Tuesday, November 22, 2016

Double sided PCB tutorial

How to make double sided printed circuit board (PCB) at home


Homebrew PCB - not as hard as you might think. Ordering the PCB from a manufacturer might be easier but the disadvantage is the long waiting time, and in case you made a mistake in board design you need to repeat the process again.
For prototyping or for a one board spin, it is more practical and economical to make the boards at home. Following this tutorial you will be able to make a nice board in just a few minutes. The tutorial looks long because of the explanations, but once you get experience it's an easy task.

STEP 1: Printing circuit diagram

You will need a laser printer for this. The toner in the laser printer has plastic particles that will melt and stick to the copper clad. The sole purpose of the toner is to protect the copper below it from the ferric chloride. I use Brother HL-1210WE laser printer.

Printed electrical circuit diagram



The circuit diagram is created using DipTrace. I find it much easier to use than Eagle. You can print directly from DipTrace but i prefer exporting top, bottom and silk layers to .png files with 600 dpi density, and then arrange them on a A4 project to be printed on a single A4 paper. For this you need an image editor like Gimp, Inkscape, etc. I use Inkscape and it's free. The project in the image editor must have the same DPI as the exported images from the PCB design software. Also the printer must be set to print with the same DPI and everything will have the correct scale.
For aligning the layers i print the board dimensions (see image above) and use the lines at the corners to align the two layers on the light projector.
Only the top layers must be mirrored. I do this in DipTrace when exporting the image but it can be done in the image editor too.

The smallest trace in this project was 0.3 mm. The distance between traces is best to be higher than 0.5 mm to prevent solder bridges and toner smearing but you can lower it if necessary.
The paper must be a glossy one. Thin paper with only one glossy side is preferred because it will peel easier in water.
Do not touch the printed side and keep it clean without dust for a better toner transfer on copper.
In the image editor place multiple copies of the PCB in case it won't work the first time you will have another to try without printing another paper.

STEP 2: Toner transfer on copper clad

You need an iron, or a laminator if you have one.

A some sort of light projector for aligning the layers. I have made one consisting of a box made out of wood, 4 aluminum cans, a piece of glass on top from an old wall clock and a halogen light bulb inside. The shape of light bulb was chosen to prevent the light going directly to eyes.

Homemade light projector for aligning PCB

Cut the copper clad to necessary dimensions. I have used a cutter and a ruler and scored multiple times on both sides and then snap it. If the tip of the blade gets dull, just snap it with pliers on the line next to the tip with the rest of the blade retracted.



Prepare the copper clad using a sandpaper sponge and circular motions for the toner to stick better. After is nice and shiny, wash the board with soap and water. Rubbing alcohol or acetone, can leave residue on the copper and the toner won't stick properly. The board must be free of dust and finger grease for the toner to stick. Feel the edges of the board with your fingers to be sure that are not raised from when you cut the board. Insist with sandpaper more on the edges.
Aligning top and bottom layers: cut the top layer shorter in height than the bottom layer like in the picture bellow. Have a piece of scotch tape prepared and use the corner lines to align the two layers and then stick them together using scotch tape. If you have heat resistant Kapton tape, use this instead because it will not melt on the iron.

Aligning top and bottom PCB layers

Place the board in the middle using the dimension lines to align it and then place the iron on top gently not to move the PCB.
For ironing find a solid surface that can take a bit of heat. If you use the floor and you have thermal isolation beneath, use some thick wood board to prevent melting the floor isolation. Set the iron to maximum temperature. The laser toner melts at around 200 degrees Celsius. I iron it for 8 minutes flipping the board two, three times and also rotating it. The pressure on the iron must be high but without breaking the iron. If the iron surface has holes in it for steams it will not make a good contact with the board and some areas might not stick, so it is better to flip the board once in awhile. A sheet of regular printer paper can be used on top to keep the scotch tape away from the iron.
A board in the water. After the ironing is complete put the board in a soapy water.


Peeling the paper. Wait a few minutes for the paper to soften (5, 10 minutes) and then gently peel the first paper layer. Then rub with the fingers the remaining paper immersing it in the water often. If the toner is fixed properly it will not come off. Using a brush like a toothbrush might remove the toner. Be sure to remove all the bits of paper from copper area or there will be a possibly short circuit. I do this for both sides then i dry the board using towel paper. When the board is dry rub it gently with a finger to ensure there is no bits of paper left on the copper.

Toner transfer on copper clad
Copper clad after toner transfer
On the edges i had an area where the toner didn't stick so i have used a marker to repair the traces.

Step 3: Etching time

For etching i use ferric chloride in a plastic container and another plastic container with water for rinsing the board.



Immerse the board in the ferric chloride and shake the container every 1 or 2 minutes to remove the dissolved copper layer on the surface of the board and speed the process. Some people use another container with hot water in which they put the container with ferric chloride to speed up the oxidation but i did it at room temperature and it took about 15 minutes. The time depends also on how much the acid was used because the more you use it the more will become weak. Carefully not to spill it because it stains stuff. The use of latex gloves is recommended.
Checking the board. After 5 minutes i pull out the board using tweezers (the plastic ones are better but metallic ones will do too) and check if the copper is gone. After 10 minutes i check the board every 2 minutes and in case the marker used to repair the traces is gone i clean the board in water and apply the marker again. Be careful not to leave the board to much in the solution or it will eat the traces after a while.

Copper clad after it has been etched
Copper clad after it has been etched

Step 4: Removing the toner

I use paint thinner and towel paper for removing the toner. Rub it multiple times and the shiny copper will show up.

Step 5: Drill the holes

I use 0.8 mm drill for most of the parts and a screw/drill machine. Ideally a PCB drill press is better but i don't have one now. Next i use a sandpaper sponge and towel paper to clean the board once again. 

Step 6: The silkscreen (Optional)

If you want you can add a silkscreen by printing the silk layer and use the toner transfer method described above. I have done this on the top side because it helps me to have some reference points when soldering. It shows better on areas with copper.





Step 7: Protecting the copper

To keep the copper nice and shiny and prevent oxidation, next you can apply some protective coat. I have used solid rosin crushed in a powder like and dissolved in isopropyl alcohol then using a small brush i have applied a coat on both sides. It's been a few days and the copper is still shiny. This helps the solder to stick as well. The board can be suspended using crocodile wire clips until it dries out. I let it sit for 2-3 hours to dry out because until then it is sticky.


Now the board is ready for soldering components.
Please share if you find this post useful.

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