Low Temperature Processing of Highly Integrated Assemblies by Selective Microwave Heating

This poster presents the principles and state of the work of a joint research project "MICROFLOW", funded by the German Department for Education and Research (BMBF). The usually used simultaneous reflow soldering processes were optimized in the past for a minimum of temperature difference between small and large or heavy components. A further development of soldering processes is demanded because of the increasing requirements for polymer electronics, electrical-optical assemblies or high temperature electronics. The technology developed should allow a direct heating of the solder paste while the components are at a considerably lower temperature. This is possible only by sequentially working selective soldering processes at present. The selective heating method for a simultaneous process presented here is possible using microwaves provided the energy penetrates the assembly and heats certain regions only, depending on specific material characteristics. Today's conventional solder pastes are already able to be heated with microwave, but only very slowly and using a high power density. It is possible to increase the absorbed heat considerably by mixing additional materials referred to as suszeptors into the solder paste which are absorbing microwave power. A main task for a save microwave application was to guarantee the operators safety and to ensure the electromagnetic compatibility to the printed circuit boards, integrated circuits and other components as well. This was possible by means of minimizing of volumetric microwave power, optimizing microwave frequency and above all the ensuring of field homogeneity