Homemade spot welder keeps blowing MOSFETs

[costi_b]

I used 9 pieces P140LF4QL-5071, Housing
LF (similar MO235B), 140A 40V, driven by a TC4422, 4R7 in each gate, mounted on an aluminum PCB. I have recovery diodes between drain and +.
As a power source I use a 12V 100Ah gel battery connected by 16mm2 cables in parallel with a capacitor pack of approximately 0.6F.
The soldering cables are 6 mm2, flexible.
I've done several hundred welds in this setup, the only thing that gets hot are the welding leads. I use about 30ms welding time.