Building PCBs

A lot of things happened since my last post! I got a lot of new modules needed by my mowing robot (RF, distance sensors, 3-axis compass, charging IC, and so on). I have to investigate them before I can use them. This will be very interesting and a lot of fun ;-)

Unfortunately my breadboards are full and I have to bring all the stuff to circuit boards. So the next step is to learn how to produce those boards.

First of all you have to design your board. There is a lot of free software available. I tested Fritzing and Eagle and both do a good job. The disadvantage of Fritzing is it's limited elements-library and the disadvantage of Eagle is it's complexity since this is a professional tool. So I will use Fritzing for simply boards and Eagle for complex ones.
Both programs offer to order boards for professional production. This is pretty expensive although the boards offered by Fritzing and Eagle are low-cost-boards. So what is the solution? DIY!

Usually the nets of circuit boards form by etching copper of non-conducting areas. But how to "tell" where copper has to be removed and where not? Professional boards are created by using light exposition: the copper is hidden by a layer of light sensitive paint, the inverted board will be printed on a transparent film used to expose the board and acid will remove those exposed areas (and the nets resist).
This also can be done at home but at home it is not that easy:

• The exposition can get blurred due to shadows if the film is not fully plain.
• You see errors and mistakes not until the board is etched. So erroneous boards only can be trashed.
• Boards coated with light sensitive paint are much more expensive then boards without that layer.

So DIY guys figured out another way: The toner transfer method. This technique is taken from bringing images to T-shirts or cups. The layout is printed to some sort of media using a laser printer and afterwards pressed under heat onto the target. The toner melts and goes to the target. This works best if the target's surface binds the toner better then the transfer media does. For T-shirts and cups special transfer paper is available.

For copper boards it is a little bit more complicated but not much. Most guys out there use any sort of glossy advertising media. After printing (mirrored) it has to be ironed onto the board. Afterwards it has to be put into water (for about 10 minutes) so the media can be rubbed off. There is no instruction which media works best - it depends on your laser printer. Some media is too thin and will not work. Some media binds the toner too much and paper cannot be removed completely. Additionally you put paper into your printer that isn't meant to be used in a laser printer and may damage the printer.
Instead of advertising paper transparent film for overhead projectors can be used. This works good but not reliable good. So at the end I managed to get this special toner transfer paper (used for cups) which works best for me.
Anyway: you have to find out what works best for you. This may be a little bit frustrating but you can be sure that there is a way for every laser printer.

How it works:

1. Clean your board
   
   This is essentially for the binding of the toner. Any bit of dust will lift the paper and the toner won't bind! Any fingerprint brings tiny bits of oil onto the board and the toner won't bind! So clean the board precisely and keep it clean.
   
   I do a three-step cleaning. First, I use stove cleaner and scrub the copper in small circles. After this the board has to be washed and dried. Second, I use steel wool to polish the copper. Third, I use Acetone to remove any oil. It seems to be essentially to use a clean board otherwise the toner does not bind well.
    
2. Prepare the transfer media
   
   I use that special transfer paper. In my case there are four things to take care of:
   
   First, print the layout mirrored and tell your printer to use as much toner as possible. Second, toner transfer paper is very soft. I can see patterns of the laser printer's transport-wheels on the paper after printing. These patterns decrease the quality of the printed layout in this regions so you should avoid printing in this regions by choosing a proper position. If the layout is too big to avoid this then try to reduce the number of SMD-elements or other small things in this regions. Third, the paper cannot be reused so print the layout as many times as possible on one sheet. Maybe your first try fails and you need another print for the second take. And fourth, avoid touching the transfer media to reduce oil of your fingerprints.
    
3. Iron the layout onto the board
   
   Put the transfer media on the board with the layout down to the copper. Put some sort of cloth on it. Preheat your iron. The toner starts to melt at approximately 180°C. This is usually the setting between cotton and linen. Which temperature is properly also depends on the thickness of the transfer media and the cloth. Put your iron onto the stack and wait 5 seconds. Now the layout sticks at the current position and you can start to move around. The bottom of the iron is not hot equally so you also have to turn the iron around. Put some pressure on the stack. How much depends on the thickness of the transfer media and the cloth. Thicker means more pressure. The whole layout should contact the copper equally over the time of ironing. You should stop ironing if all regions of the layout are at 200°C. You might ask when this is achieved and I have no answer for you. You will see whether it was too hot, too long or too much pressure if you remove the transfer media. If so the toner has melt to blobs, especially at bigger areas of toner, and the borders are not clear.
   
   One trick I use is to vary only one aspect: I always use a timer to count the time of ironing and stop always at the same time. On different tries I only change heat or the pressure. This should help to figure out what your personal best settings are.
    
4. Remove the transfer media
   
   Wait for cooling down the board and peel off the transfer media. If your media is transparent film you can use some ice because the toner and the media decreases differently on cooling down rapidly and the toner will lift off from the film almost completely.
   
   Review your board carefully! Some failures can be corrected by using a permanent marker. Take care that all nets are separated to avoid shortens. If there are nets together (for example near SMD-sockets) you can scrub the toner away by using the spot of a sharp knife.
   
   If the result isn't satisfying then try again. Remove the toner using Aceton and start at step 1. It is better to repeat the process then trashing a board after etching it. It took me several approaches the get a useful result! So don't be inpatient.
    
5. Etching the board
   
   This is quite easy. I use Natrium-Persulfat. It is easy to handle and easy to store. Some acids are gassing so you cannot close the canister completely which means you have to store it outdoor since the gas is poisonous. Natrium-Persulfat can be stored in a bottle made of glass or plastic. But keep it away from children - it is an acid!
   
   The only disadvantage of Natrium-Persulfat is that it works best at 50°C. So I put the liquid into microwave oven for some seconds and put it on a towel afterwards to keep it warm as long as possible. If it cools down too fast you can but the tank into hot water until temperature is properly again.
   
   After about 10 minutes the board is ready. Take a closer look a nets which are close to each other to be sure that all copper is gone. I use my cell phone's camera which takes nice pictures much more detailed than I'm able to see without any optical support.
    
6. Remove the resisting toner
   
   The last step is quite easy. Use Aceton to remove the toner. And after this the board is ready for further steps (for example drilling the holes).

Here some pictures I've taken:

My first try using a transparent film as transfer media

Another try: You can see light spots where dust was between the transparent film and the copper

Sometimes toner resists on the transparent film.

Special toner transfer media. Click to enlarge to see the patterns of the laser printers transport wheels.

A layout on copper using toner transfer media. Good and reproducible quality.

Etching the board - after 5 minutes.

Etching the board - after 8 minutes. Watch the bubbles!

Etching the board - after 10 minutes. It is nearly done - there is some copper left (click to enlarge).

Waiting for etching the last bits of copper.

The final result after remove the toner - pretty nice!

Another try using toner transfer paper: I wanted to know how good it is using bigger layouts.

The board you see is a breakout-board for TSSOP-14 ICs. I made a couple of them to get some practice in soldering SMD ICs. But this is another story...