Rigid-flexible interconnection realized by laser soldering through polyimide

The transmission laser soldering technology described in this paper enables the realization of rigid - flexible interconnection after mass reflow soldering of the surface mount components to the rigid printed circuit board (PCB). The novelty of our approach is that the solder paste is heated up selectively by an Nd:YAG laser through the flexible substrate, which is in most cases polyimide (PI). The amount of transmitted laser energy can be controlled and concentrated so precisely to the metallic parts (pad and solder), that it enables the application of solder paste with higher melting point as the temperature tolerance of the flexible substrate, thus solder joints can be formed on polyethylene-terephtalate (PET) and polyethylene-naphtalate (PEN) substrates as well. The paper describes the transmission laser soldering process parameters used for PI, PET and PEN substrates and SnAgCu and SnBi solders. The metallographic analysis and the pull strength measurement results are also presented. Based on our experiments it can be stated that our laser soldered joints of rigid to flexible printed circuits show similar quality and reliability properties to the reflow soldered ones.