Tim Wescott wrote:
> OK, but why does the mounting tab have to go to the center pin? If it's
> all being stamped out of sheet anyway, why not have the mounting tab
> connected to the right pin, or the left?
>
> I assume there's a good reason, because AFAIK the center pin = tab is
> just about universal.
I think it is all about physical stability of the base and
emitter lead frames. If they are tucked into a notched
corner of the collector frame, they are stronger than if
they are just hung off the side of the collector frame, as
the far corner lead would be with the base and emitter in
the center and corner positions. Since they are held in
place only with the epoxy cast, having metal around two
sides of the end of the base and emitter lead gives added
rigidity and strength. At least, after blowing a few up and
seeing how they are made, this has been my assumption.
Perhaps Phil or someone who does investigation of explosive
failure has some good photos of the exploded construction.
A dental X-ray or two would be neat to see, also. I know
that a common failure mode I have seen with the TO-126
(smaller than the TO-220 power tab, with the hole through
the epoxy and no exposed metal tab) is the base or emitter
lead frame breaking lose from the epoxy so the bond between
the lead frame and the die breaks.
http://www.fairchildsemi.com/products/discrete/pdf/to126.pdf
The epoxy is just too thin and narrow to have enough
physical strength to handle thermal stresses between board,
transistor and heat sink. For this reason, I have given up
on that package in new designs.
The TO-202, however seems to be fine.
http://www.national.com/ms/TR/TRANSISTOR_OUTLINE__TO-202_.pdf
And some of those do have the collector on the end. So maybe
my hypothesis is all wet. I.e. D40 and D41 series GE
transistors and darlingtons.
But they do put the beveled (weaker) end of the epoxy over
the collector pin, and have the full epoxy thickness over
the emitter on the far corner. Still more rugged than the
TO-126.
--
Regards,
John Popelish
|
