MakePCBInstructions

Making PCBs yourself

PCB Safety

UV light will damage your eyes and skin. Keep it in a box, or don't be in the room when it's being used. A few seconds exposure while you're setting things up won't hurt.

PCB developer is sodium metasilicate, which is bad for you. Wear gloves and - if you are particulary risk-averse - safety goggles. Don't get it on your skin, in your eyes, or in your mouth. If you do (Dimwit! What did I just tell you?), wash it away with lots of water.

Ferric chloride is less harmful (though it still is a bit - keep wearing gloves, and wash away as before if you come into direct contact); also it will stain anything that even a minute quantity of it touches (including glass and plastic) bright orange. You may want to wear a lab coat, or some old clothes.

If you do use mugs to mix the chemicals, eschew drinking coffee from them subsequently...

See here for details of ferric chloride safety, and here for sodium metasilicate.

Suppliers

Copper-clad Boards
Electronics Express (97BD12) - 5" x 6" double sided - $2.75

Ferric Chloride
Electronics Express - $9.95 / lb

Ammonium Persulfate
Electronics Express - $20.65 / lb

Transfer the Design

Regardless of what method you use, the idea of making PCB's is that you want to take a blank copper clad board and turn it into a printed ciruit, ready to rock and roll. Essentially, you use a printer to transfer your design to the board as a etch-resist coating, then use an etchant to eat away the copper where there is no resist. You are then left with just the resist coated traces and then once those are cleaned up you have a board ready for drilling and use.

There are basically 2 camps of homebrew PCB making:

• The toner transfer method, where you transfer laserjet toner directly to the circuit board
• The UV photo resist method where you use a transparency and photo sensitive resist to transfer the design to the board (like developing a photograph)

Laserjet Toner Transfer

The toner transfer method is arguably the easiest, cheapest, requires the fewest tools, and has the fewest steps. It can also be prone to failure if you do not have the right type of paper. Also, the transfers are generally (although not always) lower quality than the UV transfer method. If you only plan to do a few boards (and fab the rest on your RepRap) this is probably the one to use.

• You print the PCB designs on a certain type of paper.
• You iron the paper ink down onto your board.
• You soak the board and eventually remove the paper leaving the ink behind.
• You dissolve the exposed copper with ferric chloride and finish the PCB guide.

UV Photo Resist Transfer

The UV transfer method is a bit more complicated and requires more steps. It is also a bit more expensive and requires more specialized tools. It generally has a higher success rate, and the boards are of a bit higher quality. You can also re-use the transparencies to to multiple boards. If you plan on doing lots of boards, this is the route to take.

• You print the PCB designs on overhead projector transparency sheets using a good laserprinter (inkjets aren't black enough).
• You sandwich a photoresist-covered PCB blank between the sheets and expose it to ultraviolet light.
• You put the PCB in a developer solution, which removes the photoresist wherever the UV light fell.
• You dissolve the exposed copper with ferric chloride and finish the PCB guide.

Etching the PCB

Now mix the ferric chloride solution. Hot and strong is the rule here. The concentration I use is 200 g per liter.

Boil the kettle. Measure out the required volume into a jar and add the ferric chloride. Take care not to let it splash. Stir it till it dissolves, keeping it as hot as possible. Pour it into the tray and add the beads. Put the PCB in (splashes again...).

Once more, gently agitate the tray, turning the PCB over from time to time. The copper will take longer to react than the resist did to dissolve, but it will be quicker the hotter and more concentrated the solution is.

Rinse the PCB under the tap, and once more with distilled water if you have it. Stand it on edge to let it dry.

Drilling the PCB

The small holes in the centres of the pads act as drill centres. The easiest way to do the drilling is with a minidrill in a stand, with the PCB resting on a small piece of wood.

Drill the four corner pads 3mm - these are the mounting holes.

If you're a perfectionist drill the holes out at 0.7 mm (you can leave the breadboard area if you don't intend to use that). Then drill the holes for the power Darlingtons, the power and signal connectors, the diodes, and the L298N out to 0.9 mm.

If you're in a hurry drill everything at 0.9 mm...

When you drill the first hole you will find out how well you did the obverse/reverse registration with the sticky-tape hinge...

You can check if you've missed anything by holding the PCB up to the light - it's then obvious which holes haven't been drilled.

Drilling may leave some burrs. Remove these carefully by hand-twisting a bigger-diameter drill in each hole to clean the burr away.

The finished result should look like the picture above.

Don't remove the photoresist till you are ready to solder up the circuit - it will preserve the copper against oxidation. However treat with suspicion claims by the photoresist people that it acts as a soldering flux - in my experience it just gets in the way. To remove it use a Q-tip to wipe it off with alcohol. Ethanol or methanol (methylated spirits) both work well. Just wipe off the resist on the solder pads where you will solder. If you leave it everywhere else it will stop the copper oxidising.

-- Main.AdrianBowyer - 30 Dec 2006