A study about direct laser reflow for forming stable and reliable C4 bump interfaces on semiconductor substrates for Flip Chip applications

The scope of this study is the introduction and qualification of a contactless laser-assisted reflow process (LAR) for forming stable and reliable C4 bump interfaces on semiconductor substrates as a possible solution to overcome the high nitrogen and energy consumption rates of conventional oven reflow processes. In contrast to laser assisted bonding (LAB) as an advanced flip chip technology, as described by Fettke et. al. [1], where the laser is primarily irradiated on a uniform and planar backside of a semiconductor die, the described laser process in this study will interact directly with the metal surfaces of the round solder preforms, the flux film and the substrate surface. Consequently, the process has to cover significant differences in absorption characteristics, reflection angles, topographic inhomogeneities and thermal thresholds in parallel.Two different test vehicles had been manufactured. The effect of the LAR process on the formed solder bumps was characterized and the results were compared with the solder bump quality on reference samples reflowed conventionally in a vacuum oven. The underlying test materials include flexible FR4 substrates in the form of up to 10 mm x 12 mm stripes with a CuNiAu pad interface and 10 mm x 10 mm Si chips with 5 μm ENIG plated pads. The fluxed samples were populated with 100 μm and 400 μm SAC305 and Sn42Bi58 solder-ball preforms. For the laser reflow process a NIR fiber laser system was used. The laser spot was optically modulated to irradiate the whole ball matrix in one step. To characterize the temporal and thermal feedback of the process a contactless thermo-optical sensor device was utilized. The herein identified process windows will be shared, as well as a general power density recommendation for such applications. Bump height measurements and shear tests were correlated to laser energy and spatial power distribution for the verification of the homogeneity of the optically induced thermal energy. The progression and form of the intermetallic compound (IMC) and the metallurgical quality of the solder bulk have been analyzed by X-ray, cross-sectional polishing, scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDX) and optical microscopy.To demonstrate the potential for upscaling, a 40 mm x 36 mm ball array laser reflow process was set up. Potential applications and future prospects will be finally outlined.