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Peroxy-Sulfuric Etching Module
Operation/Calibrating the Etchant

Calibrating the etchant
Before we start etching, we need to determine how aggressive the etchant is and how long it takes to remove all of the exposed copper in our board.

Note on preparing test coupons: Regardless of whether you use immersion, bubble-assisted, or spray etching, always etch a test sample to see how long it takes to totally etch copperclad with the same weight foil as you will be using. If possible, it is a good idea to image, and develop a set of copperclad panels whose resist geometry is representative of the minimum sized feature in your circuit design. In most cases, mixed blocks (1 in. x 1 in.) of horizontal, vertical and crossed (cross-hatched) 0.010 in. (0.25mm) traces on 0.020 in. (0.51mm) centers act as very effective probes for measuring many facets of etchant performance. This is referred as an "imaged test coupon", or simply a "test coupon".

If you do not have time to make test coupons, you can use a thoroughly cleaned piece of double-sided one-ounce copperclad to determine the etching speed. It will not yield any information on how well the bath will etch your circuit pattern, but it will give a good first estimate of how long the board needs to stay in the etchant.

Read the chemical handling safety procedures until you understand them. FOLLOW THEM TO THE LETTER!

Proceed as follows:
  1. Analyze all components of the etchant and make additions as necessary.
  2. Heat the bath to 115°F (46°C)
  3. Mount a test coupon in the board clamp. Obviously, you do not need to use a full sheet of copperclad. All you need is a panel 2 in. x 2 in. (or an imaged test coupon).
  4. Immerse the coupon into the bath and etch for 1 minute. Lift the coupon out of the bath and examine for signs of copper removal. If copper remains, return the sample to the etchant.
  5. Lift the test sample out and examine the degree of etching every 30 seconds.
  6. When 99% of the copper has been removed, or when the test patterns in an imaged coupon are totally etched, thoroughly rinse and dry the sample and set it aside.
  7. Record the total amount of time the coupon was in the etchant. Denote this time as "T" and record it in your bath maintenance log under the results of your chemical analysis for the day. Later, as you gain experience, you will use test coupons to test your bath and tell you whether or not you need to run a complete chemical analysis.
  8. If you used an imaged coupon to test the bath, examine the test patterns under a microscope if available. If the image elements do not appear to be totally etched, return the sample to the etchant for 30 seconds. Remove, rinse, dry and inspect. If the image quality meets with your approval (no shorts, near vertical side walls, minimal undercutting), record the total etching time as "T" in you maintenance log. If more etching is needed, continue as above until you are satisfied with the end results.

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