Surface Graft Copolymerization Enhanced Lamination of Poly(tetrafluoroethylene) Film to Copper and Epoxy-Based Print Circuit Board (PCB)

Lamination of poly(tetrafluoroethylene) (PTFE) film to a copper foil or to an epoxy-based print circuit board (PCB) Substrate (FR4®) was carried out. Lamination was achieved during surface graft copolymerization of glycidyl methacrylate (GMA) on an Ar plasma pretreated PTFE film at elevated temperature and in the presence of an epoxy resin adhesive. The plasma pretreatment introduces peroxides which are thermally degraded into free radicals to initiate the graft polymerization of GMA on the PTFE surface. The graft copolymerization with concurrent lamination is carried out in the complete absence of a polymerization initiator or system degassing. The modified surfaces and interfaces are characterized by X-ray photoelectron spectroscopy (XPS) and atomic force microscopy (AFM). The adhesion strength between the PTFE film and copper or the FR4® substrate was assessed by the T-peel strength test method. The adhesion strength was affected by plasma pretreatment time, as well as the grafting and curing temperature. The PTFE/GMA-epoxy resin/Cu and PTFE/GMA-epoxy resin/FR4 assemblies exhibit significant higher interfacial adhesion strengths compared to those assemblies in which only epoxy resin or GMA was used. They also exhibit better interfacial adhesion reliability. The PTFE/GMA-epoxy resin/Cu and PTFE/GMA-epoxy resin/FR4® joints delaminated by cohesive failure inside the bulk of PTFE film. The results suggest that the enhanced adhesion between the graft-modified PTFE film and copper or FR4® surfaces is attributable to the formation of covalent bonds between the tethered GMA graft chains on PTFE and the network of epoxy resin.