A fluxless flip chip bonding for VCSEL arrays using silver coated indium solder bumps

Alloys of lead-tin system are the most common solder alloys used today. However, there are environmental and health issues concerning the toxicity of lead present in these lead-tin solder alloys. Also the flux residue removal is mandatory and leads to environment threats. More important, the use of flux may contaminate the optically active surface by organic residue leftover, and a conventional cleaning method may not be effective for optoelectronic assemblies. Therefore, it is necessary to look for fluxless soldering processes for soldering optoelectronic systems. In the present study, we have conducted low-temperature flip-chip bonding of vertical-cavity surface-emitting laser (VCSEL) arrays on a glass substrate that provides propagation paths of laser beams and also supports a polymeric waveguide. Using indium solder bumps, an optimum condition of flip-chip bonding is determined as a heating temperature of 150°C and a pressure of 500gf. The fracture occurs between the indium solder bump and the VCSEL chip pad during the die shear test. It is inferred that both the low bonding temperature and the oxide layer which is formed on the surface of the indium solder prevented the bump from interacting with the chip pad. To decrease the melting temperature of the indium solder and protect it from oxidation without using flux, we try also coating of a thin silver layer onto the indium surface. An eutectic reaction occurs at 97 wt.% of In with eutectic point of 144°C and the outer silver layer interacts with indium to form a AgIn₂ compound layer due to the high interdiffusion coefficient. As a result, the thin silver layer coated on the solder bump is very effective to enhance the adhesion strength between the indium bump and the VCSEL chip pads by decreasing the melting temperature of the indium solder bump locally.