Why do capacitors fail?

Bad capacitors are the bane of generations of consumer electronics. They plagued early 90s Amigas and Macs, early 2000s PCs, and cheap hardware even today. So why do capacitors fail? And how can you tell when a failed capacitor is a problem?

Capacitors fail when the electrolyte dries out, or when the gas inside them builds up to a point that it opens a safety valve and the electrolyte leaks out. A good capacitor takes decades to dry out, but a cheap capacitor can leak within a few short years.

Why capacitors fail

Capacitors can fail for any number of reasons. Many of them simply die of old age. The most common electrolytic capacitors consist of three layers: a layer of metal foil, some kind of paper insulator, and a liquid electrolyte.

When a capacitor fails of old age, it’s usually because the electrolyte dried out, or enough of the electrolyte leeched into the paper that it lost its insulating properties. When this happens, there are no visible signs from the outside. But when you test the capacitor out of circuit, you find it no longer behaves how it did when new. Once it drifts too far from its original properties, other components malfunction, since the capacitor is no longer doing the same work it used to do.

How long it takes varies. The caps in old tube radios, which usually date to the 1950s or earlier, are almost always shot. The caps in ZX Spectrum computers are usually worn out and need replacing for the computer to work right. But the caps in the similarly-aged Commodore 64 are usually fine.

Heat can accelerate this process. This could also account for the difference between the 64 and the Spectrum, as the Spectrum came in a smaller case, and its hot-running voltage regulator tended to cook the components inside, including the caps. It’s also possible Commodore used higher-quality capacitors, but the heat seems a more likely reason.

But sometimes capacitors fail because the electrolyte leaked out. This kind of failure is more common in slightly more recent equipment, and it’s very visible when it happens.

Why capacitors leak

Early surface mount capacitors, common in early 90s Amiga and Mac computers, very frequently leak, and when they do, they spew electrolyte all over the circuit board beneath them. The electrolyte is corrosive, so it can eventually eat through the coating on the motherboard and damage the traces underneath them. It can also damage the pins on any chips it comes into contact with. Left unchecked long enough, it can damage the board to the point where it’s no longer practical to repair it.

In the early 2000s, many PCs fell victim to leaky capacitors. This drove motherboard maker Abit out of business, and also severely hurt Dell. The problem in this case was botched industrial espionage. One company stole a formula for electrolyte from another company, but got the formula wrong. These capacitors became very popular because they were cheap, but because the formula was wrong, the electrolyte proved unstable. Within a few short years, the capacitors failed.

Dell learned its lesson, and so did its main competitors, HP and Acer. But in consumer electronics where price is very sensitive, such as routers, sometimes a manufacturer will use cheap caps. When a piece of network equipment fails shortly after its warranty expires, that’s frequently the reason.

Capacitors leak and fail because capacitors create gas in the process of working. Usually this gas dissipates harmlessly. But when the gas builds up too much, the safety vent in the capacitor opens in order to keep the capacitor from exploding. With the vent open, the electrolyte leaks out.

Why capacitors bulge

Sometimes the safety vent opens on the underside of the cap. Other times it opens in the top. When this happens, the top will bulge, sometimes very visibly. This is easy to spot, since the top of a capacitor should be flat and level. While it’s possible for a cap to leak out the bottom and still look OK, a bulged capacitor is always failing.

Avoiding capacitor failure

There are several ways to avoid capacitor failure, or at least leaking and bulging. The first is to use tantalum capacitors instead of electrolytic capacitors. They are more expensive and a little bit bigger so they might not fit quite as well, but if you want your Amiga 600 or 4000 or 90s Mac to last as long as possible, tantalums are a good way to go.

The other thing you can do, even with electrolytic, is to use a good brand. Nichicon, Panasonic, and Rubycon are three popular brands, but generally all Japanese-made capacitors are good. Here’s a list of capacitor brands, ranked by tier.

And since heat causes the capacitors to dry out, keeping the system cool helps extend their lifetime. Modern components tend to run cooler, so replacing hot-running parts in vintage gear can help the capacitors run longer as well.