PCB primary to secondary clearance on different internal layers?

[Flyback]

Hi,
Just checking, are there any regs against "drawing the curtains" (with copper pours) under an SMPS transformer to shield it from an underlying earthed heatsink?
ie, using layer 2 on pri, and layer 3 on sec , to totally cover it underneath with "quiet node" copper pours, eg pri and sec ground pours.

......for the 1.6mm PCB, ayk, the prepreg will be some 0.35mm thickness....thats good for many kV.
Ayk, in air, flashover distance is 160um per kV......so going through pre-preg it will be much more forgiving.

Page 53 of the below IPC-2221A, ayk, the bastion of all things PCB layout, confirms that for internal tracking on PCBs, you can get 301-500V with just 0.25mm on the same internal layer.
As such, if you are on two different internal layers, then the clearance will not need to be high at all....it certainly wont need to be 6mm as is needed for external layer primary to secondary clearance.
I do believe that IPC-2221A does not specify an "internal layer to different internal layer clearance" for primary to secondary clearance.........but it is up to the maker to use due diligence.


IPC-2221A PCB standard
https://www-eng.lbl.gov/~shuman/NEXT/CURRENT_DESIGN/TP/MATERIALS/IPC-2221A(L).pdf


The following claims 54kV/mm for pre-preg
https://www.isola-group.com/wp-cont...s/185hr-laminate-and-prepreg.pdf?t=1005241780

We only need to withstand the standard 3.5kV flash test.

 

[ChrisP58]

I assume that you're talking about a 4 layer PCB. If not, please clarify.

The internal dielectric layer thickness is not usually the same between different layers. Nor are these numbers the same for different PCB shops. In the first chart below, the core thickness is almost 3 times your estimate, while the two prepreg layers are only about 0.24mm. The second chart uses a thicker core and thinner prepreg layers.

If the internal layer to layer thickness is important to your design, then you need to specify that, and make sure that your chosen PCB vendor can make what you need.

Something else you'll need to watch is the clearance between the inner layer copper and the walls of the plated through holes.

(the two images above are random choices from google)