Thermal and gas flow characterization of a fluxless Si solder bonding oven

In this paper the technology of the “Linn” type Si fluxless solder bonding oven and the thermal and gas flow characterization of the oven are discussed. This oven is used for fixing silicon chips on metal substrates with high temperature solder bonding process. The solder is applied in a foil form which is placed between the Si chip and the metal substrate. This does not contain any flux, therefore a reducing agent has to be applied to avoid the oxidation of the joints during the soldering process. In this technology the reducing agent is the forming gas which is a mixture of 10 vol.% H₂ and 90 vol.% N₂. The key factors of this soldering process was studied; the suitable temperature (350-370 °C for 13-15 min) and the adequate H₂ concentration (8-10 vol.%). A detailed 3D gas flow model of the Linn oven was prepared which is based on the finite volume model (FVM) method. The gas flow circumstances using the basic and a hypothetic oven setting were compared by simulations applied the ANSYS-FLUENT system. The gas flow model was verified by the measurements of the temperature, the H₂ concentration and the pressure inside the oven. Furthermore the heating ability of the oven under full load was characterized by the change of the heating temperature and the time coefficient of the heating.