by Christopher Garber
When working with electronics, breadboards are a cheap way to put together circuits. Having professional Printed Circuit Boards made can be very expensive in small quantities, but the results not only make projects easier to construct but also make them look fantastic. While the cost of production is a great deterrent, for less then $20 it is possible to etch your own PCB.
There are two ways to etch a PCB: the Toner Transfer method, and the Photo Resist method. This article will examine the former method using common household items. Before you start you will need the following:
- Access to a Laser printer.
- A sheet of photo gloss paper.
- A fibre glass copper coated board.
- Ammonium Persulphate.*
- A cloths iron.
- Plastic containers.
- 0.8mm 1.0mm and 1.2mm drill bits and a drill.
- A hacksaw
- Acetone or methylated spirits.
- Some Fine grit sandpaper or a scotchbrite pad.
The following optional parts will also help improve the results:
- A fine artwork knife, or scalpel.
- A etch resistant touch-up marker.
- A kettle.
- A metal file
- Some kind of small rotary tool like a Dremel instead of a drill.
- A toothbrush
- PCB solder-through lacquer.
* Ferric Chloride can also be used. For guides on etching with Ferric Chloride select the link at the bottom of this article.
Preparing the pcb design
There are many packages such as Protel, Eagle, or expressPCB to name a few, which allow you to design your own PCBs. Ultimately the choice of software is unimportant as long as you end up with a black and white image of the traces which you want to transfer to the copper board.
Because the process of transferring the toner to the paper flips the image which has been printed, the orientation needs to be carefully considered. When putting traces on the bottom of the PCB the printout should be normally oriented as if looking through the board form the top as it will be flipped to the bottom during the transfer process. Traces on the top of the PCB require the image to be mirrored to appear in the correct orientation.
In this guide I will etch a simple stepped-on / stepped-off circuit containing a 12v regulator, a relay, and a pair of 4049 CMOS hex inverters. The small traces on this board have a width of 20mils (approx 0.6mm), and the pads have a trace width of 11mils. The smallest gap between traces is 13mils.
The toner transfer method relies on the fact that toner, being plastic based, will form a protective coating on the copper which is resistant to the corrosion of the etching agent. It is therefore important that the toner coverage is consistent and will not stick to the paper more then the copper board.
The choice of paper is critical to the entire process. Professional Press n Peel films can be used but are usually very expensive costing several dollars per sheet. Instead inexpensive Inkjet paper can be used too. Thomas Gootee’s page explains in detail how to choose paper and his favourite is Staples “Picture Paper” (Item number 471861 from Staples stores) I personally use Canon “Glossy Photo Paper” GP-401 as I do not have access to a Staples store. While I do have the “pinhole-in-dark-areas” problem that Thomas explains I still achieve satisfactory results with this paper.
If you are choosing your own paper there are several important things to remember:
- The paper will be heated to really high temperatures. Papers like Kodak Glossy Photo Paper or Canon Photo Paper Pro have a plastic film on the back which will curl up and melt when heated potentially damaging the cloths iron or even the fuser in the laser printer.
- The paper must provide consistent toner coverage with no visible blemishes in the final print. Any blemishes in the toner will cause etchant to seep through and corrode the copper underneath.
- Finally the toner must stick to the paper hard enough not to fall off during handling, yet not stick to the paper when it is being peeled off the copper board.
Once the paper has been chosen print the image as dark as possible on the glossy side of the paper to provide as much toner as possible.
The Toner Transfer
Blank copper boards are available from electronic shops in various sizes, including double sided boards. But the surface must first be prepared before the toner transfer process. I prepare the copper board by using some fine grit sandpaper, a scotchbrite pad, or something similar. Sand the board in a circular motion to create very fine scratches and remove any copper oxide which has formed on the surface of the board.
After the board has been sanded, it must be cleaned to remove any traces of dirt or chemicals which may prevent the toner from sticking to the copper. It is also important that the cleaner does not leave any chemical traces. Chemicals like acetone, nail polish remover, or methylated spirits evaporate any traces of themselves and are ideal for this purpose.
After this the board is ready for the toner transfer. In order to transfer the toner from the paper to the PCB, you will need a hot cloths iron, a flat surface, some pliers, and a container of warm water.
Toner has a reasonably high melting point, and providing the paper is carefully chosen it is best to set the iron temperature as high as it will go. This does not apply if using press n peel blue film which will melt and or shrink if heated to a high temperature. In this case follow the instructions on the sheet, otherwise, it is not uncommon for the paper to turn a light golden colour during the toner transfer process.
When applying the iron, apply it with as much pressure as possible. While ironing I typically rest my entire upper body on the iron. After the iron has been applied for a few seconds, the toner starts to melt and the paper sticks to the board. When this happens start moving the iron around the paper to ensure that all areas are properly heated and have pressure applied to them; otherwise areas under the steam-holes or other uneven surfaces may not transfer properly.
When the ironing is finished, a process which typically takes 3-6 minutes depending on board size, using pliers, drop the board in a container of warm water. Sometimes at this point the board will actually sizzle because of the heat. The board can then soak for 10-30 minutes depending on your patience and the paper used.
After the board has been soaked, it is time to carefully peel off the paper. Usually the paper will tear or won’t come off properly. It is only important to clear areas which will be etched, any paper on the toner traces can stay. To scratch off the paper either rub it off with your thumb or use a toothbrush. Toner is normally pretty sturdy and will not easily scrape off, so significant pressure can be applied.
At this point, it is important to make absolutely certain that the traces are perfect. If you are unhappy with the transferred results, simply remove the toner with acetone and start again. Small scratches can be fixed with an etch-resistant marker. [Editor: Sharpie-brand Permanent Markers in Black Ink only are a good subsitute.] Areas which have transfer not desired should be scratched off with a very fine artwork knife or scalpel. If in doubt mark the traces with the etch-resistant marker, and make sure all component pads are free of paper, as next step is irreversible.
Etching the board
There are two etchants commonly available for this process, Ammonium Persulphate and Ferric Chloride. I use ammonium as it is a clear liquid when mixed with water making it easy to monitor progress. Ammonium needs to be mixed with hot water, and during etching needs to be kept at temperatures greater then 70 degrees C to etch properly. The water will slowly turn blue during the process.
To etch the boards I start by filling a plastic sink with hot water, and placing a plastic container (in my case a pot plant dish), that will not melt, into the water. I then fill the dish with boiling water, add ammonium crystals according to instructions, dump the circuit boards into the solution, and use the remaining boiling water for a cup of coffee.
The etching process takes 10-40min depending on the size of the board, the temperature and how much solution is used. If the solution is mixed correctly the brushed look from the sanding process will disappear in a matter of minutes. To increase the speed of etching, stir the solution with a wooden stick. At no point should you use any utensils which will be used for cooking or eating, or any items made of metal. If the etching process seems to stop completely check the solution. If the solution is cool then add boiling water and more ammonium. If the solution has turned blue then dispose of the solution and mix a new one.
After all the copper visibly disappears from the board remove it and wash with soapy water. Dispose of the solution by soaking it up with paper towels or magazines, and throw away. Do not pour ammonium persulfate down the sink as it may corrode the piping and is poisonous.
Finishing the board
At this point some people strip the toner off the copper with industrial cleaner, acetone, or a specialised stipping agent. I however leave the toner on the board until the last step.
The PCB needs to be cut to the correct size with a hacksaw and the edges can be smoothed with a metal file. Once the board is the correct size drill the component holes. Standard hole sizes require 0.8mm, 1.0mm, and 1.2mm drill bits. Bits this size snap easily so it may be a good idea to buy a few of them. Then can usually be bought in a complete kit which will include 4×0.8mm, 3×1.0mm, 2×1.2mm, and 1×3.5mm bits. The copper traces generally guide the drill into the middle of the hole however it is very easy to damage the holes at this point. To avoid this hassle some people drill the holes before touching up the toner transfer with the etch-resistant marker. I find this causes drills to become blunt and useless very quickly. If sufficiently large solder pads are used they won’t easily come off the board during drilling anyway.
After drilling the board I carefully sand off the toner. The reason for sanding as apposed to acetone is that any sharp parts left over from the drilling process are also smoothed. Care must be taken not to damage fine traces during sanding. Finally clean once more with acetone, meth, or PCB cleaner.
Adding a professional touch
At this point you will have a perfectly useable circuit board. However it is also a good idea to seal the PCB with some specialised lacquer. [Editor: a clear acrylic spray finish sold in art stores can substitute for the lacquer.] The lacquer not only protects the finished PCB from corrosion, but it also provides a non conductive flux type coating to aid soldering, and at the same time provides some resistance to the accidental bridging of solder joints often caused by excess flux from the soldering process.
The lacquer can was nearly empty and sprayed unevenly which is why the final board looks lumpy.
The entire process can take some time to master; however once you are comfortable with etching a PCB you will never use a proto board for a project again. The double sided boards can be easily etched by drilling guide holes into the board to help line up the paper accurately on both sides. Some people use toner transfer to also provide a silk screen on the component side of the board.
More information about the toner transfer method is available form the following sites:
Thomas Gootee’s Easy PCB Creation – This page provides a lot of information and is very descriptive about the process and the paper.
Etching PC Boards – A guide to using Ferric Chloride to etch the PCBs.
Printed Circuit Techniques – There is lots of information here about various manufacturing methods.
Homemade PCBs – Lots of information about the photoresist method of fabrication. Other information about through plating and good PCB layout is also available on this site.
c. 2005 Christopher Garber (remove _nospam_).