Solder pastes for microwave applications

A special development of solder pastes was demanded for the development of an innovative reflow soldering process by means of microwave heating. The presented results are showing a part of the joint research project ?MICROFLOW?, funded by the German Department for Education and Research (BMBF). Usual simultaneous reflow soldering processes like convection soldering or vapor phase soldering were optimized in the past for a minimum of temperature difference between small and large or heavy components on electronic assemblies. Especially the increasing demands for polymer electronics, for electrical-optical assemblies or high temperature electronics require are further development of soldering processes. Such a process should allow a direct heating of the solder joints up to soldering temperature and have to save all other components at the same time. Today this is possible only by application of sequential working selective soldering processes like hot bar soldering or laser soldering. But for a cost effective industrial application it is necessary to realize a selective and as well simultaneous soldering process, which is indeed not available at present. The wanted selective heating method for an effective simultaneous process is possible in principle by using of electromagnetic fields, when the energy is penetrating the assembly and is launching heat in certain regions, depending on specific material characteristics. Electromagnetic fields in the microwave frequency region are able to treat various sizes and shapes of assemblies with large capacity and with a high throughput. Indeed it is possible to heat conventional solder pastes only with very slowly and with a high microwave power density. It is possible to increase the launched heat considerable by mixing of additional materials, so called suszeptors, into the solder paste, which are absorbing microwave power with a high efficiency. Such suszeptors must have suitable polar or dielectric properties and can be added to the solder paste as a powder, solvent or fluid. In this way it is possible to accelerate the heating of the solder paste significant in comparison with the remaining assembly, whereby the demanded microwave energy can be minimized. Beside of this fundamental work, to generate a microwave suited heat source in the solder paste, the usual works of paste or flux development and qualification are also very important. First of all the wetting quality, especially for lead-free solders and no less the behaviors of flux residues, have to be investigated. Therefore the solder melting and wetting was investigated, including solder balling tests on different substrate materials. In the next step various corrosion tests like humidity storage and surface insulation measurements were carried out. The test results were leading to a goal-directed selection and optimization of flux systems for a new microwave suited solder paste.