PCB slots/cutouts... when? - Page 1
Forum section: electronics / projects

author: fivefish
author_rank: Frequent Contributor
timestamp: October 26, 2015, 02:08:15 am
content: I know with high voltage/line voltage stuff, PCB slots and cutouts are used for safety reasons. Are there any other reasons where one may need (or beneficial) to use PCB slots/cutouts. I'm interested in protection against RFI, i.e. use of caps and PCB slots. Are slots/cutouts useful in this regard?

author: TiN
author_rank: Super Contributor
timestamp: October 26, 2015, 04:24:26 am
content: Slots also used to reduce mechanical stress from PCB to parts such as precision voltage references.Also to reduce leakage effects, as usual PCB is still worse insulator than dry air.As of protection for RFI/EMI, you would be much better with shielding, faraday cages around sensitive components and massive ground pours or traces around your sensitive components. Take a look on any high-frequency front end, such as spectrum analyzer boards, and so to get overall idea of methods used there. How serious you need to go would be dependent on your particular application.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 26, 2015, 04:54:31 am
content: For EMC, slots inground planesare more common (although, used carelessly, they're as oftensources!), but if you need that extra teensy fraction pF of isolation, it can be mechanically slotted as well. Of course for high voltage, it helps even more (creepage, clearance).That said, most often, around an isolation barrier, the absolute minimum capacitance isnot desirable. Rather, a modest amount (perhaps ~pF up to several nF), in combination with damping, helps to somewhat reduce very high frequencies and dV/dt at the isolation barrier.The damping might be an R+C across the isolation barrier, or a ferrite bead somewhere, perhaps on the isolation capacitor, or on the cable or connecting traces going from the isolated sections to their respective main circuits.The goal isn't really to filter high frequencies, because that's practically impossible (it would probably take ten kilos of ferrite, and huge cable lengths bound up in it, to filter e.g. a high side gate driver's full isolation voltage), but just to take the edge off, and control ringing.If low isolation capacitance is a priority, it'd be worth buying some medical grade power supply modules (e.g. from Meanwell, etc.) and inspecting their design. They achieve impressive results with little capacitance, probably through a combination of tricky circuit design and construction (e.g., balanced and/or shielded transformers), and optimizing the filtering (so it does as well as it can, with a minimum of capacitance).Oh, and the slots used in said power supplies are for safety/isolation, of course.The thing to realize about RF is, it's all about controlling impedances. There isn't really an open circuit. Short circuits are reasonable. So, you can't really have an "open circuit", you can only filter (into a controlled impedance, because filters only work that way), or shunt or shield it away. That's why the best RF kit is made out of machined metal bricks...Tim

author: tron9000
author_rank: Frequent Contributor
timestamp: October 26, 2015, 09:02:14 am
content: There are Thermal considerations to slots/cutouts:SHT21 - fig.10but yes primary applications are isolation/clearances and some cases trimmed capacitance.

author: Jeroen3
author_rank: Super Contributor
timestamp: October 26, 2015, 09:29:34 am
content: The clearance you need to withstand surges according to spec is large. If you use slots you can do it in less absolute distance.

author: Berni
author_rank: Super Contributor
timestamp: October 26, 2015, 10:11:36 am
content: There are tons of reasons why you might want to slot a PCB:-High voltage isolation: Most common and obvious use of slots by preventing arcs creeping over the surface-Leakage prevention: Sometimes you don't want surface contamination leaking current in a critical high impedance node, in some cases this can be used instead of or in combination with a guard ring.-Mechanical isolation: You might have a precision part that works in ppm precision like a reference of precision amp where mechanical stress on the component can cause deviations. Slots prevent force being put on the sensitive area to bend or stretch it.-Thermal isolation: Sometimes you have precision circuitry close to a power supply or high power amplifier/driver that gets hot. You might want a slot between the two to keep the heat from getting to the sensitive precision circuitry.Alternatively if you are ovenising your precision circuitry then you might want slots to keep the heat in there.-Ventilation: Sometimes you have a board mount heatsink or large power resistor that gets hot and nothing under it. In that case it might help to slot the pcb under it to allow more fresh air to get to it-Vibration: Its possible to isolate vibration by slotting a spring shape in the PCB, however this needs careful design to be effective rather than make it worse and might make the PCB easy to snap.-RF voodoo: Rarely slots might be used in fancy pants RF circuitry because for whatever reason its better to have air as a dielectric there. Only likely makes sense if you worked extensively with microwave for the last 10 years.-Creative mechanical solutions: Sometimes you simply want to stick something trough the board somewhere, or perhaps something more creative like clamping a magnetic core around a PCB area to create a inductor/transformer(Seen it done before) or something odd like thatBasically mostly just use common sense with slots and don't overuse them.

author: Phoenix
author_rank: Frequent Contributor
timestamp: October 26, 2015, 10:30:08 am
content: Quote from: Jeroen3 on October 26, 2015, 09:29:34 amSlots add to creepage distance but not to clearance distance.

author: TerraHertz
author_rank: Super Contributor
timestamp: October 27, 2015, 11:22:19 am
content: Quote from: Phoenix on October 26, 2015, 10:30:08 am[pedant^2]Depends what is in the slot. If it's polypropylene like in this microwave inverter board, the breakdown withstanding voltage is probably a *lot* better than with no slot.[/pedant^2]

author: fivefish
author_rank: Frequent Contributor
timestamp: October 28, 2015, 02:37:22 am
content: Great information! Thanks for the replies y'all.^^ wow... haven't seen PCB with part of enclosure going through the pcb slots like that.