Low temperature soldering on biopolymer (PLA) Printed Wiring Board substrate

The applicability of decomposable biopolymer (polylactide - PLA) as Printed Wiring Board (PWB) substrate was investigated. A novel approach for low temperature reflow soldering was applied. Different types of PLA boards were prepared and each type was qualitatively characterized. The realized circuit was designed for the use of Surface Mounted (SM) components. The low maximum process temperature of PLA boards was a key aspect to set soldering temperatures. 58Bi42Sn solder alloy was chosen to match the lower soldering temperature range required by the PLA board material. Steps of reflow soldering technology (stencil printing, component placement, reflow soldering) were applied using 0603 discrete components and were inspected during the circuit assembling process. Hot soldering iron, IR radiation, hot gas method and Vapour Phase Soldering (VPS) were applied for heat transfer during the experiments. VPS was found the best for heat transfer to melt the low temperature (138 °C) solder alloy. The results verified the flexibility and usability of the VPS in the field of special applications. The quality of boards was examined with cross-section optical inspection; the solder joints were evaluated with shear force tests. Typical failures were also analyzed.