What is meant by "coating" when PCB track spacing is concerned?

[treez]

What is meant by "coating" when PCB track spacing is concerned?

Hi
The track spacing calculator on this page….
https://www.smps.us/pcbtracespacing.html
…where it says “coating”, does that mean special conformal coating, or does it just refer to standard solder resist?

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Also, on my desk I have a PCB. This PCB is sold the world over in the 100’s of thousands (EU, UK, Aus NZ etc etc) . It is an offline 220-240VAC Flyback with a drain voltage which goes up to 600V. It is for a general consumer product. The flyback FET is a TO220 with straight legs into the PCB…..the bare solder joints of drain and source are no more than 0.9mm apart…….the IPC2221B standard clearly states that this requires 3.5mm.
Do you know what is happening here?

 

[prairiedog]

Coatings mean conformal coating (UL/CSA/EN/BS approved) to keep dust and moisture off the PCB and allows you closer spacings. PC board solder-mask is not a suitable insulator towards safety, as the coating thickness, application method, curing etc. is not controlled and the solder mask's properties are not (safety standard) tested. They have no UL certification if you do research on it.

A TO-220 package does not meet the HV spacing requirements in two ways- between pins and sometimes also the tab insulator and bushing. Most fullpack insulated TO-220 parts are rated for SMPS voltages.

If you don't meet a safety-standard's spacings requirements, they will consider it shorted and proceed to do their single fault injections to try make the product unsafe.

If there are shorts between TO-220 pins, it is not an electrocution hazard. Just an arc starts and the fuse would protect against fire. This failure is acceptable.

If the TO-220 tab (drain) shorts to the heatsink, there could be an electrocution hazard. Not if the heatsink is PE grounded and this is the product's method of protection against electrocution.
Your insulator and bushing need to be considered. Otherwise consider the heatsink as hazardous live and see if electrocution is possible, does the heatsink and enclosure have proper spacings for that.

 

[AmitThePCBGuy]

Re: What is meant by "coating" when PCB track spacing is concerned?

Conformal coating will unlikely help you if you don’t follow the standard requirements as mentioned in IPC publications. A few international standards have suggested the use of conformal coating for limiting issues related to clearance. The conformal coating is applied as per the design requirement. There are several factors at the design stage that will determine your line spacing requirements. However, you can add special insulation at various components to avoid generation of noise.

 

[marce]

Drain and source will be at the same voltage when the device is on...