This is the second post in a series of posts I am planing to write about making printed circuit boards at home. The first post explains all the things you will need to make a PCB at home using the UV exposure method.
The purpose of the UV exposure is to get the PCB layout image onto the Copper board. The pre-sensitized board we are using has a thin layer of photoresist coated on top of the Copper layer. When exposed to light (near the UV region) photoresist becomes soluble by a positive photoresist developer. If unexposed, photoresist becomes insoluble to photoresist developer and forms a strong bond with the Cu layer.
As you will see in the steps below this property of the photoresist is used to put an etch-resistant image of the PCB layout onto the Cu board.
So let’s begin!
Printing the layout onto a transparency sheet
For a double sided PCB, either the top side or the bottom side needs to be mirrored when printing. When the PCB is exposed the layout will get mirrored again. Since most of my components are on the top side I will mirror that side so when the board is developed the components will go on the top side. Otherwise some components, where orientation matter, will have to go on the bottom side.
Tip: The layout editor has a setting where you can mirror the layout when printing. Use the transparency setting on the printer for better results.
Step 1: Print a mirrored image of the top side layout onto a transparency sheet.
Chances are, if you are using an inkjet printer like I am, when you print your layout on the transparency the traces and component pads won’t be completely opaque. So I usually print two copies and align one on top of the other to increase the opacity.
Step 2: Print another mirrored copy of the top side and align it with the previous copy. Use pieces of clear tape at the edges to hold the two transparencies together
Step 3: Print two copies (not mirrored) of the bottom layout on transparencies and align them as in Step 2.
Step 4: Carefully align the top side on top of the bottom side (with the printed sides facing each other) by hand such that the component pads on the top side perfectly lines up with the corresponding component pads on the bottom side. This is possible only if one side is mirrored as in Step 1.
Step 5: Secure the aligned top and bottom transparencies with a piece of clear tape on one side such that the board can be slid in between the two transparencies making a sandwich.
Preparing the pre-sensitized board
Now is a good time to remove the pre-sensitized board from the wrapper (but don’t remove the white cover yet). The white cover sheet on the board is there to prevent accidental exposure. Chances are the board you got is not the correct size for the PCB you want to build. The 150 mm x 250 mm board I already had at home is a bit larger for my 100 mm x 160 mm layout. So I want to break a piece off the board to make it close to the size I want. That way I can use the extra piece for another PCB.
If you don’t need to break a piece off your board then you can skip to Step 9. To break a piece off the board,
Step 6: Draw a line across the board on both sides where you want to break it.
Step 7: Use a knife and a straight edge to make a groove (about half a mm) along the line on both sides of the board.
Step 8: Use a table edge to bend the board along the groove until it snaps clean.
Setting up the exposure table
Since I didn’t want to spend money on an expensive exposure kit, I found a way to hack a couch table to do the exposure. The table has a sheet of glass in the center where I can place the transparencies and the board. This allows me to expose both sides of the board at the same time.
The exposure table should be setup on a relatively dark room with no direct sunlight or a light source. It doesn’t have to be pitch black (basement is perfect!). But darker the better. If it is too dark to see anything you can use a red light source. Photoresist is only sensitive to wavelengths near the UV region.
Step 9: Place two desk lamps (with fluorescent light bulbs) on either side of the glass sheet facing each other as in the image above. The light bulbs should be 5″ to 6″ away from the glass surface.
Tip: To turn on both lamps at the same time, connect them to a power bar with the switches turned on and then plug the power bar to the wall.
Step 10: Place the transparency stack assembled in Step 5 on the glass sheet centered between the lamps, and secure with clear tape on one side such that you can slide the board in between the top and bottom transparencies.
Exposing the board
Steps in this section are time sensitive, and they must be done in one sitting. The exposure time depends on the power of your lamps and how far away they are from the board during exposure. I am using 13 W bulbs 6″ away from the board and a 8 minute exposure time is enough. I have found previously that a 10 minute exposure is a bit too much for my setup.
You may also want to gather the following items beforehand and keep them in a easy to find location,
- A second sheet of glass (I took mine from a photoframe)
- Positive photoresist developer
- Plastic container large enough to place your board
- Tissue paper or soft cloth
- Safety gloves and safety glasses
Before you begin, ensure both lamps are off and the room is relatively dark. You can use a red light source.
Step 11: Carefully remove the white protective covering from both sides of the pre-sensitized board taking care not to touch the board surface (hold from the edges).
Step 12: Slide the board in between the top and bottom transparencies and ensure the complete layout falls within the board edges.
Step 13: Place the second sheet of glass on top of the transparency-board sandwich and apply weight to press it down as in the image below.
Step 14: Turn both lamps on at the same time and set the timer for 8 minutes.
Meanwhile, prepare the developer solution.
Warning: Positive developer is corrosive. Wear safety gloves and safety glasses when handling chemicals.
Step 15: Add 10 parts water to 1 part positive developer solution to the plastic container. Ensure you have enough depth so when you put the board inside, the solution will completely cover it.
Step 16: When 8 minute exposure is done, turn off both lamps. Carefully remove board from the setup and place in the plastic container with the developer solution.
Step 17: Tilt the container back and forth to move the solution around the board. After a few seconds you will start to see the image of the layout appear (like magic!). Keep agitating the solution until all the excess photoresist is removed from the board and only the image of the layout remains. The board should be in the solution no more than 30-40 seconds. If you keep the board in the solution for too long it will start eating away at the layout image as well.
Step 18: Once all the excess photoresist is dissolved, remove the board and rinse with plenty of cold running tap water.
Step 19: Dry the board with tissue paper or a piece of soft cloth.
So the UV exposure and development of Music and Lights board turned out not too bad. However, if you notice closely, the left side of the top layout got a bit over exposed. This happens because the glass sheet on top of the board wasn’t pressing hard on this side of the board. When the transparency is not pressing tightly on to the board during exposure, light could seep through.
This is bad because the faint traces on the left side will not be able to completely protect the Cu during the etching process. Fortunately, it can be fixed by going over the traces and pads with a ultra fine tip sharpie (or permanent marker).
Finally, the board is ready for etching! Etching is easier than the UV exposure part but it takes a bit longer. So I will leave that to the next post. Stay tuned and Thanks for reading!