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An open-ended microwave oven in the form of an open waveguide cavity partially filled with a low loss dielectric material is proposed for the curing of polymer materials during flip-chip assembly. The open-ended nature of the design allows for simultaneous precision alignment and curing of devices for fast flip-chip assembly, direct chip attach (DCA) or wafer-scale level packaging (WSLP). A well defined resonance can be achieved within the dielectric filling with evanescent fields in the open end of the cavity. A new optimisation scheme, based on a quarter wave impedance controlling section, is presented for the maximisation of the evanescent (heating) fields. In order to optimise the fields (and therefore the heating capability) a thin layer of low-loss dielectric material is inserted at the interface. Adjustment of the thickness and permittivity of the dielectric insert allows the phase of the reflection coefficient at the dielectric-air interface to be controlled. Experimental results show that a tenfold improvement in heating times can be achieved within a lossy polymer sample.