MakePCBInstructions

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Here we describe etching PCBs, the final step in several different Automated Circuitry Making techniques.

Contents

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

These are boards that are plated with copper. We will be transferring our design to them.

USA
Electronics Express (97BD12) - 5" x 6" double sided - $2.75
[[1]] 24" x 18" - $2.00

UK
(RS part number 551-277)


Etchant

Ferric chloride
This chemical etches (removes) the copper from the board where it is exposed.

Electronics Express - $9.95 / lb

Ammonium persulfate
This is an alternative etchant to ferric chloride. Clear, but more expensive. Good for demonstrations.

Electronics Express - $20.65 / lb

On a related note about etchants in general, Seagrol mentioned:

Might want to look at something even cheaper/more efficient as a PCB etchant. Might I suggest something self replenishing, hideously cheap, and readily available? Hydrochloric acid and copper chlorate.

There is also a description of several methods of etching copper at: http://chestofbooks.com/reference/Henley-s-20th-Century-Formulas-Recipes-Processes-Vol2/Etching-Fluids-for-Copper.html

There is an article on recovering copper from etching solutions at: http://www.circuitree.com/Articles/Column/b4d49de37efe7010VgnVCM100000f932a8c0____

An explanation of copper electrowinning at: http://electrochem.cwru.edu/ed/encycl/art-m02-metals.htm


Paper

Multi-Project Photo Paper
This photo paper is the best. It peels off really nicely. No scrubbing of boards needed.

Jet Print - $15.99 / 60 sheets.

User:FiremanDecko: I didnt have much luck with the Jet Print paper. It didnt peel off easily at all. PCB Fab In A Box is a great all in one solution. You can get the individual components of the "box" kit on mouser or digikey too.

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)
  • (highly experimental) Use a RepRap to put some etch-resistant coating on the copper-clad PCB Automated Circuitry Making#Etching .2F Resist Route

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 photoresist 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 laser printer (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 grams 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.

Tips

  • Put down plastic underneath where you're doing this. It can be messy!
  • Only etch one board at a time!
  • Don't save the used ferric chloride. It's not worth it.
  • Set a timer!! A watched pot never boils, and a watched circuit never etches. =)

Drilling the PCB

Tools you'll need

  • Drill press (or access to one)
  • Carbide drill bits (3mm, 0.9 or 1.0mm, and optionally 0.7mm)
  • Some wood to drill on

The small holes in the centers of the pads act as drill centers. The easiest way to do the drilling is with a drill press, with the PCB taped to a small piece of wood.

Process

  1. Drill the four corner pads 3mm - these are the mounting holes.
  2. If you're a perfectionist, drill all the holes out at 0.7 mm (you can leave the breadboard area if you don't intend to use that).
  3. Then drill the holes for the power Darlingtons, the power and signal connectors, the diodes, and the L298N out to 0.9 mm.
  4. If you're in a hurry drill everything at 0.9 mm...


Tips

  • Drill slowly!!! Put your drill press on its slowest speed. These drill bits are easily broken, so use smooth movements.
  • Drill out connected pads, not the islands. That way if they're off, you have a guaranteed good hole.
  • When you drill the first hole you will find out how well you lined up your two-sided board. As long as they are close and your holes aren't going into other connections, you should be fine. If it's off, find out which way it's off and drill in that quadrant on the pads.
  • 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.

If you use photoresist don't remove the resist - it will preserve the copper against oxidation, and soldering just melts and displaces it. But if you used the laser printer toner transfer process you may have to remove the toner. Alcohol (ethanol or methanol - methylated spirit) or acetone (nail polish remover) should work.


User files and further info

Some user files, for toner transfer of Gen3 Electronics can be found here : http://www.reprap.org/wiki/User:NoobMan