hahahahaha I finally solve the problem in COMSOL myself. all these damn things come from the physics induced sequence.
next time cannot converge the model., right click mesh edit the physics induced sequence, then set the minimum order to be smaller then you can solve any model.
https://www.comsol.com/forum/thread/53241/ivar-kjelberg-problem-on-joule-heating-analysis-over-a-thin-geometry
Ivar kjelberg after so many years you are still the best mentor on the forum. thanks for tips.
