packages
This is the TO-252 package.
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.
It's a (granted, fugly) TO-220 derived package used by Allegro for current measuring parts like the ACS756, datasheet. (I don't know if other manufacturers also use this kind of package.) The thick bent connections are for the current to be measured, in the case of the ACS756 up to 50A. This requires careful PCB layout (yes, the heavy pins are soldered in a PCB \$-\$ obviously not suited for paste-in-hole reflow, and hand-soldering will be tough as well :-))
Possibly other manufacturers prefer screw connections for low resistance current shunts like these ones from Isabellenhütte:
I don't know why Allegro prefers soldering over screw mounting.
I found this application, also based on an Allegro device (ACS715) which uses indeed screw connections.
What I find strange is that they pump 30 A through two ordinary SOIC pins. I would at least have replaced pins 1-2 and 3-4 by two tabs, I think. 15A through an SOIC pin seems like a lot.
It looks like SON and DFN refer to the same package. JEDEC documents like this one talk about "SON/QFN", but on the whole JEDEC site I couldn't find one reference to "DFN".
On the other hand, among its dozens of packages Linear Technology lists DFN and QFN, but there's no trace of SON.
JEDEC describes the USON as:
The Plastic Ultra-Thin Small Outline No-lead Package(USON) is a rectangular semiconductor package with metal terminals along two sides of the bottom of the package. The terminals are either flush with bottom or protruding slightly below bottom of package. The main body of the component is generally a molded plastic.
So a SON package, having its terminals on two sides, is much like a DFN. So 4 pins on 2 sides would be SON/DFN. A QFN-12 may have 4\$\times\$3 terminals, but other configurations like 2 \$\times\$ (5 + 1) are also possible.
This may cause some confusion if we look at the following package:
which appears to be a DFN-16 and not a QFN-18.
Best Answer
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.