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04-29-2025, 12:36 AM
Hi Guys
Above we saw a conservative voltage spacing for PCB traces, which works out to 8V33 per mil.
UL (Underwriters Lab, a third-party approval agency) in the USA uses an aggressive trace spacing formula of 40V/mil. Where our conservative 40 mil trace spacing allowed 40 / 0.12 = 333V, the UL formula allows 40 x 40 = 1600V. This is 4.8 times the conservative limit.
I generally use the conservative approach, but for a trace here or there I have resorted to less conservative spacing and had no problems.
There are complex formulae which take into account many factors, beyond what we have considered here. The table above is a practical average of many factors saving PCB designers a lot of grief trying to slog through the detailed equations and look-up tables for minute surface details, contaminants, etc.In any case, we can see that there is quite a bit of freedom to have tight spacing when needed and that likely no harm will befall us.
Over time, the surface of the PCB may become dirty with substances that may be slightly conductive themselves, or that may compromise the solder mask and its innate insulating ability. The PCB itself is hydroscopic, which means that it can absorb moisture through raw edges. PCBs are separated using a router bit, which leaves raw edges. We can seal these edges before we install components, or we can simply design the board so there are no traces or components right near the edge. How near to the edge is too near? Probably smaller than we need to worry about.
The boards are usually cleaned at the PCB house before they are shipped. As long as you handle them carefully, wash your hands first, and try not to get too much oil from your skin onto the surface of the board, then they should remain relatively clean for a while.
As we mentioned in the TUT-series and Technical Articles, it is best to avoid he use of flux remover chemicals after stuffing the board. The best chemicals for the task have been banned, leaving types which must be used in huge amounts and flushed as well as possible. Otherwise, the flux remover breaks the solder resin into chemicals that will eat the solder mask and sometimes the PCB.
Our advice is then to let the resins cool and harden, then chip them off if you find them ugly. residue from Kester 245 dries clear, where tradition Kester 44 dries brown.
Above we saw a conservative voltage spacing for PCB traces, which works out to 8V33 per mil.
UL (Underwriters Lab, a third-party approval agency) in the USA uses an aggressive trace spacing formula of 40V/mil. Where our conservative 40 mil trace spacing allowed 40 / 0.12 = 333V, the UL formula allows 40 x 40 = 1600V. This is 4.8 times the conservative limit.
I generally use the conservative approach, but for a trace here or there I have resorted to less conservative spacing and had no problems.
There are complex formulae which take into account many factors, beyond what we have considered here. The table above is a practical average of many factors saving PCB designers a lot of grief trying to slog through the detailed equations and look-up tables for minute surface details, contaminants, etc.In any case, we can see that there is quite a bit of freedom to have tight spacing when needed and that likely no harm will befall us.
Over time, the surface of the PCB may become dirty with substances that may be slightly conductive themselves, or that may compromise the solder mask and its innate insulating ability. The PCB itself is hydroscopic, which means that it can absorb moisture through raw edges. PCBs are separated using a router bit, which leaves raw edges. We can seal these edges before we install components, or we can simply design the board so there are no traces or components right near the edge. How near to the edge is too near? Probably smaller than we need to worry about.
The boards are usually cleaned at the PCB house before they are shipped. As long as you handle them carefully, wash your hands first, and try not to get too much oil from your skin onto the surface of the board, then they should remain relatively clean for a while.
As we mentioned in the TUT-series and Technical Articles, it is best to avoid he use of flux remover chemicals after stuffing the board. The best chemicals for the task have been banned, leaving types which must be used in huge amounts and flushed as well as possible. Otherwise, the flux remover breaks the solder resin into chemicals that will eat the solder mask and sometimes the PCB.
Our advice is then to let the resins cool and harden, then chip them off if you find them ugly. residue from Kester 245 dries clear, where tradition Kester 44 dries brown.
