Life-time test of laser and reflow soldered flexible substrates

We examined the quality and the reliability of lead-free solder joints which were made with a selective laser soldering process. The three types of applied substrate materials - polyimide (PI), a kind of polyester (PET) and polyethylene-naphtalate (PEN) - withstood the local heat dose even if the allowed temperature limits of them were lower than the melting points of the applied solders. We used substrates with different adhesives and cover layers. The two types of solder pastes were SnBi and SnAgCu (SAC) alloys. Altogether 16 solder-substrate-adhesive combinations were tested and compared to similar reflow soldered combinations when the implementation of the technology was possible. The testboards were submitted to HAST (highly accelerated stress test) and the degradation was monitored. Optical inspection and shear force measurements were used for comparative analyses. As a result, the combinations were put in order on the basis of these data. With these achievements; the processing, materials, product performance and reliability could be improved.