| dc.description.abstract |
In the semiconductor industry, low thermal budget is a critical topic. A low thermal budget is achieved by using a polymer substrate, a molecular machine, and 3D integration technology, which are associated with a low glass transition point, low decomposition temperature, and severe warpage, respectively. In conventional assembling technology, low-melting-point solder is an extremely economical solution for a low thermal budget. However, low-melting-point solder used in industry cannot solve the aforementioned problems. According to two eutectic points in an In-Bi phase diagram, solder alloys with the compositions 33In-67Bi wt.% and 68In-32Bi wt.% were selected in this study. Both alloys were reflowed on a Cu substrate 1, 3, 5, and 10 times at a temperature that was 30 oC higher than each melting point. To investigate the intermetallic compounds (IMCs) and phase formation in diffusion couples, samples treated with one reflow cycle were aged at different temperatures. The microstructural evolution and phase formation of two diffusion couples (33In67Bi/Cu and 68In32Bi/Cu) were observed through scanning electron microscopy, X-ray diffraction, electron probe microanalysis, and grazing incidence X-ray diffraction at the National Synchrotron Radiation Research Center.
In the 33In67Bi/Cu system, the rate of IMC growth at the interface was quite low. The phase of the IMCs was identified to be Cu2In, and those in the solder were determined to be Bi-rich and InBi phases.
The IMC growth rate of the 68In32Bi/Cu system was higher than that of 33In67Bi/Cu. After long-term reflow, IMCs that formed at the interface of 68In32Bi/Cu served as a self-diffusion barrier so that the thickness of the IMCs would not increase. The phases of the IMCs that formed at the interface of the 68In32Bi/Cu diffusion couple were Cu11In9 and CuIn2, and the phases formed in the solder were In-rich and BiIn2 phases. Furthermore, CuIn2 was a metastable phase that could transform to Cu11In9 spontaneously. Consequently, both CuIn2 and Cu11In9 were found at the interface.
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