Power HBT is comprised from multiple fingers. Overstress may cause some fingers to fail in sequence. It can be seen as progressive degradation of the performance. Your observation of Pdet signal may be a sequence of a ‘fused’ signal runner (single or a small close group) that may disconnect the Pdet node from the other functional output fingers.
In many cases, poor grounding leads to oscillation failures. Maintaining a good interface between the transistor and the circuit is essential to prevent oscillation related ruggedness failures. Typical RF board provides multiple vias along the active traces and all arownd the board to provide RF path between top/bottom ground layers.
The next two reasons are: poor thermal management and operating at high VSWR conditions (no load or short circuit).
Typically a fail will be accompanied by physical damage evident on the die itself which can be observed by microscopic inspection. If you would like analysis on the failed devices, they can be returned to NXP for continued assistance. Please, contact the distributor who sold you these parts to submit the request to NXP.
I hope you will find these information useful. If we can be of additional assistance, please let us know.
Have a great day,
Pavel
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