Assembly process reliability for Optical Multi-Chip Modules (MCM) is studied and improved. In the optoelectronic (OE) chip assembly for the Optical MCM, the OE chip with Au stud bump is joined with Sn-Ag-Cu (SAC) soldered in a through-waveguide via on an organic substrate to obtain high optical coupling efficiency. Since solid-liquid diffusion of Au to molten SAC is rapid, and formation of brittle intermetallic compounds such as AuSn4 is observed by an energy-dispersive X-ray analysis, and as a result the temperature and the dwell time for the chip assembly process should minimized. Furthermore, if OE chips are underfilled, resin could infiltrate into the total internal reflection mirror cavity, and it will not reflect anymore. On the other hand, Au - SAC joints are not mechanically stable without underfill because of a large thermal stress from the coefficient of thermal expansion mismatch between the OE chip and the optical waveguide-integrated organic substrate. The issue is solved by using sidefill encapsulation instead of underfill. Appropriate material selection of a high viscosity and high thixotropic index prevented infiltration under the chip. The effect of the sidefill process is verified by simulation and experimental results. The chip assembly with sidefill passes more than 1500 deep thermal cycles from -55 °C to 125 °C.