A comparison of FBAR oscillators with standard resonators and stress relieved resonators

One of the issues facing many types of oscillators is the transference of stress from the external world through the die to the resonator. Typically, resonators are clamped at more than one location. Integrated FBAR oscillators (referred to as FMOS^1,2), uses a resonator that is clamped on all sides and integrated into an all-silicon, chip-scale package with integrated circuits. For filter applications, this is quite reasonable, but, for oscillators this may not be acceptable. Mounting FBAR oscillators (epoxy die attach, over-molding and soldering) onto a customer board exposes the FBAR oscillator to stresses that occur during the assembly, plus additional stresses that occur during the wear and tear (as well as temperature induced stresses) that the customer board experiences. Although intuitively, applied stress will change frequency of the resonator, this is hard to quantify. This paper does a first cut model of the stresses and then matches that to the changes in the measured frequency. Next, we discuss several designs that help mitigate the effect of external stresses on the resonator.