TECH TIPS: Anatomy of a Solder Joint

Have you ever tried to solder to the back of a volume pot, only to watch the joint pop off like a champagne cork? Though we may not want to admit it, we’ve all had them: bad solder joints. A good solder joint should look nice and shiny. A bad solder joint will look … well, bad. It will be cloudy and have a dullish gray look about it. But most importantly, it will be a bad electronic connection. It shouldn’t look like someone stuck a piece of cauliflower in your control cavity.

A bad solder joint is (obviously) the result of improper soldering. Specifically, a bad solder joint is the result of not properly heating the surfaces that you are trying to solder together. That’s why they call them “cold solder joints.” Solder flows and adheres best on surfaces that have been properly heated. How hot should each surface get? Hot enough to melt solder. How long it will take to properly heat a surface depends on the type of surface. A piece of wire, for example, is physically small, and does not have a large surface area to heat up. You can usually be on an off in about a second.

The back of a typical potentiometer, though, is clearly a larger, wider surface, and will require a little more time to heat. This is because as the heat is transferred from your soldering iron to the back of the pot, the heat gets absorbed by the entire potentiometer casing, which (at first) draws heat away from the spot that you are actually trying to heat. Sometimes, with a hot iron, you can get on and off the back of the pot quickly, and get a good joint because the pot casing doesn’t have time to absorb the heat. You get localized heat quickly enough to do your business. But with most irons, you’ll want to stay in contact with the back of the pot for 5 to 10 seconds before bringing the wire in or applying any solder. When you heat your surfaces properly, you will see the ease with which the solder flows, and you will find that it is much easier to get it to flow where you want it to without getting lumpy, gunky, cloudy … maybe there’s a Seven Dwarves joke in there somewhere.

We all get impatient at times, and try to solder quickly, but taking a few extra seconds to heat things properly can eliminate tons of frustration down the road from cold solder joints becoming intermittent or failed connections. Patience is key.

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