Saturday, March 14, 2009

Mechanical Etching

In response to the inexorable tightening of environmental regulations and the difficulty of obtaining strong acids and oxidizers in many locales, "mechanical etching" has seen unprecedented growth over the last 10 years. In spite of high up-front costs and lack of plated through-holes, its complete freedom from toxic chemicals has made this technique very attractive to many PCB prototyping shops.

As the density and complexity of circuit designs have increased, forming reliable electrical connectivity between the top (component) and bottom (solder) circuit patterns has evolved from a mere convenience to an absolute necessity. While some mechanical etch vendors offer manual and semi-automatic machines for inserting eyelets into all of the designated through-holes, the cost of the equipment, the penalty in PCB real-estate (holes must be drilled oversized to accommodate the eyelets), and the relative sluggishness of the process render it undesirable for many applications. As a result, many shops have adopted simplified electrolytic plating processes (like Green CirKit) to accomplish this critical task.

As mentioned above, mechanical milling involves the use of a precise numerically controlled multi-axis machine tool and a special milling cutter to remove a narrow strip of copper from the boundary of each pad and trace. There are a number of configurations currently available for these special mechanical etch bits, but most users report that bits with spiral flutes (vs. a flat "spade" geometry) are the most effective at removing copper debris and tend to stay sharp longer at higher cutting rates. Tip angles of 60° and 90° are the most common, with 90° seeming to offer the best combination of minimal substrate penetration and longer cutter life. If the circuit design also requires that some (or all) of the non-circuit copper be removed (clear milling), conventional carbide end-mills can be used to accelerate the copper milling process. Typical diameters range from 0.010" (.25mm) to 0.050" (1.27mm).

Mechanically etching a PCB proceeds as follows:

  1. Lay out the circuit using a compatible PCB layout package (often proprietary to the milling machine vendor).
  2. Run the layout through a post processor to generate the boundary paths that the cutter will need to follow to define the circuit elements and any cutter paths needed for clear milling (always vendor specific).
  3. Mount the substrate (or flex circuit) in the machine as instructed in the user's manual.
  4. Insert the drill size indicated by the operating software into the chuck and adjust the height (if no set ring is present) to insure that the bit drills all the way through the substrate and a short distance into the backing material.
  5. Drill the first hole size.
  6. Repeat the previous 2 steps for each drill size. If you will be plating the through-holes, remove the board from the machine for hole wall activation and electroplating. After the through-holes are plated, return the board to the milling machine for further processing.
  7. Insert a mechanical etch bit and adjust the depth setting foot to the approximate cutting depth desired for your design. The cutting depth sets the width of the channel milled through the copper foil, so you must know the tip angle of your cutter prior to setting the depth.
  8. Off to one side of the substrate, cut a couple of test channels and carefully measure the width of each. Adjust the height of the foot until the desired width is consistently achieved.
  9. Mechanically etch (and clear mill) the first side of the board.
  10. Carefully inspect the cutter. If the cutting edge shows signs of wear or of excess copper buildup, replace it with a new bit before proceeding.
  11. If your design requires a double sided PCB, flip the substrate and repeat steps 8-9 for the second side (making sure, of course, to load the second side artwork into the mechanical etch software).
  12. Mechanically etch (and clear mill) the second side of the board.
  13. Remove the "etched" board from the machine.
  14. Carefully examine both sides for copper debris that may have become wedged (or smeared) in the milled channels. You cant bet that any such material will short out the most expensive and unobtainable component on the entire board so you need to be very diligent during this inspection.
  15. If any copper strands are found shorting out circuit elements, use the tip of an X-Acto knife to remove them. Be very careful not to cut thin traces during this operation.
  16. After any shorts have been removed and the through holes cleared of plugs of copper and milled substrate detritus, the board is ready for further processing.

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