Re: Cold Solder Joints
|Originally posted by Mr_Happy
Whenever I'm assembling a DIY amp, I'm always worried about having a cold solder joint somewhere.
If you're soldering properly, and taking care over each joint, then the chance of producing a bad joint becomes extremely small.
The trick to making a good joint is having just the right amount of heat to let the solder "flow" properly. You can see the effect with a piece of copper clad board: Put your soldering iron into the middle of a big area of copper, and apply solder. As the temperature rises the solder begins to stick. At first it forms a bead that sits up high on the board. If you take the iron off at this point the solder may appear to have stuck to the copper, but it's probably not a good joint. Leave the iron on a little longer, and the bead of solder will appear to sink down into the board and flow outwards. That's when all the metalurgical magic happens, and that's when you get a good joint. You need to get the same thing to happen in every solder joint that you make. You will learn to notice the way the solder flows across the pad and up the component leg. (Of course, it happens much faster in most joints because a small joint heats up much more quickly than a big area of copper board does).
Leaving the iron on the joint for too long can also cause problems. When you do that the flux evaporates off, the solder becomes thick and pasty and crystalline, and you can drag it about with the tip of your iron. This is another recipe for bad joints. Sometimes you can save the joint with a touch of fresh solder (and the flux that comes with it) but often the best thing to do is to suck it clean and start over.
Of course, you must ensure that you allow the joint to cool slowly (don't blow on it) and that the parts don't move as the joint cools.
Sometimes when I'm done soldering everything I will go back over all the connections and reheat them just to be sure I haven't inadvertently produced a cold joint.
This could possibly cause more problems than it solves. Most of the flux evaporated when you made the original joint, and you risk creating bad joints by heating the solder up again. Far better to practise until you are confident that you can get the joints right first time.
Are the symptoms of a cold solder joint always noticeable in the sound (hum, buzz, etc.)? Or are there subtler effects? Am I just paranoid?
I don't know. I don't make cold joints.
It is often said that a good joint is bright and shiny and a bad joint looks dull and grey. Clearly a dull grey joint on a board full of bright shiny joints would merit investigation, but I'm not sure that this is always the case: I've just started using a roll of solder which is unlabelled (it could be an odd Pb/Sn ratio, or perhaps even lead-free) which makes every joint look dull and grey! I've checked them carefully, and they're fine - they just look different.
I have heard that joints can go bad over time and that old equipment (tens of years old) can benefit from having cold joints remade. Does anybody know what causes these joints to go bad? Does it indicate a manufacturing fault, or can a good joint spontaneously turn bad?