PCB Circuit Design - First time

Discussion in 'General Electronics Chat' started by deefactorial, Jul 18, 2008.

  1. deefactorial

    Thread Starter Active Member

    Jun 11, 2008
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    I have created a Schematic diagram of the circuit I wish to design from the schematic diagrams of the Explorer 16 Microchip test Circuit board. I converted the schematic to the PCB application and designed the PCB board. I have put a lot of work into this circuit board and I was wondering if someone could take a look at my schematic diagram and my PCB design to see if there are any mistakes or things that could be improved. I am very new to this and this is my first circuit board.

    I have attached a picture of the schematic diagram, a picture of the finished PCB and the Gerber files. Please have a look, Thank you.
     
  2. SgtWookie

    Expert

    Jul 17, 2007
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    Not bad, particularly for a first time :)

    I noticed that there are a number of traces that go at odd angles for no apparent reason. It helps to keep your angles at 45° or 90°, particularly when making revisions later.

    Some of these odd-angled traces have "squiggles" in them. These squiggles will add inductance.

    You might consider increasing the keep-away or isolate or whatever your copper pour syntax is for increasing the distance between the copper pour and the pads; otherwise it may be difficult to keep from shorting the pads to the ground plane when soldering.

    Sprinkle 0.1uF capacitors around to go between V+ and the ground plane. It's hard to get too much decoupling for supply noise. It's very easy to have too little; and too little = big problem. You are better off having extra pads and vias for decoupling caps and not using them, than not having the pads and needing them.

    I really don't have time to analyze your entire board, unfortunately.
     
  3. deefactorial

    Thread Starter Active Member

    Jun 11, 2008
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    Thats ok, I don't expect you to look over the entire board.

    when you say odd angles do you mean when I go through a via and then go in another direction. I also found that I had to take slight off angles so that there was enough space around the vias so that they would not get too close to another signal line. I tried to make sure that none of the angles were less than 90° unless they went through a via. What do you mean by squiggle inductance ?

    How do you increase the keep-away or isolate size in the PCB application ?

    the power is supplied by the CN1 ezLCD connector It has a 3.3v out @ 0.5 amps. There are about 10 X 0.1uF capacitors connecting the V+ to the ground plane located all around the board. I was contemplating connecting the top and bottom ground plane to each other in some spots because the ground plane has to go trough some really tight spaces in certain spots.

    Thanks for taking a look at the board. It is really important to me that I get this right.
     
  4. SgtWookie

    Expert

    Jul 17, 2007
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    No, I mean angles that aren't evenly divisible by 45. A 90° angle is fine, a 135° angle is fine. But you have a number of areas that aren't "square" to the edges of the board. See the attached image; a couple of those areas are outlined in blue. Notice all the jagged points on the traces that aren't square with the board? That'll come through in the finished board. There are some more traces on the other side of the board that are not square as well. Perhaps you had the "snap to grid" function turned off. That will lead to problems that quickly get compounded. I always have my grid on, and usually have it set to 0.1", 0.05", or if I'm routing pretty tightly, 0.025". It helps a lot to keep the increments evenly divisible.

    OK, try to move things around a bit so that you can make the angles 135° or 90°, and keep them square to the board edges.

    Take another look at the attached image; there is a squiggle circled in green. At high frequencies that is an inductor, and can cause strange things to happen. There are more squiggles in the middle of the reverse side of the board.

    I can't tell you that; I'm only familiar with the Eagle Layout Editor v4.16. I don't know what schematic capture or PCB routing software you used; you haven't mentioned it.

    Yes, tie your ground planes together at regular intervals. Consider putting a couple of low-ESR capacitors at your power connector, one 10uF and one 220uF, rated for 10v or higher. This will help keep your bypass capacitors charged, and will help compensate for inductance in your power connection harness.
     
    Last edited: Jul 18, 2008
  5. SgtWookie

    Expert

    Jul 17, 2007
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    There also seems to be a lot of "white space" on your board; expanses where there aren't any components.

    If you could put things closer together, it would help to reduce parasitic capacitance and inductance for those long runs. It would also save you money, particularly if you will be buying the boards in quantity.

    Most of the cost for getting boards produced is the set-up fees; it takes time to get everything set up, and they have to pay someone to do it. Making copies of the same thing is very inexpensive. However, if the board is a lot larger than it needs to be, those costs will add up.

    Sometimes, extra "white space" is necessary, such as when you're using a driver IC. They frequently require a large area to dissipate heat. But unless you really need it, omit it.

    Really, you did a VERY nice job, particularly since it's your very first effort.

    Your first looks a heck of a lot better than my first effort did. I'm not kidding!

    I wound up completely trashing mine and starting over. Yours just needs some minor "tweaks". You should be quite pleased that you've done so well. Your hard work shows.
     
  6. deefactorial

    Thread Starter Active Member

    Jun 11, 2008
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    The board size is fixed. I have those mounting holes in the four corners where the board bolts onto the ezLCD; it also lines up with the ezLCD connector. The bottom of the board lines up flush with LCD panel so I can design a case that will line up with the serial ports. Otherwise of course I would have designed the traces that much more tighter with very minimal board space. But since space wasn't an issue I decided that I would just spread everything out. Next time I do some editing/design a board I'll put everything as close as I can get away with.

    I straightened all the traces and now I'm just looking for a footprint for the surface mount 220uF capacitor to add to the 3.3v power supply. I was also intending on adding an option to power the board independently with two LM1117 chips. That way I could use the board without the LCD for other projects. It can't hurt to have extra pads for components that could be useful. I was also wondering if the 5v power supply should have a large capacitor attached to it too. I'm only using the 5v power supply for one of the serial ports. I have two jumpers attached to it so I can chose between 3.3v or 5v for that serial port. the 5v power supply has 2amps available on it from the ezLCD connector.

    Thank you for your help it has been a invaluable resource.
     
  7. Wendy

    Moderator

    Mar 24, 2008
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    What software did you use?
     
  8. deefactorial

    Thread Starter Active Member

    Jun 11, 2008
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    gschem, PCB, gsch2pcb, footgen, vi, bash, ubuntu
     
  9. Wendy

    Moderator

    Mar 24, 2008
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    Thanks. What I could look at (.png files) looked pretty good.
     
  10. SgtWookie

    Expert

    Jul 17, 2007
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    That looks a lot better!

    Just a few more tweaks on the front, and one on the rear.

    Are you going to do a copper pour on the top layer?
     
  11. deefactorial

    Thread Starter Active Member

    Jun 11, 2008
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    Yes I am going to pour the front layer. The PCB application has a bug in it where I was getting segmentation faults and it wouldn't draw the poly plane accurately. I had to recompile it from source and now it works.

    Although for small projects the PCB application is sufficient. I do feel that I am reaching the limits of this application though.

    Ok I think that I have straightened out all the lines now. I'll double check them again. Next time I'm not even going to think about moving lines off angles.

    I created a footprint for the 220uF capacitor and added a 1206 for 10uF capacitor, I found a Aluminum Electrolytic Surface mount Capacitor. I was wondering if it was difficult to hand solder surface mount capacitors. Would it make my life easier if I extended the footprint out a mm or two.

    Thank you for all the pointers. It really helps to have someone point out all the imperfections. And now I know what to look for.
     
  12. SgtWookie

    Expert

    Jul 17, 2007
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    It's looking pretty darn good. ;) Spotted a couple minor things on the back near the uC; see attached.

    On my first board, I made the mistake of setting the grid resolution to 0.001" and didn't stick to the angles. Boy, what a mess I made of it! I might as well have been using a crayon freehand. :rolleyes: I didn't understand the copper pour feature, either - so I laid that out by hand, using a zillion rectangles. It looked like the losing side in a war zone!

    Double-check all of the library layouts you used against the manufacturer's recommended layouts before you commit the design for manufacture - or you may have some very nasty (and expensive) surprises coming your way. It takes time to check it beforehand. It takes more time and more money to fix it after the fact. There are less expensive ways to replenish your supply of coasters than having faulty PCB's made.
     
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