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I am trying to solder arduino nano onto a perfboard/solderable breadboard using rosin core solder. The connection is only working some of the time, when I check with a multimeter (+) at pin 29 (in image below) I and (-) at pin 17 there is power at the top of the pin sticking out of the via/hole but there is 0 volts if I check at the actual joint (lower portion of the pin). I see a little flux residue around pin 29 that could indicate a little burning around the area could that be causing this, it is only very minor .. anyway I can fix it?

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Also is it normal to have a little yellow residue left over on the joints? I am new to soldering so not sure if a little flux reside is a bad thing on the joint...

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  • If all you are going to do is solder wires to the perfboard, why not eliminate it and just solder to the Arduino Nano?
    – Milliways
    Aug 31 '16 at 12:20
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Fair play, that is a lot of flux residue. I am assuming you are using some quite chunky solder with lots of flux in it.

Flux is non-conductive, so the residue will be forming an insulating layer on the outside of the solder. That doesn't mean the joint is bad, only that you can't get to the joint because the flux residue is in the way.

If there is a conductive path between the tip of the pin and the pad next to it then all is fine. The fact that you can't see the signal through the outside layer of flux is neither here nor there.

If you want to clean it off you can use isopropyl alcohol (AKA PCB cleaner, AKA "rubbing alcohol") and use, for instance, a tooth brush to scrub it off.

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  • Would a hotter iron burn off the excess flux, or otherwise dispel it better from the solder as it solidified?
    – CharlieHanson
    Aug 31 '16 at 22:01
  • Not really, no. That will just bake it on more and risk lifting the annulae of your holes.
    – Majenko ♦
    Aug 31 '16 at 22:04
  • Fair point. I have used 'rework solder' before which is bursting with flux and has a lower melting point, designed to flow as much as possible to help lift awkward components. That required lowering the tip temperature otherwise the flux would disappear too quick. It makes sense that it's more likely to bake on than bake off.
    – CharlieHanson
    Aug 31 '16 at 22:17
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“The connection is only working some of the time” does seem to indicate bad solder connections. (Another possibility is wires in wrong holes.)

To locate problems, clip (rather than hold) one lead of an ohmmeter (or a DVM set to Ohms), to a wire soldered to the beginning of the problematic chain of connections. Then advance step by step along the chain, looking for an open junction.

To fix problems, obtain a solder sucker and/or solder wick, a hot fine-point soldering iron (preferably temperature controlled), and thin (eg 1-2mm) 60-40 tin-lead rosin-core solder. Use the solder wick or solder sucker to remove the old solder from the joint. Apply new solder to make a nice shiny proper solder joint.

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  • Unless you're in the civilised parts of the world where you're not allowed to use lead... I use 0.7mm 97.1, 2.6, 0.3 Sn, Ag, Cu
    – Majenko ♦
    Aug 31 '16 at 16:50

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