Recently, I had a project that required a reliable thermal fuse. There was little space to accommodate the classical axial versions, so I did some investigations and settled for the L33 type fuses. These come in various temperature versions, here, the 130°C limit, rated 2 Amps, 250 Volts.
The main components are the 2 wires, a plastic case, and some resin. Having never studied one open, I disassembled a few good ones.
Clearly, the resin is filled to certain level. At the top, there is a bridge between the wires, made by low-temperature melting alloy (having tin, bismuth, indium and related metals).
The alloy is quite substantial, likely to be able to handle 2 Amps of current.
Triggering it with heat gun, the alloy melts, and there is enough space in plastic case that it forms a drop, effectively interrupting the circuit.
While the devices studied showed very good consistency of construction, a little overfilling with resin may result in the thermal fuse not opening, I hope the manufacturer has this parameter under strict control. For the 10 pieces I have, the weights measured on a precision balance where quite uniform at least. But if you get such critical parts from some doubtful sources, better you do some tests first and be sure they are reliable.
