| Summary: | 碩士 === 國立中興大學 === 材料科學與工程學系 === 96 === Tin-silver-copper lead-free solder balls were welded on substrate pads by reflow soldering in this research. Different reflow temperatures were applied to study the effect of reflow temperature on the morphology, microstructure and mechanical reliability of solder joints. Experiment results showed that the reflow temperature did not have an obvious effect on the macroscopic appearance of the solder joints. The solder joints were well welded and exhibited a uniform structure. The largest wetting force increased with the reflow temperature and then stabilized at the temperatures above 240°C. From the observations of scanning electron microscopy, it was found that an intermetallic compound layer formed at the interfaces between the solder balls and the substrate pads. The thickness of the intermetallic compound layer increased with reflow temperature, as about 1.86 to 5.42 μm. The joint strength was measured by shear tests, and the average value under different reflow temperatures was 7.48 Nt/mm2, about 1.4 times the value of industry standard. As the reflow temperature increased to 240°C, the joint strength began to decrease, indicating that a higher reflow temperature resulted in the formation of a thicker intermetallic compound layer and consequently lowered the joint strength. Moreover, the hardness and elastic modulus of the eutectic and primary phases of the solder balls, the intermetallic compound layer and the nickel layer on the substrate pads were measured by nanoindentation tests, and their fracture behaviors were examined. The hardness and elastic modulus of the intermetallic compound layer were measured as about 6.49 and 193.46 GPa, respectively, and the highest among these four structures.
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