Micron Ag paste had a more affordable price, feasible large-scale synthesis, and longer storage life compared to nano Ag paste, thus it attracts much industrial interest for die attachment of high-power devices. However, the previous studies of high-temperature reliability were mainly focused on nano Ag joints, the research about reliability of micron Ag joints, especially low-temperature and pressureless, was very limited. Therefore, we evaluated high-temperature stability of low-temperature and pressureless micron Ag joint, involving in the changes of mechanical behaviors, evolution of microstructure and interfacial reliability. The average joint strength of micron Ag joints was independent of aging time and kept approximately 35 MPa after aging for 1000 h. The fracture of the micron joint was dominated by the ductile deformation of Ag grains during the fracture process. On the other hand, the microstructure of porous structure evolved greatly during aging process. Ag grains were oriented randomly before and after aging process, but the Ag grains increased slightly from 827.2 nm initially to 1178.4 nm after 1000 h aging. Meanwhile, the pores size in porous structure increased, the number decreased significantly, and the porosity also decreased slightly. Moreover, the barrier layers at interfaces of micron Ag joint remained stable and reliable during aging at 250 °C. The results would promote the large-scale application of the commercially available micron Ag paste in high-power devices.
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This work was also partly supported by the JST Advanced Low Carbon Technology Research and Development Program (ALCA) project “Development of a high frequency GaN power module package technology”. H. Zhang acknowledges the financial support from China Scholarship Council for his PhD research in Osaka University.