This Instructable is to accompany a demonstration to be given to Ipswich (UK) Makerspace in May 2014, but might be of interest to any would-be PCB producers, particularly based in the UK (due to the sourcing of materials).
It is a 'photographic' system, using pre-sensitised boards (4" x 6" at £2-40 including VAT and delivery) and a laser printer for the printing of the PCB layout image. The final results are suitable for very fine PCB tracks.
The total cost, from scratch, for all the tools and equipment is less than £25. This then sets up the whole process, enabling boards to be produced for the cost of the pre-sensitised boards themselves.
This Instructable covers the purchase of the UV light source, the chemicals, and the pre-sensitised PCB. It then details the 'calibration' of the exposure time, the developing and the etching. It does not cover the software needed to produce the PCB layout.
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Step 1: Order the Tools and Chemicals
If starting from scratch, you need to order a few things for delivery.
1) Buy a UV Nail Curing Lamp. The one I bought cost £10.47 inclusive from Ebay (free delivery). If you look at the photograph of mine, you can see dozens of vendors selling the same one. It was advertised as "36W UV Lamp Light Gel Curing Nail Dryer + 4 x 9W Blubs". I removed the sliding bottom, and did not bother using it.
2) Get some good quality tracing paper. Many people online suggest alternatives, so be prepared to experiment, I'm sure there are many different options. In the past, I have purchase some 'special' PCB tracing paper called 'Laserstar PCB Printing Film', it was brilliant, but too expensive for my current situation. This time, I went to my local art shop (Hussey Knights Ltd. Bramford Rd, Ipswich, Suffolk, UK IP1 4AS Phone: 01473 461603) and bought an A4 pad of their 112gsm tracing paper 'Gateway natural tracing paper pads 112 gsm'. You get 30 sheets, which I cut into A5 size, because all of my PCBs are smaller than A5! This paper cost £6.50p inclusive of VAT. I intended buying 90gsm paper which I'm sure would be great, but they also had this thicker paper in stock. The idea is that tracing paper passes UV easily; thick tracing paper has a very smooth surface which reproduces fine detail really well; thick paper does not crinkle; thick paper is very dimensionally-stable as it passes through the laser printer.
3) Get some pre-sensitised PCB. I use single-sided, economy board. I have had great success with economy boards from Rapid Electronics, but this time I went to CPC (www.cpc.co.uk) because they do not charge delivery, even for a tiny order. I ordered 'Economy PCB Board 4"x6" PC00348' at £2.49 inc VAT. This board has a layer of photo-sensitive etch-resist already laid onto it. It has a layer of black tape stuck over the resist, to keep the light away. This layer has to be removed, just before exposing to the UV light source.
4) Developer for the board. I ordered 'Developer Powder for 1L PC00034' at £2.09 inc VAT from CPC. I only make small boards, so I split the powder into 4 and only mix up a quarter of the powder (to make 0.25L of developer solution). The dilution varies, but I use 50g for 1 litre (the strongest recommended dilution). I add 12.5g of powder to 0.25L of water in a bottle. The developer has a shelf-life of 3 to 4 weeks.
5) Etching chemical. I prefer Sodium Persulphate to Ferric Chloride, but either are fine. Sodium Persulphate is a clear liquid, so you can watch the progress of the etching, and remove the board as soon as it is complete. The Ferric Chloride is opaque, and you have to keep lifting the board out to check it. Both chemicals come as a powder/crystals. When mixing, it is vital that you add the powder to the water, not the water to the powder (the latter can boil the tiny quantity of water and throw some REALLY nasty boiling acid in your face). To make the working solution, I add 100g of Sodium Persulphate powder to 0.5L of tap water. I bought 0.5kg of Sodium Persulphate from ebay a couple of years ago and mix up 0.5L of solution as necessary. The solution is supposed to have a shelf-life of 6 to 8 weeks, but I am currently using solution which I mixed up 2 years ago - it seems fine and still etches well. If I were buying it now, I would probably add it to my CPC order as 'KEMO ELECTRONIC ETCHANT, 100G PC01608' at £2.75 inc VAT.
These goods total £24.30p (including VAT and delivery) and set you up for years of PCB manufacture!
Step 2: Safety
I'm very careful using the chemicals. In particular the Sodium Persulphate (and Ferric chloride) is a very nasty acid.
Goggles and rubber gloves are recommended.
Young people, should not handle these chemicals without adult supervision.
I am old and do not have children in my house. If I did, then I would be doubly-careful with my working practices. As it is, I follow these rules:
- I always label the solutions carefully. Sometimes, I use old carbonated drink bottles to store solutions (which is not good practice), but they are fully labelled and kept in our basement.
- I always add the powders to the water, not the reverse.
- I have a special set of plastic containers for these chemicals and do not use our kitchen ones.
- I flush waste solutions down the outside drain with copious quantities of water.
- I work on layers of newspaper - drops of liquid invariably end up around the containers - newspaper absorbs them really well.
Step 3: Gather Other, Supporting, Materials
Other equipment is needed.
- An exposure frame (mine is a piece of glass from an old picture frame and a matching piece of plywood, with some rubber bands).
- Some plastic containers to act as chemical baths.
- A timer capable of measuring seconds.
- The PCB layout you want to produce - preferably laser-printed on tracing paper.
Step 4: Calibrate the Etching Time
With a new set-up, you need to find out the best exposure time. This should only need doing once (in theory).
A strip of PCB is placed under the PCB printout (black toner should be in contact with the photo-sensitive layer of the board) and the glass is held firmly down to clamp everything.
An opaque card (or similar) is placed on top of the assembly, leaving a small exposed area of PCB. The lamp is placed on top and switched on for one minute. Then the strip of opaque card is moved to expose a new strip of PCB and the lamp switched on for another one-minute interval. At this stage, the first exposed area has had a total of two minutes of UV light, and the second area has had one minute of exposure. This process is repeated, slowly building up a set of PCB areas, each with a different, known, exposure time.
I did 8 PCB areas, each 1cm wide.
The resulting board is developed by immersing it in the developer (for 60 seconds or so, depending on the temperature of the developer) and then rinsed under tap water. I agitate the board continually during the development. The developer should be permitted to finish its job - you do not need to 'judge' when it is right to remove the board, you cannot really over-develop unless you leave it in for 20 or 30 times the optimum time.
The used developer is returned to the stock bottle for future use (it has a shelf-life of 3 to 4 weeks).
You can immediately judge the best exposure time by looking carefully at the image which is left. Some will be very thin (too much exposure) some will be too dark (not enough exposure). You can also etch the board and see the effect of the different exposure times on the finished product.
Surprisingly, the exposure time I required was short (1 to 2 minutes). The lamps must be pretty powerful.
I should probably repeat the process using 20 second intervals, to get a more precise estimate of the best exposure time.
Step 5: Etching
Once developed and rinsed, the board is immersed in a bath of etchant and agitated continuously. The board seems to generate tiny bubbles which, I assume, would inhibit the action of the etchant. Sometimes, the bubbles are persistent, and I extract the board completely for a couple of seconds; this clears them off.
The etching process is very slow when it is cold. I always place the container of etchant, complete with the PCB, in a bigger container and pour boiling water into the outer container, to act as a hot water-bath. This rapidly raises the temperature of the etchant and brings the etching time down dramatically (eg to 5-6 minutes).
It is pretty obvious when the etching is nearing the end of the process, because the board matrix (ie white fibreglass) starts showing through (firstly at the edges of the PCB).
Once everything necessary is etched away (including the tiny drill hole marks), the board is flushed under tap-water.
In fact, the board can be removed, rinsed, inspected and put back into the etchant as often as needed. I find inspecting the PCB with a very bright light underneath the board (and hence shining through the board matrix) highlights any unwanted copper bridges, etc.
The etching solution is returned to its stock bottle for future use.
Cellulose thinner (aka acetone or nail-polish remover) is used to remove the photo-resist layer from the copper.