Sep 11, 2026Application Guides
Inside an Aluminum Electrolytic Capacitor: Foil, Separator Paper and Tab Connections
A practical look inside an aluminum electrolytic capacitor, explaining the roles of foil, separator paper, tab connections and winding control.
Most people see an aluminum electrolytic capacitor as a black can with two terminals. Once the winding is opened, the design becomes much easier to understand.
The two teardown photos show the wound element after it has been removed and carefully unrolled. The grey layers are aluminum foils, the yellow layers are separator paper, and the metal strips are tab connections. These simple-looking materials have to work together as one controlled electrochemical system.
The aluminum foils
An aluminum electrolytic capacitor contains an anode foil and a cathode foil. The anode foil carries a very thin aluminum-oxide layer that acts as the dielectric. Its surface is etched to create a much larger effective area, allowing high capacitance to fit inside a relatively compact case.
The two foils do not perform the same job, and their material condition, formation process and handling all affect the final component.
The separator paper
The paper between the foils is more than packaging material. It keeps the aluminum layers from making direct contact, holds the electrolyte and helps maintain the required spacing throughout the winding.
Paper thickness, absorption, cleanliness and alignment must remain consistent. Contamination, wrinkles or uneven overlap can change the internal electrical and mechanical conditions.
The tab connections
The metal tabs connect the wound element to the external terminals. They are the current path between the internal foils and the rest of the power circuit.
Their position, attachment and mechanical strength deserve close attention. A sound tab connection supports stable electrical contact while the capacitor experiences assembly stress, vibration and repeated operating cycles.
Why winding control matters
During production, the foil and paper layers are wound together under controlled tension. Too loose, and the element may lack mechanical stability. Too tight, and the layers may be placed under unnecessary stress. Alignment also matters because the edges, tab positions and paper overlap must remain controlled across the complete roll.
This is one reason two capacitors that look similar from the outside may not have the same internal construction quality.
What a teardown can—and cannot—tell us
A teardown is useful for understanding construction and identifying visible clues such as winding alignment, damaged layers, contamination or abnormal local marks. But a photograph alone cannot confirm the complete condition or determine one single failure cause.
A proper evaluation should combine the physical inspection with electrical measurements, production information and the actual operating history. Capacitance, ESR, leakage current and insulation condition each provide a different part of the picture.
Questions worth asking during supplier review
For engineers and purchasing teams, internal construction is a reminder to look beyond the printed label. Useful questions include:
• Are foil, paper and electrolyte materials controlled by specification?
• Is winding tension monitored consistently?
• Are tab positions and connections inspected?
• Is production traceability available?
• Are electrical results checked against the relevant series requirements?
The point is not that every capacitor must be opened. The point is that reliable performance begins long before the finished part reaches the equipment.
At COREVIA, we support application-driven selection and specification review for screw-terminal, snap-in, radial-leaded and surface-mount aluminum electrolytic capacitors.
For a project review, send the required ratings, operating conditions, dimensions and quantity through the website inquiry form.
