Selective laser soldering on flexible boards using through foil heating

In our selective laser soldering experiments, we soldered surface mount (SM) chip resistors onto flexible substrates. Taking advantage of the transparency of the substrates in the red and the infrared range, we applied an interesting and unique technique by heating the solder pads exposing them through the substrate. Our previous measurements showed that the polyimide (PI) and polyester (PET) substrates absorbed less than 5% of the 1064 nm wavelength laser light, while polyethylene-naphtalate (PEN) absorbed about 30%. The laser energy which was let through was enough to melt the two lead-free alloys and formed a perfect meniscus at the solder joint.