The next step is to apply the conductive ink and coat the inside of the through holes.
Known in the industry as "holewall activation", most of the procedures encountered in the production of commercial printed circuits are fairly complex, requiring multiple process tanks, each with it's own control and maintenance requirements.
A method that is far more amenable to the needs of the PCB prototype involves the use of a low viscosity "ink" that has been specially formulated to form a high adhesion, high-conductivity coating on the inside wall of each through-hole.
Instead of multiple chemical treatments, a single application step, followed by a thermal cure, will result in a continuous film on the inside of all of the hole walls that can be directly electroplated without further processing.
The ink is based on a resin that is essentially a very aggressive adhesive which adheres without difficulty to the most heat polished holewall so etchback is eliminated.
I use the Conductive Ink from Think and Tinker
This is a very messy process. Be sure to lay down some newspapers, wear old clothes, safety glasses and disposable gloves.
The purpose of this operation is to form a smooth, full-coverage coating of conductive ink on the inside of all of the drilled through-holes while leaving a minimum of ink on the either surface of the PCB.
- With the PCB facing up, squeeze about 1/4 tablespoon (3 or 4cc) along one edge of the stack. This equates to a quantity of ink that is about 3/8" (10mm) wide by the width of the board. There should be enough ink to fill in about 1000 ea. 0.040" (1mm) holes.
- Holding the board by the opposite edge, tilt the board until it makes a angle of about 30 to 45 degrees relative to the plane of the table, then using the squeegee, briskly wipe the ink around the surface until all of the holes are filled.
- Push any excess ink to one edge. Hold the stack up to the light and look for light shining through unfilled or partially filled holes. If a hole is filled, the shiny surface of the ink should be visible filling the bottom rim. Holes larger than 0.0625" (1.6mm) in diameter may show light even if the walls are uniformly coated.
- Examine these holes from multiple angles to insure that light is reflecting off of the smooth surface of the wet ink. If necessary, push the excess ink (or add more ink) to areas of the board that show light through the holes or where the walls of large diameter holes do not appear to have been totally covered.
- Once all of the holes are filled to your satisfaction, it is time to remove the bulk of the ink. Squeegee of the bulk of the ink, wiping the squeegee on a paper towel to remove the excess. Then use a clean paper towel to clean the remainder off the board. (You won't be able to get the surface totally clean.
- Then, using the vacuum cleaner suck the remaining ink from the holes. Run the nozzle over the entire bottom surface to suck the ink from the holes.
- Hold the board up to the light to insure that all the holes are sucked clear.
- Once all the holes are clear you can clean up the surface of the board with a cotton swap wet with acetone. Don't use too much acetone, you do not want to break the connection between the surface of the board and the fresh ink which is drying on the inside of the through holes.
- Finally, cure the ink by placing the board in an oven, preheated to 100 degrees Centigrade, for 30 minutes.
- You can clean up the squeegee and vacuum cleaner nozzle with acetone.
- Disposable gloves, safety glasses, and an old shirt or lab apron
- Conductive electroplating ink
- Vacuum cleaner (or low pressure blower)
- Compressed Air
- Cotton balls or pads.
- You absolutely must have a continuous, full coverage conductive surface through the substrate to support reliable through-hole electroplating,. Totally filling each hole is the easiest manual way to accomplish this.
- Vacuum removal of the bulk of the plating ink is most easily accomplished using a nozzle that has been modified with a ring of notches or drilled holes that act as a vacuum relief to prevent the nozzle from sucking up against the bottom of the stack. Drilling a couple of 1/4" dia. holes in the nozzle of a 2" I.D. hose commonly found on shop vacuum cleaners in the USA effectively eliminates this "vacuum clamping" while having a minimum effect on the suction developed.