Easy hole plating at home. A VIA (Latin for path or way, aka Vertical Interconnect Access) must be often plated, to interconnect lower side ground plane and tracks located on the upper side or to provide with an escape route for heat a medium-power active device. A rather simple method to plate the vias in a printed circuit board for frequencies up to microwaves, even with challenging substrates such as Teflon is described in the following lines. In the following example, a double-sided teflon substrate with a thickness of 0.5 mm and 35-micron copper (1oz/sqft) with a dielectric constant of 2.33 will be used to build a 2300MHz amplifier equipped with a SHF589 affording a gain of 10dB and an output power of 2W. Fig. 1 shows the small circuit designed with Sprint-Layout, one of the many software applications available for this purpose. Fig. 1 Layout of the RF amplifier with a SHF589 From this layout an Excellon .DRL file can be obtained to drill holes into the PCB using a CNC machine. Steps include: • Drilling holes • Graphite coating • Copper plating • Photoresist application, exposure and development • Attack by acid Drilling holes The first step is drilling holes into a virgin PCB. The holes are subsequently metallised in a copper electroplating bath. The holes should be drilled with a high-quality bit (ideally tungsten) at the highest possible speed. This will result in clean entrance and exit edges and, after plating, smooth inner hole walls. Fig. 2 The CNC machine at work. Rotation speed is 11,000 rpm Fig. 3 The drilled PCB. Each hole measures 0.8 mm Both sides need to be smoothed with a scourer in order to remove any mechanical drilling residue as well as oxide build-ups. Graphite coating Spray both sides with Graphit 33 (Kontakt Chemie’s liquid graphite), which will seep into the holes and create conductive “bridges”. Fig. 4 Graphit 33 Resistance is quite high but sufficient to trigger copper electroplating. Of course, other products available on the market can be used with comparable results. Fig. 5 – the PCB with graphite coating on both sides Next, both sides need to be scraped and smoothed with 800- or even 1000-grit sandpaper in order to remove the graphite. This operation should be limited to the surface of the PCB. The graphite should not be removed from the inner hole walls. Fig. 6 – Scraped surface and graphite-coated 0.8 mm holes, ready for copper plating Copper plating Copper electroplating involves two 99% pure copper anodes (positioned in front of the two sides of the PCB) with a current density of 0.32A/sq. inch. The electrolyte used in this example is produced by Tifoo, but there are many others. In this example, the total surface (including both sides of the PCB and 53 0.8 mm holes drilled in the 0.5 mm thick substrate) amounts to approximately 2.3 sq inch. Therefore, the necessary current is about 740 mA. The necessary tension, on the other hand, depends on multiple elements (the electroplating bath, the surface of the anodes, their distance from the object to be plated, etc.), but it’s normally between 1 and 2 volts. As the plating progresses, the current changes and the tension provided by the anodes needs to be adjusted accordingly in order to maintain an optimal value. A small aquarium bubbler was used to expedite the process, as it keeps the electrolyte in chaotic motion and ensures an even distribution. This is by no means a critical process; simply keeping the current slightly below the theoretical optimal value will ensure high-quality copper deposition. At this current density the deposition rate is about 1 micron of copper per minute. Of course, each micron will deposit anywhere in the holes and on either side of the PCB. In order to prevent too much copper depositing on the two sides (part of it will need to be removed in the last step, when it is attacked by acid), deposition is limited to 15 microns, obtainable in about 30 minutes (at first, deposition in the holes is slower due to the low conductivity of the graphite). 15 microns of copper in each hole is more than enough to ensure a resistance below 35 milliOhm. At microwave frequency, parasitic inductance inside the holes is a greater concern than resistance. Fig. 7 – Plated 0.8 mm holes. Optional: once the PCB has been copper plated it should ideally be electroplated with silver. A layer of a few microns is more than enough to improve surface conductivity for the future tracks. The skin effect also limits the penetration of alternating currents in good conductors: for silver, about 2GHz at approximately 1.5 microns; 5GHz at slightly below 1 micron; and 10GHz at about 0.6 microns. Greater thicknesses (increased by 100% for redundancy) would not bring any additional benefit, except ensuring excellent conductivity to direct current, i.e. power supply to the devices. The salts and acids normally used to attack copper also attack silver (sodium persulfate, ferric chloride, hydrochloric acid plus hydrogen peroxide, etc.). Again, the goal is to deposit silver throughout the surface of the PCB and remove all that is unnecessary (such a shame…). A silver plating bath is possible even when the tracks have already been made. However, the thickness will not exceed 0.5-0.7 microns as the free silver must bind to the substrate. Once this has occurred, no more silver will deposit. In addition, welding causes a “discolouration” of the tracks and the surrounding area. The reason for this is that the deposited silver melts and mixes with the copper, creating an alloy. To prevent this, an insulating nickel layer should be added between the copper and the silver layers. However, these are electroplating techniques that go beyond the scope of this example. Photoresist application, exposure and development Photoresist is applied to the end of protecting the tracks on the PCB, which will be obtained by subtraction, i.e. removing any unnecessary material. A different example is alumina: the metal constituting the printed circuit will deposit on it and the tracks will be obtained by addition, i.e. adding the necessary material. Besides protecting the tracks constituting the printed circuit, a crucial problem is to protect the holes from the attack of the acid. This is because the holes have sharp edges and the photoresist layer is very thin (just a few microns). As a result, it can easily chip or crack, creating small fissures through which the acid can seep in. The first step is to protect the back side of the circuit by spraying it with quick-drying paint. Direct the spray vertically on the plated holes to make sure that as much as possible of the micronised, extremely fluid paint flows into each plated hole, protecting it. Let the paint dry completely. Fig. 8 – Ground plane protected with spray paint One of the types of photoresist that can be used is dry film as it’s easy, quick and allows for very high resolutions (down to under 0.10 mm, should it be necessary). Dry films are applied and adhere through heat and (gentle) pressure, for example using a laminator. The optimal temperature is 110°, and the film should be run through the laminator four or five times. There are dozens of demo videos un YouTube. However, the same result can be achieved with an iron: just set the temperature on low and let its weight do the job. Next, expose it applying a negative mask (when using dry film, only the areas exposed to UV rays for a couple of minutes polymerise and become resistant to acid). Development can be achieved by simply using some calcium carbonate (Solvay’s washing soda is excellent) in a 1-3% solution (not at all critical) for 20-30 seconds. There is no need to buy expensive developer powders. Fig. 9 – Exposed and developed PCB At this stage, it is very important to make sure that each hole is properly protected. Fig. 10 – The edges of the two holes are protected only by a very thin layer of photoresist Quite simply, apply a microscopic drop of any nitro lacquer diluted at 40-50% to each plated hole. The best – and cheapest – solution is to use a toothpick. When a tiny, fluid drop of lacquer forms on the tip, drop it onto the hole. This is an excellent protection in view of the next step. Attack by acid The last step is to dip the circuit in a sodium persulfate solution (250 gr per litre of water) at a temperature of about 45°C (110°F) while agitating it with the above-mentioned aquarium bubbler. In about half an hour, the exposed copper parts (with a thickness of 35 microns plus the layer deposited during hole plating) will be completely removed. You can see the result below: Fig. 11 – The finished circuit with the exposed Teflon substrate Fig. 12 – The central plate with metallised holes Fig. 13 – The back side, the circuit’s Ground Plane The PCB is now ready for welding SMD components. Optional step: welding mask (it may be helpful given the extremely small size of the components). The circuit is fitted with the following SMD components: 1206 resistors, 0805 capacitors, one ICL 7660 and one SHF-589. A detailed layout is provided in a separate article.
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