Mechanisms of thermoplastics-to-metal adhesion for applications in electronics manufacture
2020-01-07T16:02:30Z (GMT) by
The Substrateless Packaging process was developed at Loughborough University as an alternative method of manufacturing electronics with an improved end-of-life materials recovery profile. The process involves injection moulding to overmould electronic components in thermoplastic polymers. Initial prototype samples manufactured in previous work exhibited undesirable small gaps around the embedded components after solidification and which were thought to be the result of adhesion problems between the thermoplastic overmould and components. The study reported here had the aims of determining quantitatively what factors affect adhesion, and to identify which thermoplastic polymers are most suitable for the process. Following a literature survey, six engineering thermoplastics, PC, PBT, PS, ABS, PA 6 and PMMA were chosen for study as overmoulding materials, and tin as the solid adherend. The literature survey also identified the mechanisms contributing to adhesion at the metal-thermoplastic interface in insert moulding as material properties, interfacial forces between the materials, wetting at the interface, temperature of the insert (consequently temperature at the interface) and insert moulding parameters. A methodology was designed to allow investigation of all these factors, with Atomic Force Microscopy (AFM) force-distance measurements being used to measure room temperature interfacial forces, high temperature contact angle for wetting, and pull-out strength tests on overmoulded tin-coated wire for overall system adhesion. Excellent repeatability was seen in the measurements obtained with all three experimental methods. Moldflow finite element simulations of the insert moulding process were also undertaken. For the AFM measurements tin particles were adhered to the probe with the aid of a Focused Ion Beam (FIB) apparatus. PA and PMMA interatomic interactions with tin were found to be noticeably stronger than the other polymers. From consideration of the different possible contributions to the measured forces, it was concluded that the trend of interatomic interactions obtained is due to a combination of electrostatic forces, capillary forces and dispersion forces acting between the materials tested. In the high temperature contact angle measurements it was observed that the contact angles for all the materials producing drops in equilibrium reduce monotonically with rise in temperature at the interface. The work of adhesion was calculated from the contact angles for PMMA using the Young-Dupre equation and values of surface tension from the literature. It was found that it does not increase monotonically with temperature as might be expected from the contact angles. The works of adhesion at 240°C for all the materials were also calculated and it was found that the materials ranking for expected adhesive strength changed significantly from that expected from consideration of contact angle alone. In the pull out tests, except for PC, the breaking loads for the materials tested rise then fall with rise in temperature of the insert. It was observed that peak breaking load for the amorphous polymers ABS, PS and PMMA occurred for insert temperature just below Tg of the polymer, and for semi-crystalline polymers PA 6 and PBT it was just above Tg. The ranking of materials by maximum pull out strength was found to be consistent with the ranking by mechanical strength (tensile strength at yield) of the thermoplastics. The Moldflow simulations yielded the significant results that the thermoplastic melt comes in contact with the insert at relatively low pressure (less than 0.6 MPa), and that the temperature of the melt near the insert drops to the temperature of the insert almost instantaneously on contact. Therefore it was concluded that the efficacy of holding pressure on assisting wetting of the insert by the thermoplastic melt may depend on the temperature of the insert interface. The results in terms of material rankings from both the material level tests (AFM force distance experiment and wetting at high temperature) did not correspond to the mechanical strength test results. It was therefore concluded that the choice of material for thermoplastic overmould cannot be made purely based on the material interactions at interface between tin and thermoplastics in solid or melt phase. It was also concluded that the observed variation in the pull-out strengths with temperature of the insert maintained during overmoulding, must be largely due to the thermo-mechanical properties of the material at the interface. Based on the results of the study, PC, PBT and PMMA were recommended as being likely to give superior performance to the ABS which was used in early trials of the substrateless packaging process. Of these, from a process economics point of view, PBT would be the most suitable.