Small acoustic components, including acoustic resonators and transducers, are employed in a number of devices, including acoustic filters used for radio frequency (RF) wireless communications, for example. Various types of acoustic resonators include surface acoustic wave (SAW) resonator devices and bulk acoustic wave (BAW) resonator devices, including thin film bulk acoustic resonator (FBAR) devices and solidly mounted resonator (SMR) devices. For example, FBARs operating at frequencies close to their fundamental resonance frequencies may be used as key components of RF filters and duplexers in mobile devices.
Some conventional RF filter devices are packaged in hermetically sealed, chip-scale “microcap” dies, which may be referred to as cavity packages. A cavity package includes an electronic circuit (e.g., acoustic resonators and corresponding circuitry forming an RF filter) arranged in a cavity formed between a base substrate and a lid separated by a seal-ring. The cavity package may be flip-chipped onto a printed circuit board (PCB) or other substrate or substrate laminate, and then overmolded with an epoxy molding compound. The molding compound protects the cavity package and provides a surface for subsequent handling of the packaged parts. However, various environmental factors, such as temperature changes, hygroscopic swelling, and/or external mechanical shocks, cause the molding compound to exert forces on the cavity package that tend to pull the cavity package apart at its seal-ring. For example, the different materials used for the cavity package and the molding compound typically have different coefficients of thermal expansion (CTE), resulting in different rates of expansion and contraction in response to temperature changes, which exerts forces (stresses) on the cavity package. In other words, the presence of the molding compound may actually work to breach the RF filter device's hermetically sealed cavity package, or even break solder joints joining the cavity package to the PCB.
It is therefore desirable to develop packaged devices, including one or more hermetically sealed micro-cap dies or cavity packages, in which forces exerted on the cavity packages by the molding compound under stress is reduced.