Home-made UV exposure box for PCB prototyping

Earliest last year I wrote this  post on how to make printed circuit boards using the method of toner transfer. Since before then I have known that the method is not consistent in the results obtained. Sometime the toner does not stick evenly on the copper clad especially for bigger designs.

During the same period I made several PCB at the University of  Nairobi Electrical Engineering department and the boards were much more superior to the ones I had made before using the toner transfer method.

good pcb made using pre-sensitized copper clad  

double sided pcb exposure box 

controls for the pcb exposure box

The department has a double sided PCB exposure box that is proven to give very good results over the many years it has been in service. Problem is I can't access this machine at my  leisure and therefore I decided to make my own machine.

First I sourced for the UV bulbs that were available in Nairobi and then enlisted the help of a colleague to get the box designed. A press fit design was quickly put together and cut from left over mdf using a cnc router we have at the fablab and some pictures are shown below!

Cutting the parts on the Shopbot

Some of the cut parts ready for assembly

Filing the edges for a nice fit

The we put together the box by press fitting as shown in the pictures below.

Press fit Box for the top cover

The bottom was also cut and assembled in the same way. The UV bulbs were then fitted and the fun began. . . TESTING!

Putting the bulb holders in place

fitting the bulbs after fitting the reflective foil(kitchen foil works fine)

the complete assembly

fitting a paper cover for a quick and dirty testing session(Just couldn't wait to fix the plexi glass cover)

 After all this hard work, it was now time to test this baby! Having recently bought a can of UV curable solder mask paint I decided to see if it will cure in this UV light.
After several failed attempts mostly due to underexposure, the results were messy.

the messy results

At the point whre I almost gave up and almost  threw the box in the dustbin, I had some of the  solder mask cured on one of the samples and my hope was rekindled! With this burst of energy and enthusiasm I experimented with different timings for the exposure and got some impressive results

pcb with solder mask after curing and cleaning with acetone

first encouraging results using an actual pcb

At this point I could predict the results and my money did not go to waste after all.

 One more experiment though PCB exposure for etching!
This is the primary reason for this box and it had to be tried as well with the following great results!

pcb  after exposure and development

pcb after etching!

pcb with Solder mask applied to it!

 Now that the machine is useful, a Plexiglas cover will be perfect for the top. Using a laser cutter this was cut and fitted and now the box is complete at least for single sided boards.

For double sided exposures I will have to finish the top, but I am impressed with the results I have got already!

THE QUEST CONTINUES!

Making good printed circuit boards using toner transfer

Many a times electronics hobbyists find themselves at a point where they need to create decent circuits. They  also realize that using vero boards is tedious especially when making circuits with many parts in them. At this point a custom PCB comes in handy. It is not just easy to put together the circuit but also the assembly is pleasing to the eye. On the same breath, some components especially surface mount (SMD) are impossible to work with. This implies that for any enthusiast who wishes to experiment with such parts must learn to design and make their own printed circuit boards with whatever materials available within their reach.

Toner transfer is a good way of making PCBs without any specialised tools. You will need access to a laser jet printer. Toner present in these printers is the material that will cover the parts we want to preserve on the circuit board. The cheicals required are:

• Ferric chloride( the etchant ),
•  bicarbonate of soda,
• nail art(for repairing broken tracks)

The materials required are

• copper clad board 

This is a copper laminate board. If you are in Kenya you can buy these boards from shops along Luthuli avenue.

• Art paper

Art paper is a special paper that gets easily mashy when put in water, it also has a glossy look. This is the best paper for the job for two reasons: It doesn't absorb the toner and it is easily removed from the board during processing.

Tools and equipment required are 

• Iron box 
• hacksaw

Procedure

• Design your schematic with a schematic capture suite. Eagle is great for this job  but  suites are also available. Try whichever pleases you.

• Ensure that the tracks are thick enough. Narrow tracks easily get ripped off during the process and make the work untidy.
• Print the PCB design on the laserjet printer on glossy magazine paper or glossy art paper. NB: for bottom copper print the design as is, for top copper side mirror the image.
• Once the design is printed, cut a piece of the copper clad board that fits your pcb design using a hacksaw blade.

• Clean the copper side of the board with steel wool till it shines. Avoid touching the copper with your fingers. The oil in the hands usually prevents toner from sticking.
• Dry the board.
• Place the printed layout on the copper clad board toner facing the copper.
• Heat the iron to the highest temperature possible.
• Iron the layout(on the copper board) concentrating on the sides. You can place another paper on the design to prevent direct  heating

1. There is no rules as to how long ironing needs to be done. It is known through practice. So get ironing and if the design doesn't come out well, go back to the beginning.
2. When the toner sticks to the copper board well, allow them to cool down. Meanwhile prepare in a trough  a solution of bicarbonate of soda in cold water.  Soda softens the paper making it easy to remove

Soaking the board to soften the paper

• Now soak the board with the design stuck on it into the solution and allow it to sit for sometime. I have found ten minutes to be enough. Remove the paper softly if it is soft else give it some more time to soak.
• Remove the paper pulp that sometime remains between the tracks using a toothpick or a pin being careful not to rip up tracks.
• Repair broken tracks with the nail art solution or with a permanent marker with a fine tip.
• Voila! Almost done

Removing paper from the board

Remove the paper from the board (within tracks)using toothpick

Copper clad board with toner on it

Ferric Chloride solution

Ferric Chloride

• Make a concentrated solution of ferric chloride and put it in an open trough. Warm Ferric chloride etches the copper faster. This can be done in a hot water bath although it is not absolutely necessary.
• Rock the ferric chloride solution slowly side to side to ensure faster etching 
• Etch until all the exposed copper is all gone.

• Remove the board from the Solution and thoroughly wash it in lots of water. At this point rubber gloves will protect your hands from the corrosion and staining from ferric chloride. 

• Use steel wool to remove the toner from the board by scrubbing thoroughly. If there are holes for through hole components make them and HURRAY! YOU HAVE MADE YOUR FIRST PRINTED CIRCUIT BOARD

At this point you are familiar with the steps taken to have a circuit board made. It takes practice to perfect
this art. Keep Practicing.

A board I made using toner transfer for atmega2560