Why the elaborate metallic case on some SMD polyfuses?

[lpc32]

What's the idea behind the shape of SMD polyfuses like the following?

[Zero999]

There is no case. What you have there is the semiconducting PTC resistor element sandwiched between two metal plates.

[Simon]

standard sheet metal

[Ian.M]

The PTC element's bulk resistivity is too high, so a large area thin layer is used to reduce the resistance to something useful for a fuse in the rating desired.  The metalwork is only what's needed to make contact to the element, spread the heat to prevent hot-spotting and premature tripping, and keep it off the board (apart from the ends) for predictable thermal behaviour.

[lpc32]

Still, oddly shaped, no? Usually components come in neat little "sealed" packages.

Also, that's not the only package for polyfuses. There are more standard looking ones, such as these:

[SeanB]

The first will have a larger current rating, while the small flat ones are low rated, and also require a specific board pad layout, traces to and from that are a specified width and a specified solder amount to get the trip rating within the specs. The others are wire ended, so that you have basically the first fuse with no shaped metal to make it SMD, just the 2 wire leads and a conformal coat for extra protection and the ability to survive a pick and place machine grippers.

[lpc32]

Presumably you could make the smaller SMD ones a smaller version of the one in the top post, or the large one a supersized version of those SMDs. So I'm still puzzled.