What is Photochemical Milling?Photochemical milling, also known as “chemical milling” is a process for producing usually small flat parts from metal foils. The process has these steps:
- 1. Pre-clean foil in preparation for photoresist application
- 2. Apply photoresist, usually dryfilm, may be liquid
- 3. Image photoresist
- 4. Develop, usually using 1% Sodium or Potassium Carbonate solution
- 5. Etch, most commonly using Ferric Chloride solution
- 6. Strip Photoresist.
Ferric Chloride is most often used, (if using Cupric Chloride etch, go here) and may be the least understood step in this process. Ferric Chloride will etch most metals, and is quite inexpensive, and fast, so it is the preferred etchant, but control of the chemistry of the etchant is crucial to consistent performance, and it is not an easily controlled chemical.
The active content of the Ferric Chloride, is easily controlled, but is not the most important component of the etch. Ferric Chloride must have some amount of free Hydrochloric (also known as Muriatic) acid to remain dissolved. When the Hydrochloric acid content goes to zero, the Ferric chloride bath turns to a brown mud. Very precise control of the Hydrochloric acid content is critical to consistent performance, because the level of the Hydrochloric acid in the etch controls the speed, and the amount of “undercut”, or sideways etch.
The ideal situation is for the etch to etch vertically through the foil, with no sideways etching (undercut), this is essentially never possible. The amount of sideways etch may be influenced by mechanical aspects, like spray pressure in the etch, but the single most important factor is the Hydrochloric Acid content. And, unfortunately, controlling the Hydrochloric Acid content is quite difficult. There have been some attempts to automate measuring of the Hydrochloric Acid content, but without success, so far. The normal way Hydrochloric Acid is controlled is by laboratory titration. The normal and standard methods of titrating Hydrochloric acid do not work on Ferric Chloride etchants, because the Ferric Chloride titrates also, giving totally erroneous results. There are methods known which overcome this problem, but they are not easy, and results can vary, depending on what metals are being etched.
See here for seven different Hydrochloric Acid titrations:
- Method 1: Determination of Free Acid in Ferric Etchants (RDM-5001)
- Method 2: Analysis of Hydrochloric in Ferric Chloride Etchant (RDM-5002)
- Method 3: Determination of Free Acidity as Hydrochloric Acid in Ferric Chloride Solution (RDM-5003)
- Method 4: HCl in Ferric Chloride Solutions (RDM-5004)
- Method 5: Analysis of Hydrochloric in Ferric Chloride Etchant (RDM-5005)
- Method 6: Determination of Free Acidity as Hydrochloric Acid in Ferric Chloride Solution (RDM-5006)
- Method 7: Determination of Free Acidity as Hydrochloric Acid in Ferric Chloride Solution (RDM-5007)
Pre-CleaningPre-cleaning of metal foils for photoresist application is a difficult challenge, only partly because the foils used in photochemical milling are particularly difficult to clean. These foils have been formed by rolling ingots under high heat and pressure, using organic lubricants (rolling oils) which react with the metals to form essentially invisible, and insoluble organo-metallic films. The situation is further complicated by the fact that most photochemical milling organizations do not want to disturb the typically shiny smooth appearance of the foil, so cleaning must be done without etching the surface.
Cleaning metal foils for photoresist application is usually a two step process, the first step being an alkaline cleaner to remove the organo-metallic film, and the second step being an acid to remove any remaining oxides.
RD Chemical offers a number of cleaners for most metals, both spray and soak (immersion) application. Please <a href="contact/rd-chemical-company-inc.html">contact us</a> for specific recommendations.
Photoresist DevelopingFor more than you ever wanted to know about Photoresist Developing, go here.
Photoresist StrippingOne of the key difficulties for the photochemical miller is that the support industries, suppliers of both photoresist, and photoresist strippers do not seem to comprehend the possibility that anyone might want to apply photoresist, and subsequently strip it from any substrate except Copper.
RD Chemical has targeted meeting the needs of the photochemical milling industry since the early 1980’s, and has figured out how to optimize formulations to avoid the common issue of dye transfer to the substrate.
There are so many variables, given the vast array of metals being processed in the photochemical milling industry, as well as issues like desired particle size, and of course, speed and cost. This cannot be discussed here, but we would be happy to discuss on-line, or on the telephone.