Chemical Milling: Matching the Correct Chemical with Right Metal or Alloy
Chemical milling is a microfabrication technique that uses chemicals such as acids or bases, to erode and remove the undesired portions of a component leaving behind a part of the desired shape. This process can be performed on a variety of materials, ranging from metals such as aluminum and steel to semiconductors and glass. The process involves using masking agent to mask the desired shape of the part. On masking the part, it is exposed to etchants either by swabbing or by immersion. The etchant dissolves the exposed portion of the part. The amount of material etched is dependant on the rate of etching of the material and the time for which the part is exposed or immersed in the etchant. Two factors that significantly affect the etching process are:
- the temperature of the etching agent in the tank
- whether the etching agent is agitated
To achieve the desired etching depth, these two factors must be carefully monitored.
A number of etching agents or etchants are used to facilitate the chemical milling process. The etchant used is very important for the chemical machining process and care is to be taken while selecting the etchant for applications. Good etchants are expected to have a few properties, some of them are:
- High rate of etching
- As low as possible undercut
- Compatibility with most masking materials
- Easy to control
- Excellent surface finish
One of the most commonly used etchant is ferric chloride, which is used to etch various metals, including copper, iron, aluminum, and their alloys. Some other commonly used etchants are:
- Sodium Hydroxide
- Hydrochloric Acids
- Nitric Acids
- Nital -- A combination of nitric acid, with methylated spirits, ethanol, or methanol
- Ammonia Persulfate
- Nitric Acid
- Hydrofluoric Acid
Selecting an etchant is based on various factors, but one factor to be particularly considered is the material of the component. This factor is significant because different etchants are suited to particular metals and alloys. A list of commonly used etchants based on the part material is given below.
Hydrochloric Acid, Nitric Acid
Ferric Chloride, Ammonium Persulfate, Ammonia, 25-50% Nitric Acid
Hydrofluoric Acid, Mixture of Hydrofluoric Acid & Nitric Acid
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