FR4 - 3kV TOP / BOTTOM - Page 1
Forum section: electronics / projects

author: harerod
author_rank: Frequent Contributor
timestamp: August 21, 2024, 03:18:29 pm
content: A client asked my about the feasibility of the following setup:- 1 or 2 layer 1.6mm standard FR4- no holes (no vias or other drillings)- SMD on TOP side only- BOTTOM no copper or solid plane (optional)- BOTTOM to TOP will have a steady potential difference of 3kVdc during normal operation- clean environment (lab use)Taking multicb as reference, one could assume that a standard 1.6mm FR4 PCB may be able to withstand this. Before I run the concept by TÜV, I'd like to hear about your experience with this kind of setup.Thanks!https://www.multi-circuit-boards.eu/en/support/download/datasheets.htmlhttps://www.multi-circuit-boards.eu/fileadmin/pdf/leiterplatten_material/isola_duraver-de104_www.multi-circuit-boards.eu.pdfhttps://www.multi-circuit-boards.eu/fileadmin/user_upload/downloads/leiterplatten_design-hilfe/e_multi_cb_material-standard-values.pdf

author: TurboTom
author_rank: Super Contributor
timestamp: August 21, 2024, 03:58:19 pm
content: I'ld say from a safety point of view, this will work unless the top SMD components heat up the PCB considerably. The effect of capacitive coupling between top and bottom will have to be evaluated but shouldn't pose a problem if it's a constant potential (no or no considerable AC component). Nevertheless, I'ld expect high-impedance or sensitive low-level circuitry (if present on the component side) to get more noisy than without a properly shielded design (multilayer).

author: Doctorandus_P
author_rank: Super Contributor
timestamp: August 21, 2024, 04:21:06 pm
content: e_multi_cb_material-standard-values.pdf states an "Electrical strenth" of 45kV/mm and that suggests plenty of margin, but I understand your caution. I also have no experience with this.And apart from that 3kV, how much energy is there in that high voltage? The range is between a bit of leakage that just makes the circuit not work, and a big explosion, and that is quite a big difference from a safety point of view.

author: harerod
author_rank: Frequent Contributor
timestamp: August 21, 2024, 05:54:11 pm
content: TurboTom - what triggered your remark regarding temperature?The SMD resistors will act as heating elements. The TOP side will include temperature sensors.The sensor temperature will be regulated to 60°C, 80°C tops. This PCB will contain the resistors and sensors, only. Control is on another PCB.Doctorandus_P - The 3kV are low energy and capacity. Assume <1µA.A fall-back concept that crossed my mind, would be a 4 layer type setup:prepreg/prepreg/core/prepreg/prepreg.Layer 1/TOP would hold the SMD. Layer 2 is flooded copper shielding, with blind vias connecting to 1/TOP. Layers 3 and 4 are empty. This would give us shielding, while the core plus 3/4 prepregs would provide insulation. This would also be my base of discussion with TÜV, simply asking them how many core/prepreg layers they would like to see. Thicker PCB wouldn't be an issue either.

author: Doctorandus_P
author_rank: Super Contributor
timestamp: August 22, 2024, 01:23:03 am
content: If there are just resistors and sensors on the PCB, then why do you want to shield it?I do not see an advantage of a 4 layer PCB here. Quite a lot of PCB manufacturers offer thicker PCB's. Often up to 3.2mm.As an extra precaution, I would design the other PCB in such a way that even if the 3kV reaches it, it does no harm. That should not be too difficult if the maximum current is only 3uA.With such a low current, leakage is probably a bigger concern then safety. Avoid any sharp corners and points on the high voltage side, as these increase the field strength in the surrounding air and promote corona discharge.And I still have no experience in this area. Just some gut feelings.Out of curiosity I had a short look at a few websites for PCB materials for high voltage. When you go over a few kV, then long term stability of FR4 is apparently a problem. I suggest you do some more reading into that direction.

author: TurboTom
author_rank: Super Contributor
timestamp: August 22, 2024, 10:41:45 pm
content: Quote from: harerod on August 21, 2024, 05:54:11 pmThere are several scientific papers that cover the dielectric characteristics of polymers with regards to temperature,see here for example.I had been involved in design considerations for a multi-phase AC power meaqsurement system and the electrically isolated data transfer from the "Pickup section" to the processing section was a quite difficult problem, provided it's got to be done with off-the-shelf components. Most manufacturers of high-speed data couplers don't warrant a "safe isolation voltage" since isolation barriers tend to degrade over time (ion conduction, dendrite formation...) and can only be considered safe for a certain amount of time. Heat tends to accelerate this progressively. Hence my question... Yet, at the relatively low dielectric strength @ 3kV and temperatures below 100°C, I wouldn't be worried too much. As long as the glass transition temperature of the epoxy isn't reached, things should progress rather slow. But you don't have ionizing radiation around by coincidence, do you?

author: ganfetsic
author_rank: Newbie
timestamp: August 24, 2024, 05:33:02 pm
content: Beware of creepage round the board edge to the bottom. You can solve this to an extent with conformal coating.Avoid sharp corners on copper ect to reduce chance of flashover.Should be no problem, the detonator PCBs have way more kV than this from top to bottom.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 24, 2024, 05:41:24 pm
content: If 3kV is the worst-case peak value, that's not very much; mains equipment is normally rated about this.Real problem is if you need vias, that pretty much screws it up and you need an insulator on the bottom -- a modest thickness of most any plastic will do, with PET, PP and polyimide being likely options.Also, being lab equipment, does this need any particular requirements? Technician operated exemption, functional type insulation, etc.? (You probably know TÜV better than I do, just asking for completeness.)Tim

author: floobydust
author_rank: Super Contributor
timestamp: August 24, 2024, 06:03:56 pm
content: Quote from: ganfetsic on August 24, 2024, 05:33:02 pmYes, leave extra room (spacings) around the perimeter and don't forget transitioning from FR4 to air, or between layers is where mistakes get madePic is a design I tested, that consulting firm was not happy I found that lol.Everyone uses the safety standard spacings numbers to achieve transient rating... Problem is steady-state ozone buildup after a while (even 1 minute) decreases the air breakdown voltage. For PCB inner layers, I have this table snippet as a guideline. Minimum thickness is 0.4mm.I don't hear safety is a concern if this insulation breaks down?If there was a safety issue, certifiers will want PCB laminate data for its insulation test/rating/quality. "Conformity is checked by inspection and either by measurement of the separation or by inspection of the manufacturer’s specifications".

author: free_electron
author_rank: Super Contributor
timestamp: August 24, 2024, 06:59:33 pm
content: Saying FR4 is like saying i used wood. What wood ? oak ? spruce ? birch ? plywood ? osp ?FR4 = Flame retardant class IVIt is NOT fiberglass-Resin. That is a common misconception. People call it that but if you use that in front of a pcb fabricator they will give you the eye-roll.You need to know what is the material being used. Manufacturer and type. Some people will go , oh i used Rogers for my RF project. That is still saying "i used wood". Rogers has hundreds of different materials. So does Isola and anyone else.ok, i used Isola 370HR. That's still saying you used wood.What is the resin content ? 370HR material comes in different resin/fiberglass ratios.ok, 63% resin. That's still wood.What is the weave of the fiberglass ? Which fiberglass ?Specifying : 1 need a board made from 17micron rolled copper foil with 4 sheets of 2116 glass 53% resin 370HR-type. Is a specification. the 2116 tells you the strand desnity, strand type and what kind of glass you are dealing with) If you are doing controlled impedance you will need some other numbers too, as well as the post-press thickness of the prepreg or core.All that being said, there is a difference between creepage and clearance.Creepage is the path along a surface. Clearance is the path in open air. There are rules for both. Rules that change based on contamination levels ( this can be dust , but this can also be ionic contamination that happens in the production process. ) They also change depending on surrounding air pressure ! ( height. a board for aircraft that goes above certain altitudes has different rules, and it's not uniform, there are classes based on actual height)Then there is the soldermask. Soldermask can have a lower breakdown voltage than the raw pcb material. In some applications it is necessary to not put soldermask near high voltage components.I'm working on a 800 volt battery system for an aircraft. We need to pick our materials VERY carefully. Especially because altitude is involved. Soldermask is a disaster.the problem is we are working with surface mounted, small parts so copper features are relatively close together. we need a good isolator. So we need a material with a high CTI (comparative tracking index). Tracking is the creation of conductive paths (creating a track) on the surface and inside the pcb material based on temperature, aging, and other factors. The higher the CTI the longer the life of the board will be and the higher the voltage it can hold.Your household "FR4" (i know, i know, but that's what people call it) has the WORST possible tracking index.it can hold about 275 ~ 300 volts -edit- in the CTI test. -end edit-. Go above that and pathways will start forming that will start discoloring the material, reacting and create conductive paths.clarification. the CTI is determined using a standardized test with electrodes spaced a certain distance apart and a "contaminant" dripped on the pcb material. its part of IEC 60601-1:2005 and the test is explained in IEC 60112.The CTI has a direct impact on the creepage needed for a given voltage.You can meet all the regulatory clearance and creepage rules you want : the board material itself will give out. Clearance and creepage only defines copper to copper spacing. it does NOT look at what happens in the pcb substrate itself. the CTI tells you how to adapt the creepage.For 1 kilovolt you will need a material in PLC0 (Performance Level Category 0) >600VDCThe curing of the material is also important. Most fiberglass boards use the DICY process (dicyano-mumbojumbo i can never remeber how it is spelled, look it up) . Phenolic curing is much more resilient but more expensive.Another enemy is the formation of CAF in the board. This is heavily dependent on the glass weave and resin content pre-curing. Another factor is how they drill the boards. Especially for larger holes that are made using endmills instead of a true drill , and how many strikes they allow per drill bit.CAF is a conductive anodic filament. It grows between two points of different voltage. CAF can form with as little as 12 volt ! It grows along the glass strands and spreads like wildfire. it can form in seconds under the right conditions. boards can fail spectacularly. (Especially in big battery systems that have lot of "oomph" behind them. The filament forms, vaporizes, an arc forms and the arc carbonizes the resin even further. it's a cascade effect)It always forms along the glass strands and in voids. So if you use a material with big glass strands with a loose weave and there is not enough resin you risk creating voids that do not fill during the lamination step. Drilling pcb material is a violent action. You are literally drilling through glass. The fibers shatter and it creates voids. Depending on the wear of the drill bit this gets worse. a new bit drill cleaner than a bit with 1000000 piercings behind it. your PCB fabricator knows this. If you specify this board is for high voltage you can pay a bit more to change drills more often.The same is true for milled holes. Holes larger than 1.5 mm diameter are typically milled, not drilled. The collet in the spindle simply cannot hold anything larger than a 2mm shaft.(depending on the machine. 2mm used to be the limit , but newer machines are going to 1.5). It matters what kind of milling bit they use. again : if you tell the fabricator they can use dedicated milling bits for those holes. They may use a cheapo bit for the mounting holes but an expensive and newer bit for the important holes.if the hole wall is damaged and the glass pretty broken up there is a risk for moisture entrapment during electroplating. Power up the board and you get spectacular CAf induced failures.Copper : the surface finish of the copper matters too. Rough copper tends to arc easier than smotth rolled copper. Even the plating finish becomes important.For high voltage : Panasonic R-3566D / R-3551D, Shengyi S1600None of what i have stated here is the end-all. You need to do your own homework and understand that every board is different. What works for me may not work for you and vice-versa. If you build everything on teflon standoffs using long-body thru-hole parts in dead-bug construction then none of it matters. Air is a better insulator than the pcb material. (provided it's not 99% humity and dusty)I'm just providing some insight into PCB materials and factors that are important when selecting materials and processes when dealing with high-voltage boards.

author: gnif
author_rank: Unknown
timestamp: August 24, 2024, 10:02:30 pm
content: ganfetsic was banned, this is farringdon again

author: harerod
author_rank: Frequent Contributor
timestamp: August 25, 2024, 12:13:32 pm
content: floobydust: thanks for the pictures. I regularly design against 61010 and I am familiar with PCB specifications. Your pictures would be a great answer, when some client asks, why components and traces are not routed to the edge. I also have seen, never photographed, creepage arcs around the edges when testing 3rd party designs.T3sl4co1l: We are building equipment for scientists to use. We electron pushers must take care of our biologists...All: Thanks for your insights. I gave a pretty concrete description of the actual PCB setup and gave basic datasheets for a selection of material. I explicitly specified no drills. I think we all understand the design basics regarding creepage and arcing.I was hoping for concrete experience with using a PCB as a 24/7 3kV insulator.I have forwarded the requirements to multicb and wait for their recommendations.