MakePCBInstructions
Contents
Making PCBs yourself
Overview
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 put the PCB in ferric chloride solution, which dissolves away the copper where the resist has been removed.
You clean the resist off the resulting finished PCB with alcohol (or by re-exposing it).
You will need:
- Double-sided photo-resist PCB (for example from RS - part number 3970160)
- A UV lamp or UV lightbox (Maplins UV lamps part number L31AF)
- Ferric chloride (RS part number 551-277)
- PCB developer (RS part number 690-849)
- A dish or plastic tray in which to do the etching
- Some mugs/jam-jars to mix the chemicals
- A measuring jug
- A reasonably accurate means of weighing a few grams of chemicals
- A kettle
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, 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...
What you are going to do
The example we will use is the Universal PCB that is used for both the RepRap motor controllers and the extruders. It is in the KiCad project file ~your-id/workspace/electronics/Universal-pcb/stepper-and-extruder.pro. The PCB looks like this in KiCad:
The red tracks are on the obverse (component) side, and the green ones are on the reverse. Here is what you are going to make:
Printing and taping up the designs
Postscript files for both sides of the board are in the files stepper-and-extruder-Copper.ps and stepper-and-extruder-Component.ps in ~your-id/workspace/electronics/Universal-pcb/. These files were created by KiCad, and you can get it to write them again, if you want; if you modify the design you'll need to re-write them, of course. Use the KiCad mirror check-box in its print options to ensure that the two files are printed such that the side of the print with the toner is the side that will be against the PCB when it is being exposed.
Print the files on overhead projector transparent sheets on a good-quality laserprinter (as mentioned above, inkjets don't produce a black enough print). The scale in the postscript files should be right, but check it as some print drivers rescale prints to fit them to their idea of a page. The simplest way to check is to hold the pins of an 18-pin DIP socket (which you'll need to put the PIC in) against the pads on the print where it will be located. The pins should fall exactly in the centres of the holes.
Cut about five 30 mm lengths of sticky tape and place them where you can get them.
Then cut about 2 cm off the long side of one of the prints. Put the two prints together toner-side to toner-side and get the circuits into registration. If you have a light-box this will help, but it is quite easy to do just resting them on a white sheet of paper. If (like mine) your close-up vision is blurred by the passing of the decades, use a magnifier to check that things are exactly aligned.
Without breaking the alignment tape the cut sheet to the uncut one to make a hinge:
The tape, being clear, is not very clear... So here's what it looks like with the hinge unfolded:
Preparing and exposing the PCB
Leave the plastic covering on the double-sided photoresist PCB until you are ready to expose it.
Find a dark place to put the PCB after you have exposed it. I use a black document folder placed in a drawer.
Cut a rectangle of PCB 135 x 75 mm. This is larger than the finished result will be by about 5 mm all round. This will allow a margin so you can tape it down.
If the cutting has left burrs or raised edges remove them with a fine file.
Cut two 30 mm lengths of sticky tape and place them where you can get them.
I have never found out how much ordinary light (as opposed to UV) the PCB can stand without becoming exposed. If you know, tell me and I'll put that here.
Under a low light (I use a photographic red light in a darkened room) peel off the plastic protective film from both sides of the PCB, holding it by its edges. Try not to get fingerprints on the flat surface.
Place the PCB on the lower layer of the hinged transparencies so the design is central under it (close the hinge to check alignment), then tape it to the lower transparency, taking care to restrict the tape to the edges beyond the active area.
Expose the PCB to UV light on both sides. I have placed it under a sheet of glass (to flatten the transparency film right against it) under an old sun-tan lamp; that works well. Strong sunlight (beware clouds...) should do too.
But best is a light box. These seem (for no good reason) to cost an absolute fortune, so I made two:
They are on the left. They are just aluminium-foil-lined chipboard boxes with the UV lamps from Maplins (part number L31AF) and sheets of glass on top held by double-sided tape. Centre is the darkroom red lamp, and on the right is a rocking tray for agitating the solutions during etching. That is more chipboard, driven by a crank from a small 60 RPM DC motor like the one used in the extruder. Note the professional-grade ice-cream tub for holding the etching chemicals, the plastic tongs for fishing out the PCB without dissolving one's fingers, and the ferric chloride stains (it gets everywhere...).
Place the transparencies with the PCB taped between them on the bottom box, invert the other box, and put it on top. You may need to put a scrap or two of waste PCB offcut between to keep the glass sheets parallel.
With the materials and lamps listed above an exposure of three and a half minutes seems to work well.
Back under the low light, untape the PCB and put it in its dark place.
Preparing the etching solutions, developing and etching the PCB.
Get yourself organised - you really don't want to answer the phone while holding a beaker of ferric chloride solution. One spill or splash will corrode lots of things (including you) and stain anything it touches.
Fill the tray that you will use for developing and etching with water to a depth of about 20 mm. Pour that into the measuring jug to find the volume of each solution you will need.
Weigh out the sodium metasilicate developer. With the materials I specify above you need 2.5 g per litre (which is half the strength that it says on the developer bottle; the PCB specified uses half-strength developer).
Boil the kettle. Measure half the volume you will need of cold water into a jar (chemist's conical flasks are ideal for this if you can get them, incidentally), and top it up with the other half of boiling water. Dissolve the sodium metasilicate (which should be quick in the warm water).
-- Main.AdrianBowyer - 30 Dec 2006
