Building PCBs - Part 2: Two sided PCBs

In my last post I used the toner transfer method to build a PCB. Sometimes the arrangement of nets of a layout get pretty complicated or impossible to figure out. Even auto-routing fails or results in arrangements hard to work with.

In this case it might be better to use a two sided layout: Whenever a net cannot reach its target you can use vias to switch to the other side and go on easily. For the H-bridge IC I use to run motors (see Control motors - Part 4: H-Bridge) I need a two sided PCB so let's see how this can be done using the toner transfer method.

Two sided PCB

First of all you need PDF-files of the mirrored layout of the top and bottom layer. They have to be printed to toner transfer paper. Afterwards I punched the 3mm holes at the edges of the layout which are needed to screw the board into a box. Now I put one layout on the copper and used a permanent marker to amplify the holes made before. After this you can remove the layout and you will see the positions for drilling those holes. The next step is to drill the holes.

Now you have two layouts printed to toner transfer paper and the board - all parts with holes at the same position. After cleaning the board the next step is to iron the layouts onto the board. Now I arrange the first paper using the holes: they have to match exactly. Next I put the iron on the stack for some seconds to make the paper stick on the copper. After this the board can be flipped to repeat the procedure for the second side using the remaining print. If both papers stick well you can flip the board several times to ensure each side is ironed as explained in my last post. The rest of the story is the same as for single sided PCBs: Peel off the media, etch the board and remove the residing toner.

The upper side of the board after toner transfer...

...and the lower side.
(the dark areas are minor defects I fixed using permanent marker)

After etching the board you can drill the holes needed for common elements or vias. Do not drill through the board. Sometimes the position of the layout is not as exact as it should be and drilling through could break nets of the second side. I drill from both sides to the middle of the board - similar to  digging a tunnel. If the hole does not match exactly it does not matter. The elements can be soldered even if the hole is displaced a little bit.

After etching and drilling you can see how exact your work was. I'm satisfied :-)

First note: If you prefer transparent film you can orientate the layout as shown in this post: http://www.instructables.com/id/Two-sided-PCB-using-toner-method/.

Second note: You might asked yourself which kind of vias I use. None! I build vias by soldering strand on both sides of the board. Before doing this I put the board on two stencils. This builds a distance which helps to ensure that both sides can be soldered well. After soldering the excrescent strand can be cut off.