Years ago, my monitor broke. Back then we had repair shops for such things. I called one and described the symptoms I was having. “Sounds like it could be a cold solder joint. Bring it in,” he said. But what is a cold solder joint? Isn’t solder hot?
A cold solder joint doesn’t refer to temperature, but rather, lack of electrical conductivity. A cold solder joint is a solder connection that isn’t conducting electricity the way it should.
Types of cold solder joints
There are several types of underperforming solder joints, all of which tend to give the same symptoms. The cause can be different, but the overall effect is the same. These are cold, cracked, and dry solder joints. All of them result in intermittent or lack of electrical conductivity, which means unreliable operation.
Cracked solder joint
A cracked solder joint is just what it sounds like. The connection cracked under stress, and now there’s a crack in the solder. This allows the pin to move around in its hole in the circuit board. Depending on where it moves, the electrical connection can stop or start.
Heavy components on boards mounted vertically are more prone to cracked solder joints, due to gravity. Vibration exacerbates the problem. This is one reason it’s so difficult to ship a CRT monitor without damaging it. Padding the box decreases the vibration of internal components, but it can’t completely eliminate it.
Dry solder joint
A dry solder joint is a solder joint that doesn’t have enough solder in it. Solder is metal heated to liquid, and the goal is to flow it into the joint and wet the entire connection before the solder freezes back into solid form. A dry solder joint doesn’t have enough metal. These look dull, so they are easy to identify.
Cold solder joint
A cold solder joint just is one of the two. It doesn’t work. All it means is you’re not sure if it’s dry or cracked, but it’s not working, regardless. If you want to be pedantic, a cold solder joint can be cracked, dry, or both. And technically, not all cracked or dry solder joints are necessarily completely cold. But they’re likely to be or become problematic, so it’s best to fix them when you find them.
What does a cold solder joint look like?
Let’s start with the opposite question. A good solder joint is shiny and cone-shaped, ideally slightly concave, like a volcano, and smooth, not lumpy. That means it doesn’t have too much solder, but it also has enough that it’s making good contact with all the nearby metal.
A cracked solder joint has a crack in it. The solder can be either shiny or dull. The crack is the bigger problem, usually.
A dry solder joint is dull, and likely to be the wrong shape.
How to fix a cold solder joint
The quick fix for a cold solder joint is reflowing. Add a bit of flux to the cold solder joint, heat up your soldering iron, and touch the soldering iron to the joint to remelt the solder. Hold it there for a second or two, then pull the soldering iron away. If all goes well, the result is a smooth, concave joint. If it doesn’t go well, it still won’t look right after it cools.
If you know what kind of solder it is, you can try again, adding a bit of fresh solder. If the board was made prior to 2006, it probably had lead solder, so the safe bet is to use 60/40 or 63/37 leaded solder. It should mix well with the existing solder. If it was made after 2006, use a lead-free solder. Add flux, heat up the joint, and touch just a bit of solder to the joint. You don’t want too much.
The safer thing to do is remove the solder and completely resolder the joint. Add flux, then use a desoldering wick, desoldering pump, or desoldering gun to remove the old solder. A trace of solder will remain on the surfaces, but the pin will move freely in its hole. Add fresh flux, then solder with whatever solder you like. You’ll probably find 63/37 the easiest to work with, with home equipment. Apply heat to the joint, not the solder. The heat from the joint pulls the solder in. Let it cool, and examine the joint afterward. It should be smooth, shiny, and concave. If it is, clean it with isopropyl alcohol, contact cleaner, or mineral spirits to remove any remaining flux. The board may still have problems, but that cold solder joint won’t be one of them.