Avoid unintended capacitance, planes on a PCB

Discussion in 'General Electronics Chat' started by ke5nnt, Apr 26, 2011.

  1. ke5nnt

    Thread Starter Active Member

    Mar 1, 2009
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    I'm sure each designer is different and has their own standards, but I'm wondering, when designing a PCB, particularly a dual layer one, what is the best way to design ground and power planes while avoiding unintended capacitance between the layers? Is a ground plane more important than a power plane?
     
    Last edited: Apr 26, 2011
  2. tom66

    Senior Member

    May 9, 2009
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    A typical plane introduces 18pf capacitance per square inch. Even for moderately high speed designs, this should not be a problem.
     
  3. Wendy

    Moderator

    Mar 24, 2008
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    And unless there is something critical about the circuit that needs it most ground planes are unnecessary.

    Going to Dallas HamCon in June?
     
  4. John P

    AAC Fanatic!

    Oct 14, 2008
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    Now hold on a minute here--capacitance between ground and power is a good thing, and in fact 18pF per sqin is nowhere near enough, so we add filter caps by each IC. Having the ground plane makes sure that every "ground" connection has as low an impedance as possible to whatever point you define as "real ground", and likewise with a power plane, and of course we locate the filter caps so they have minuscule trace lengths to the ICs, and as short as possible to the planes, though once in a while we may connect power to a filter/IC via a small resistor. What we always strive to avoid is "ground bounce" where transients in a chip get carried outside the IC to make the ground conductors in that area show more or less than 0V. And if you have sensitive analog circuits, placing all your traces over a ground plane is the first easiest way to protect them from radiated interference. (It may still not be enough, but it's a start.)
     
  5. ke5nnt

    Thread Starter Active Member

    Mar 1, 2009
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    Ok, thank you all for your input on the subject. I guess I was generally more concerned that, if you have a 2 layer PCB made, and one side has a large ground plane, while the other has a large power plane... that essentially 2 sheets of copper separated by a dielectric material would in essence be a giant capacitor.

    I've been doing a lot of research on doing PCB design/layout, and looking at different software available for the task (there are a lot, with huge variances in opinion about each one). One thing seems to be fairly consistent though, which is "make sure you add planes". I like the idea of basically making the entire board surface a plane, then just etching the copper between traces so as not to have shorts, but I have yet to find a program where I can figure out how to do this easily. Oh well, thanks for the feedback.

    @Bill, probably not. I'm taking some time off in June to go to Indiana, then again in July to go to Michigan. I always work on the weekends, and seeings as how the ham cons are always typically fri/sat/sun... I always get the shaft in being able to make it. :(

    If you see a nice oscope though, let me know! lol
     
  6. tom66

    Senior Member

    May 9, 2009
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    You may as well add them for your home made PCBs though. They save etchant.

    I use ground and power on the same signal layers for 2-layer boards and on inner layers for 4-layer boards.
     
  7. Wendy

    Moderator

    Mar 24, 2008
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    Heh, the etchant I use is extremely cheap, and not worth saving. Quite the opposite, it continues to react chemically with the two ingredients and saving of this chemical is contraindicated. No matter which chemical you use the PCB stock is the expensive part, and single sided is much easier to work with than double sided, not to mention cheaper.

    Getting a clean transfer between computer to copper requires specialty tools, be it photographic or toner. This is where the money goes. It is a bad idea to base your layout on expense of something that runs well under a $1, better to make it exactly the way you want it.

    HamCon always has an abundance of test equipment for very good prices. I bought several variable power supplies last time for around $10 each, and there was a wide selection of oscilloscopes. I bought my new used on for $90, dual trace 60Mhz. I still have the antique if you want it, I'll probably have it for many years. Forget Sunday though, Friday afternoon and Saturday morning (early) is the best times. People start packing up and going home after that.
     
  8. SgtWookie

    Expert

    Jul 17, 2007
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    One item not discussed yet is the low inductance that ground and power planes present. A narrow trace can have a fair amount of inductance (around 10-15nH per 10mm), where a 2-dimensional plane of copper has, for practical purposes, zero inductance. This can be significant when dealing with high speed signals, particularly square waves. A theoretically perfect square wave implies unlimited bandwidth; inductance limits bandwidth.

    I like to use traces that are as wide and short as possible.
     
  9. ke5nnt

    Thread Starter Active Member

    Mar 1, 2009
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    Thanks for the feedback Sarge.
     
  10. tom66

    Senior Member

    May 9, 2009
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    Okay; while it may not matter to you, some use other methods... Also, it has uses in production as using a lot of etchant may cost more money and increase per unit cost. That's why you sometimes see planes on a PCB with no electrical connection.
     
  11. JMac3108

    Active Member

    Aug 16, 2010
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    As John P. said ... you want capacitance between power and ground. Remember those bypass capacitors between power and ground that are all over your board?

    In very high speed design, the inductance of traces, vias and even the connections on SMD components can make bypass capacitors ineffective. In extremely high speed cases its common use the capacitance between the power and ground plane for bypassing.

    So don't worry aboutt his capacitance. It won't hurt you and in fact is a good thing.
     
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