Notes on capacitors, part 2: aluminium electrolytics

This is part two of my “convert a bunch of notes I made about capacitors into a blog post” series. If you missed part one, which covered ceramics and MLCCs, you may wish to read it first:

Again, this article is mostly going to be useful to you if you already know a bunch about capacitors and want a big infodump to pick up some new facts from, and it comes with a caveat that I am not a capacitor expert.

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I’m primarily going to talk about wet aluminium electrolytic capacitors in this post, since they’re the most common type and they’re what most people are familiar with.

Wet aluminium electrolytic capacitors are constructed from aluminium anode and cathode plates separated by a liquid electrolyte. The electrolyte is typically soaked into a paper layer. A dielectric oxide layer is grown on the anode plate. The cathode plate is also sometimes thinly coated in order to protect it from being corroded by the liquid electrolyte. The difference in the coating is the primary reason that aluminium electrolytic capacitors are polar.

Most low voltage (<100V) aluminium electrolytic capacitors use a water-based electrolyte with salt additives to aid conductivity. Others use electrolytes based on ethylene glycol or organic solvents depending on the voltage and temperature requirements.

The capacitance of an aluminium electrolytic capacitor is determined by:

Where ε0 is the vacuum permittivity, εr is the relative permittivity of the dielectric, A is the surface area of the dielectric, and d is the thickness of the dielectric layer.

Aluminium electrolytic capacitors use an aluminium oxide based dielectric with an εr typically somewhere between 9.6 and 14. This is vastly lower than the εr of ceramic dielectrics, for Class II MLCCs is typically between 200 and 14000. However, aluminium oxide has a trick up its sleeve: incredibly high dielectric strength.

The dielectric strength of the dielectric layer inherently determines the voltage withstand of the capacitor. An aluminium oxide dielectric only needs to be a few nanometers thick per volt. This vastly reduces the value of d in the equation above, which more than makes up for the small dielectric constant.

The dielectric layer is grown to the minimum thickness required to provide some safety factor of voltage withstand above the nominal maximum rated voltage of the part, e.g. 450V of withstand for a 100V rated part. Cheap capacitors often skimp on this safety factor in order to produce higher rated capacitors at a lower cost.

The next way in which aluminium electrolytic capacitors achieve high capacitances is by having a large dielectric surface area. Making the electrodes (and therefore the dielectric layer) physically larger is the most obvious method of increasing the surface area. A large dielectric-coated electrode sheet provides a large surface area. However, increasing the physical size of the capacitor has its drawbacks. The amount of space it takes up is one. ESR and parasitic inductance also suffer in physically larger capacitors.

The size problem can be partly solved with optimised packaging of the foil. Most aluminium electrolytic capacitors are constructed as a rolled-up sheet of foil-sandwiched electrolyte in a cylindrical package. This allows for very long sheets to be packed into a relatively small volume. However, if you tried to build a capacitor from these raw sheet materials, you’d find that it performs rather poorly.

An acid etching process is critical to improving the capacitance density. This process is carefully controlled to ensure that high aspect ratio pits are created in the surface of the aluminium and dielectric, to vastly increase the surface area at the microscopic level. This results in surface areas that are many multiples of the geometric area implied by the dimensions of the foil sheet.

While the electrodes are primarily made from aluminium, they are typically alloys that contain some other metal additives such as copper. These details along with paper composition, electrolyte composition, material thicknesses, encapsulation materials, and packaging methods, vary depending on the application and are often trade secrets.

Unlike ceramic capacitors, wet aluminium electrolytic capacitors do irreversibly age. There are four primary mechanisms behind this aging. The first is that the electrolyte slowly erodes the dielectric layer. This occurs continuously, which is why these capacitors have a shelf life. The second is that an applied voltage will induce electrolysis inside the capacitor, slowly wearing down the dielectric layer until it reaches a thickness that corresponds with the dielectric strength matching the applied voltage. While this second process is occurring, oxygen in the electrolyte reacts with the aluminium to reform the dielectric layer, resulting in a self-healing effect. However, this eventually leads to the third type of aging: when oxygen in the electrolyte is depleted, the capacitor experiences increased ESR and a loss of capacitance. This is commonly referred to as “drying up”, even though it does not actually mean that the liquid electrolyte has evaporated. However, the increased ESR from this effect tends to cause the capacitor to run hotter, which can lead to the fourth and final type of aging: the electrolyte boiling off. This typically leads to complete failure.

The electrolyte boiling off is also a common failure mechanism in damaged or abused wet aluminium electrolytic capacitors. Any event that leads to heating of the electrolyte above its boiling point has a potential to cause the electrolyte to leak out of the capacitor. In a catastrophic failure case, the electrolyte boils so quickly that an over-pressure safety release scored into the top of the capacitor, usually visible as a little X shaped indent, splits and the capacitor bursts. In extreme cases, or in capacitors where the safety release is missing or improperly formed, the boiling electrolyte rapidly generates enormous internal pressure and quickly causes an explosion. This can release significant energy – enough to cause hearing damage and physical injury, and even warp metal.

As a quick side note, people often conflate the boiling electrolyte failure mechanism with the capacitor plague of the early 2000s. However, that particular problem was caused by faulty electrolyte compositions causing a gas-releasing corrosive reaction between the electrolyte and the electrodes. The end result is the same (gas build-up leading to rupture) but the underlying mechanism is not.

I’ll end this post with a brief overview of some other variants of aluminium electrolytics.

“Solid” aluminium electrolytics use a solid electrolyte instead of a liquid. The two main types are manganese dioxide (MnO2) and solid polymer (also known as “solid-poly”).

A manganese dioxide aluminium electrolytic capacitor uses a thin manganese dioxide layer between the anode oxide layer and a graphite and silver cathode. These are relatively uncommon, and were mostly sold as a tantalum capacitor replacement until we figured out how to make better tantalum caps.

Solid polymer capacitors are very similar except they use a conductive polymer layer instead of manganese dioxide, and can have either the graphite and silver cathode construction or a more traditional construction that integrates a paper spacer. These have exceptionally low ESR and high ripple current capability compared to other aluminium electrolytic capacitor types.

Hybrid aluminium electrolytic capacitors take elements of both solid and wet designs. They use a solid conducting polymer layer between the electrode oxide layer and the liquid electrolyte. These are usually used when lower ESR and higher ripple current handling is required than a regular wet aluminium electric can handle, but when leakage current must be kept lower than that of a solid polymer capacitor, and when the “self healing” effect of the liquid electrolyte is desirable.

All of these technologies are capable of delivering 1000μF of capacitance. Solid polymer capacitors can reach around three times that. Water-based wet electrolytes can reach around 20mF. All of these technologies are capable of voltage ratings of up to around 100-125V. Organic wet electrolytes and other speciality electrolytes can reach as much as a few farads of total capacitance and may be rated for many hundreds of volts, albeit at a much higher ESR.