Question about Prepreg and high voltage on mid layers

According to the old 1010.1 standard
"There are no CLEARANCE or CREEPAGE DISTANCE requirements for the interior of void-free moulded
parts, including the inner layers of multi-layer printed circuit boards."

Other references state 0.2mm or 0.25mm minimum

I have always taken a conservative approach and used 25mils minimum for <=300V, and never had an internal failure during Hi-Pot tests.

Yesterday I read this in an Infineon note about electrical safety and layout (section 4.2.11):
"Therefore, it is always suggested to keep the internal layers (separated just using few μm of prepreg) as common or ground layers which have a voltage drop not higher than 100V. Using higher voltage layers may be possible, but any overlap of inner layers must definitely be avoided."

This is the first time I have read about a 100V limit for layers insulated with prepreg, I assume they are concerned about voids in the Prepreg. I also read some specify a double layer of Prepreg to minimize the chance of a void.

Has anyone ever run into this 100V rule?

Comments welcome.

Link to paper: https://www.infineon.com/dgdl/Infin...N.pdf?fileId=db3a30433d1d0bbe013d20e0cbf017fe

The good point of the Infineon application note is that it lists all possible creepage and clearance issues involved with high voltage design. Regarding PCB design, it's not up to date with safety standards, particularly IEC/EN 61010-1.

Since IEC 61010-1:2010 respectively EN61010-1:2011, there's a detailed Annex K covering the internal insulation of PCB. There a choices of either using certain substrate thickness or individually test the isolation strength.

I don't see a "100V-rule" for prepreg. You'll check the actual substrate and prepreg thickness, possible prepreg voids must be considered.

- - - Updated - - -

As an additional comment, in contrast to the PCB stackup discussed in paragraph 4.2.11, the commonly used design of four layer PCBs is using a single core with prepregs on the outside. But if you refer to two cores with center prepreg, there's still a wide range of possible prepreg thickness.