On the integration of flexible circuit boards with hot embossed thermoplastic structures for actuator purposes

In this work, the integration of flexible printed circuit boards (FPCB) on thermoplastic structures has been studied for the first time. The process was developed with the objective to provide thermal paraffin-based microactuators with local heaters. Essentially, such actuators consist of rigid cavities sealed with flexible membranes deflecting on the melting of enclosed paraffin. Following this concept, 100 μm deep and 2.5 mm wide, circular cavities were fabricated by means of hot embossing of polycarbonate (PC) and joined thermally and by gluing to FPCB or blank polyimide (PI). The bond strength was measured by cavity bursting. The adhesion between PC and PI with thermal bonding was too low to allow any mechanical post processing, whereas gluing facilitated satisfactory joining. Here, the bond strength, measured with cavity bursting was found to depend heavily on the curing conditions. For instance, the use of an intensive UV-exposure could increase the adhesion with a 100% compared to curing with an ordinary UV-lamp. Investigation of channel cross-sections revealed an overall glue thickness of 2-15 μm and that only a minor amount of glue migrated into the channels. Two embossing tools of different resilience were investigated for the embossing of the microstructures. A polydimetylsiloxane (PDMS) mould replicated from SU-8-patterned silicon was compared to a more conventional nickel mould replicated from dry-etched silicon. The embossed samples were inspected in polarised light and it was found that PDMS embossed samples contained no interference fringes. This indicated that ridges, otherwise occurring just outside the cavities, were eliminated in these samples. Electron spectroscopy for chemical analysis (ESCA) revealed a slight difference in silicon contents between surfaces resulting from the two moulds. The nickel embossed surfaces were essentially free from silicon, whereas the PDMS embossed surfaces typically contained a significant concentration of silicon. A couple of actuators with FPCB joined to PDMS embossed PC cavities were fabricated using the developed process. These devices facilitated heating far beyond the melting point of paraffin but failed through paraffin leakage at the bond interface after a small number of activation cycles.