PCB trace spacing for isolation resistance test - Page 1
Forum section: electronics / projects

author: coatiDave
author_rank: Newbie
timestamp: January 08, 2015, 10:51:12 am
content: Hi guys!I have trouble finding information on how to dimension PCB trace spacings for isolation resistance, can someone point me in the right direction, please? :-)I need to design a front panel board for an assembly group. The board itself is passive. It only takes line level audio signals, composite video signals and some 12V switches from 'the outside' to connectors on the inside. The devices inside the assembly group are all certified, have input protection etc. so I don't have to worry about that. Still, the board needs to withstand a 500V resistance test.The test definition itself is quite vague. The resistance has to be 'high enough' (great, so I guess a handfull of MegOhms for a 500V test?) and shall not change significantly if the test is repeated. The test is conducted in a climate chamber. Again - great. ^^ So, I guess I need a nice conformal coating and the right trace spacing.But: what is 'right' in this case? I can find plenty information on spacing for a 500V working voltage. (IEC-60950-1; IPC-2221) (The spacings seem a little big, though. I would rather use smaller ones if I can justify that with other norms. It's a quite close thing with the 1.27mm pitch THT connectors I need to use.)Should I use those as a guideline, or do I have to look at a different place?Please note that while I am ok with reading and understanding English for the most part, I am not so good at expressing myself in it. That cripples my internet searching abilities quite a bit. Often enough I just search with the wrong terms.So, if this is already discussed somewhere, please let me know. :-)Thank you! :-)Dave

author: T3sl4co1l
author_rank: Super Contributor
timestamp: January 08, 2015, 11:27:09 am
content: PCB resistivity is extremely high. Resistances in the gigohms are practical in circuit, as long as contamination from flux or ambient (humidity, dust, etc.?) is kept clean.UL standards for isolation will require a bit over 1mm trace separation (when covered by soldermask, or double when exposed), but that's because it includes transients over 3kV. To do 500V and nothing more, you really don't have to worry.Also, as long as the climate chamber test does not include condensing humidity, it should be fine.I would recommend against 1.27mm THT connectors if at all possible. There's just no meat on them, so you can't make a good PCB. The pin diameter, annular ring and spacing are marginal in any fabrication process. SMT connectors that fine are preferred, but they aren't robust: you don't want the connectors handling large stresses (heavy cables, vibration?) or lots of cycles (definitely avoid SMT connectors where end users have access). If changing them is an option, you will have better results with dual (or more) row 2.54mm headers, which are almost as dense anyway (including the connector and shrouded header), or perhaps some types of "high speed" mating connectors (but they will probably be more expensive).Also, what is the 500V applied between? If it's between adjacent pins, you're screwed. If it's between separate connector blocks, you'll be fine (following the usual rules so the groups of traces don't get too close).Your English isn't bad, BTWTim

author: coatiDave
author_rank: Newbie
timestamp: January 08, 2015, 12:05:04 pm
content: Hey Tim, thank you very much for your answer! :-)Oh, sorry, I ment 2.77mm (Sub-D THT, mandatory), don't know why I wrote 1.27.Still, with annular rings, there is not that much space left. I think I can meet the ~1.3mm spacing I read up, but not much more.The test is between adjacent pins and with ~90% humidity, so I guess I have to conformal coat the bottom side of the board and apply some 'isolation gunk' between the connectors and the top side. If the mandatory connectors become the weak point then, it would be good enough. ;-)So I guess if I keep the ~1.3mm spacing and cover all surfaces, I should be fine?Thank you! It took me well over half an hour to write the OP, so it is a relief if it is understandable. ;-)Thank you again!Dave

author: T3sl4co1l
author_rank: Super Contributor
timestamp: January 08, 2015, 03:28:53 pm
content: Oh, D-sub type, yeah, those are usually rated for 300 or 500V, something like that.If the voltage is actually required, I would still recommend against that... it should be possible, but that doesn't mean it will be reliable. What is the nominal operating voltage?You can try using oddly shaped pads, like oblong or offset pads, to improve clearance.Tim

author: coatiDave
author_rank: Newbie
timestamp: January 08, 2015, 04:11:31 pm
content: The nominal voltage is only 12V DC for the power pins and audio line level/ composite video for the signal pins. The 500V is really only for the insulation test.The d-sub connectors would not have been my first choice either, but it is supposed to be a simple swappable replacement for an older system, so I cannot change that.Yeah, I already experimented with creatively shaped pads.I think that will work.Thank you!Dave