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An image of the product

The product soldered

Soldering specification

Product: QSB35024S24

Images:

  1. An image of the product I'm working on. The image depicts pins -Vo,+Vo,-Vin,+Vin- these four pins are the troublemakers.
  2. A sample image of the product as it was soldered in the past. As you can see, while past employees do not remember running into the trouble I'm dealing with, the solder joints do look poor at a glance.
  3. A specification sheet I received when communicating with the manufacturer about wave soldering.

Problem: I am soldering wires to a DC-DC converter that is intended to be wave soldered onto a PCB. Soldering wires directly to it is how it was done in the past and I do not have PCBs readily available or a wave soldering machine. There are nine pins- the five "center" pins are thin, run of the mill terminals that solder easily sticks to. There are four voltage carrying pins (-Vo,+Vo,-Vin,+Vin) that are thicker. According to the specification sheet (image 3) they are 3.5mm non plated through hole pins. Solder will not stick to these troublemakers. Solder does notwet or flow but instead balls up.

Using my "regular" soldering iron operates between 50F and 104F (not sure which mine is set to; Im new here.) Using my regular, I cannot get solder to stick at all. Using a Weller with a variable temperature set to maximum (850F,) I am able to get the solder to stick somewhat, but the solder joints are poor, with poor connections and at least one came off. Re-soldering the joints with more patience (heating the pin for much longer) and liquid flux, I was able to get a firmer, cleaner looking solder joint. I suspect the solder isn't really sticking to the pin, but instead is flowing around it like a donut. Also, the pin looks worn; the heat is likely damaging it.

Do you have any instruction on how to solder non-plated pins?

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    \$\begingroup\$ Those pins are plated, I think you are misinterpreting the datasheet. What they are calling non plated through-hole is for the mechanical mounting holes on a PCB. As you can see, those pins are certainly not 3.5 mm thick. Also, I doubt any soldering iron operates at 50F to 104F. \$\endgroup\$ Commented Apr 13, 2022 at 20:57
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    \$\begingroup\$ Also consider finding a wire crimp-on terminal which fits these pins. Might be easier and cleaner than trying to solder them. \$\endgroup\$
    – rdtsc
    Commented Apr 13, 2022 at 21:03
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    \$\begingroup\$ "sing my "regular" soldering iron operates between 50F and 104F" Eeh? No, that's not a soldering iron. \$\endgroup\$
    – Lundin
    Commented Apr 14, 2022 at 9:49
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    \$\begingroup\$ Considering low temp solder is around 200C or 450F, no way your soldering iron is 100F. Your hot water faucet is 120 to 140 F! \$\endgroup\$
    – Passerby
    Commented Apr 14, 2022 at 10:50
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    \$\begingroup\$ At such high temperature you burn away the flux too quickly resulting in a poor joint that breaks easily. Lower the temperature, use a larger tip (if possible) and add more flux as the joint comes up to temperature. \$\endgroup\$ Commented Apr 14, 2022 at 13:47

3 Answers 3

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Those large pins conduct the heat away from your iron too fast. Use an iron with a larger tip. The extra thermal mass will help greatly.

Using flux is a good idea, and you should keep doing that.

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The pic you link says that the pins are plated, I'm guessing tin or some tin alloy.

  • Ensure that the peeled wire sits in a loop covering approximately 300° of the pin. Bend it tightly around the pin using pliers.
  • Ensure that the wire is fixed and won't move during soldering. Tape is useful for this.
  • Optionally add some flux to the pin in advance.
  • Use the thickest solder wire you got, ideally 1mm or so.
  • Important: use a large, flat solder tip. Some manufacturers call them "screwdriver tips" because they kind of look like one.
  • Aim the tip so that the side of it rests against the pin. Most of the heat should go to the pin, not the wire, or you'll melt the insulation as was done in the picture.
  • Your datasheet says above 250°C for 3-6 seconds. The normal recommended setting is to have the solder iron to 350°C/662°F - the surface temperature will never get anywhere near that high in this case. You can use 450°C/850°F even, if you are having a hard time, but that's a "pro" setting which shouldn't be used for long.
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There were a lot of helpful tips in the answers/comments, but unfortunately they made no difference. I tried a moderately hot iron (650F once and 750F once) with a wedge/chisel tip. I used a thick, 1.5mm solder (only "thicker" solder I have availible). I drenched the pin in liquid flux. I soldered in such a way to mimic the tips in the comments/answers. The solder joints were removeable by hand with moderate force.

What worked for me was taking an Xacto knife (no sandpaper at the moment) and scratching the pin till I saw copper 360 degrees around the pin. I then heated the pin with a hot (850F) iron with a wedge/chisel tip. I used regular, thin unleaded solder.

The solder did not "flow" as well as a normal thin pin, but the solder did flow and stick better than it ever had previously. The connections are solid, with a tight mechanical connection showing the solder properly mated with the pin. Attempting to remove the joint with my fingertips or pliers and moderate force is impossible, also showing a strong joint.

If anyone coming across this would like to comment as to why removing the surface metal is a bad idea, please let me know.

Edit: Also note that, after scratching off the outer plating the pin will oxidize. The pins I scratched and immediately worked on were good. The pins I worked on last/later had the same issue I originally contended with. But scratching the surface again allowed the solder to flow. So the gist is, work one pin at a time.

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    \$\begingroup\$ "If anyone coming across this would like to comment as to why removing the surface metal is a bad idea, please let me know." The most obvious would be oxidation, though you can counter that by covering the whole exposed surface in solder. Or in case the end result is just too messy for that (sounds like), maybe just cover it with PCB lacquer or similar. \$\endgroup\$
    – Lundin
    Commented Apr 19, 2022 at 14:41
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    \$\begingroup\$ Btw if you don't have to conform to RoHS etc then always use leaded solder for "lab junk" kind of work: things that aren't supposed to be placed on the market as a commercial product. It's significantly easier to work with since it melts at some 30°C lower temperature than RoHS solder. \$\endgroup\$
    – Lundin
    Commented Apr 19, 2022 at 14:46
  • \$\begingroup\$ Thanks, I'll consider leaded solder. I've never worked with it, so I'll just have to look up safety precautions first. Yes, the oxidation was a problem, I'm going to update my answer to reflect that. \$\endgroup\$
    – jfwork
    Commented Apr 19, 2022 at 15:31
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    \$\begingroup\$ It's not any different to work with than RoHS solder, just easier. Both are harmful to your health, but only through the vaporized flux. Use a fume extractor and wash your hands afterwards, no matter solder type. \$\endgroup\$
    – Lundin
    Commented Apr 20, 2022 at 6:26

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