I was working on an ARP Omni 2 and came across a really bad soldering job. Someone had replaced the gating capacitors, but all of their solder joints were “cold solder joints”. What’s a cold solder joint? Well, first let me explain what a good solder joint is. With a good solder joint, both the pad on the circuit board and the leg of the component you’re soldering are sufficiently clean (both clean before soldering and from the flux in your solder) and hot for the solder to form a solid bond between the two parts you’re soldering. A good solder joint looks nice and shiny, like my work on this ARP Omni 2 below:
Now that we understand what a good solder joint is and how it’s made, we can understand that a “cold” solder joint is made when the two metals are not sufficiently heated or clean enough for the solder to truly bond between both metals. In a cold solder joint, the two parts may be stuck together, but may not have good electrical contact, and since the solder is really just a blob sitting on the surface of the pad or component, it’s very prone to crack or break off. You can typically spot a cold solder joint by its dull appearance, like I saw in the ARP Omni 2 below:
On the closeup photo you can really see the dull solder blobs just caked onto the pads and leads of the capacitors. And to no surprise, several of them just cracked off when I was going through removing and replacing all these capacitors.
Hopefully this explains what a cold solder joint is and how to spot one. To avoid making them yourself follow the following Synthchaser tips:
- Clean all surfaces before soldering
- Use flux cored solder
- Make sure your soldering iron is at a high enough temperature (typically for synth rework I use around 700 degrees F)
- Tin your soldering iron tip before soldering a part (apply a small amount of solder to the tip of the iron)
- Position your soldering iron tip so it touches both the pad on the circuit board and the component and allow a little bit of time for both to get sufficiently hot
- Feed the solder onto the component or the pad, not the tip of your soldering iron
If you find yourself melting the solder on the tip of the soldering iron, or “painting” solder onto the component from a blob on the tip of your iron, you’ll probably wind up with a cold solder joint.
Hopefully this helps, please comment if you have any other tips to avoid cold solder joints that I missed!
