Guard traces/isolation slots: pointless because the package sets the creepage? - Page 1
Forum section: electronics / projects

author: rs20
author_rank: Super Contributor
timestamp: May 17, 2015, 02:12:15 am
content: Firstly: I know I'm wrong, I just want to understand why.Whenever there’s mention of guard traces or isolation slots, I feel like there’s a giant epoxy elephant in the room -- the plastic package of the device itself. Take this still from EEVBlog #731:Those amazing, intricate slots increase the creepage distance (shown in blue) to prevent leakage currents from travelling through surface contamination and affecting high impedance nodes. Furthermore, that guard trace shunts away those leakage currents, giving the leakage current no path to the node (shown as blue question mark). And so ends the typical explanation of slots and guard traces.Except, I’m always yelling at the screen at this point because the very moment you install the chip on the board, the blue creepage drawing is now wrong. The creepage is now the red line. Right? How is that not a creepage path? Are the epoxy packages somehow intrinsically more immune to surface contamination than sticky solder mask? Or do all the isolation slots and guard rings in the world only buy you a halving of leakage currents (remember, for a surface with an even coating of contamination, the point-to-point resistance is equal no matter what the distance between the points is). Or are these things entirely probabilistic in nature: "if a drop of water falls into the case and lands near this isolation slot, the device will still work perfectly; but if it lands on the package, we're screwed either way. But that's still a halving in failure rates, so worth doing."I confess I posted a similar question a while back in response to EEVBlog #659, and got a response from Dave (which I never got around to replying to, so I will now):Quote from: EEVblog on September 06, 2014, 12:18:59 amBut the creepage path from primary to secondary is exactly as long as it would be without the isolation slot! See the two straight red lines, not to mention the creepage along the body of the transformer itself! The isolation slot isnotimproving primary-to-secondary creepage, surely? To be clear, where I've written "deadly spark" on the picture, I now rewrite as "surface contamination arcing path".EDIT: Fixed broken image link.

author: xrunner
author_rank: Super Contributor
timestamp: May 17, 2015, 02:20:33 am
content: I think they will say without the slot it will be worse ... but I don't know. Interesting question.

author: c4757p
author_rank: Super Contributor
timestamp: May 17, 2015, 02:23:14 am
content: The PCB is the surface onto which dust and other contaminants will fall, because it's wide and flat.

author: rs20
author_rank: Super Contributor
timestamp: May 17, 2015, 02:35:29 am
content: Quote from: c4757p on May 17, 2015, 02:23:14 amSo mounting the PCB upside-down would be almost as effective as all these guard rings and isolation slots? (I guess the answer is "PCBs have 2 sides, and designing so that no senstive nodes/through-vias end up on the side facing up is more difficult than just chucking in some isolation slots". I can buy that if that's really all there is to it.)

author: Psi
author_rank: Super Contributor
timestamp: May 17, 2015, 02:58:24 am
content: It's about mitigating the issue, not eliminating it

author: DanielS
author_rank: Frequent Contributor
timestamp: May 17, 2015, 03:01:16 am
content: While the package may determine the ultimate absolute minimum leakage between its pins, the cut-outs and guard traces help minimize the PCB's contribution to additional leakage terms and effects.

author: rs20
author_rank: Super Contributor
timestamp: May 17, 2015, 03:21:15 am
content: Quote from: DanielS on May 17, 2015, 03:01:16 amSo in other words, all these isolation slots and guard rings buy you less than an order of magnitude improvement? Obviously any improvement is worthwhile, I'm just starting to understand now that my feeling that isolation slots and guard rings gave 10x-100x reductions in total leakage (and were thus absolutely, completely essential even for a one-time use, prototype fA-input op-amp circuit) is where I was going wrong.Moving on to the second picture, are we in agreement that the "isolation" slot there is largely pointless? I don't quite follow the "mitigation" argument in this case, since all the parallel potential breakdown paths don't really parallel up in any detrimental way like leakage currents do. Unless it's some sort of probabilistic thing, like I mentioned in my first post. (But probabilistic safety?? How does that work??)

author: Mechatrommer
author_rank: Super Contributor
timestamp: May 17, 2015, 03:25:04 am
content: to improve dielectric coefficient?to improve voltage breakdown point?to improve crosstalk?to improve leakage current?why dont you ask the person who designed the board?nobody knows better than him. its keithley afterall.

author: Someone
author_rank: Super Contributor
timestamp: May 17, 2015, 05:47:39 am
content: Quote from: Mechatrommer on May 17, 2015, 03:25:04 amThermal gradients and isolation too, can be important in low voltage applications.

author: SL4P
author_rank: Super Contributor
timestamp: May 17, 2015, 06:54:11 am
content: In some cases the pins to be 'isolated' are not those pins at the end of the gap - but those nearer to the middle with the path along and around. e.g. pins near the end are 'other non-associated parts of the circuit' etcYes - they could fail in other ways, but the design often considers what you've said - and others are deigned by *numties (*technical term)plus all the above comments!

author: T3sl4co1l
author_rank: Super Contributor
timestamp: May 17, 2015, 07:24:22 am
content: The package is generally assumed to be ideal. Not like you have any choice in the matter, so yes, to that extent, it's merely a matter of making it as good as possible, not that it can ever be perfect.If you really wanted to pay for it, you could presumably get them made in ceramic or metal packages (glass bonded lead-outs). Which aren't that far out (SMTs and THTs have been made in this way; look at oldish military computers for instance).Washing the board until it's exquisitely clean, and sealing it (conformal coat, IP6x sealed, hermetic, vacuum, etc.), or going hybrid (ceramic substrate, die bonded chips) for some or all, would be best practice; though it would seem even Keithley didn't see fit to go quite that far.Tim

author: Someone
author_rank: Super Contributor
timestamp: May 17, 2015, 09:32:22 am
content: Quote from: T3sl4co1l on May 17, 2015, 07:24:22 amBob Pease discussed in "What's All This Femtoampere Stuff, Anyhow?" that the plastic packages were in fact vastly superior for leakage than the ceramic packages.

author: Fungus
author_rank: Super Contributor
timestamp: May 17, 2015, 09:34:14 am
content: The board is where all the crap happens. It's where they put all the liquid goo and pastes during manufacturing.

author: AndyC_772
author_rank: Super Contributor
timestamp: May 17, 2015, 11:48:12 am
content: If two nets have to be kept apart for safety compliance purposes, then you're right, the minimum creepage distance would be taken to be the distance between the pins of the IC. Most likely, it would fail. That's why components like opto-isolators have the 'hot' pins on one side of the package and 'cold' pins on the other.Here, though, the object of the exercise is to minimise leakage currents. There's no high voltage and no safety issue, the slots are there for purely functional purposes. Eliminate the PCB, and any crud that might collect on its surface, and you eliminate a leakage path.Yes, there's another path through the IC package itself, but you're stuck with that.

author: DanielS
author_rank: Frequent Contributor
timestamp: May 17, 2015, 12:22:34 pm
content: Quote from: rs20 on May 17, 2015, 03:21:15 amIt depends on what that bottom trace actually is. If it is ground, then the likelihood of anything happening due to the slot not extending that far down is slim to none - that bottom transformer leg could very well be to ground shield layers/windings inside the transformer.

author: Phoenix
author_rank: Frequent Contributor
timestamp: May 17, 2015, 01:06:08 pm
content: Quote from: rs20 on May 17, 2015, 03:21:15 amYou need to measure the distance copper to copper, not solder to solder. The slot is located where there are copper tracks that extend closer to the other side of the transformer than the slot itself. If you re-measure based on that the slot has provided some benefit.It may not be a shinning example of isolation slots, but there is a bit that goes into calculating the required creepage and clearances on a PCB. Sometimes it's better to be safe than sorry (especially if it doesn't cost any extra).

author: Neilm
author_rank: Super Contributor
timestamp: May 17, 2015, 06:57:00 pm
content: There is one term I haven't seen in all these replys - Comparative tracking index (CTI). The creepage required for safety between two conductors depends on what the material is between those two conductors. For instance, I have seen the creepage requirement treble between group I and group III materials when doing the calculations as described in IEC61010. PCBs will tend to be group III and so require longer distances to provide the same protection as the chip package which will tend to be group I material.