the bonding quality evaluation. Fig. 9 shows the SAM images of sintered silver and SAC305 solder joints. We can see that before the thermal cycling test, the voids are barely observed in both samples. This observation confirms that the sintering pressure of 5 MPa used here was uniform. It is worth mentioning that only a few voids can be detected at the edge of sintered silver after 600 thermal cycles, as shown in Fig. 9b. In comparison, both the edge and the middle of SAC305 solder appear the voids after 600 thermal cycles (Fig. 9d). We believe this is because the SAC305 solder might be easier to creep at high temperatures due to its much lower melting point than that of the sintered silver.
Fig. 7. Thermal resistances of two samples showing (a) the initial states, and (b) the states after 600 thermal cycles.
In this study, the thermal cycling test was used to evaluate the reliability of sintered silver and SAC305 solder joints. The temperature range of thermal cycling was from −40 °C to 125 °C, and the dwelling time at extreme temperatures was 15 min. Fig. 7 depicts the comparison of thermal resistances between the two samples. The thermal capacitance (Cth) versus thermal resistance (Rth) of the initial states is plotted in Fig. 7a. Before thermal cycling, the thermal resistance values of test and calculation for the sintered silver were 0.331 K/W and 0.315 K/W, respectively. Meanwhile, the thermal resistance values of test and calculation for the SAC305 solder were 0.495 K/W and 0.443 K/W, respectively. It is worth to notice that the sintered silver showed an obviously better thermal conductivity than the SAC305 solder. After 600 thermal cycles, the test value of sintered silver increased to 0.351 K/W, as presented in Fig. 7b. However, the test value of SAC305 solder decreased to 0.441 K/W after 600 thermal cycles. This may be because of the test error from equipment. The electrical properties of sintered silver and SAC305 solder joints are presented in Fig. 8. The electrical characteristics were measured at a same gate turn-on voltage of 15 V. It can be seen that both samples exhibit similar electrical performances. And the on-state resistances of samples increase only slightly after 600 thermal cycles. The
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Fig. 8. Electrical properties of two samples showing (a) the initial states (inset: enlarged sections of the same curves), and (b) the states after 600 thermal cycles (inset: enlarged sections of the same curves).
IV. CONCLUSIONS In this study, silver sintered samples were synthesized by bonding IGBT dies on silver-plated Cu substrates. We found the samples had a good printing quality. And a porous and crack-free structure with uniform thickness was observed in the cross-sectional morphology image. It was found that the sintered silver provided a better thermal conductivity compared to the SAC305 solder. This result imply that using sintered
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silver as a die-attach material can improve the thermal performances of power modules. In addition, the sintered silver possessed stable thermal and electrical properties after 600 thermal cycles. Further study on the reliability of sintered silver for bonding high-temperature power devices will be carried out in our next research.
Fig. 9. SAM images showing the initial states and the states after thermal cycling for (a, b) sintered silver joint, and (c, d) SAC305 solder joint.
ACKNOWLEDGMENT The authors would like to thank colleagues Zengsheng Wang, Haibin Xu, Ganyu Zhou, Xin Zou, Weijiang Li, Xuetao Guo, Zhanghua Jiang, Shili Wu, Shouyu Hong and Kai Lu at Delta Electronics (Shanghai) Co., Ltd. for their help in the preparation processes and measurements. REFERENCES [1] C.M. DiMarino, R. Burgos, and B. Dushan, “High-temperature silicon carbide: Characterization of state-of-the-art silicon carbide power transistors,” IEEE Industrial Electronics Magazine, vol. 9, pp. 19–30, September 2015.
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