DIY printed circuit boards – printing and milling. Part 1: photo-etching.

There are several ways that an amateur can go about making their own printed circuit boards. They include: the toner transfer method, photo-etching and isolation milling. Their is another alternative – sending your design off to a commercial supplier. For a long time, I have used very basic software (Circuit Wizard) for designing my own circuit boards. More recently, I have used KiCAD and EasyEDA. Of these two, I have found EasyEDA very easy to learn and use. It also has, should you need it, the facility to send a design off and have the boards made at a very reasonable cost. For that reason, and because it also incorporates a simulation package, I have now concentrated on using it rather than any other package. The only ‘down-side’ to EasyEDA is that it is web-based and your designs are created and remain on the EasyEDA server. However, you can download Gerber files and PDFs to enable photo-etching and isolation milling.

I am not going to explain how to use it here, you can find everything you need at: www.easyeda.com. Here, I am just going to explain how I used to make and how I now make PCBs at home. I have no experience of the toner transfer method so it won’t be discussed. Suffice it to say that it involves transferring the plastic toner used by laser printers from glazed paper to the surface of a copper-clad board where it protects the copper from the etchant during the etching process. This, the first of two posts, explains how I photo-etch PCBs. A later post will explain how to use a milling machine to do the same.

Photo-etching:

What you need

To make your own photo-etched PCBs you will need an inkjet photoprinter – I have used both an Epson R300 and a Canon Pixmar 4950 – both worked very well. In addition, you will need a hairdryer, some pre-sensitized photoboard  (Maplin can supply this), a UV light box – I made my own (see below), some OHP film designed for use with an ink jet printer (I use the stuff you can buy from WH Smiths), some developing trays, a pair of broad plastic forceps, a measuring cylinder or jug, something to weigh out the chemicals, and some software in which to create PCB designs (see above). To drill the boards, you will need a Dremel or other small hobby drill.

Printing your design

I have found that ink jet printers can do a first rate job.  However, the settings are crucial. Print your design onto inkjet OHP film. You can buy this in WH Smiths or get it on the web. Results may vary between manufacturers. With WH Smith’s film I print using the ‘Glossy photopaper’, ‘high quality’, and ‘photo printing’ options. Print your design to the film using these settings and remember to use the rough side of the plastic sheets. The rough side carries a coating designed to absorb the ink. Once you have printed your design, dry the film with a hairdryer holding the dryer about 45 cms away. Don’t let the film get too hot – it will curl and distort. Drying takes about 2 minutes. Return the film to the printer and overprint your design and then dry it again with the hairdryer. Remarkably, with my Canon printer, the two prints line up perfectly. If you hold up the film to bright light after the first printing it will appear slightly transparent, but after the second printing, it should be jet black. Tiny details can be reproduced and I have produced boards with tracks only a fraction of a millimeter wide.

A caution, the OHP prints may appear totally dry but actually they are slightly hygroscopic and will forever by slightly sticky to the touch. They can be stored for a few months but eventually blur as the ink diffuses through the paper’s coating. I haven’t found this to be a problem – you can always print another mask.

Mask printed on a photo-printer.

Exposing the printed circuit board

This step requires some experimentation. However, it will not take long to establish for exactly how long the board needs to be exposed. I have a homemade UV light box. It consists of two Sylvania blacklight 350 tubes mounted in a wooden storage box bought from Homebase. It has a 5mm glass platen and a switch. I used some aluminiumized radiator reflector material to make a reflector for the tubes. The whole thing cost about £20 to make. When in use, it is covered with sheet of card and a black cloth. NEVER let any UV light enter your eye! If I were making the box today, I would probably use half a dozen 3W UV LEDs.

To expose the board I attach the mask to the copper surface using a tab of sellotape and place it on the platen. I put a heavy book on the board to make sure there is a good contact between the mask and the board. I cover the whole thing with a black cloth and then switch the tubes on. For me, the exposure times are about one minute and fifteen seconds. To establish the best exposure times you need to make a test strip – i.e. expose a piece of board through a photo-mask while using a piece of card to block the light from a portion of the board. Move the card along and expose it again etc. I used intervals of 15 seconds for this and then, when I had checked out how well this had worked by developing the board, I made a second strip using 5 second intervals. Exposure time is crucial and I have found that you need to be within 10% of the correct time to get a perfect board. My lights may be a little bright or too close to the board because ideally one might like exposures to be around five minutes. However, it works perfectly for me so I’ll stick with it.

My DIY UV light-box – you really don’t need anything more complicated!

Once the board has been exposed, you will be able to see your design on the photoboard’s surface.

Developing the PCB

Probably the wise thing to do is to buy some purpose made PCB developer from a supplier like Maplin. I made my own. It consists of 2.5 grams of sodium metasilicate (anhydrous) and 5 grams of sodium hydroxide in 375 ml of water (see below for a link to the origins of this formulation). Both these chemicals are dangerous and need to be handled with care. Add the sodium metasilicate slowly to the water and protect yourself appropriately from splashes. Then add the sodium hydroxide again taking great care. Shake until everything has dissolved. The developer keeps for a few weeks. If in doubt, make it up fresh every time.

To develop the board, immerse it in the developer with the design side uppermost. I use a discarded plastic tray from supermarket food packaging for this. Gently rock the dish. After about a minute you will see the tracks remain green and the exposed areas floating away in swirls of black. When the design is fully developed, remove the board from the developer using plastic tongs and place it a tray full of water. Rinse it off and then rinse it some more under a tap. A light stroke with some wet tissue will remove any remaining etch resist from the parts of the design exposed to light.

Top – Board in process of development. Bottom – Board after development and washing.

 

 

Etching the board

There are two etchants in common use – ferric chloride and ammonium persulphate. Both are nasty chemicals but ferric chloride is the nastiest! Be very careful using it. You can buy ferric chloride already made up and I suggest that you do. You can also buy it as granules; cheaper but nastier. Most people use a solution at about 40% concentration. The solution takes about 30 minutes to etch a board at room temperature. It will corrode everything in sight, burn you, and splashes will stain and rot your clothing and also anything else they touch! You have been warned.  Ammonium persulphate is cleaner to use and less nasty. A 20 – 25% weight to volume solution works best at about 40 oC. You can keep the solution at this temperature by placing the tray you use for etching in a water-bath.  Etching time will be about 15 minutes at this temperature. With both ferric chloride and ammonium persulphate you should rock the bath to keep the etchant moving over the surface of the board.

When all the copper has been removed from the spaces between the traces you can remove the board and wash it with copious amounts of water. When dry, you can remove the etch resist from the tracks either by exposing the whole board to UV, developing it again, washing and drying it, or by rubbing it with propyl alcohol. Actually, on the boards I have used the resist can stay there and you can solder through it.

Finished boards ready for drilling and cutting out

Drilling the board

I use small diameter tungsten carbide drills and my Dremel for drilling the pads.  You can do it without a drill press but it is a bit fiddly. The most important thing is that your board has nice holes etched in each and every pad. If it doesn’t, it is hell’s own job to get the drill to go through the pad without skidding all over the place!

 

Useful links

A rapid etching method using ferric chloride – looks amazing, haven’t tried it:

http://www.instructables.com/id/Sponge-Ferric-Chloride-Method-Etch-Circuit-Bo/

A different etchant; copper chloride and hydrochloric acid:

http://www.instructables.com/id/Stop-using-Ferric-Chloride-etchant!–A-better-etc/

Using ammonium persulphate:

http://www.youtube.com/watch?v=TLf4w1zTpkw

A survey of methods using an inkjet printer to print directly(!) to a pcb – haven’t tried it but looks fantastic:

http://www.pabr.org/pcbprt/pcbprt.en.html

Someone using the same technique described here with excellent results:

http://projects.dimension-x.net/archives/77

Some further info for those wishing to make their own developer solutions:

http://forum.allaboutcircuits.com/showthread.php?t=16675

 

 

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About petermobbs

Inveterate meddler.
This entry was posted in electronics, Photography and electronics and tagged , , , , , , , , , . Bookmark the permalink.

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