CHARACTERISTICS OF CATHODE FOILS OF ALUMINIUM ELECTROLYTIC CAPACITORS

Thickness of the cathode foil in DC capacitors—particularly aluminium electrolytic types—is primarily determined by several factors:

• Mechanical strength and handling: Foil must be thick enough to withstand mechanical stresses of winding and assembly, but thin enough to minimize volume and cost. Typical cathode foil thicknesses range from 15 to 60 microns, depending on design and voltage class.
• Electrical conductivity: Cathode foil acts as electrical contact to negative electrode. Optimum thickness ensures low series resistance and reliable current carrying capability.
• Capacitance optimization: Capacitance is determined mainly by the anode foil and its dielectric. The cathode foil is usually not “formed” to high voltages and has a naturally grown oxide; a thinner foil is acceptable without substantial loss in device capacitance, except in very low voltage or high capacitance designs.
• Manufacturing technology: Advances in corrosion and etching techniques have enabled the use of increasingly thinner cathode foils without compromising reliability. Hard aluminium foil is often used to further enhance surface area and capacitance per unit area.
• Application voltage and physical size of capacitor: Lower voltage and higher capacitance favour thinner cathode foils, whereas higher mechanical or surge requirements may call for thicker foils.

Cathode foil thickness in DC aluminium electrolytic capacitors generally increases with rated voltage and decreases with increasing capacitance, and is governed by specific design needs for mechanical integrity, reliability, and electrical performance.

Effect of Rated Voltage

• Higher voltage capacitors require thicker cathode foils to ensure mechanical strength, electrical reliability, and resistance to voltage-induced stress or breakdown.
• For low-voltage capacitors (up to about 63V), cathode foil thickness often ranges from 15 to 25 μm.
• As the rated voltage increases (above 100V), cathode foil thickness is typically increased to 25–50 μm or more to prevent physical damage and ensure safe operation.

Effect of Capacitance

• For higher capacitance values (especially at low voltages), thinner cathode foil may be used to increase volumetric efficiency and fit more foil into a given size.
• In small, high-capacitance devices (like those for power filter applications), cathode foil may be thinner to maximize available surface area for electrolyte.
• Lower value capacitors, usually for higher voltages, involve relatively thicker cathode foils.

Manufacturing Considerations

The minimum cathode foil thickness in DC capacitors is determined by several manufacturing limits:

• Mechanical durability: The foil must be thick enough to withstand rolling, slitting, winding, and assembly without tearing or excessive deformation. If made too thin, the foil can break during high-speed production or device winding, leading to failure or shorting.
• Handling and processing constraints: Manufacturing equipment such as rollers, slitters, and winders require foils with enough thickness to prevent wrinkles, folding, or jams, which are more likely with ultra-thin foils below about 10–12 μm.
• Surface quality and uniformity: Producing ultra-thin foil with consistent flatness and minimal defects becomes more challenging as thickness decreases. Defects such as pinholes or scratches can increase dramatically in foils approaching the minimum thickness.
• Electrical conductivity: Even at minimum thickness, the foil must maintain adequate current-carrying capacity and low resistance to ensure reliable electrical contact over the capacitor’s lifetime.
• Economic yield: Thinner foils have a higher scrap rate due to increased manufacturing defects and handling issues, which can make ultra-thin foils uneconomical for mass production.

As a result, while advances in foil rolling and production have enabled foils as thin as 10 μm for some applications, the practical lower limit for stable, high-yield, and reliable processing is often in the 12–15 μm range for cathode foils in electrolytic capacitors

Electrolytes for etching characteristics of cathode foil

Electrolytes used in etching process of cathode foils for aluminium electrolytic capacitors are typically aqueous solutions containing chloride ions. These solutions assist in dissolving the aluminium surface to create an etched rough texture.

• Etching method and current type: Etching is performed electrochemically using AC, DC, or pulsed currents. For cathode foil, AC etching is commonly used to create porous pits that increase surface area. The choice of AC or DC affects pit shape and distribution, impacting surface roughness and area.
• Electrolyte composition: The etching bath typically contains chloride ions and other additives such as sulfate or phosphate ions. The concentration and chemical makeup of the electrolyte significantly influence the etching rate, surface morphology, and pit formation.
• Current density: The intensity of the applied current alters the rate of aluminum dissolution and growth of etched pits. Higher current densities increase etching speed but can cause rougher or deeper pits.
• Temperature of the electrolyte: Elevated temperature generally increases the etching rate and affects pit size and depth uniformity. Precise temperature control is needed to ensure consistent surface morphology.
• Chemical pretreatment: Prior cleaning or acid treatment of the foil surface (e.g., with phosphoric or hydrochloric acid) prepares the surface for uniform etching and pit formation.
• Metallic impurities and alloying: The aluminum foil composition, specifically trace elements or alloying additions (e.g., copper, silicon), can influence the uniformity and nature of pit formation during etching.
• Etching time: The duration of the etching process controls the pit size and surface area increase. Excessive etching can damage foil integrity, while insufficient etching limits capacitance gains.

Gain of cathode foil

The gain of cathode foil in aluminium electrolytic capacitors refers to the increase in effective surface area achieved through etching, which directly enhances the foil’s capacitance per unit area.

• Cathode foil gain is typically expressed as a multiple of the original flat foil surface area after electrochemical etching. For low voltage capacitors, this gain can be as high as 80 to 100 times the original area, while for medium and high voltage capacitors, the gain tends to be around 30 to 40 times due to different etching techniques and voltage requirements.
• This gain translates into higher capacitance because the increased surface area allows more electrolyte contact and higher charge storage capacity without increasing physical size of foil.
• For example, cathode foils etched to a capacitance of about 240 μF/cm² can result in volume gains of 12% compared to thicker foils delivering 100 μF/cm² capacitance.
• Total capacitance of a capacitor is mainly determined by anode foil capacitance, but a high gain cathode foil capacitance significantly improves overall capacitor performance by reducing the series capacitance effect of the cathode.

Typical surface gain values for cathode foil in aluminium electrolytic capacitors, expressed in terms of surface area increase, usually range from about 30 to 100 times the original flat area.