The high-temperature storage test was conducted at 200℃ for100,500, and 1000 h to evaluate the bonding reliability of the TLPS joints under high-temperature conditions. The cross-sectional microstructure was analyzed using FE-SEM with an increasing aging time in Fig.1. At Cu-90(Sn-58Bi), the eutectic Sn-Bi lamellar structure gradually disappeared as the aging time increased.However, Kirkendall voids were formed with the increasing aging time and were located between Cu and the Cu3Sn, resulting in the weakening of the mechanical properties. The microstructure of Cu-70(Sn-58Bi) showed that the Cu3Sn and Bi phases were gradually coarsened with the increasing aging time. The scattered voids merged with each other to create large voids with the growing Bi and Cu3Sn phases. The Kirkendall voids were generated less compared with those in the Cu-90(Sn-58Bi) case between Cu and the Cu3Sn under high-temperature conditions. At Cu-70(Sn-58Bi), Sn phase was already transformed to IMC after the bonding process, so that the Kirkendall voids were less formed after the high-temperature storage test. The morphologies of Cu-50(Sn-58Bi) was maintained after the high-temperature storage test.
Fig.1.Cross-sectional micrographs of the TLPS joints after the high-temperature storage test.
The fracture morphologies were investigated using FE-SEM(Fig,2) after the high-temperature storage test of 1000 h.A coarsened Bi fracture region(a) was observed in Cu-90(Sn-58Bi).and the fracture(b) occurred between the Cu substrate and the Cu3Sn because of the Kirkendall void formation. On the contrary, the fracture occurred inside the TLPS joint at Cu-70(Sn-58Bi).Likewise, the fracture at Cu-50(Sn-58Bi) occurred inside the joint.In summary, the bonding reliabilities were expected to be low at Cu-90(Sn-58Bi) considering the fracture between the Cu substrate and the Cu3Sn caused by the Kirkendall void formation .Fig.3 shows the mechanical properties with increasing aging time. With respect to the tendency of fracture morphologies, the decreased shear strength ratio of Cu-90(Sn-58Bi) were the highest as the aging time increased because of the Kirkendall void generation between the Cu substrate and the Cu3Sn. The bonding reliability of Cu-70(Sn-58Bi) was the highest among the three conditions at the end of the high-temperature storage test.
Fig.2.Fracture morphologies of the 1000 h high-temperature storage tested sample after the shear test.
Fig.3.Shear strength of the TLPS joints with varying high-temperature storage times.
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