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