| Summary: | 碩士 === 國立中興大學 === 化學工程學系所 === 101 === Damages on material are unavoidable in the production process and application of electronic device. One of these damages is mechanical strain which is resulted from the difference between the coefficients of thermal expansion (CTE) of materials. It may affect the reliability of solder joints. The objective of this experiment is to simulate the effect of mechanical strain on Ag/Sn interfacial reaction. To investigate the effect of mechanical strain on solid-solid and solid-liquid Ag/Sn interfacial reactions, intermetallic compound the Ag3Sn phase growth rate and morphology were observed under applied compressive and tensile strain respectively and then compared with those on the reference sample with no strain.
To evaluate the solid-solid interfacial reaction, thermal aging test was carried out on 3 individual sets of samples, which were applied compressive, tensile strain and without strain, with the same structure of 20-μm-Sn/30-μm-Ag/400±50-μm-Si. Aging temperatures in this experiment were set to be 150℃, 170℃ and 200℃ for the aging times of 24h, 72h, 120h, 240h, 360h and 480h. With the increasing aging time, the IMC thickness increased while the Ag3Sn phase was the only phase observed. From the results, it was that the applied strain enhanced the nucleation rate of the Ag3Sn phase to obtain grains in a smaller size and a larger amount. Thus, activation energy for the Ag3Sn grain growth under strain was increased and leaded to the decrease in growth rate. On the other hand, the application of strain showed no significant effect on the average thickness of the Ag3Sn phase. This can be explained by the rapid release of strain in the Ag3Sn phase due to its low melting point and small elastic modulus of silver.
Reflow soldering was also conducted on the samples which has a structure of 1.3±0.07-mg-Sn/30-μm-Ag/400±50-μm-Si, under strains and with no strain at 255℃ to examine the effect of mechanical strain on the solid-liquid Ag/Sn interfacial reactions. The reflowing times were set to be 8 min, 10 min, 12 min, 15 min, 20 min and 60min. The Ag3Sn phase was also found as the only phase. The grain morphology was proved to follow the Wulff construction theory. Drastic change in morphology and increase in grain size were showed all sets of samples with reflowing time of 8min and 10min. Crystal structure became faceted under strain and the grain size showed no change at reflowing time of 12min, 15min, 20min and 60min.
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