Soldering wires to non-plated pins?

Question

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?

Answer 1

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.

Answer 2

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.

Answer 3

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.