Question: Do I have to use the thermal via of the exposed paddle underneath FX2LP 56-pin QFN package part?
Yes, you should have to use the thermal via when designing the PCB layout for FX2/FX2LP 56-pin QFN package part. The thermal via is used for the thermal design consideration of FX2/FX2LP 56-pin QFN package part. The design of the land area for the exposed paddle is critical to proper thermal transfer. A copper fill is to be designed into the PCB and under the QFN package in order to assist thermal transfer. The heat is transferred to the solid signal ground plane of the board over these vias.
You can diminish the number of vias for better routing the trace of other signal layers, but it will affect the performance of thermal efficiency. Normally we use a 5×5 array of 25 thermal vias in the exposed paddle. We recommend that there should have to be at least 4×4 array of 16 thermal vias in the exposed pad.