can't get the solder to flow

Thread Starter

clangray

Joined Nov 4, 2018
261
Hi, I am a beginner and having trouble (very basic) soldering. I've got my iron on 480deg and I've got a resistor plugged into a practice board where the leads pass through. I am applying the tip of the soldering gun to the resistor leg and hold it there for 4-5 seconds. I then apply the solder to the leg that I've heated. Sadly no solder melts. What am I doing wrong?
 

sagor

Joined Mar 10, 2019
903
Make sure your solder has flux in it, or use separate flux to coat the leg of the resistor and the pad you are soldering it to.
 

DickCappels

Joined Aug 21, 2008
10,153
If your soldering iron tip if very dirty, if for example it has a lot of oxidized solder on it, the heat of the soldering iron will not transfer to the joint well. In that case wiping or shaking off the old solder may be justified. With a newly wetted tip you should be able to make a joint. Experiment a little. It is not difficult to get a good joint thought it may take some practice.
 

cmartinez

Joined Jan 17, 2007
8,218
Here is a film from Pace, a leading soldering equipment manufacturer. It is old, but the fundamentals haven’t changed.

Here is a film from NASA, it includes basic theory as well as practice. It is also old, but also fundamentally correct.

I've just learned something new ... I didn't know you're supposed to not solder adjacent pins on a chip consecutively so as avoid concentrating too much heat on a single spot on the chip ... makes lots of sense. Thanks for posting!
 

Audioguru again

Joined Oct 21, 2019
6,672
We use soldering irons with temperature contol, not a soldering gun.
Maybe your solder is made for plumbing or is the newer "lead free" that needs a higher temperature.
 
Last edited:

KeithWalker

Joined Jul 10, 2017
3,063
Hi, I am a beginner and having trouble (very basic) soldering. I've got my iron on 480deg and I've got a resistor plugged into a practice board where the leads pass through. I am applying the tip of the soldering gun to the resistor leg and hold it there for 4-5 seconds. I then apply the solder to the leg that I've heated. Sadly no solder melts. What am I doing wrong?
You didn't mention whether the iron is at 480deg. C or F. If it's Celcius, it's far too hot. If it's Farenheit it's not hot enough. I find that the best temperature for me is about 360deg.C or 680deg.F.
 

djsfantasi

Joined Apr 11, 2010
9,156
You also only mention holding the tip to the resistor leg. It should touch both the leg and the pad to which it is being soldered. You may already be doing this. If you only hold it to the component leg, the PCB board (or other item) will act as a heat sink pulling heat away from the joint.
 

cmartinez

Joined Jan 17, 2007
8,218
680deg.F.
:oops: Waaaaayy too hot for my taste ... I normally soldier things at 550°F, 575°F tops. Otherwise the soldier's flux/rosin dries up too fast. And when it comes to delicate parts, I turn down the heat all the way to 450°F even if it means I have a harder time putting things together. ADC parts are especially sensitive to high soldering temps.
 
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MrChips

Joined Oct 2, 2009
30,706
First of all, the tip of the soldering iron must be clean.
Clean the tip with the proper brass scourging pad.

1671138634600.png

Next step is to tin the tip with some rosin core solder. Melt the solder all around the tip. If the solder melts easily then you know the soldering tip is hot enough. Clean the tip again on the scourging pad. The tip should be shiny silver with very little trace of solder left behind.

Now you are ready to solder. Make sure that you are using rosin core solder designed for electrical work (not plumbing solder).
The operating temperature for Sn/Pb solder is 700°F and 800°F for lead-free solder (370°C and 430°C).

1671138698369.png
 

DickCappels

Joined Aug 21, 2008
10,153
Another thing is that the resistor lead and solder pad should be clean and shiny for the solder to adhere. You can do this by scraping, as with a scouring pad of sand paper. A little extra (non-acid) soldering flux might also help as well.
 

Reloadron

Joined Jan 15, 2015
7,501
Somewhere on your spool of solder it should tell you things like diameter, core type for example rosin core. it is lead free (PB Free).and what id the actual solder composition such as for example 60/40 tin lead . All of those figure into how easily the solder will melt and stick.

Ron
 

Ian0

Joined Aug 7, 2020
9,667
Is that a soldering iron, or is it a dart?
You'll ruin the board with soldering iron with a point that sharp!
Use a soldering iron with a 2.4mm or 3.2mm "screwdriver" bit.
A really sharp point doesn't have enough surface area in contact with the board.
If you need to use lead-free solder, use the type called TSC (Tin-Silver-Copper), it is much better than the Tin-Copper alloys; and those with too high a tin content can suffer from Tin Pest (where the solder turns to dust at low temperatures)
 

ThePanMan

Joined Mar 13, 2020
773
the tip of the soldering iron must be clean.
Next step is to tin the tip with some rosin core solder. Melt the solder all around the tip.
With a newly wetted tip you should be able to make a joint.
You also only mention holding the tip to the resistor leg. It should touch both the leg and the pad to which it is being soldered.
All valid points.

When soldering, you need a heat bridge. That is an iron that has molten solder on it. If you put a dry iron against a dry lead then you're not going to have sufficient heat bridging. A little solder on the tip before you solder will help bridge the gap between the iron, lead and board. More solder means greater heat mass, more thermal energy transfer. A dry iron just won't do.

One last thing: A dry lead and dry pad don't transfer heat very well. A pre-tinned lead will help immensely. As for pre-tinning the pad - only in surface mount would that be valid. But either way, surface mount or through hole soldering, if the iron and lead are tinned properly then soldering will easily occur. It's not absolutely necessary to have the pad tinned. And if your lead is not tinned, it will still solder. But tinning it before you solder it will help.

The biggest obstacle to soldering is oxidation. Flux is used to remove that oxidation. So after you've tinned your lead and iron, when you make thermal contact you then want to bring fresh solder to the hot joint. When the joint is full but not over full then you'll have a good joint. Just don't move the component while the solder freezes. Yes, solder freezes. As a matter of fact, when you're holding it - it's frozen. Depending on the type of solder it becomes plastic at a given (higher) temperature and becomes molten at an even higher temperature. When the solder is in a molten state it is not frozen. But when it solidifies - it's frozen. Don't think of freezing in terms of water/ice.

Clean your iron
Clean the lead
Clean the pad
Tin your lead
Insert the component through the hole.
Tin your iron
Bring the iron (tinned) to the joint
Heat the joint briefly and apply solder

It should flow.

Of course all this depends on whether you're using the proper materials. As others have said, there are different types of solders and different purposes for them. Same is true of flux.
 

panic mode

Joined Oct 10, 2011
2,715
Another thing is that the resistor lead and solder pad should be clean and shiny for the solder to adhere. You can do this by scraping, as with a scouring pad of sand paper. A little extra (non-acid) soldering flux might also help as well.
i would say avoid anything that is harsher than brass scrounge pad shown in post #12. so no sand paper or metals tools (file etc.). otherwise tip can easily be damaged and once the thin coating of tip is ruined, tip will tend to erode due to acidic nature of flux. but then if tip is so poor shape that it cannot be easily cleaned, it is time to get another anyway
 

MrChips

Joined Oct 2, 2009
30,706
Hi, I am a beginner and having trouble (very basic) soldering. I've got my iron on 480deg and I've got a resistor plugged into a practice board where the leads pass through. I am applying the tip of the soldering gun to the resistor leg and hold it there for 4-5 seconds. I then apply the solder to the leg that I've heated. Sadly no solder melts. What am I doing wrong?
Applying solder to the heated metal part is the correct technique.

However, if you are new to soldering and just trying to get the hang of it, bring the solder to touch in between the heated joint and the soldering tip at the same time. Being able to wet the soldering tip will assist in the transfer of heat to the joint.

Once you get the feel of it with a properly cleaned and tinned tip you can go back to doing it correctly.
 

BobTPH

Joined Jun 5, 2013
8,804
Is that a soldering iron, or is it a dart?
You'll ruin the board with soldering iron with a point that sharp!
Use a soldering iron with a 2.4mm or 3.2mm "screwdriver" bit.
A really sharp point doesn't have enough surface area in contact with the board.
If you need to use lead-free solder, use the type called TSC (Tin-Silver-Copper), it is much better than the Tin-Copper alloys; and those with too high a tin content can suffer from Tin Pest (where the solder turns to dust at low temperatures)
I use that type of tip and it solders quite well for through hole and SMT parts.

Also I use the brass cleaning pad every time the iron comes out of its holder.
 
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