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High Reliable and High Bonding Strength of Silver Sintered Joints on Copper Surfaces by Pressure Sintering Under Air Atmosphere
Keywords: Silver Sintered Joints, Pressure Sintering, Copper Surfaces
In recent years, the trends in power electronics are higher efficiency, longer lifetime, and lower manufacturing costs. In addition, at the expense of achieving higher power density, there needs to have better thermal management to conduct the heat generated away. Hence, silver sinter materials have attracted rising attention as interconnect materials in the power electronic devices. A number of studies have indicated the feasibility of die attachment on silver or gold surfaces by either nonpressure or pressure sintering [1, 2]. Nevertheless, at present, sintering on copper surfaces is still a challenge. The main problem is oxidation of the copper during sintering process. It is difficult to create a bond between silver and copper oxide and as a result, sinter joints between both materials will be weak. Several studies [1, 3, 4] have demonstrated that good bonding quality on copper surfaces can be obtained by performing sintering process under reducing atmosphere. However, sintering performed under reducing atmosphere is more complex and cost is higher compared to sintering performed under air atmosphere. In this study, we present results of sintering on bare Cu Si3N4 AMB in air atmosphere by pressure sintering using a newly developed silver sinter paste. A comparative study on the influence of Ag metallization thickness (0.05μm, 0.15μm ad 0.3μm) on bonding strength was also performed in this work. For die attach we used Ag metallized dies with sizes of 4 mm x 4 mm and 10 mm x 10 mm. The pressure sintering process was performed at 230°C with pressure of 10 MPa for 3 min under air atmosphere. Temperature cycling test (TCT) with a condition of -40°C/+150°C and high temperature storage (HTS) at 250°C were carried out to evaluate the reliability of silver sintered joints on Ag and Cu surfaces. Die shear measurements on the assembled samples before and after TCT as well as after HTS to measure variations in bonding strength of silver sintered joints. The average die shear strengths of Ag plated AMB with different silver thickness before TCT (approximately 35 N/mm²) are rather similar, which is higher than the average initial die shear strength obtained on bare copper AMB (about 13 N/mm²). This can be attributed to the fact that the diffusion rate between Ag and Cu is lower than between Ag and Ag. However, the particles continued to diffuse during TCT and HTS resulting in higher shear strength. This explains the observed increase in shear strength obtained after TCT and HTS as can be seen in Figure 1 and 2, respectively. After 2000 cycles, the average die shear strength of bare copper AMB increased to 44 N/mm² which is fairly similar to the average die shear strengths of Ag plated AMB. The die shear failure mode also verifies the increased die shear strength with increasing temperature cycles and storage hours. Adhesive break was observed on the bare copper AMB before TCT and HTS, however, cohesive break was observed after 2000 cycles. Interestingly, cohesive break in Cu layer was observed after 250 hours storage at 250°C indicating that Cu particles diffused into silver sintered layer. In summary, high reliable and strong sintered joints on copper surfaces were achieved by pressure sintering under air atmosphere using the newly developed silver sinter paste. The special formulation in this sinter paste is able to prevent copper oxidation during sintering process. This new developed sinter paste is suitable for sintering on Ag and Cu surfaces.
Ly May Chew,
Heraeus Deutschland GmbH & Co. KG
Hanau, D
Germany


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