Friday, 20 January 2012

DIY Printed Circuit Boards

PCBs or printed circuit boards are used in almost every electronic device to mount electronics and to connect different components with metal tracks. Most PCB's that are made commercially use a subtractive process where a laminate with a thin layer of copper on top is used. A plastic coating in the shape of the circuit prevents the copper from being etched while the rest of the unwanted copper is removed using an acid. The most common way to place the protecting plastic in the desired circuit shape is by using a photoresist. The design is printed out onto a transparent plastic sheet and placed on top of the board. Ultraviolet light is then used to break down the plastic coating where the mask is not covering. The board is then etched leaving the circuit design.


Figure 1 | Printed circuit board (source: Cschirp)

Another method to place the protecting layer onto the copper is by using the toner transfer method. This is a much simplier method. A laser printed is used to print the curcuit design onto a piece of shiny paper (magazine paper works well). The ink is then transfered to the copper covered laminate and fused on using an iron or a laminator. The paper is then wet and rubbed off leaving the ink on the paper. The metal not covered with the ink can then be etched away leaving the circuit.

At the Photon Factory in University of Auckland we decided to use this method to make our own PCBs in the lab. We initially attempted using a cheap iron but the results were not very good with the ink not transferring over to the copper and the ink spreading in some places.

The laminator was the next option however as the laser printer ink melts at around 150 degrees C we had to modify the laminator to run at higher temperatures. The laminator had two thermostats that kept the temperature around 145 degrees C by cutting power to the heaters when the temperatures went above 145 degrees C. The other thermostat was used to light a green LED to tell the user when the laminator was hot enough to use. The simplest hack would be to replace the thermostat with a higher temperature thermostat (see link). However we wanted to be able to finely adjust the temperature of the laminator so we bought a temperature switch control board from Jaycar. This allowed us to control the temperature up to 245 degrees C. We set the laminator to sit between 160 and 180 degrees C. (Jaycar temperature switch)


Figure 2 | Modified laminator mounted onto a piece of MDF

  

Figure 3 | Temperature control switch with red and black cables turning the heater rails on and off and the grey cable going to a thermistor to measure the temperature.

  

Figure 4 | Top of the laminator was cut and covered the electronics. This meant we could put the PCB straight into the rollers and were able to see what was going on.

The laminator cost NZ$40 and the temperature switch cost the same. So the whole system cost NZ$80. This was a very quick and easy hack that will let us now make our own PCB's in the lab.

Steps to making a PCB

1. Design circuit on the computer. I have used the free version of Eagle CAD a very good program for doing circuit diagrams and then easily transferring the design to a PCB .

Figure 5 | PCB design on Eagle CAD ready to be printed

2. Print the circuit board onto a magazine we used our trusty student magazine Cracuum. The best paper needs to be very shiny and is easily wet. So most magazines will do the trick. Print the design using a normal laser printer and make sure the toner density is set to its highest value no eco-saving mode. Then cut out the design ready to transfer the toner. As this will flip the design (mirror the design) you can print the circuit as is on the computer if you have designed the pads and circuit on the top layer they will be on the bottom of the pcb when transferred.

Figure 6 | PCB design laser printed

3. Clean the PCB using a metal cleaner we used Brasso. A scrubbing brush is used to clean the PCB. This is very important as even a finger print can stop the toner from transferring.
 
  

Figure 7 | Brasso and scrubbing cloth to clean copper on the PCB.

4. Put the paper onto the PCB and run it through the laminator. We found doing this 10 times worked well switching between length ways and width ways. Be careful to catch the PCB as it will shoot out the back due to the pressure from the rollers. 

  

Figure 8 | Copper board and magazine with design going through the laminator

5. You want to put the board straight into water to start wetting the magazine. It takes around 5 minutes for the water to soak in. Then the paper can be gently rubbed off exposing the toner transferred to the copper. If you have done it right the toner should be bonded strongly with the copper.

  

Figure 9 | Toner bonded to the copper on the board.

6. A solution of one part potassium persulfate  to 5 parts water was used to etch the copper around the toner. The hot water tap was run making a hot water bath which is needed to speed up the chemical reaction. 

 

 Figure 10 | Etching solution in hot water bath


The solution needs to be stirred and after around 5 minutes you will see a blue solution of copper 2+ forming as the acid dissolves the copper into aqueous ions.


Figure 11 | Copper being etched leaving the fiber glass underneath

7. Clean off the toner using acetone or some similar solvent and you are left with a copper printed circuit. 


Figure 12 | Printed circuit board this board is for doing nanosecond to microsecond delays of nanosecond lasers more on this in another post. One hundred micron features were able to be made on this board.


Figure 13 | Header pins drilled and mounted onto an Arduino clone. Nano delay shield.

So in summary a PCB laminator was constructed to transfer laser printer toner to a copper board. This was found to produce features down to around 100 microns. I will post soon about the circuit that is being constructed when I have drilled and mounted all the components.

Links

There are many web pages on PCBs, Google DIY PCB or toner transfer PCB. 

3 comments:

  1. What a great project! I've never seen this "quilt as you go" technique before. I love how this
    turned out!pcb prototype

    ReplyDelete
  2. i have enjoyed this great information so much. This was really very interesting and helpful to read. I cant wait to read more from this site.

    ReplyDelete
  3. Wing Tse

    Excellent idea. You dont have to buy the special blue film from electronic store. Bravo.

    ReplyDelete