As expected, there were many things that went wrong in the process from the prototype to the first PCB version of the project. I would like to share them here, maybe it will prevent similar mistakes in one of your projects.
While drawing the circuit diagram, I copied all values from the components into this document. But later, I experimented with the prototype and changed components – but forgot to update the values in the circuit diagram.
When I soldered the components onto the PCB, I just read the values from the diagram and did not cross check them with the prototype. I soldered the wrong capacitor on the board and it took a while to find the problem. It was hard to remove the wrong component and replace it with the correct one.
How to fix later: If the shape of the components match, you can easily unsolder the wrong component and use the correct one. But often you have to destroy the wrong component to be able to unsolder it.
How to avoid: This is very easy to avoid and there are two safe strategies. First I keep a sheet with the printed circuit diagram beside the prototype. So I can easily note any changes I do and update the diagram later. Second, before I order a new PCB, I check all values in the diagram with the prototype.
Not Connected Copper Pour
I created a copper pour on both sides of the PCB, but somehow I did not connect the pour on the bottom side with
GND. This is not easily visible, because usually while drawing routes you keep your board free from the pours and just add them for the final touches.
How to fix later: You can not fix this entirely, but you can prevent the worst. Just scratch off the solder mask from the largest pours and connect them to ground with a small wire.
How to avoid: If you do the final checks, check each layer individually. So you will spot this problem immediately.
Not Accessible Solder Pads
I had this brilliant idea to place the potentiometers on the bottom side of the board to keep them out of the way. But I never spent any attention to the space around the solder pads on the top side of the board. It was almost impossible to solder the potentiometers into place, without melting the components on the top side of the board.
How to fix later: Maybe it is possible to solder this components first. This was not possible in this case, because the potentiometers on the bottom side cover solder points from the top components.
How to avoid: You should always think about the dimensions of the components on the board. Imagine or construct a 3D version of the board to check if there are any such problems.
When I create circuit diagrams, I often create own symbols. So I have exact the look I like to have in the diagram. The disadvantage is, I have to be very careful to make sure the packages are correctly connected to the pins of the symbols.
Here I used a own symbol for the potentiometer I used in a previous project. The package had only two pins connected which was perfect for the previous project, but not for this one. Even the circuit diagram was perfect, on the PCB one pin was not connected. I did not found this problem while my final checks of the board.
How to fix later: At least missing connections are easy to fix, just use some wire to connect the two pads or points.
How to avoid: I am not sure if something like this can be easily avoided. I checked the PCB design thoughtfully, but did not spot this problem.
Not Enough Space for Knobs
I planed to add two nice knobs to the two potentiometer to control the volume and speed, but I forgot to plan enough space for them. As engineer I usually working with 3D software to design the whole device, then thinking about electronics and where to place them. Because the thought this is still a prototype, it seems it turned off my common sense. 😅
How to fix later: There is not always a fix, but in this case I will just unsolder them and use some wires to connect the potentiometer further apart. So I even can mount them horizontally, which was my original plan.
How to avoid: First design the device and its case, them the electronics, adjust the case, then adjust the electronics, repeat until everything fits into place.
Wrong Pin Assignment
A fatal mistake happened with the MOSFETs on the board. I created a own symbol for the
2N7000 MOSFETs for the circuit diagram, because I did not like the way how the connections and labels were designed on the ones I found in the Eagle library. For the package I copied one from a similar component – and here I did a fatal mistake.
The package was called
TO92-3L, I just saw
TO92 which is the package I use for the MOSFETs and ignored the
3L at the end. Assuming it is just some graphical variant for the package. I checked the package and the solder pads and everything looked as expected, so I never even thought there could be something wrong.
After soldering all 25 MOSFETs into place and started the first tests, the MOSFETs seemed to act strange. If there is a nice guide printed on the PCB, you would never suspect they are just soldered in the wrong orientation into the board, wouldn’t you?
But it turned out to be exact the problem. The suffix
3L for this package meant, pin 3 left which seems to be a quite unusual. The
2N7000 has the usual pin numbering.
It is already difficult to unsolder components with two pins, it is simple not possible to unsolder the MOSFETs without special tools. So, back to start, I will do all mechanical tests I can with the failed board and start using fresh components on a new board.
How to fix later: I think there is no easy fix for this.
How to avoid: Just stopping to do any assumptions. From now on I will check each pin of each package manually, just to make sure they not only look as expected but also are numbered as expected.
It is interesting how many mistakes I could made on this single PCB and I am glad I started with a simple prototype PCB. The used components are really cheap, especially in larger quantities. I also used sockets to the chips, so I can simple reuse them. I came up with some additional points on my checklist:
- Check each value of the prototype and compare it with the circuit diagram, before you start building a board.
- Check the copper pour on both sides hiding all other layers to make sure they are connected correctly.
- Make a rough 3D model of the circuit board and check for not accessible solder pads.
- Check the solder pads of each component and compare them with the circuit diagram to make sure all pads are connected.
- Make a design of all operated parts of the device first to make sure the components have the required spacing for the knobs.
- Not only check copied packages visually, also check if the pins are numbered as expected.
I will continue working on this project and let you know, as soon there is a working PCB version.