My First Circuit Board Design - Will this print out well for a transfer?

[Steve Peart]

Hello All,

I am putting together a circuit board for a project that will plug into my Raspberry Pi with a ribbon cable to provide motor control for 3 separate motors using a hobby battery that's 7.4v 1900maH.

I have gone through 2 other iterations, attempting to build it first with all jumper wires (fail), then I used a blank protoboard with copper tape circuit lines (worked ok but crossed a wire and fried my RPi)

Now I am attempting to build the circuit using the Copper Board with the Ferric Chloride solution, transferring the design to the board with a printout on tranparency paper, and ironing it to the board before immersing it in the chemical solution.

I have no clue how to use the PCB software that's out there, but I am ok with Adobe Illustrator, so I decided to design it there.

Here's the resulting design, I have put in a legend to show what is what on the board, knowing I will have to print it out all black before transferring it to the PCB:



Couple of questions:
1. When I print this out, will it transfer to the board well and also come out with clear line separation after the chemical bath? I am worried about the lines being so close together.

2. In my last board iteration, I failed to implement circuit protections which is likely how I fried my Raspberry Pi. I looked up how people do this and came up with the Zener Diode + Resistor method, so the lines can't spike above the 3.3v Zener as it will ground any extra voltage. The resistor will limit the current going through the line so as to not allow a current spike to destroy things as well. Is this sufficient?

Don't hesitate to ask for more info if you want, I really want to get this right before committing it to the PCB with the solution. The actual size of this circuit is roughly 2in width X 1.5in height.

Any help is appreciated!

Thanks,
Steve

 

[(*steve*)]

Since you're making your own board, you need to consider that you'll be drilling holes after you've etched it.

With small pads, this can easily result in the pad being ripped from the board. So the first advice is to make the pads significantly larger. Also due to this, it is better to have a trace go past a pad with a line going to it, rather than through the pad. You'll have half the work if the pad gets ripped off.

However, once you've done this, passing connections between the pads will become more difficult (and possibly not what I would consider doing for my first board.

Toner transfer is an inexact science. I would recommend that you use wider traces, and keep them separated a little more. Any movement of the paper whilst you're ironing it on can smudge the tracks. Too much pressure can cause the traces to widen. Wider and more spaced traces make this less of a concern.

I would also recommend against sharp right angle turns. Make it a pair of 45 degree turns.

Five parallel traces between rows of pins with what appears to be 5 mm spacing is extremely ambitious, as is a pair of conductors between pins with 2.54mm spacing.

In several places, you could have routed traces along alternative paths so as to have better clearance and/or improved the clearance for other conductors.

If there are places where you need a fine trace, or where traces need to pass very close together, try to keep this portion of the trace small -- where there is room, widen and separate the traces. This will reduce the places where you will have trouble or need to do manual fixes before or after etching.

Wherever you have a pad which needs to be drilled, etch a small pinprick in the centre of the pad. This will help centre the drill bit.

As for the resistor/zener arrangement, please draw the circuit for us. One thing you need to be aware of in low voltage zeners is that they have a very soft knee and will start to conduct well below 3.3V. With a high value resistor you can easily end up with the signal being clipped around 2V. You may be better off using a pair of schottky diodes connected to the supply (3.3V) and ground rails.

 

[Steve Peart]

For the protection parts of the circuit, I was referencing this: http://www.thebox.myzen.co.uk/Raspberry/Breakout.html#id8

That's where I found the circuit diagram for it. It's primarily used for GPIO pins that you are using as inputs, but the protection is supposed to work both ways.

This is the datasheet for the L298N, for reference: http://www.st.com/web/en/resource/technical/document/datasheet/CD00000240.pdf

All of the traces routed between pins were simply to avoid jumper traces and I wasn't able to find ways around that to get all of the traces to where they needed to go. I can revisit this though, and see if i can space them out more and make the pads larger. I printed it out for reference in fitting the components in the small space, but had no idea if the thin, close-together traces would be ok.

I'll attempt some re-routing, to see if i can widen the traces, and maybe stop being afraid of the jumpers that might be needed, to ensure that the transfer to the copper wont get smudged. Though I imagine if i Iron on the transfer and it gets messed up, I could clean it off with some alcohol and try again?

 

[(*steve*)]

OK, the resistor/zener should be safe -- as it says "for moderate abuse".

Try the transfer, and if you get it looking OK, try to etch it. Then try to drill it. This will give you an idea of how much trouble you're in.

It's not unusual to go through a couple of iterations before you're happy -- especially on your first board!

 

[BobK]

What is the width of your traces and the minimal spacing. I would not go for less then 15/1000 in (or 0.4 mm) on either of these for a first attempt.

Bob

 

[Steve Peart]

I have created a newer design based on @(*steve*) 's suggestions. @BobK : The traces are now 0.66mm thick. Is this better? I had to make jumps on the board to make this all route ok, but that's fine. Let me know what you guys think!

 

[(*steve*)]

It looks a lot better.

you may be able to get rid of the kink in the top left corner, and be careful about how the line segments job.

other than that, it's probably worth a go.

 

[Steve Peart]

@(*steve*) what do you mean by "be careful about how the line segments job"? Never heard "job" used as a verb before

 

[(*steve*)]

Aaagh, "join". We've been trapped by auto-corrected text!

To explain more fully now I am using a real device, the line segments you have drawn do not fully join at the corners. This will likely lead to problems similar (and maybe worse) than having right angle joins.

It may be worth trying to find a proper tool to draw the layout.

 

[Shlomo Schwarz]

Nice job, looks good. I also had some diffculties with PCB design software. But then tried Eagle . It's so easy to use and saves you so much time compared to Adobe Illustrator. There is a good community to help you get started and the tutorials help a lot. Besides the professional version there is a freeware to get started.

 

[Arouse1973]

I have created a newer design based on @(*steve*) 's suggestions. @BobK : The traces are now 0.66mm thick. Is this better? I had to make jumps on the board to make this all route ok, but that's fine. Let me know what you guys think!

View attachment 22811

That's a good effort. You have some acid traps that might be ok but it's not good practice to have them. Look at the green tracks where the stubs join. They are better to have 90 degrees connections. Sorry if it's too late.
Adam