I'm working on a 4 layer PCB with internal 2 layers being power and GND... 2 outside layers are signal. Is it better to have a pour on the outside layers or just leave them empty except for the signals?
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1\$\begingroup\$ I "usually" put GND pours on the outer layers and stitch them to the inner grounds. \$\endgroup\$– Dejvid_no1Commented Jun 17, 2014 at 14:09
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\$\begingroup\$ I don't think there's much point in doing pours on the outer layers unless you're using them as an extra power plane, but I don't think it makes much difference either way. \$\endgroup\$– Spehro 'speff' PefhanyCommented Jun 17, 2014 at 14:41
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\$\begingroup\$ I guess what I'm asking is - is there a reason (benefit or disadvantage) for doing or not doing a pour on the external layers. \$\endgroup\$– PugzCommented Jun 17, 2014 at 15:31
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1\$\begingroup\$ I think having pours on external layers is good as they shield internal layers from ambient noise. The point is that you'd better use internal layers for signal and external ones for power. \$\endgroup\$– Vladimir CraveroCommented Jun 17, 2014 at 15:41
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1\$\begingroup\$ @Pugz There's a slight advantage for reducing impedance and better shielding, and a slight disadvantage in decreasing yield because of more possibility of shorting. Probably not worth the trouble except in special cases (eg. RF front end etc.). Solid internal ground planes make the board pretty quiet anyway. \$\endgroup\$– Spehro 'speff' PefhanyCommented Jun 17, 2014 at 15:57
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2 Answers
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Back in the days, pouring all layers was done in order to minimize the amount of copper etched during the PCB manufacturing process. The more homogeneous amount of copper across the PCB the better is the etching process.
Nowadays there is no point in pouring with copper all layers
Pouring outer layer doesn't improve the shielding and the impedance necessarily. These are critical points and they must be considered in a proper way.
In general, you don't need to pour outer layers.
answered Jun 17, 2014 at 17:01
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Ground pours on layers other than the ground plane is generally not needed. One exception occurs if you are using SMT power devices (and even some thru-hole) the do their heatsinking via the pc board. In that case, having a nice big copper area directly under the device can be a very good thing.
answered Jun 17, 2014 at 17:13