Soldering On (Part 3)
Printed Circuit Boards
Further hints and tips for project builders everywhere. This month, Tim Edwards describes PCB production.
Probably the simplest way to make up a circuit is to use one of the several commercially available plug boards. These consist of a grid of metal contacts, commonly on 0.1" pitch, into which the component leads are pushed. By linking appropriate rows the circuit is made. Components can be used over and over again and the method is quite easy to master. It is not, however, suitable for permanent work.
There are several ways to make your circuit more permanent. A popular method amongst hobbyists, which is also very versatile, is the use of 'Veroboard'. BICC-Vero make several types of board, but for general use the standard 0.1" pitch matrix board is best. The first photograph shows a fairly complex circuit built on Veroboard and the second the reverse side. Note the break points in the track to prevent short circuits. These must be worked out as the circuit is built up and preferably cut before soldering as the solder can make this difficult. The track can be neatly sliced away using a special spotface cutter, or a sharp twist drill about 3/16" diameter.
Try to keep the circuit neat by placing components and link wires in parallel lines, either vertical or horizontal. This applies to all circuit construction, since although tidiness does not affect the performance, it does save space on the board and makes faults very much easier to trace.
In transferring your circuit from the diagram to the board, try to lay out the components in a similar pattern to the diagram. This becomes more difficult as the circuit increases in complexity, but again the aim is to simplify any subsequent fault diagnosis.
You may decide to have a go at making your own printed circuit board (PCB), in which case all the materials needed can be found by looking through electronics advertisements or component suppliers' catalogues. Follow any directions carefully and treat the chemicals with respect, as they are corrosive.
A PCB is a copper clad board that has been selectively etched to leave the desired track pattern. This means that where the copper is not to be removed, it must be protected from the etchant. There are a number of ways of doing this:—
(a) Use an etch resist pen to draw the shape of the track directly onto the copper clad board. The thick ink in these pens seals the surface of the copper and prevents the etchant (ferric chloride) from attacking it.
(b) Use adhesive transfers and rub these down directly onto the copper. This has the same effect as in (a).
(c) Use a board coated with a photosensitive lacquer. Make artwork of the track ie, a full size replica on translucent or transparent film (the artwork must be opaque). Tape this to the board and make sure it is the right way up. Keep the board in the dark until ready to expose it. Expose for a few minutes under UV light or sunlight.
Develop the board in a developing solution. A copy of the track should now remain covered in an etch resist lacquer. Again there are several choices as to how the artwork is produced.
(i) Drawing freehand with an etch resist pen onto the translucent film (ii) Using a magazine foil pattern as a template and trace this onto the film with a pen and/or tapes, pads and transfers.
(iii) Make up your own artwork from the circuit diagram using tapes and transfers.
Once the image of the track has been transferred to the board, the excess copper must be removed by etching in a bath of ferric chloride. Use a container (plastic) as close to the board size as possible to keep the amount of solution required to cover the board to a minimum. Make a small hole in the board and attach a nylon thread for lifting purposes. Hang or lay the board in the bath. Etching will take around 20 minutes depending on the temperature of the solution. Agitate the solution gently to disperse the copper compounds.
Once etching is complete, wash thoroughly in water. The board is now ready for drilling. All that remains is to mount the components and solder in place.
Don't be surprised if the first attempts don't turn out too well. Like most things it requires practice, so it is advisable to start with simple circuits to avoid disappointment.
Feature by Tim Edwards
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