Why might deep N-well isolation fail in preventing latch-up in a multi-domain analog layout?

I think deep N-well isolation can fail if the guard rings are not placed properly or aren’t strong enough. Even with a deep N-well, if the P-substrate injection paths from high-current domains aren't blocked effectively, latch-up can still occur. You’ve got to make sure the P+ guard rings are tied correctly and surround sensitive blocks. We faced this in one of our mixed-signal chips, and just increasing the width of guard rings helped.

You know, sometimes we rely too much on deep N-well and forget about layout floorplanning. If two domains share a common substrate or if there's a short return path for current, latch-up can happen even with isolation in place. I always recommend separating domains physically on the layout and keeping noisy blocks away from analog ones. Deep N-well is just one part of the strategy—it’s not a magic fix.

From my experience, deep N-well helps, but it’s not a complete solution when power domains operate at different voltages. If you have fast switching digital blocks near sensitive analog domains, the noise can still couple through the substrate. You also have to consider how close the wells are placed. So, we usually combine deep N-well with deep trench isolation or extra shielding in critical areas.