Holewall Activation - Through Holes and Vias
There are numerous methods for establishing an "electroplating-receptive" surface on the walls of the holes drilled through the various substrates. 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. In large measure, this complexity is needed as a result of the drilling process. As the bit cuts its way through the copper foil and underlying substrate, the heat generated melts the resin that forms the "matrix" of the insulating composite (in most cases FR-4). The melted resin is carried up the hole with the rest of the drilling debris, where it is smeared onto the freshly exposed hole walls in the foil. This effectively spoils the surface for subsequent electroplating. The melting of the resin also leaves behind a heat glazed hole through the substrate that exhibits poor adhesion to most activation agents. Hence the development of such things as desmearing and etchback chemistries.
A method that is far more amenable to the needs of the PCB prototyper 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.
The following procedure assumes that you have "formatted" your copperclad as described in the chapter on drilling.
The process consists of three basic steps:
- rubber gloves, safety glasses, and an old shirt or smock
- conductive electroplating ink
- 4" to 6" squeegee
- single-edged razor blade window scraper
- 320 grit "wet-or-dry" sandpaper
- PCB scrub cleanser
- abrasive scrub brush
- vacuum cleaner (or low pressure blower)
- curing oven large enough to accommodate the entire stack
- lab timer
- PCB drying/cooling rack
Conductive electroplating ink is applied using a manual process that can be a bit messy during the learning phase. When trying any new procedure, a well laid out laboratory/shop is highly desirable, if not absolutely essential. Every effort should be made to insure that tools and cleaning supplies are within easy reach and that safety procedures are followed religiously. In the absence of a semi-infinite cleaning budget and clothing allowance, the following preparations are strongly advised.
Now is the time to dig out those old 60's bell bottoms and long-sleeve camouflage skivvies. The first time you use the ink, you are going to get some on your clothes, so be prepared. In addition to your old wardrobe, you should wear:
- safety glasses with side shields
- chemically resistant gloves (kitchen gloves work just fine if you can find a pair that fits relatively well)
- an old lab smock if available
- old shoes
- spread a drop cloth (or old newspapers) on a waist-high table with at least 9 square feet ( 1 square meter) of open work area
- hang a paper towel dispenser within easy reach of your work table
- have an approved eyewash handy in case you splatter ink into your eye
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 stack.
For designs that include blind or buried vias, each layer must be separately activated as described below. Wherever you see the word "stack" substitute the word "layer".
The following description assumes that the stack being activated is about 12" by 12" (305mm x 305mm).
- With the drillmaster facing up, squeeze about 1/4 tablespoon (3 or 4cc) along one edge of the stack. For new users, this equates to a quantity of ink that is about 3/8" (10mm) wide by 12" (305mm) long. There should be enough ink to fill in about 1000 ea. 0.040" (1mm) holes.
- Holding the board by the opposite edge, tilt the stack until it makes a angle of about 30 to 45 degrees relative to the plane of the table.
- 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. WHAT? YOU GOTTA BE KIDDING! Nope. The reason that it was necessary to totally fill all of the holes is that it is the easiest way to guarantee that 100% of the surface area of every holewall is coated with ink. 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.
During application, some of the ink has probably seeped between the various layers. This is usually not a problem since such wicking will not occur unless the gaps are very small. Any ink that penetrates such a narrow gap will cure to a very thin film that is easily removed during pre-plating board cleaning.
Ink extraction can be accomplished using either compressed air or vacuum. While the compressed air method is easier to master, it can cause more ink to be forced into interlayer gaps resulting in more challenging board cleaning. The use of a vacuum cleaner on the other hand, actually helps to extract any invading ink, resulting in boards that are easier to clean. Both are presented in this section, so use the one that works best for you.
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. This is definitely one of those cases where a little experimentation will quickly yield near optimum results.
- With the bottom surface (exit foil) facing up, brace one edge of the stack firmly against a block clamped to the table and tilt to an angle of about 45 degrees relative to the plane of the table.
- Briskly run the modified vacuum nozzle over the entire surface to suck the ink from the holes.
- Do not tarry. The purpose is to remove the bulk of the ink, not to dry it. The ink in the holes should still be very wet and shiny when extraction is complete.
- Hold the board up to the light to insure that all holes have been sucked clear. Repeat step 2 to clear any blocked holes.
- If pools of ink remain on either side of the stack, take a few moments to vacuum them off. Free running ink is likely to flow into holes during curing and form hard to remove plugs.
- When all of the holes are clear, the stack is ready for oven curing.
A good source of low-pressure, high-volume compressed air is the exhaust port of a conventional vacuum cleaner, if precautions are taken to insure that no debris enters the air stream during ink removal. If possible, it is usually a good idea to have a small portable vacuum cleaner or hand blower that you reserve exclusively for this purpose. Do not modify the nozzle as described in the vacuum extraction section.
- With the bottom surface (exit foil) facing up, brace one edge of the stack against a block clamped to the table and tilt to an angle of about 60 degrees relative to the plane of the table. There is a pretty good chance that blowing ink from the holes will result in a plethora of silver/black spots all over you and your table. To minimize this, pinch a paper towel between the bottom edge of the board and the clamped block. Hold the other edge of the towel with the hand holding the stack so that it forms a barrier as the ink is ejected from the holes.
- Holding the air nozzle close enough to the surface of the stack to form a surface-effect air-bearing, move the hose briskly back and forth across the board to blow the ink from the holes.
- This step should be done quickly. The purpose is to remove the bulk of the ink, not to dry it. The ink in the holes should still be very wet and shiny when extraction is complete.
- Hold the board up to the light to insure that all holes have been blown clear. Repeat step 2 to clear any blocked holes. Remember, do not dry the ink with the blower!.
- If pools of ink remain on either side of the stack, take a few moments to squeegee them off and blow any blocked holes clear. Free running ink is likely to flow into holes during curing and form hard to remove plugs.
- When all of the holes are clear, the stack is ready for oven curing.
It seems like every part of this process has generated a schism among users. Oven curing is no exception. On one side you have the "Church of Stack Curing" and on the other there is the "Holy Order of the Separated Layers". The first camp insists that, as soon as the holes are blown or sucked clear, the stack should be placed in the curing oven. The other points out that, if the layers stick together in the oven, the ink in the holes might be fractured when the stack is separated. While the "separatist's" position is very reasonable, the main difficulty with disassembling the stack while the ink is still wet is that you must be very careful not to smudge surface ink into any of the holes. It is very important that every hole, in every layer be clear during plating. The best approach is to use a small test board to try each method and use the one that works best for you.
To cure the ink:
- Cure the stack, or separated layers, for 30 minutes in an oven (convection if available) preheated to 100 degrees Centigrade.
- When the time has elapsed, use "oven mitts" to CAREFULLY remove the stack, or layers, from the oven and allow to cool to room temperature.
- Disassemble the stack if necessary.
- Carefully inspect all of the holes one last time to insure that no blockages have formed. If some of the holes are occluded, use a small drill bit to remove any debris or ink plugs. Try not to scrape the walls of the hole. The less you disturb the holewalls, the more like you are to have 100% coverage during electroplating.
- The layers are now ready for cleaning and subsequent processing.
included by permission of Think & Tinker, Ltd copyright 1986 to 2012