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I am trying to solder some 2N7000 MOSFETs onto a PCB that I got manufactured, but the pads are very close together. When I try soldering the first leg on the MOSFET, I always end up shorting it to the other pad through the solder. The soldering iron I am using is relatively thin and heating at a temperature of 300 C.

Could I be applying too much solder or are there other things you should do to prevent this?

Below are images of the top and bottom of the MOSFET pads, marked as Qx on the top of the PCB:

Top

Bottom

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    \$\begingroup\$ do you have flux? add a little if you do to make it flow \$\endgroup\$
    – Hatman
    Commented Jan 16, 2023 at 16:44
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    \$\begingroup\$ Smaller soldering iron tip and smaller diameter good quality solder would help. The design isn't great, it would be better to have the pads spaced out more (plenty of room) and to have to form the TO-92 leads. \$\endgroup\$ Commented Jan 16, 2023 at 16:45
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    \$\begingroup\$ its an easy fix to make sticky solder flow better, alternatively you can use good quality tin/lead with rosin core \$\endgroup\$
    – Hatman
    Commented Jan 16, 2023 at 16:56
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    \$\begingroup\$ There are a bunch of TO-92 footprints in KiCad 6. I actually don't like any of them. I prefer the middle lead out in front (staggered) so that the part doesn't waggle back and forth under vibration and break off. KiCad has staggered footprints but they put the middle lead in back for some reason. Maybe it's a European thing, IDK. In any case, you can make your own footprint and you only need to do that once. \$\endgroup\$ Commented Jan 16, 2023 at 17:09
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    \$\begingroup\$ In addition to what Hatman says, there are liquid, paste, and solid rosin-based or synthetic fluxes for PCB soldering work. I suggest a liquid variety - these tend to be more-suited to board soldering. Just google "liquid solder flux" and start looking through reputable sources. It will make a night-and-day difference. \$\endgroup\$
    – rdtsc
    Commented Jan 16, 2023 at 17:26

2 Answers 2

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A smaller soldering iron tip and smaller diameter good quality solder would help. 0.8mm is a good compromise for through-hole parts, maybe with some 1mm and some 0.4mm for coarser/finer parts and SMT.

The design isn't great, it would be better to have the pads spaced out more (plenty of room) and to have to form the TO-92 leads. KiCad has such footprints defined.

Here, for example, is a tape containing pre-formed transistors (the once-ubiquitous 2SA1015 in a Y beta bin). They are formed to fit 3 pads on 2.54mm (0.1") centers. You may prefer to use a footprint with staggered leads.

enter image description here

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  • \$\begingroup\$ 0.8mm is massive. I use 0.5 for regular through-hole work and for SMT 0.25 or thinner. \$\endgroup\$
    – Lundin
    Commented Jan 17, 2023 at 7:37
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There's two ways you can do this:

  • Either use a very fine tip, fine solder 0.5mm or thinner, and carefully solder each via.
  • Or (easier) just use a chisel tip, focus on making 3 good joints regardless of if they short with each other or not. Then apply plenty of flux with for example a flux pen and heat all 3 joints a second time.

Could I be applying too much solder

Very likely since this is a classic mistake. Particularly using far too thick solder wire. There's a tendency to focus too much on which tip to use instead of just use thinner solder.

But also the layout isn't good and the CAD tool should warn against placing pads this close - the plated area looks too large. As mentioned in another answer there's all manner of TO92 leg shapes you can pre-order.

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  • \$\begingroup\$ Solder I am using is 1.2mm so this doesn't help. In future I will be more careful about the layout of component pads that are too close together. But will take everyone's advice here with thinner solder and some flux to try and use up the PCBs that I have bought. \$\endgroup\$
    – David777
    Commented Jan 17, 2023 at 9:59
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    \$\begingroup\$ @David777 You can still use that for now, but then you must be extra careful not to apply too much of it, since the flux method won't work if there's too much solder to just wet along each leg and both sides of the hole like it should. Or otherwise you have to remove excess solder with the tip, which is strictly speaking questionable practice (yet I do it all the time). \$\endgroup\$
    – Lundin
    Commented Jan 17, 2023 at 10:56

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