| Summary: | 碩士 === 國立交通大學 === 材料科學與工程學系 === 101 === Microbumps are adapted as the interconnect between chips in 3D-IC packaging. With the interconnect shrinks to micrometer scale, the cross interactions across the two ends of the microbump may become an important issue, since the bump height is below 20μm
SnAg solders with Cu metallization and Ni metallization was used in this study. The two sets of samples were aligned and jointed together at 260°C for 3 min to form sandwich structures of Cu/SnAg/Cu and Ni/SnAg/Ni. Included the 3 min of forming the joint, reflow times of 3, 8, 13, 23, 43 min at 260°C were performed for the Cu/SnAg/Cu structure. For Ni/SnAg/Ni structure, additional thermal paste and a Si chip were put on the cold end of the structure to form a larger thermal gradient and then additional 5, 10, 20, 40, 100 min reflow at 260°C were performed. After that, scanning electron microscope (SEM) was employed to observe the thickness of Cu6Sn5 and Ni3Sn4 intermetallic compounds (IMCs) at the SnAg/Cu and SnAg/Ni interface.
We observed asymmetrical growth of Cu6Sn5 intermetallic compound (IMC) at the two interfaces of Cu/SnAg/Cu solder joints during reflow at 260˚C hotplate. For Cu/SnAg/Cu, the Cu6Sn5 IMC grew to 12.3 μm on the cold end and 3.5 μm on the hot end after reflow for 43 min. The consumption of Cu on the cold end is less than that on the hot end. We propose that rapid thermomigration of Cu is responsible for the asymmetrical growth of the IMC. As to the Ni/SnAg/Ni microbumps, we also observed the asymmetrical growth of the Ni3Sn4 IMC after reflow for 100 min on hotplate: 4.61μm on the cold end and 2.12 μm on the hot end. Furthermore, the consumption of Ni on the cold end is slightly less than that on the hot end. It inferred that the thermomigration of Ni also occurs in liquid SnAg solder.
No electrical currents were applied in the tests, and only the thermal gradient in molten solder joint should be responsible for the asymmetric IMC growth. Thermal gradient across the molten solder was simulated by finite element analysis due to the measurement difficulty. Based on thermomigration flux calculation and the simulated thermal gradient, the heat of transport (Q*) of Cu and Ni was calculated to be57.97 kJ/mole and 0.84 kJ/mole, respectively.
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