opinions on copper pours on PCBs
opinions on copper pours on PCBs
(OP)
I wanted to get input on the advantages and disadvantages of using copper pours to fill in all unused spaces of outer layers of PCBs with ground in cases where plane layers already exist. It seems to me that there is the possibility of providing additional shielding if there are internal signal layers, but that this may be offset by the fact that it is chopped up into so many small pieces. It also seems that, unless you're really careful about putting ground vias in the appropriate places, it's easy to end up with pieces of copper that are connected to ground at one end only, which would allow them to act as antennas for any noise in the ground plane.
I suppose the other advantages copper pours may have is some additional heatsinking or using less etchant when fabricating.
I tried looking for any info on copper pours that actually had testing or simulations done, but didn't find anything. The info that I've found has been more anecdotal. If anyone knows of any books or white papers or other sources that would be helpful, let me know.
I don't have any particular issues with copper pours right now. I am just interested in the reasons that people use them.
Glenn
I suppose the other advantages copper pours may have is some additional heatsinking or using less etchant when fabricating.
I tried looking for any info on copper pours that actually had testing or simulations done, but didn't find anything. The info that I've found has been more anecdotal. If anyone knows of any books or white papers or other sources that would be helpful, let me know.
I don't have any particular issues with copper pours right now. I am just interested in the reasons that people use them.
Glenn





RE: opinions on copper pours on PCBs
I never heard about that problem with traces connected to ground in just one place radiating noise from the ground plane. I would have thought the prescence of the ground would conveniently capacitively couple noise away from adjacent traces away to ground.
RE: opinions on copper pours on PCBs
Sometimes ground pours are done to reduce the possibility of PCB warping during reflow by balancing the amount of continuous copper on each side of the board.
But, adding pours to a poor PCB layout may achieve nothing.
Remember - patch antennas, at first glance, resemble a ground plane or ground pour.
RE: opinions on copper pours on PCBs
1. If there's a four-layer board (signal-plane-plane-signal) that passes all EMC tests with no copper pour on the outer layers, will adding grounded copper pours show any reduction in radiation? Will it stay about the same? Can it make it worse?
2. If there's a six-layer board (signal-signal-plane-plane-signal-signal) that passes all EMC tests with no copper pour on the outer layers, how will the addition of grounded copper pours affect the tests?
Glenn
RE: opinions on copper pours on PCBs
I never bother with pours unless at a glance the board appears to have acres of wasteland, as it were. Most of my boards are so specialized or dense that pours would be folly. Often they're 4 layer and would have planes anyway.
Keith Cress
kcress - http://www.flaminsystems.com
RE: opinions on copper pours on PCBs
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However, I don't know if this only becomes a problem if the chunk of copper is long. i.e. Is there some length below which it becomes such an inefficient antenna that a single ground via would be acceptable?
Keith, I agree that retesting would be wise. I guess I'm just trying to figure out cases where it makes sense to do the ground pour in the first place. It can be a considerable amount of work to do the pour, put a bunch of ground vias in the appropriate places, and make sure that there are no isolated or single-via areas of copper.
Glenn
RE: opinions on copper pours on PCBs
Additionally, if the copper is thick, the full perimeter heat sink makes desoldering in repair and service operations more difficult, or more skill-intensive, and may interfere with soldering at first assembly. I've seen more than a few power supply boards with full copper pours that were actually etched all the way around the vias, except for a few radial spokes of copper.
Mike Halloran
Pembroke Pines, FL, USA
RE: opinions on copper pours on PCBs
Keith Cress
kcress - http://www.flaminsystems.com
RE: opinions on copper pours on PCBs
My first task at my first for-pay job ever was replacing a loudspeaker at the top of a telephone pole, using a Bernz-O-Matic to do the soldering.
Not a Bernz-O-Matic torch with soldering tip; just the torch.
Mike Halloran
Pembroke Pines, FL, USA
RE: opinions on copper pours on PCBs
Mike, thermal reliefs should be added as a matter of habit in PCB designs. Imagine something as simple as no reliefs on a part, but it will not be reworked, just IR soldered. That part will have a significantly reduced life as the heat retained in the remaining copper will continue to transfer to that component long after the IR oven has cooled down, killing off the original heat ramp spec.
I used a Weller from the time I first grabbed my father's off of the shelf at the age of 8 until just a year ago, when I switched to a Metcal. The Weller did it's job quite reliably, but I found myself trying to work around problem components (such as large components with no thermal relief pads!). Now that I've spent some time with the Metcal, I'll never voluntarily go back... inductive heating is the way to go! Even beefy 10W components are no match for the smallest of tips.
Dan - Owner

http://www.Hi-TecDesigns.com
RE: opinions on copper pours on PCBs
Remember, the high frequency return currents will follow the path of least impedance, which at their frequencies is almost certainly the path of least inductance. To get low inductance at high frequencies on a PCB you need a wide, flat surface and one full of holes is better than a set of serpentine traces.
You need to also remember that the high frequencies / high speed signals have nothig to do with logic clock speed. Rather it is the rise and fall times of the devices that are doing the switching that is important.
I had a two layer board desgin that suffered terribly from EMI problems. I redesigned the board using 'finger traces' off a wide trunk for the power and a copper pour on the top side of the board for a ground plane. I also switched from through hole to SMT components because of the flatter - wider tabs over long thin pins (lower inductance). Cleared the EMI problems right up and never had another failure in the 3 years I remained at that company.
The website sigcon.com has a lot of information on this subject if your interested.
RE: opinions on copper pours on PCBs
I had looked at Johnson's first book, but didn't think of checking his website. His advice is to use copper pours on 2-layer boards, but not on boards that have power planes already. However, his advice is from a signal integrity point of view, and not with respect to heat or manufacturability.
By the way, I also have high praise for the Metcal soldering irons. When I'm soldering 0402 components under a microscope, a Weller tip looks as big as a club. And I suspect a Bernz-o-matic torch might be a little bit of a problem as well.
Glenn
RE: opinions on copper pours on PCBs
Dan - Owner

http://www.Hi-TecDesigns.com