Capacitor Polarity on a PCB: Markings, Mistakes and Consequences
Reversing a polarized capacitor is one of the rare assembly errors that damages the board as well as the component, and it is also one of the easiest to prevent. The marking conventions are consistent once they are understood, and the failure mode is predictable once the physics is understood. Both are worth knowing before the first board is assembled.
How Polarity Is Marked on the Board
Most boards carry a plus and a minus sign in the silkscreen next to the two pads of a polarized component. Where the signs are absent, the convention is to place the positive pad in a distinctive shape, most often a square, while the negative pad remains round. A filled or larger pad typically indicates the negative terminal, and a small dot indicates the positive one.
The printed body of an electrolytic capacitor carries its own marking: a band with minus signs runs down the side that connects to the negative terminal. On a finished board, the side of the capacitor that shows the white stripe is the negative side, and the silkscreen marking should agree with it. Confirming both, on the board and on the component, is faster than reasoning about which convention the designer followed.
Why Reversal Destroys an Electrolytic Capacitor
An aluminium electrolytic capacitor is built on an anodised oxide layer that only forms in one direction. Applied with the correct polarity, that layer insulates the plates and the capacitor behaves as intended. Applied in reverse, the oxide layer is electrochemically removed, the leakage current rises sharply, and the part heats internally. Depending on the available current, the result ranges from a loss of capacitance to a vented or ruptured package.
Reverse voltage is sometimes described as harmless at low levels, and small excursions are indeed tolerated by some parts. The safe position for a production design is that any sustained reverse voltage across a polarized capacitor is a fault, and the layout should make the correct orientation obvious to the assembler.
Tantalum Capacitors Behave Differently
A tantalum capacitor is also polarized, but its failure mode is more energetic. A tantalum that is reverse-biased, or that experiences a voltage spike above its rating, can fail as a short circuit. In a supply rail with significant available current, that short becomes a thermal event. This is why tantalum parts are often specified with a voltage derating, commonly to about half of the rated voltage on a low-impedance rail, and why the polarity marking deserves more attention than it usually receives.
The same failure mechanism explains why a tantalum should never be used where the polarity of the applied voltage can reverse, such as a node that swings below the negative rail, or an input that can be back-fed by a charged load.
Finding the Polarity in the Layout
A polarity error is much easier to prevent in the layout than to find in the finished board, because the silkscreen is generated from the same library as the pad geometry. Check the component footprint against the manufacturer’s drawing when it is first created, and check it once more when the library part is modified. A footprint with the polarity marking on the wrong side will reproduce that error consistently across a production run.
It also helps to place all polarized components in the same orientation where the circuit allows it. An assembly operator who sees every electrolytic capacitor facing the same way will notice immediately when one is rotated, whereas a board with parts in arbitrary orientations invites a mistake.
Other Polarized Parts and Non-Polarized Failures
Electrolytic and tantalum capacitors are not the only polarized parts. Diodes, light-emitting diodes, transistors, integrated circuits and some connectors all have a defined orientation, and the conventions for those are different again. Establishing a single marking convention for the whole design, and applying it consistently, is more useful than following several conventions in different parts of the same board.
It is worth adding that not every capacitor failure is a polarity problem. A non-polarized ceramic part cannot be fitted backwards, but it can fail from mechanical stress if the board flexes during depanelling or handling, and the symptom, a short or a loss of capacitance, is easily mistaken for a polarity fault. Checking whether the damaged parts cluster near a breakaway edge is a quick way to separate the two causes.
Design and Assembly Practices That Help
Keep the silkscreen marking clear at the smallest component size in the design, and avoid placing the marking where a neighbouring component will hide it. Where the space is too tight for a marking, put the polarity convention on the assembly drawing instead, and check that the contract manufacturer has it. Verify the first article visually before the whole batch is built; a polarity error caught on the first board costs a rework, while the same error caught after a full run costs a production lot.
Our component tolerance and reliability notes cover the derating decisions that apply to polarized parts, the design release checklist lists the footprint verification steps, and the solderability material covers the assembly checks that should accompany the first build.
The Cost of Getting It Wrong
A reversed capacitor is rarely an isolated cost. On a prototype, the board may survive with the part replaced, provided the capacitor failed without taking the supply with it. On a production assembly, the failure appears as a yield loss, and the diagnostic effort is spent on boards that are otherwise good. Where the reversed part is a tantalum on a low-impedance rail, the short circuit can also remove the copper of an internal plane, which turns a component replacement into a board scrapped at the end of the line.
The economics therefore favour prevention over inspection. A marking that is unambiguous, a footprint that has been verified against the manufacturer drawing and an orientation convention applied across the whole board cost nothing at design time and remove an entire class of defect from the assembly line. That is a better trade than adding an inspection step later to catch the same error.
FAQ
Will a reversed electrolytic capacitor always fail immediately? No, and that is what makes it dangerous. A part that is reversed at low current may simply lose capacitance and leak, which shows up later as a supply problem that is difficult to diagnose. The failure may also appear only after the capacitor has been stressed thermally for some time. Treating any reversed part as a scrap item, rather than testing it and returning it to the board, is the practical rule.
How should a tantalum capacitor be derated? A common practice is to limit the applied voltage to around half the rated voltage, and to choose the lowest equivalent series resistance that the application tolerates. The derating addresses the voltage spike that occurs when the part is connected to a low-impedance source, which is the condition under which a tantalum fails as a short circuit rather than degrading gracefully.
Does gopcb check polarity conventions before fabrication? We check that the silkscreen and the pad geometry agree with the footprint library and that the marking is legible at the smallest package size used. Polarity errors are almost always library errors rather than layout errors, so the check is applied when the part is first introduced and repeated whenever the footprint is revised.
