Effect of insert temperature on integrity of a thermoplastic circuit board

At Loughborough University an environmentally friendly manufacturing process for electronic circuits, involving the embedding of electronic components in thermoplastic resins via insert injection moulding, is under development. Once embedded, interconnection between components is achieved by subsequently plating or printing metallisation patterns on the moulding surface. The environmental benefits of the process arise from the enhanced separability of components, metals and matrix at end of life compared to conventional solder mounting of components on a thermoset polymer based circuit board. The separated components then form concentrated waste streams allowing improved efficiency of material recovery. It was observed in earlier work that intimate contact between the overmoulded thermoplastic resin, and the legs of the electronic components, was crucial for the integrity of the electrical interconnection. If small gaps open up around the embedded components after solidification these will either act as weak points in the electrical interconnect pattern, or prevent electrical interconnect being achieved at all. The quality of the component/overmould interface is likely to depend both on material-material interactions, i.e. adhesion between overmould and insert surfaces, and on the solidification history of the overmould material. What reports there are in the literature come to conflicting conclusions on the importance of wetting for adhesion and joint strength obtained in insert moulding. In principle the wetting of metals by polymers, and hence material-material adhesion, is expected to improve with rise in the temperature of the interface. At the same time, an increased insert temperature is expected to reduce the cooling rate of the thermoplastic after injection, and so increase the crystal fraction and degree of shrinkage of the solidified material in the vicinity of the insert. The study which is reported here was undertaken to determine quantitatively the imp- - ortance of these two factors on the joint strength obtained by overmoulding metals, and in particular tinned surfaces, with thermoplastic resin. Data from systematic measurements of the variation with insert temperature of the joint strength measured by pull out force measurements, for tin coated wire overmoulded with six different thermoplastics, are reported. In most cases the peak strength occurred for insert temperature near the glass transition temperature of the overmould material. The maximum strength variation with temperature observed for a single material was 42%, which occurred for PMMA between room temperature and 80°C. By contrast the wetting of the same set of polymers on tin, in high temperature contact angle measurements, was observed to uniformly improve (reducing contact angle) with increase in system temperature. The results are of wider interest than electronics, because of the sparsity of reports in the literature on factors affecting overmould/insert joint strengths in the industrially important process of insert moulding.