Guard-ring circuit is designed aroung high sensitive (low current high impedance inputs). What is the technical impact if a guard-ring circutry is fully masked versus unmask ?
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3\$\begingroup\$ A guard ring is normally used to prevent leakage currents through surface contamination of the PCB. If the ring is isolated from the contamination by soldermask, it can't do its job. \$\endgroup\$– Dave TweedCommented Dec 1, 2014 at 13:22
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\$\begingroup\$ @DaveTweed You should make that an answer. \$\endgroup\$– Matt YoungCommented Dec 1, 2014 at 13:34
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\$\begingroup\$ @DaveTweed Reference here from LTC says the same thing. \$\endgroup\$– Spehro 'speff' PefhanyCommented Dec 1, 2014 at 13:45
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2 Answers
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A guard ring is normally used to stop leakage currents that flow through surface contamination of the PCB, by shunting them to a low-impedance node. If the ring is isolated from the contamination by soldermask, it can't do its job.
answered Dec 1, 2014 at 14:16
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The solder masks generally used to reduce moisture infiltration to the PCB (thus can prevent leakage in some degree), but there may be surface-charge problems when using large area solder-mask. So the better approach is to use bare-copper guard area around sensitive nodes. Using solder or plating the guard with gold or tin to prevent oxidation. The guard trace is to provide a path for the leakage to the low impedance node, so they should not be covered by solder mask.
And There are some similar discuss: http://www.eevblog.com/forum/reviews/6632a-teardown/
... you can clearly see the guard traces ( silver ) . the reason for removing the soldermask over the guard is to remove that pathway as well. Soldermask has a specific dielectric constant that is worse than air. By removing the soldermask you make a lower leakage capacitance across the guard ring.
Notice on the left bottom pin that there is again a guard ring present. For some reason they forgot to open the soldermask ( i looked carefully and it is a guard ring ... )
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\$\begingroup\$ Are you confusing soldermask with conformal coating? Normally, soldermask has so many openings through it that it can't have any significant effect on moisture. \$\endgroup\$ Commented Dec 1, 2014 at 14:22
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\$\begingroup\$ Yes, in a big board, there may be many openings. But if we want to protect a small area, we can indeed cover it with a mask. \$\endgroup\$– divergerCommented Dec 1, 2014 at 14:30
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\$\begingroup\$ It doesn't matter how big the board is, there are at least two openings for each component on it. \$\endgroup\$ Commented Dec 1, 2014 at 14:57
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\$\begingroup\$ @DaveTweed: Yes there many openings, and they have limited effect on moisture. But limited or insignificant not equal to no-use or nothing :). \$\endgroup\$– divergerCommented Dec 1, 2014 at 15:06