How to Make a Flexible Printed Circuit Board

A few months ago I started developing my own bio-impedance measurement system and I planned to use a flexible printed circuit board as a sensor. Since I haven't used this type of board before, I started looking for a manufacturer specializing in flexible PCBs. I found that I can order one from PCBWay.com but to my surprise, the automatically calculated price was way too expensive (111.98$ for the board used in this instructable).

For a minute I thought that maybe I can make a flexible PCB by myself so I started to surf the internet on how to make a flexible PCB at home. I found that many people use a material called "Pyralux". Unfortunately, I couldn't find this material in my country and I didn't want to order it from China, since the delivery will take another 6 weeks. So the only way to go was to create something new from materials that are easily accessible to everybody. Ladies and gentlemen buckle up your seatbelts and get ready to jump into this instructable on how to make your own flexible printed circuit board!

Supplies

Laser printer
White A4 paper
Glossy magazine paper
Transparent sheet (200 microns thickness)
1200 grit sandpaper
Copper tape (50mm wide)
Kapton tape (50mm wide)
Paper tape
Scissors
Pencil
Cotton ear picks
Protective gloves
Plastic container
Microfiber towel
Acetone-free nail polish remover
Ferric chloride etchant
Nail polish remover with 75% acetone
Spray bottle with water

All these supplies you should be able to find easily at your local stores. For example, I got a transparent sheet at a local office supply store for only 0,29€ also I got 1 metter of copper tape at a local electronics component store for 1,30€. The most expensive material was a good quality Kapton tape which cost around 14,59€ at a local electronics component store. I recommend using only a good quality Kapton tape because the cheap one will deform from heat when you will solder.

Step 1: Creating Flexible Printed Circuit Board Layout

Before you start designing your PCB, it’s a good idea to make a schematic of your circuit. The schematic will serve as a blueprint for laying out the traces and placing the components on the PCB. Most of the time I work with Altium Designer, but when I'm in a rush, I use EasyEDA because of its simplicity. If you are new to printed circuit boards I highly suggest you to start with EasyEDA.

You can download EasyEDA for free!

Step 2: Printing Flexible Printed Circuit Board Layout

When you finally have a printed circuit board layout now it is time to print it. The most important thing in this step is to get a glossy magazine paper. You can easily find it at your local supermarket. While using this glossy paper the only thing that will transfer to your flexible printed circuit board is going to be the actual toner.

First, I printed the PCB layout on white A4 paper. Then I placed the glossy magazine paper on top of my print and secured it in place with paper tape. Some people might think that you can print straight on glossy paper and that is true, but I printed on white A4 paper first just to have a rough idea of where I should place my glossy magazine paper.

Step 3: Creating Flexible Board From Scratch

Now in this step, we are going to make an actual flexible printed circuit board material. When I was experimenting with different materials, at first I used only one layer of copper tape and one layer of Kapton tape. I ended up getting a really thin and weak flexible printed circuit board, so I decided to use the transparent sheet between those two layers to make the board a bit stronger.

So I started by cutting a transparent sheet with scissors. Keep in mind that the transparent sheet must be larger than your printed circuit board layout. Next, I sprayed the transparent sheet with water. This trick is undoubtedly a life-saver because water prevents the adhesive on Kapton tape from sticking to the surface of a transparent sheet. After spraying the transparent sheet with water it is time to apply a Kapton tape. Using a microfiber towel push all the water out and fix all imperfections. Let the board dry for a couple of minutes, so the Kapton tape will bond with the transparent sheet Don't forget to repeat this process on the other side of the transparent sheet!

The Next thing you will need to do is apply the copper tape to your board. While experimenting I found out that the easiest way to do it is by placing a piece of copper tape on your table (The copper side must be facing downwards to the table). Unfold the protective paper just a little bit on one end and secure it in place using paper tape. Repeat it on the other end. Remove the protective paper from the copper tape completely and spray the sticky side of the copper tape with water. Apply your board with a transparent sheet and Kapton tape on both sides to the copper tape and using a microfiber towel push all the water out and fix all imperfections. If the copper tape or Kapton tape is sticking out on the ends cut it with scissors.

Step 4: Toner Transfer

Toner transfer is a really important step because it will decide how good the result of the final product is going to be. You must be very diligent in this step. I used this video as a guideline, maybe it will help you too.

The first thing you will need to do is to make some marks where to cut your glossy paper with a printed circuit board layout. Remember your glossy paper with a PCB layout must be larger than your board, so you can easily fold your glossy paper around your board. When you are done cutting, place your board on top of the glossy paper. The copper side of the board must be facing downwards to the PCB layout. Then simply fold all the ends and secure them in place with paper tape. The most important thing here is that the glossy paper must be sitting flush with the board. This will guarantee success.

Now it is time to use some chemicals. When working with chemicals always use a plastic container and wear protective gloves! Place your wrapped board into the plastic container and pour a little bit of nail polish remover. I used Oversa nail polish remover which contains around 75% of acetone. You should be able to find a similar product at your local supermarket. Now when the paper will get wet using medium pressure rub the board with your fingers to successfully transfer the ink. When you notice that the paper is dried, that will be the sign that you are done. Remove the board from the plastic container and remove the paper tape with glossy magazine paper from your board.

Step 5: Etching

After successfully transferring the ink now it is time to etch your board. This process involves removing copper from the surface of the flexible PCB to reveal the desired circuit pattern. To remove the copper I use ferric chloride, but you can also use other etchants.

First, place your board into a clean plastic container. Secure the board in place using Kapton tape. It is important to secure the board with Kapton tape because when the flexible PCB will be getting close to being finished it can start to float. Then pour your etchant and wait until the flexible PCB will be etched out. This process should take around 25 minutes, but this time may vary due to using different etchants. After you are done etching, give your board a good rinse and clean your plastic container.

Step 6: Cleaning!

Now it is time to do some cleaning. Fill your container with fresh water and get 1200 grit sandpaper. Carefully sand down the black ink on traces. Then cut your board to the size you want you so will have less glue to remove. I used Oversa acetone-free nail polish remover and a cotton ear pick to remove the remaining glue on the board. Take your cotton ear pick and soak it into acetone-free nail remover. Then gently scrub the remaining glue. You have to be careful while removing the glue because you can accidentally remove a trace from its position.