Impact of thermo-mechanical stress on the TCF of WLP BAW filters

For BAW filter structures, the Temperature Coefficient of Frequency (TCF) specifies the frequency shift of the filter skirts over temperature. The lion's share of materials used in BAW filter stacks exhibit negative TCF characteristics, which means as the temperature rises, the filter passband shifts downwards in frequency. At the expense of performance, TCF compensation can be achieved through application of materials with positive TCF characteristics. Therefore, the extent of compensation has to be carefully increased only up to the point where it becomes sufficient for the given application. The intended TCF is usually verified through calculations based on S-parameter measurements on wafer at different temperatures. However, recent performance studies have shown a non-negligible TCF degradation of up to 5 ppm/K, which occurs after flip-chip module assembly. This paper describes the investigations that revealed the main cause for degraded TCF characteristics in a band 7 Tx filter application.