Step 6: Etching
There are several options for etching or removing the exposed copper from the PCB. I had a lot of ferric chloride (FeCl3) solution, so I didn’t look beyond it. FeCl3 is available from chemical shops in the form of crystals and liquid. Please use protective gloves and spectacles for this step. If you have ferric chloride in crystal form, prepare the etching solution by adding 200 grams of FeCl3 crystals in one litre of distilled water. Be cautious while handling it and use either glass or plastic to stir. Ferric chloride should not come in contact with skin. Do not spill or throw it away here and there as it will leave ugly, brown colour stains.
Remove the developer solution from the container, wash it well and pour around 200ml of ferric chloride (etchant) in it. Immerse the developed PCB into the etchant and continuously shake it slowly to ensure that the solution is swished from the PCB surface and fresh solution replaces it. Green-coloured areas on the PCB resist the etching solution and the exposed copper gets dissolved in it. This takes around 10 minutes. Remove the PCB and rinse it under running water. The etched board would look as shown in Fig. 28.
Step 7: Removing the masking material
Use a piece of cotton cloth dipped in nail polish remover to remove the blue-coloured toner polymer coating from the PCB.
Some nail polish removers are acetone based, and though these serve our purpose well, they are less commonly available. Many nail polish removers use a mix of ethyl acetate (solvent for nail polish) and isopropyl alcohol. This is a milder chemical but is commonly available and delivers equally good results.
You may also use a paint thinner. This step will result in a PCB as shown in Fig. 29.
Step 8: Drilling holes in the PCB
This is a critical step. The size of the holes and their precise location on the PCB are two most vital aspects you need to address. For most components, 1mm hole diameter is sufficient; some terminal blocks and diodes may need a hole diameter of 1.2mm. This aspect is not difficult to take care of. Maintaining the exact distance between holes required for multi-pin components like ICs and header blocks, though, is very crucial. The slightest misalignment of holes will make it very difficult to even push the component in its destined place.
Tips, pitfalls and workarounds. I use a good-quality general-purpose PCB as a former for drilling holes on the newly etched PCB by aligning two farthest placed pins of the component pad centre. This gives excellent results. The boards have 1mm diameter holes, which are spaced 2.54mm apart. Eagle uses a pitch of 0.05 inch or 1.27mm by default in the board layout view. Interestingly, most of the components have pins spaced by multiples of 2.54mm. This makes these boards ideal for use as a former.
Let’s go through this step by step. Get a general-purpose PCB like the one shown in Fig. 30. Now let’s say you want to drill holes for an 8-pin single-in-line header like the one shown in Fig. 31. Place the copper side of the general-purpose board against the copper side of the PCB and align the two holes you just made with the corresponding holes on the general-purpose PCB (see Fig. 32). It is important to use this PCB orientation because it ensures that the drill bit makes almost perfect holes without damaging the copper pads.
Cut two pieces of a 15mm long and 0.9mm thick wire. If that’s not available, use 0.8mm thick tin wire as hookup wire. Push one piece of the wire through the first hole and the other one through the eighth hole. Now squeeze the two PCBs using a small bench vise; preferably one with rubber lips, as shown in Fig. 33.
Now using a mini push hand drill, create holes between the first and eighth pins by placing the drill bit in the six holes on the general-purpose board, one at a time. Release the vice, remove the two pieces of the wire and release the two PCBs. Now place the 8-pin header in the holes just drilled. It should fit perfectly.
To drill holes for a 26-pin dual-in-line header like the one shown in Fig. 35, identify the diagonally opposite pads as shown in Fig. 36. Now drill 1mm holes at the centre of the two pads as precisely as you can. Hold the two PCBs together with their copper sides facing each other. Cut two pieces of 0.8mm tin wire, used as hookup wire, 15mm long. Now push one piece of the wire through the first hole and the other one through the 14th hole, which is diagonally opposite to the first hole as illustrated in Fig. 34. Now squeeze the two PCBs using a small bench vise; preferably one with rubber lips. Now using a mini push hand-drill drill the balance 24 holes as before.