This is the TO-252 package. enter image description here

It's got a tab, and two leads. There is, however, a "stub" of a lead, usually connected to the tab. What function does this serve? Is it a vestigial pin that mutated and was kept by selection bias?

I am aware of a few other questions concerning this; however, none of them seem to address why exactly this is here.


1 Answer 1


Several different packages (both SMT and through-hole) are all based on the same lead frame. The only difference is in how the lead frame is trimmed and bent after the epoxy encapsulation has been added.

It is less stressful on the adhesion between the epoxy and the lead if the trimming is not flush to the epoxy, so a small stub is left.

  • 1
    \$\begingroup\$ To a related example, on some boards, SOT89 voltage regulators only have their tab soldered, but the middle pin is left unconnected and resting on the solder mask. \$\endgroup\$
    – Fizz
    Oct 10, 2015 at 18:45

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