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Wednesday, August 29, 2012

DYI PCB

Today I made my first homemade Printed Circuit Board (PCB).

In the past I was lucky enough I could get access to some lab to produce them for me and today, does it still make sense to etch your PCB at home?

You probably get a much better result by using one of the many available online services that can produce professional grade PCBs for a reasonable price.... but that might eventually work if you know what you are doing... which, sadly, it's not my case.

By that I mean that any circuit I design and then test on a breadboard, has good chances it will not work once transferred on a PCB.
Why?
The answer is really simple : lack of experience which you can build up with the most classical process called "trial and error".

Now, if for every try and probable error I need to wait weeks for the pcb to be delivered to me I would consider that sub-optimal, it would not provide me a really steep learning curve.

I remember some years ago I saw a great BC comic strip :
BC (or was it Thor?) is pushing a square stone with a hole in the middle and says something like : "I had a wonderful Idea! This thing is called wheel and it will change the world as we know it".
In the next panel he comes back with  a triangular stone and goes : "I made a great improvement to my invention" and the other guy : "Uh? Where's the improvement?".
The answer is "I eliminated one of the four bumps".

So, to set the stage, I am pretty much there now : bumps optimization.
Not much, I agree, but it's always a good start to know were you stand.

I usually build circuit prototypes with breadboards, but it 's becoming more and more cumbersone since most of the components are now available in SMD packages.
I know there are adapters, but in the end I figured it's easier to be able to etch a quick PCB and use it instead... would have been fun anyway, so I thought.


Before I dig in further more : it actually worked, better than I expected (maybe my expectations were pretty low).
The board in the picture is 4 x 2.5 cm, pretty small... those pads on the bottom are for smd 805 components.
This should give you an idea of the size of the traces.

This one is just a test, my very first test, so I am pretty sure every step, from design onwards can be improved and yes, I could have started with something "simpler", maybe with bigger traces etc, but the purpose of this one was really to see where's the limit.

I still need to apply solder mask, so the process is not over yet.

How this was achieved :

I designed the pcb using Eagle Cad.
Nice tool, I chose this one mainly because you can type commands instead of clicking icons, I know it might sound silly to you, but it helps me a lot since my eyes make it difficult for me to use icons.
I created my own library with the two packages I needed, in that way I was able to design the pads with the shape I thought best to ease the soldering afterwards.
Soldering those tiny chips (and mainly the LDO on the right) will be difficult, hopefully I will have a good solder mask soon, without that I would not even try.
I designed the pads a bit longer than the suggested measure, I probably should have done more than that.
The idea is that it should be used to drag the solder away after applying it.
I suppose I will need to use the solder wick to remove shorts between the pins, but that's still to be discovered (wow, isn't that fun? So many things to try out!)
  
The circuit itself is really simple, it's a RS485 driver with a 3.3V LDO voltage regulator, nothing fancy, it should replace the one I am using on a breadboard.
It is also a part of a bigger circuit I am designing to host a MSP430G2553 (TSSOP). 



Once the PCB design was done I "panelized" 6 circuits (they are tiny, makes sense to run a batch of 6 at least) and printed some tests on plain paper, checking also that the solder mask overlapped nicely to the pads once printed.
Another check I did was to place the two chips on the paper and verify the contacts were sitting nicely on the pads (I designed the footprints in Eagle, had to test it).

To transfer the design to the copper I found two options you are probably familiar with :
- Photo resit and UV  light
- Toner Transfer

I have the equipment for the UV thing, a built myself a nice bromograph (I am quite proud of it :) Simple but apparently reliable), with 4 8W UVA lamps with ballasts and nice reflecting white inside. 
Yeah, told you I was proud of it, no? :)
But finally opted for the toner transfer as I found in the web people very happy with the results.
I will still use the bromograph for the solder mask.


   
The panel is laser printed (mirrored) on the toner transfer sheet (I had A5 sheets, managed to cut one in half for this test, to reduce the wasted material).
After cleaning the copper surface (dish soap first to remove finger prints etc, then a metallic wool sponge -gently- and water) I placed the pcb on a tempered glass (you can get them really cheap at Ikea, just look for shelves) with a bit of bi-adhesive.


Using a glass there helps a lot, it's a smooth solid surface, thermally suitable for this job and helps a lot also in aligning the toner transfer sheet with the board, I left on purpose some black borders around, to easily overlap the two parts.
Once the board was placed on the glass, I cleaned it with isopropilic alcohol.


You cannot see it properly in the picture, because of the flash, but placing the glass against a light source it is possible to use the black border to center the sheet and than secure it with some tape.


I initially overlapped a piece of fabric as I feared the yellow paper might stick to the iron, then I realized it is made specifically to be ironed... so it does not stick, nor it burns.
Thanks to the tape and the bi-adhesive the paper did not move even if I feared it might once it was really hot.
I then realized I might have overheated it, especially in the center.
Since everything is stuck to the glass, it's quite easy to handle, I used some cold water to cool down everything before peeling off the toner transfer sheet... which proved to be quite an easy task.


As you can see, in the center there is still a bit of toner, it probably happened because I overheated that area or because of the bi-adhesive tape on the back which absorbed part of the pressure I applied.
Checking the copper side revealed further signs of overheat, the copper appeared "reddish", darker than in the other area and  the toner was not covering completely the copper in some points.


I initially thought that the two boards in the middle would have been useless, but turns out I was wrong.


 I then etched using the classic FeCl solution, if you never used it, that thing is really messy, better use protection gloves and to keep it away from any metallic thing you are not planning to corrode.
To speed up the process, while gently rocking the container with one hand (while not holding he camera), I heated the solution with a hot air gun.
Combined with the fact that I tried to optimize the design to reduce the amount of copper that needed to be removed, the process was quite fast, probably around 5 minutes.


 The board is then rinsed with a lot of cold water.
A this point you can remove the remaining toner, but, if you are not planning to go ahead with the next steps, it's probably better to leave it there at it somehow protects the copper from further oxidation.



In my case I need to first check the result and then proceed with the solder mask part.
Since I am not sure at all of the outcome of the solder mask process, I decided to split the board in two to have two chances to run through the process, I then cleaned one.


I used a regular cutter, which did the job... but was probably not the best solution.
Using an aluminium guide to ensure the cut was traight helped a lot, specially since it was secured with two clamps.
These things make the job safer and easier, they come in extremely handy and can be easily found in hardware stores.


cleaning the toner away revealed that even the middle board, which I thought was useless due to the overheating, was pretty good.
In fact the first picture on top is the middle board, probably the worse in the batch.
Ok, I know, it could have been better, but you should realize those traces are really tiny, I could not find any short or broken trace after inspecting the pcbs with a magnifier.
The worse point is the one shown in the circle below.
It's the overheated part of the middle board, not perfect, but not too bad either in the end.
Check the pad on the right (red rectangle), it's 0.4 by 0.9 millimeters, that should give you an idea of the sizes in the picture.


Overall, for being my first attempt I am quite satisfied with the result, now I need to work out the solder mask part and then... the hand soldering.
   


1 comment:

Alvin Smith said...

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