Good day!

I have designed a little circuit and got some (5) PCBs printed and assembled in china and unfortunately I made a mistake when I transferred my prototype circuit into my schematic. Even less fortunate I only realized that mistake after I tried to test the boards at home, not before I ordered them.

I have the following subpart of a schematic: faulty schematic

I mistakenly confused the emitter and collector here. The INP should go into the collector and DT into the emitter.

printed board

since I already have some readily printed and assembled boards here this poses a challenge to me. I don't want to spend another 140 bucks for a minor mistake (and I'm not even sure if I didn't make another mistake here).

On one board I already removed the transistor. I only have normal, let's call it breadboard sized transistors at home. I tried to bend one's (TIP120 - Not the correct specs but worth a short try if it's feasible at all) connectors into the correct pinout positions and solder it to the board with more flexible wires in between, but I think my motoric abilities are not sufficient enough for that kind of task (don't know if you can grasp the tiny dimensions from that picture, but it's super small).

A second idea was to find a transistor that has its emitter and collector flipped (where the emitter is on the top center and collector is on the bottom left). I thought of a PNP transistor but that wouldn't suit my circuit either. As for NPN I couldn't find a suitable replacement for the given transistor and it's form/package (SOT-23).

I want to try a smaller transistor as opposed to the TIP 120 (like the SS8050-TA in TO-92). In the meantime, does anyone have experience with that and maybe even know a transistor (or type of transistor) that i could search for? I would also be happy to try different approaches to solve that problem.

Thank you very much for your help!

** EDIT **

for completeness and to make it easier to read in future I tried to visualize the pin situation.

  • BLUE is the correct transistor SOT-23 pin out
  • RED is my faulty connection, aka how the transistor should be connected., resulting into the flipping of C and E, indicated by the blue arrow

pin situation

  • 2
    \$\begingroup\$ Did you try mounting the part upside-down? Then you wouldn't have to stretch the leads too far to reach the correct pads. \$\endgroup\$
    – The Photon
    Apr 16 '20 at 17:07
  • \$\begingroup\$ hi thanks for your comment, since i have to flip the middle top pin with the bottom left i don't know how mounting it upside down would help. wouldn't this just flip the base with the emitter pins? \$\endgroup\$ Apr 16 '20 at 17:18
  • 1
    \$\begingroup\$ Flip it, and rotate it so the base on the correct pad and the other two (more or less) end up where they belong. It'll look funny, but it'll work. \$\endgroup\$
    – JRE
    Apr 16 '20 at 17:21
  • \$\begingroup\$ ok and i guess expand the pins with a piece of wire? definitely worth a try, thanks \$\endgroup\$ Apr 16 '20 at 17:24
  • 1
    \$\begingroup\$ SOT-23 is unambiguous in transistor pin order, unlike TO-92. What probably happened is the schematic symbol for NPN was incorrect, or use B C E for pin numbers. Big mistake. I always check this stuff, and got so tired of transistor layout mistakes that I went to the trouble of making my own transistor symbols that show the device 'in' the SOT package, with the numeric pin numbers. And I never use TO-92 anymore. \$\endgroup\$ Apr 16 '20 at 22:46

A way to rework this is to unsolder the part, tack it back down on the base lead (pin 1) but rotated, then blue-wire the collector and emitter to the right pads.

Another way is to unsolder the C and E leads, bend them up, then blue-wire them to the pads. This isn’t so bad if you use magnet wire and looks ok.

MORE: I'm not in favor of dead-bug and blob solder as suggested. Doing it this takes away any 'give' the leads have when the board flexes, which can lead to cracked joints. It's not an acceptable rework.

For reference, the SOT-23 package: enter image description here

One great thing about the SOT-23: once you get it right, you’ll never get it wrong again. The SOT pin out sequence is standardized for all transistors (FETs too). No more fretting over B-C-E or E-B-C and other such nonsense like the old TO package.

The downside for you, at least in this moment, is there isn’t a different SOT-23 pin sequence to choose from.

One more thing. That’s a really beefy transistor (2A). Do you need that? Can a more common one like a 2n3904 or 2n2222 work? Or a FET, like BSS138 or 2n7002? All are available in SOT-23, share a common footprint, and are very cheap (like, 1-2 cents in volume.)

  • \$\begingroup\$ thanks for the answer. i will try both approaches and see what i can handle better, since it's actually quite a delicate work. i got enough boards to practice with. as for the transistor itself, i use it because i started with a smaller transistor that didn't work that well. I just started over with that topic ("electrical engineering") last year, first time after vocational school so i was going for something rather safe ;) \$\endgroup\$ Apr 17 '20 at 7:26

If you squint just right SOT23 looks a bit like an equilateral triangle...

enter image description here

So, flip it upside down, rotate it, and either bend the pins or just add big enough solder blobs... and you just swapped pins 2 and 3.

enter image description here

Relax, you're not the first to solder one of these upside down because you screwed up the pinout 😁

  • \$\begingroup\$ thanks for the answer (and the heads up :P) i will try both approaches and see what i can handle better, since it's actually quite a delicate work. i got enough boards to practice with. \$\endgroup\$ Apr 17 '20 at 7:21

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