Guard Rings Without Solder Mask?

[Hi-Q]

Hey,

if there is a high impedance circuit node, it's a good practice to surround the connected PCB pads and traces with guard ring structures and driving them with an as close as possible voltage with a low source impedance in order to prevent charges or leakage currents from reaching the high impedance node.

I wonder if it would be wise to not cover the guard ring traces with solder mask, so any surface currents (dust and moisture on the board) would really get caught by the guard ring instead of being able to cross it above the solder mask layer.

Thoughts?

Regards,
Hi-Q

 

[Ronald smith]

The solder masks are generally used to reduce moisture infiltration to the PCB (thus can prevent leakage to some degree), but there may be surface-charge problems when using large area solder-masks. So the better approach is to use bare-copper guard areas 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 a solder mask.