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I swapped the base and emitter of my NPN transistor land pattern. If I flip over the part, the pins are in the correct orientation, although there is a 1mm between the pins and the pads. When I tried to solder one I just made a mess. I soldered wire wrap wire in the vias and then tried to solder the wire to the pins. Is there a better technique?

Here is a photo of the part before I would desolder:

enter image description here

Here is the mess I made of a different one:

enter image description here

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  • \$\begingroup\$ The dirt on the lower PCB, is it the resin core of the solder or is it from flux? \$\endgroup\$
    – jippie
    Commented Oct 21, 2012 at 15:30
  • \$\begingroup\$ Resin core from the solder. \$\endgroup\$
    – Bill
    Commented Oct 22, 2012 at 21:56
  • \$\begingroup\$ Try using a little bit of flux, It'll make soldering a lot easier. \$\endgroup\$
    – jippie
    Commented Oct 23, 2012 at 5:45

5 Answers 5

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One way is to leave the SOT-23 soldered to the board, simply cut those two traces on the PCB, and solder jumper wires to the base and emitter on the part, swapping the wires over to the correct vias. Cleaner, less risk of mess-up.

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  • \$\begingroup\$ I vote for the clean solution, which is also more reliable. \$\endgroup\$
    – D.A.S.
    Commented Oct 21, 2012 at 11:47
  • \$\begingroup\$ ... and avoids risk of damage to pins or traces while desoldering. \$\endgroup\$
    – Anindo Ghosh
    Commented Oct 21, 2012 at 12:47
  • \$\begingroup\$ yes risks & stress degrade reliability a statistical number. Routine ECN methods used by IPC are best. \$\endgroup\$
    – D.A.S.
    Commented Oct 21, 2012 at 12:50
  • \$\begingroup\$ If one is so concerned about the small risk, he/she should use the correct tools. Such as a SOT remover tip. I don't think cutting the traces is a cleaner way. \$\endgroup\$
    – abdullah kahraman
    Commented Oct 21, 2012 at 13:04
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A quick and simple way of doing this is to flip the SOT23 on its back then solder the side with two pins onto the pads. This will leave the third pin up in the air, and you can then run a short wire down onto its pad. A picture should be worth a thousand words, but this one only runs to about 250 because of the focus, sorry:

enter image description here

The main advantage of this method is that no pins are bent, and only a single short and straight wire is needed, so it is easily removed and the component recovered. Sits within the existing footprint too.

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  • \$\begingroup\$ Your answer was very good and I will use it in the future. But since I don't have a easy way to desolder the old part (like a hot air gun), I am going to try Anindo's idea. \$\endgroup\$
    – Bill
    Commented Oct 22, 2012 at 21:59
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    \$\begingroup\$ If you have two irons you can use one to bridge the two pins and the other to heat the single pin, like a pair of tweezers, and it'll come off in short order. A big blob of solder helps and is easy to clean off afterwards. \$\endgroup\$
    – dunkers
    Commented Oct 24, 2012 at 1:12
  • \$\begingroup\$ I never had any problem removing SOT-23's from a PCB with one iron. Add some more (hot) solder to one side, the touch the other side and give the component a gentle nudge. Clear up the mess with solder wick. \$\endgroup\$
    – Wouter van Ooijen
    Commented Jun 10, 2014 at 18:34
  • \$\begingroup\$ For desoldering SMDs (also SO with more than 8 pads) I use a SMD shim blade (a very thin but strong steel foil) to desolder pad by pad with minimum mechanical and thermal stress for both the part and board. I made good experience with that tool. \$\endgroup\$
    – Ariser
    Commented Nov 10, 2014 at 11:21
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If you are careful you can bend the lead frame and invert them. It's entirely possible to break the leg off. Using two tools and you can bend the outer portion of the lead down.

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    \$\begingroup\$ Yes, I've done this. In my experience, you have only one chance to bend the leads on SOT-23. Fortunately, SOT-23 NPNs are pretty cheap parts. \$\endgroup\$
    – markrages
    Commented Oct 21, 2012 at 5:00
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If you have the space around the transistor's location in the layout, you can leave it in its normal orientation and solder any two of the transistor leads directly to their pads, and then run a single wire between the remaining lead and its pad.

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Here is a crazy idea.

There is likely plenty of clearance on the other side of the board for a small SMT component. If this is a simple two-sided board, and pads of this surface mounted device are not over any traces on the other side, I'd just drill them through to the other side to create a mirror image footprint on the opposite side of the board. (I have a set of very fine drill bits, and a thumb drill just for this sort of thing.)

To create pads, I would find the smallest brass nails I could get my hands on. Check out the following link: these mini nails have only a 20 mil diameter: http://www.micromark.com/mini-nails-3and32-inch-long-x-020-inch-dia-pkg-of-100,6561.html

Push these through the drilled holes from the opposite side until flush with the board, then solder from the original side, exactly if these nails were through-the-hole components being installed in a one-sided board. Then trim them, just like component terminals.

Presto: the nail heads give you brass pads.

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