The Terminology Chain: Light Foil, Etched Foil, Formed Foil
Aluminium foil for capacitors passes through three states that are often confused. Light foil, also called plain or electronic foil, is the rolled material as delivered by the mill and is the true raw material of the industry. Etched foil is light foil whose surface has been electrochemically attacked to multiply the effective area. Formed foil, sometimes called chemical foil, is etched foil on which an anodic oxide has been grown to serve as the dielectric. Electrode foil is the collective term for etched and formed foil, and it splits commercially into high voltage anode foil, low voltage anode foil and cathode foil.
Purity and Alloy Selection
Anode foil needs high purity aluminium, generally 99.99% or above, because iron, copper and silicon residues raise leakage current and reduce the withstand voltage of the grown oxide. The standard grades used for anode stock are 1050, 1060 and 1070 type material, with high purity variants where the application is voltage critical, while cathode foil can be made from lower purity stock because the dielectric is not grown on it. Compositional limits and designation of wrought grades are set out in ISO 209 and in the corresponding national designations, and foil thickness is conventionally classified as product below 0.2 mm, with the tolerances for delivery governed by EN 546-1 and equivalent national foil standards. Capacitor requirements, including electrical test methods, are specified in IEC 60384-4.
From Rolling to Etching and Forming
Light foil is rolled in soft temper to thicknesses of typically 0.020-0.120 mm for anodes and 0.015-0.050 mm for cathodes, then slit to the widths required by the winding line. In etching, the foil is passed through a chloride based electrolyte where a controlled current dissolves aluminium and generates a tunnel or pit structure; surface area gains of roughly 20 to 100 times the geometric area are achieved, and the exact gain is traded against mechanical strength, because heavy etching makes the foil brittle. Forming then grows the dielectric in a solution chosen for the voltage class: growth consumes about 1.2-1.4 nm of oxide per volt, so a foil formed at 500 V carries an oxide of roughly 0.6-0.7 micrometres per face.
Product Stages and Typical Parameters
| Stage | Typical thickness (mm) | Aluminium purity | Function |
|---|---|---|---|
| Light foil (anode stock) | 0.020-0.120 | 99.99% and above | Raw material for anode foil |
| Light foil (cathode stock) | 0.015-0.050 | 99.5% and above | Raw material for cathode foil |
| Etched foil | as above | unchanged | Surface enlargement |
| Formed foil | as above plus oxide | unchanged | Dielectric for the rated voltage |
Low voltage anode foil is formed at a few tens of volts and optimised for very high area gain, while high voltage anode foil is formed at several hundred volts and needs a coarser tunnel structure that will not break down under the applied field.
Incoming Inspection and Handling
Capacitor producers inspect light foil for thickness profile across the width, camber, burr height, pinhole count, surface cleanliness and residual rolling oil, since any of these defects propagates through etching and forming. Rolls are supplied on cores with tension controlled winding and a stated maximum number of splices, and they are handled in clean areas because fingerprints and moisture produce localised etching defects. Where the foil is delivered in the annealed condition, hardness or tensile testing confirms that the mechanical state matches the etching process window. Burr control is particularly important: a raised edge can perforate the separator in the wound element and cause a short circuit.
Frequently Asked Questions
Q: What is the difference between light foil and electrode foil?
A: Light foil is the plain rolled foil before any electrochemical treatment. Electrode foil is the same material after etching and, for anodes, after forming, and it is the form actually wound into the capacitor.
Q: Why do anode foils require 99.99% purity aluminium?
A: Impurity elements such as iron, copper and silicon create defects in the anodic oxide and increase leakage current, so high purity is needed to hold the rated voltage and keep losses low.
Q: Can the same foil be used for anode and cathode?
A: Not normally. Anode stock is higher purity and thicker, while cathode foil is generally lower purity and thinner because no dielectric is grown on it.
Q: How does etching affect mechanical strength?
A: Etching removes aluminium and creates a porous structure, so tensile strength and elongation fall as surface area gain rises. The process window is set where the required capacitance gain still leaves enough strength for winding.
Q: What thickness tolerance is normal for electronic foil?
A: Mill deliveries are certified against the applicable foil standard, with tolerance bands typically in the range of a few percent of nominal thickness, and profile rather than single point thickness is what the capacitor line measures.
Q: How should electronic foil be stored?
A: In the original packaging, at stable temperature and low humidity, away from direct contact with bare hands, because moisture and contamination change etching behaviour and can cause localised defects.