I have seen transistors with the middle pin cut used in computer equipment. What do they do?
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2\$\begingroup\$ kinokijuf - Maximising light on subject helps such photos immensely. You have a shadow line across most of the IC. Also, using the camera at the minimum distance that it WILL focus at, even if th image size is smaller, is superior to a larger but out of focus image. Some webcams have a focus adjustment which can be would down to decrease focusing distance. \$\endgroup\$– Russell McMahon ♦Feb 7, 2012 at 19:03
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1\$\begingroup\$ @RussellMcMahon Replaced the picture, complete with freehand circles! \$\endgroup\$– kinokijufFeb 7, 2012 at 19:07
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1\$\begingroup\$ I remember reading about TO-3 packages, that only had two pins protruding through the metal can. The third terminal was the case. \$\endgroup\$– polemonFeb 7, 2012 at 20:13
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3 Answers
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Because it isn't required. On most of those parts the middle pin is internally connected to the heatsink tab, and that one is soldered anyway, for cooling reasons. BTW, the pin isn't cut, the short tab is the way the part is supplied.
Note that the PCB can accommodate both parts with or without the pin.
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\$\begingroup\$ @kinokijuf - No, that page has only through-hole components. This is an entirely different part, it comes precisely like that. \$\endgroup\$ Feb 7, 2012 at 18:47
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\$\begingroup\$ @KevinVermeer But it shows that the heatsink tab is connected to the middle pin, which I meant. \$\endgroup\$ Feb 7, 2012 at 18:54
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They're called D2PAK or TO-263 packages (DPAK = TO-252 is similar but smaller), and they're designed that way on purpose.
As stevenvh states, the middle pin of the leadframe is internally connected to the heatsink tab -- it's actually one piece of metal! (Sometime try dissolving away / removing the packaging compound and you'll see this.)
The reason the middle pin is short is to allow for increased creepage and clearance distance. Some of the transistors in TO-263 packages are rated at several hundred volts, and getting the middle pin out of the way is a huge deal because the other two pins are relatively close to each other in voltage, whereas the middle pin is the drain/source pin which is often at a very different voltage.
Why they have the middle pin stick out at all is a little strange to me (it's still an unwanted voltage near the other two pins, and it would seem like if they didn't make it stick out of the package at all, there would be even better creepage/clearance distances) -- although I'll admit it seems just right, as you can hook a probe clip on it if you need to.
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2\$\begingroup\$ The photo shows solder on the unused pad. Would it be better not to include that pad in the solder paste mask so that it stays bare? \$\endgroup\$– Matt B.Feb 7, 2012 at 21:28
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\$\begingroup\$ @Matt - (a late reply) You could do that, but you gain little, and it may cause errors in other designs, if you would use that same footprint for parts which need to have the center pin soldered. Better be consistent, and always assume the third pin present. The unused solder paste doesn't do no harm. \$\endgroup\$– stevenvhJun 7, 2012 at 9:53
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1\$\begingroup\$ "The reason the middle pin is short is to allow for increased creepage and clearance distance." Doesn't make sense. Removing the pin would likely to increase any creepage distances even further. \$\endgroup\$ Jul 19, 2018 at 8:27
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\$\begingroup\$ Yes, that's what I said in this answer. ("Why they have the middle pin stick out at all is a little strange to me (it's still an unwanted voltage near the other two pins, and it would seem like if they didn't make it stick out of the package at all, there would be even better creepage/clearance distances)") \$\endgroup\$– Jason SJul 19, 2018 at 16:19
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1\$\begingroup\$ The sentence you quoted refers to the creepage and clearance distance on the circuit board as compared to a TO-220, where the limiting distance is the distance between pads. TO-220's have 100mil pin centers, so by the time you put a pad on the board, the spacing between pads is only maybe 20 mils or so, which is really small. With the TO263, there are two possible creepage/clearance distances: the one on the board is roughly 120 mils (you get an extra 100 mils because the middle pin does not attach to the board), the one on the part itself is maybe 50 mils. \$\endgroup\$– Jason SJul 19, 2018 at 16:19
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History. (As in: Why, after all, is the middle pin still there, in its strange, stub-like nature?)
The Evolution goes like this: TO-220, TO-262, TO-263
(Source: Infineon; via http://www.powerblog.de/2011/12/06/bleifreie-mosfets-in-standard-to-gehausen-fur-die-automobilindustrie/, hopefully o.k. as per fair use.)
TO-220 has been the classic THT power package for, like, forever.
TO-262 is shorter and may be handy in height-restricted mounting space. The smaller tab is not a big drawback when no heat sink is required or when you use a clamp instead of a screw to mount the part onto a heat sink. Clamping is a better choice for a number of reasons, anyway.
TO-263 could be called the surface-mount version of these THT parts. The tab and the middle pin are connected, and the tab offers a path with less inductance compared to the middle pin. With no solder pad for the middle pin, board space may be saved (other components may go where the pad would be), and insulation distance may be increased (good for high voltage parts).
Note that the plastic molding of all three cases is exactly the same. Thus, much of the same machinery can be used in the packing process of the parts, it's likely just a different lead frame.
