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[2]-1 Surface Roughing (Etching)

The raw foil for the anode uses a high-purity aluminum foil (a minimum purity level of 99.99%) that is normally 50 to 100 m thick. The cathode foil material uses an aluminum foil that is at least 99% pure and about 15 to 60 m thick. Because the capacitance is proportional to the surface area of the electrodes, the effective surface area is increased by roughening (etching) the surface of the aluminum foil before growing the dielectric film. Generally, this surface roughening is referred to as "etching."

There are two typical etching processes. The first option submerges the aluminum foil in hydrochloric acid (physical etching). A secondary option is electrolysis where the aluminum as the anode is placed in an aqueous hydrochloric acid solution (electrochemical etching). In electrochemical etching, the etching profile will vary depending on factors such as the waveform of the electrical current, the composition of the solution, and the temperature. The etching method can be determined by the desired capacitor performance. Generally, it is possible to achieve etching multipliers (the ratio between the surface area of the smooth foil and the effective surface area of the etched foil) approximately between 3 and 120.

The foil is then rinsed thoroughly with water. Any residual chlorine ions on the foil's surface after etching can corrode the foil and damage the capacitor. After etching, the foil's surface can be categorized broadly as shown below by the selected voltage at which the capacitor functions properly. See the magnified view of the surface in below photograph.

		Foil Surface (3500x Magnification)	Cross-section of Capacitor (350x Magnification)
Types of Etched Foils	Low-voltage foil
	High-voltage foil