Pour Or Not — Just What Is My Opinion?

I posed a question about using copper pours (AKA flood) a not long ago. The premise was a simple microcontroller board with a 20MHz clock and no special requirements.

I had a couple of different comments on the post with some very good insight. Myself, I generally don’t use copper pours. My only reason is that I think it usually looks better without (although I do like the look of the cross-hatch pour on the Arduino). A well done flood can be pretty cool, but still my inclination is to only use it if it’s needed. If it’s a shop doing the PCB, the metal will be recovered and recycled, so the conservationist in me is pleased.

If it’s a home-etched deal, then a pour is probably a better idea because it will reduce the amount of etchant needed. Although you do need to be careful to keep plenty of space between things to prevent solder bridges. Solder bridging isn’t such a big deal on a PCB with a good solder mask, but it certainly is on a board with no mask or thin mask.

If there is a good reason, I will. Like a high-current motor driver — I use the pour to keep the current capacity up and the kelvons mellow. Heat-sinking is a good reason for a pour. High-speed stuff usually benefits from a flooded plane of some sort too and in four-layer boards, using the innerplanes for power and or ground is nice and convenient. But you all know that. I’m just rambling now.

Duane Benson
Does high speed stuff on a flooded plane require a speed boat?
Will too much heat sink it?

http://blog.screamingcircuits.com/

About Duane

Duane is the Web Marketing Manager for Screaming Circuits, an EMS company based in Canby, Oregon. He blogs regularly on matters ranging from circuit board design and assembly to general industry observations.
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3 Responses to Pour Or Not — Just What Is My Opinion?

  1. Mitch says:

    Hi Duane,

    Been designing board since the early 80s and have seen it plenty both ways. Back when PCB design software wasn’t so accomidating, there usually wasn’t pours on the board layers. When PADS came out with an easy pour routine, it became pretty common to pour layers. Now I see it both ways on many different board designs. I’ve come to believe it’s the user’s preference. Me, I pour my internal layers (like you mention, it saves etchant – or does it? I’d have to confirm that, since I believe etchant is also recycleable.) As for your comment that copper is recycleable, does that mean your fabricator can charge for the copper; etch it off; re-use it; and make a bigger profit? I doubt that. I’ve never heard it mentioned, by any of my fabricators (and I’ve probably dealt with 10-15 companies), that they would like to have the layers having only the copper ‘necessary’.

    Again, I firmly believe it’s all user’s preference. Like they say “Have it your way.”

    Good day.
    Mitch

  2. Hi,

    On boards with buried power planes, a copper pour is not normally
    necessary. (rf boards an exception)

    On two layer boards (with 5V CMOS logic in particular) it is very difficult
    to pass CISP-22 EMC tests without a well grounded flood plane.

    In the 1980s it became a requirement for more and more categories
    of electronic equipment to be tested for EMC compliance.

    From first hand experience, I have seen how just adding a flood plane
    to two-layer boards can reduce EMC by 20dB. (We had to revise existing
    boards to pass EMC tests during the late 80s and 90s).

    The ability of Protel and PADS to pour flood planes arrived at around
    the same time as the need to pour the flood planes (for a while,
    I used to lay down lots and lots of crossed-tracks)

    My (very) ad-hoc experience was that any microprocessor clock
    speed above 10MHz needed a flood plane, and from 4-10MHz
    it may have needed a flood plane, and below that it seemed OK.

    With 3.3V logic, the problem doesn’t seem so bad, I think the
    current spikes that 5V “HC” logic had, dont seem so bad in LVCMOS
    and LVTTL?

    I was amused to find, though, that in a 4-layer board I’ve just
    designed, which is a mixed rf (VHF) and digital (>100MHz clock)
    board, the worst offending part for noise feeding into my rf
    was a single 74HCT367 buffer being used as a level translator
    (3.3V -> 5V). Some things remain the same! :-).

    So flood planes “arrived” in the 80s, and like so many things,
    have remained, when the reason for their arrival has been
    mostly removed.

    This is because that these days, using a single core, and two
    pre-preg layers, a 4-layer board costs not that much more than
    two layers – if board cost savings are important, then probably
    one should consider a single-sided, hole-punched paper phenolic
    board.

    It has become engineering folk-lore that a flood-plane will (somehow)
    always help improve EMC. So, I tend to add them automatically just
    to save a (possible) argument. (we contractors know that the
    “customer is always right”).

    (Note that on an rf board, from around 100MHz and up, a well
    grounded flood plane is necessary for rf coupling reasons)

  3. John Lambert says:

    As stated .. depends..

    Today a good pcb designer / engineer has to be aware of many, many issues.
    as related to pours.. heat spreading/sinking, impedance control, shielding, copper balancing, warp-age control, etc…..

    I don’t think the 80s have much to do with any of this though.
    The FCC has ALWAYS required ALL products to meet min. standards of non-interference …. basically part of their reason for being.
    As more electronics were in people’s lives .. the more likely a company became aware of it’s responsibilities… (reports of interference).

    Yea, CAD tools have influenced how many designers bothered to create pours on outer layers of a board..
    But many designers were doing pours .. before they had a CAD system (hand tape ups)… count me as one of them.

    As far 20mhz or some other frequency for determining when a design needs pours or ground planes… not really relevant.
    The real issue is rise/fall times of signals.
    It is this characteristic of signals in a design that causes the need for planes/pours.
    The assumption that the frequency will generally track with rise/fall times is what causes a lot of board turns (because of failing EMI/EMC)

    a 20 mhz clock … will often generate harmonics that are excessive for FCC/CE limits for a given frequency.
    generally 5th ,7th ,13th as strongest… so 100mhz, 140mhz, 260mhz freq content will be present…
    nearly any “odd” multiple of original frequency well up into 25 x will be present.

    And you will have a real problem if the harmonic matches a radio receiver’s IF frequency.. in the same unit (kills all reception).

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