Adhesion of Flexible Printed Circuit Substrate to Overmoulded Polymer and Characterization of Overmoulded Electronic Components

Next generation of smart systems in different application areas such as automotive, medical and consumer electronics will have various electronic, optical and mechanical functions integrated in polymer packages. We have studied the in-mould integration of electronic and optoelectronic modules with injection moulding and film-overmoulding technologies. A test set was made to find out adhesion strength between various overmoulding thermoplastic materials and thermoplastic films that could be used as substrates for flexible printed circuits. By overmoulding we mean here that the film is set as an insert into injection moulding mould and thermoplastic polymer is cast on the film in injection moulding process. Both overmoulding and polymer materials were mainly transparent. It was found out that generally adhesion between the substrate and overmoulded plastic was poor, but some good material pairs could still be found. Situation is especially difficult with materials which can withstand high temperatures. These materials can be bound together only by special means. Adhesion was tested by ISO 8510-1 standard in addition to visual inspection. It was noticed that another problem beside poor adhesion was warping. Some of the tested overmoulding material pairs are not suitable for overmoulding, because of their remarkable high difference in coefficient of linear thermal expansion, which leads to distinct warping effect of the foil and overmoulded plastic package. Deformation takes place also after demoulding when plastics are cooling to room temperature. PET, PEN, and PC -type foils were tested as flexible printed circuit substrates. The overmoulding materials were PC, COC, PMMA and PS. Investigated high temperature resistant foil materials were PI and LCP and corresponding overmoulding materials LCP and LCP/PPS blend. A test structure was also made, in which bare die and SMT electronic components were assembled to foil with different interconnection methods before overmoulding process. Characterization of overmoulded components was carried out by visual inspection and by functional testing. It was found out that with correct injection moulding process parameters, all components survived the challenging conditions of the injection moulding process.