Authors: Mathias Nowottnick, Dirk Seehase and Heiko Huth Company: University of Rostock Date Published: 9/28/2014
Abstract: In a normal reflow process the entire assembly is heated simultaneously in a continuous furnace up to the soldering temperature, which is above the melting point of the solder alloy (usually 217°C for SAC solder). The larger the differences between big and small components, the greater the temperature differences on the assembly. While uniformly constructed assemblies can be soldered safely at temperatures of 235°C, for very inhomogeneous assemblies, peak temperatures above 260°C can be required, if the largest components have to be heated above the melting temperature. For certain materials and components are these temperatures not admissible, they can be damaged or their service life can be reduced. By exothermic reactions the additional need of heat for heavier components can be provided in principle. Previous solutions of exothermic joining processes are working with thin films or foils, which must be selectively ignited, e.g. by laser or current pulses. With the presented development it is possible to print a particular exothermic reacting paste on the circuit board, which increases locally the temperature to achieve reliable melting and soldering even at low process temperatures. The paste consists of a dispersion of organic and inorganic components, so it can be applied by stencil printing or dispensing like a solder paste. It is even possible in principle to solder the whole assembly at a furnace temperature below the melting point of solder. The exothermically reacting components are set in such a way, that the reaction will be initiated below 200°C and the temperature at the solder joint increases over 250°C. Therefore, the furnace temperature can be reduced significantly, multiple or re-soldering processes can be saved. Furthermore a noticeable energy saving can be achieved in the soldering process by decreasing of soldering temperature. The exothermic reacting pastes can be stored and processed safely at room temperature.
reflow soldering, exothermic reactions, solder paste, energy saving