Cleaning Copperclad Substrates

The proper cleaning of copperclad substrates is so important to the ultimate success of your PCB prototyping effort that it is understandably worth the effort to learn to do it well. Luckily, it is a process which is easily mastered given the ready availability of chemically optimized copper cleaners and easy to use cleaning tools. Be patient. It may take a couple of trial runs before you get it right. As your skill develops, you will find that this step goes quite rapidly and that your boards emerge uniformly clean, activated, and usable.

Equipment needed

• small sink or washtub with a spray wand if possible
• abrasive cleanser / copper brightener (scrub cleanser)
• abrasive scrub brush
• rubber gloves, safety glasses, and an old shirt or smock
• plastic cutting board (or 12" x 24" sheet of plastic)

Preparation

1. Thoroughly clean a work area adjacent to a utility sink
2. Clean the sink with a commercially available cleanser and insure that no residue remains that might contaminate the surface of the board. Since a number of the chemical cleaners used in PCB fabrication are mildly toxic, DO NOT USE A SINK THAT WILL LATER BE USED FOR FOOD PREPARATION. If a utility sink is not available, set up a stand with a plastic washtub and a source of hot water.
3. Have an approved eyewash handy in case you splatter some of the cleanser in your eye
4. Wipe down, and thoroughly rinse the cutting board (or sheet of plastic). Since this surface is where the copperclad will be cleaned, it is essential that it be as clean as possible.

Board cleaning

1. Spray both sides of the substrate with warm water to remove any loose grit.
2. Lay the substrate of the cutting board.
3. Sprinkle a liberal amount of scrub cleanser on one side and spread evenly with a slightly wet abrasive brush. The cleanser should form a loose slurry not a thin foam. The copper will immediately brighten upon exposure to the mild etchant in the cleanser.
4. Holding the board along one edge, scrub the surface using moderate to heavy pressure. A 2" (5cm) diameter circular wiping motion seems to work well and is less tiring than an equivalent linear motion.
5. Scrub the entire surface (except where your fingers are, of course).
6. Rotate the board 90° and repeat steps 3 and 4.
7. Continue in this fashion until the board has been rotated a full 360°. The intent of all of this diligent scrubbing and rotating is to impart a random pattern of shallow scratches to the surface of the board. Combined with the etching action of the cleanser, this abrasion will yield a chemically and mechanically activated surface that is ready for subsequent processing.
8. Flip the board over and repeat steps 2 through 6.
9. Flip the board back over to the first side and quickly wipe with the scrub brush to remove any discoloration that may have formed.
10. Very quickly rinse the board with warm water while lightly scrubbing both surfaces with the brush to loosen and remove any cleanser that might remain on the board.
11. Spray the board with cold water and carefully examine both sides. A properly cleaned and activated surface will support a uniform, break-free sheet of even the coldest water (cold water has much higher surface tension than warm or hot water). Any trace of beading or uneven sheeting may indicate surface contamination that will require further cleaning. Use your judgment. A perfectly cleaned board is not necessary for reliable PCB production, but an adequately cleaned one is. By and large, a good motto to use here is, "The Cleaner The Better (TCTB)" or "Anything worth doing is worth over-doing".
12. Once the board is cleaned to your satisfaction, blow dry with clean, dry, oil-free air and wipe both sides with a lint free cloth moistened with methanol (denatured wood alcohol).
13. Set the clean, dry copperclad aside until it needed. If the next step in the process is dry film lamination or multilayer lamination, it is a good idea to put the board into an oven for 5 minutes at 100°C to insure that all of the water is driven from the surface. This is also true if the board will be sitting for more than a couple of hours before further processing.

Clean up your work area!

• Thoroughly rinse the scrub brush and cutting board and set aside to drain. Always store both the brush and cutting board vertically to allow them to drain completely and to minimize the accumulation of airborne contaminants.
• Thoroughly rinse the sink. If you always leave your work area ready for the next use, PCB fabrication will flow more smoothly and your results will be more predictable.