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I'm trying to do some soldering on a PCB with a 20 watt iron and 1.2 mm 63/37 solder. From what I can understand, I'm supposed to touch the iron (has been tinned) to the point and heat it up then touch the solder to the point and it should start melting. But it takes like 20 seconds for it to heat up enough to melt that way instead of like 5 seconds in the videos I see. I end up having to touch the solder to the iron and try to let it flow on to the board.

Would 0.8 mm solder work better?

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IMO you should consider switching to thinner solder wire to get more control over the amount of solder you use. But this is not an answer to your actual question. A 20W soldering iron should not have that much of a problem when you gave it time to heat up properly, but I second the suggestion to get you a temperature-controlled soldering iron with more power: more power to heat up and if you encounter large parts (ground plane, connectors) and temperature controlled so the raw power does not mean your iron gets too hot. – 0x6d64 Jul 1 '12 at 11:17
1.2 mm is really fat solder. I use 800 um for ordinary soldering, and have thinner stuff around for special uses. – Olin Lathrop Jul 1 '12 at 12:16
I use 1mm for realy coarse work (through hole with large pads), 0.7mm for finer work (TH with just a ring, SMD). 20W sounds a bit low. If this is a lasting hobby, go for a >= 40W temperature controlled iron. – Wouter van Ooijen Jul 1 '12 at 14:28

5 Answers

up vote 8 down vote accepted

I used a 25 W soldering iron for some time before I got a 80 W temperature controlled one and one thing I've noticed that helps is to have a bit of solder on the tip of the iron.

The story told to beginners is not to try to transfer solder from hot iron to the joint and to instead apply solder directly from the wire. I won't say that that is incorrect, but it often helps to have a bit of solder on the tip. That solder will improve thermal connection between the tip, device and the pad. The amount of solder should be just enough so that once the tip is in contact with the device and the pad, solder from the tip is in contact with all 3. At one point you'll see that the solder from the tip is starting to flow into the joint. That is the moment when you should add solder from the wire to the joint. If you've done everything correctly, it will easily melt and flow into the joint and the flux will have the chance to clean the joint this way.

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I would go as far to say it is wrong. On through hole your soldering iron should contact on one side and the solder wire on the other. Gorgeous joints every time. Ohh, and I pretend my board is possessed and flux is my holy water. – Kortuk♦ Jul 1 '12 at 16:58
An ordered list of directions could probably make this much more readable. – Kortuk♦ Jul 1 '12 at 17:12
I agree with AndrejaKo's method. So I used this method to instruct a client's contract manufacturer to use this method for an irregular reason. My client's design was tight for space and chose to solder the leads directly beneath the base of the 5mm LED's, Even though this violated the soldering instructions given in my spec, I indicated the discrepancy and suggestion they use the above method to solder. My only stipulation was to do so in 2 seconds. My method as above might have read as follows. – Tony Stewart Jul 2 '12 at 2:31
(1) melt solder instantly and touch tip in same motion. (2) hold tip and then remove solder only after enough to surround the lead joint with no bulge (3) remove tip instantly after solder surrounds the joint. ( I demonstrated this repeatedly in 2 seconds and asked them to never exceed 3 seconds. using a 600'F iron, which is a fairly hot tip) Then asked Engineering to see if possible design a 5mm lead keep out zone for solder from the base and maintain the 3 second max rule.This reduces risk of damaging epoxy seal around leads (moisture ingress) and also stress on gold wire bond. – Tony Stewart Jul 2 '12 at 2:43
up vote 5 down vote

When iron tip touches the leg and pad, heat transfers inefficiently as there is not much surface contact between iron and solderable part. One trick is to add a little bit of solder so that the heat transfers more efficiently and then add more solder to form a good joint. The process would be as this:

  • Clean the tip
  • Touch it to both leg and pad
  • Put a little bit of solder onto tip so that it flows and contacts both leg and pad
  • Wait a bit
  • Put more solder onto leg and pad simultaneously

When pad gets hot enough, solder will flow on it. Sometimes I just touch solder onto pad/leg and wait.

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That's doing it the wrong way. You have to apply the solder wire directly to leg and pad, otherwise the flux won't work. – Federico Russo Jul 1 '12 at 11:44
"Put more solder onto leg and pad simultaneously" - this makes flux to work – miceuz Jul 1 '12 at 11:46
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@Federico: Micuez is absolutely right. I do the same thing, and everyone else I know that does this routinely. There is lots of bad soldering advice out there. Just the iron touching something won't transfer much heat. You use a little solder first just to fill the some of the air space between the iron and what it is heating. Yes, that flux largely goes up in smoke. Then you apply more solder once everything is hot. That second application is the bulk of the solder left in the joint, and the flux from it will do its job. – Olin Lathrop Jul 1 '12 at 12:13
@Olin - I always work with a clean tip, and to be honest I'm positively surprised that it transfers heat so well. My solder joints look ever so pretty! Weller WES51 – stevenvh Jul 1 '12 at 16:37
@OlinLathrop and miceuz, I think he was critiquing how you wrote that. I understood what you were getting at when I read it but was going to write a comment to ask you to clarify that you should touch the solder from another point onto the join, not onto the soldering iron for best results. I would love pictures also. I always tell people from the opposite side use the solder then you know you have heated the joint. This is similar to my coworkers always using the minimum functional heat and myself using a much higher temp so that only the join gets heated to solder. – Kortuk♦ Jul 1 '12 at 17:09
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up vote 4 down vote

Probably not. What are you soldering on? 20 W is rather low power for a soldering iron, and mostly used for SMT components. If you have a large copper plane the 20 W won't be sufficient to heat it up; it will lose too much heat compared to what you apply. The solder thickness of 1.2 mm will take far less heat than that, so switching to 0.8 mm won't really help. Besides, from your last sentence I understand that the iron doesn't have trouble melting the 1.2 mm solder, so the problem seems to be the heat capacity of your object to solder.

Applying the solder to the iron will evaporate the flux, which is needed to clean the surface to solder. Without flux you may have bad soldering.

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There's a 40 watt setting but I read that it's too much for soldering boards and could damage it. I'm soldering on a red pcb from a nixie clock kit. – Jack Jul 1 '12 at 10:57
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A temperature-controlled soldering iron won't damage the board, as it won't go higher than the set temperature. The difference is that it will go to that temperature faster. Non-temperature controlled irons will continue to heat up while they're unused, and the first solderings may be at a way too high temperature. Still may not cause damage other than a bad soldering. If you don't have one I'd suggest you look out for a 40-50 W temperature controlled iron. – stevenvh Jul 1 '12 at 11:03
Oh ok, so that means as long as I don't put keep the 40 watt iron on too long, it should be fine? How long is too long? I tested the 40 watt setting on some of the scrap legs I cut off and it heated it up within 6-7 seconds. But I don't know how that would be like on the PCB. Cause the various guides I read says that 20 is enough and 40 is for heavier duty stuff and could damage electronics so I didn't risk it. But I am having difficulty using the 20 setting. – Jack Jul 1 '12 at 11:15
I guess it's not temperature-controlled? Then the important thing is not to use it on small SMDs after it has been sitting in its stand for a while. Better touch some larger copper plane first to take the temperature down a bit (copper planes can absorb quite some heat). With my 50 W temperature-controlled iron I usually can apply solder after about 3 seconds, depending on what I'm soldering. A more massive object takes more time. It comes with experience. – stevenvh Jul 1 '12 at 11:21
@Jack, I cannot easily describe why without a picture but a hotter iron often heats the chip itself less, this is because you get your join spot up to melting temp far before the conductors have carried full temp to the chip or across the plane so you can solder and get your tip away long before you heat the chip. – Kortuk♦ Jul 1 '12 at 17:12
up vote 2 down vote

Yes, 0.8mm solder should work better. Besides that consider your working temperature, using flux and maybe using a fine tip (if you do not do that already).

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A too fine tip will transfer even less heat from the already underdimensioned 20 W iron. – Federico Russo Jul 1 '12 at 11:46
up vote 1 down vote

The problem seems to be thermal resistance not power of heat source. Often professional techs only need a 25W iron with a dimmer set to 60% with a well tinned clean tip. Rule of thumb, if it is not clean or shiny, then the surface is insulated with an oxide coating.

If you have mechanical screws for a gun or threads between the heater and tip, make sure they are clean and tight to reduce the thermal resistance. Then you can reach the 550~600'F required.

FWIW, stained glass leaded wire soldering irons are not hotter, but have more thermal mass and low thermal resistance when properly maintained so they transfer heat efficiently.

Once these steps are understood resin core flux can solder any leaded part in < 3 sec. SMT may take longer and preheat the board is often recommended on a hot plate for rework or a vacuum solder station. Thermal controlled tips work better because they can compensate to some extent for variable thermal resistance in the tip from oxide coating.

The very best irons use RF to heat the surface metal of the tip, rather than a heating element so the response time is <0.1 sec. Of course, they are more expensive and only commercially used. They employ the "Peltier Effect" to regulate temperature in the metal clad coating and can desolder a flatpack SMD IC at the rate of 2~3 IC's per second with special adapters and skill.

So my rule of thumb is, if does not transfer heat enough to solder in < 3 seconds, your tool is oxidized or has loose fittings. Expect ~ 1~3 seconds for a well-maintained iron, to stay as a good as new. This maintenance is before and after every use to keep it tinned to prevent corrosion (oxide). A moist sponge may be used to remove excess solder and oxide. Abrasives may remove the special plating on the tip so be careful and try not to inhale the solder & flux fumes using a fan or vacuum exhaust system for daily work.

You are correct to add solder to pre-heat the joint, but this is due to poor maintenance and can work in a pinch, but then move the solder to the other side to all flow throughout the centre of strands or to wick up the edge.

FYI "60/40: melts between 183–190 °C (361–374 °F)" Contamination from oxides will raise this temperature significantly.

1/16" dia. tip is standard and you may consider larger or smaller depending on the size of the solder pad.

The velocity of heat transfer depends on the metal, mass and heat sink effect if a ground or power plane is attached to the pad. But for steel wire LED's resistors with flash tin plating have a thermal velocity of about 2mm / second, so keep that in mind when soldering.

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A number of things here are specific to lead solder, things such as how to tell a good joint will be different with a lead-free solder. – Kortuk♦ Jul 1 '12 at 19:28

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