When you learn to solder, you are warned about the pitfalls of creating a solder joint. Too much solder, too little solder, cold joints, dry joints, failing to “wet” the joint properly, a plethora of terms are explained if you read one of the many online guides to soldering.
Unsurprisingly it can all seem rather daunting to a novice, especially if they are not used to the dexterity required to manipulate a tool on a very small-scale at a distance. And since the soldering iron likely to be in the hands of a beginner will not be one of the more accomplished models with fine temperature control and a good tip, it’s likely that they will experience most of those pitfalls early on in their soldering career.
As your soldering skills increase, you get the knack of making a good joint. Applying just the right amount of heat and supplying just enough solder becomes second nature, and though you still mess up from time to time you learn to spot your errors and how to rework and fix them. Your progression through the art becomes a series of plateaux, as you achieve each new task whose tiny size or complexity you previously thought rendered it impossible. Did you too recoil in horror before your first 0.1″ DIP IC, only to find it had been surprisingly easy once you’d completed it?
A few weeks ago we posted a Hackaday Fail of the Week, revolving around a soldering iron failure and confirmation bias leading to a lengthy reworking session when the real culprit was a missing set of jumpers. Mildly embarrassing and something over which a veil is best drawn, but its comments raised some interesting questions about bad solder joints. In the FoTW case I was worried I’d overheated the joints causing them to go bad, evaporating the flux and oxidising the solder. This was disputed by some commenters, but left me with some curiosity over bad solder joints. We all know roughly how solder joints go wrong, but how much of what we know is heresay? Perhaps it is time for a thorough investigation of what makes a good solder joint, and the best way to understand that would surely be to look at what makes a bad one.
So before we reach for a lot of pins and stripboard to abuse with inadequately heated hot metal it’s time to ask you the Hackaday readers: what do you think makes a bad solder joint? How should we investigate the conditions required to go against decades of soldering experience and create a bad joint on purpose instead of a good one?
We’d expect the variables to be temperature, solder, and movement while setting. Temperature is obvious, it’s easy to dial in successive readings on a temperature-controlled iron. But solder, that bears some investigation. It’s tempting to believe that all solders are created equal, but as we all know nothing could be further from the truth. Decades ago for example a friend of mine bought a reel labeled as American MIL-spec solder on the premise that if it was good enough for fighter jets it was good enough for him, and came away with the conclusion that if that was what kept the American war machine running then all the Soviets needed to do would be to gently vibrate whatever super-weapons they faced and watch all the electronics fail.
In truth he probably picked it up cheap because it was a dodgy batch, maybe even a fake, but it taught us an early lesson about solder quality. It’s not a problem limited to dubious 1980s bargains though, in recent years the arrival of lead-free solders has thrown this question into sharp relief. Do you find lead-free solder makes it more difficult to create a good joint? And once you’ve created a joint, how do you know it’s a bad one? Sometimes it’s obvious, it looks grey rather than shiny, and granular on its surface. If you solder a lot, you’ll just know that look from experience. But what electrical properties should it have? If we pull out a milliohmmeter, what resistance should we expect to see? It’s easy to spot a high resistance, but at what level of low resistance does a joint become bad?
We’d like to investigate these factors, but before we do so we want to tap into your collective knowledge on the subject. The Hackaday readership never cease to amaze us with their collective ingenuity, knowledge, and experience, it is probable that among you lie some of the world’s most accomplished practitioners of the soldering art. Help us answer the question: Just how hard is it to make a bad solder joint?