PC Board Construction - 1


1. The 'Low Tech' Method

I have used this method for single or double sided boards for years with very few problems but it does require a fairly steady hand and some practice - the second time is much easier than the first! You are recommended to print off and read all of these instructions before starting work.

You will need graph paper, 2H drawing pencil, soft pencil rubber, two pots of different coloured cellulose paint, cellulose thinners, modellers small and medium size paint brushes, modellers scalpel, rubber gloves, electical modellers drill and stand, assorted small high speed drills to match the component hole sizes, single or double sided printed circuit board as required, hacksaw, flat file, Ferric Chloride etching solution and some cleaning tissue. Typical PCB material is usually 1/16 inch thick G10 (fibreglass) material with 1 ounce copper on one or both sides.

Draw the electronic circuit for your board making sure that it is tidy, signal flows are clear, inputs and outputs are not next to each other and with the minimum of cross-over connections. Decide on the imperial or metric dimensions for all components including lead centres for wire ended items and sketch more complex parts like coil assemblies.

Using a piece of graph paper (I prefer 0.1 inch squared paper but metric is fine), roughly lay out the components of your circuit as viewed from the solder (non component) side and decide on an initial mechanical arrangement. At this stage you may need to consider any physical limitations that will affect the size of layout of your board - the enclosure, positions of connectors etc.

Here is the trick - if you have drawn out your electronic circuit properly then your component and track layout will be much easier as it will follow the circuit diagram in terms of track and component positioning.

Now draw in the mounting pads and centres of all drilled holes at a 1:1 scale - I use a dot for the drill centre and a small circle to outline the pad. Lightly join up the connections that are not 0v (ground or common) and mark the 0v connections with an earth or chassis symbol - joining these up just creates more track on your layout with little benefit at this stage. When you have finished you should have all components in place but the layout may not be ideal. If anything is missing or the layout is poor or some components cannot be connected up with track then you need to go back and repeat the layout process, it gets easier as you do more, until you are satisfied. Also check that all 0v points can be connected together on the track side or using the top side ground plane if a double sided PCB.

When you are satisfied, outline the edges of the layout, dot the drilling points with an ink pen and then photocopy the layout without too much contrast - the edges and drilling points should stand out clearly. Cut a piece of PCB to the required size, file the edges straight and remove any burrs.

Lay the photocopy onto the piece of PCB and line up the edges by looking through the paper towards a good light source. When in line, fold and fix the paper in place with Cellotape or similar. Now drill through all holes with a pilot drill - I use 1/32 inch (0.8mm) for most component leads - and then repeat the process for holes that are larger in diameter. With care you should only need three or four hole sizes for most layouts.

Remove and discard the paper, copper/PCB swarf and gently rub down the copper surface(s) with fine emery cloth or a flexible rubbing down block - a worn one is much better as it minimises marking the copper. Wash and dry the PCB.

Now paint each pad using a small model paintbrush and one of your colours and then paint in the interconnecting, non 0v track in the same colour. If you make a mistake, leave it to dry, scrape away the incorrect paint with a small model makers scalpel and make the necessary corrections. Now paint in all of the 0v track using your other colour paint and leave to dry. If you are making a double sided board paint in the track on the component side - usually ground plane, leaving small space around component leads that are not connected to the top 0v track.

Examine the finished article to make sure that all tracks are correct and do not have any hairline cracks or overlaps onto other tracks - the two colours significantly ease this process.

When you are satisfied, etch the PCB using a warm ferric chloride solution (about 20 minutes at 40 degrees C solution temperature), rinse and examine to check that no unwanted copper remains - put it back in the etchant for a few more minutes if required. Rinse in clean water, thoroughly wash and dry the board, remove all of the paint with thinners and a cloth, and then give it a clean with a copper polish (Brasso etc), clear out any clogged holes and wash, rinse and dry again. Tin if required and you are ready to mount your components.

Practice each of the steps until you are proficient. You should note that Fibreglass is very abrasive and wears out drills and files quite quickly.

Keep your layout, circuit and design notes for the future.


2. I have recently been experimenting with the One4All acrylic pens instead of cellulose paint with some success. The non-metallic colours appear to work as an etch resist as long as the pens are well shaken, the board is clean and well degreased and the etching solution is just warm - if it is too hot then the acrylic softens. Make sure that all burrs around drilled holes are fully removed before you commence painting with the acrylic pens as the ink is fairly thin and may splash if the pen tip catches on a burr.

These pens are easier to use than small paint brushes and can produce thinner tracks but the acrylic material when dry is not as rugged as the cellulose paint so more care is required.

If in doubt, conduct an experiment on a piece of scrap PC board - if the acrylic paint finish, when dry, smudges when rubbed then there is a problem.