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74 DESIGN007 MAGAZINE I AUGUST 2024 e challenge is that drilling creates heat. is tends to melt and possibly smear epoxy over the internal copper connections of the circuit board. Smeared epoxy creates a pos- sible insulator between copper connections and the through-hole, eliminating or making an unreliable connection. e manufacturer needs to remove these smears chemically. It's called the de-smear process. is results in a uniform, clean surface—perfect for the appli- cation of electroless copper. Next, we remove other remnants from the drill process. e drill can sometimes produce copper burs on the surface, like when drilling through a 2×4. Any bumps or protrusions need to be removed. Maintaining the smooth sur- face plane throughout this process is crucial. We use a mechanical scrub to remove these irregularities on the surface created by the drilling process. Designers may have concerns about rough surfaces le by this step, primarily because the scrub uses a nylon brush to roughen both sides of the panel surface. Copper is porous but very smooth. It needs to be roughened to get good adhesion during the dry film lamination pro- cess. Copper roughening is a great equalizer to keep the adhesion of the dry film photoresist during the plating processes. Time for Electroless Aer being de-smeared, smoothed, and roughened, the boards go into the baskets that enter the electroless copper process. ey first go into a chemical bath to help slightly swell the epoxy. As it swells, it becomes more porous, allowing the copper and catalyst to enter. When the swelling decreases, it has a more uniform copper coverage. e catalyst is a palladium-based process. Palladium is not cheap, but it works very well. A catalyst initiates a reaction but is not con- sumed. e catalyst gets into all the through- holes and deposits in the epoxy. Later, the pal- ladium will attract the copper ions in the elec- troless copper bath and act as a nucleation site for a reaction to occur. e copper bonds to the palladium, creating a copper layer about 30 to 70 micro-inches thick. Two forces act against the copper plating. First, all the air needs to be removed from the through-holes. Manufacturers use mechani- cal processes such as vibration, thumping, and agitation to increase the fluid flow through the holes and move the air out. ere are other tricks to help break the surface tension and increase the opportunity for getting copper in all those holes. If you fail to get copper in every hole, it creates a void, which may lead to a missed connection later, which means a scrapped panel. Another potential challenge to the electro- less copper process is in the aspect ratio—the thickness of the panel to the smallest drill size. For example, a 100-mil thick panel with a 10-mil drill creates a 10:1 aspect ratio. is is common and shouldn't challenge too many manufacturers. But as you get thicker pan- els and smaller holes together, the increased aspect ratio becomes more problematic. Even- tually, manufacturers must rely on capillary action to get fluid in and out of the holes. However, capillary action won't be enough. Once you place the solution in the hole, you need to continually replace it to ensure enough " Heat tends to melt and possibly smear epoxy over the internal copper connections of the circuit board. "