Ahne,

 You mentioned "I would put in a generous safety factor"

 I have been looking for a realistic "Safety Factor" for some small high isolation voltage coils. Any idea how "Safety Factors" are computed? Are safety factors just wild guesses for engineers to make up themselves?

Thanks,
Bob K.

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Ahne Oosterhof
Sent: Monday, June 23, 2014 12:54 PM
To: [log in to unmask]
Subject: Re: [TN] z-axis voltage separation

After a long search, http://advancedmaterialscience.com/products/NEMA%20FR4.pdf
Dielectric strength:  550 volts/mil

An example (from a vendor).
Semi-Flex FR4, 2 Layers Prototype PCB FR4, 0.3mm Thick Board thickness: 0.3mm Electric strength: >1.3kv/mm

Dupont Pyralux: Dielectric Strength,  ASTM-D-149,  6-7 kV/mil

FR4 is not a homogenous material, so I expect that the dielectric strength for thinner layers will be lower than for thicker layers. And if you measure it, I would put in a generous safety factor.
Many vendors provide some kind of number for dielectric strength, but rarely add the complete description of and conditions under which the material was tested.

If your concern is rooted in safety you will end up testing it yourself for UL (or something like that) approval.

Have fun,
Ahne.


-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Jack Olson
Sent: Friday, June 20, 2014 8:38 AM
To: [log in to unmask]
Subject: [TN] z-axis voltage separation

maybe I'm having a "not enough coffee yet" morning, but I was asked how much separation I need between layers for high voltage.
I tried to search the TechNet Archives, but it doesn't seem to work as well as it used to!
anyway,

We have a design that may have 1700V in several places.
Since we are looking at a clearance into the board, layer-to-layer I'm pretty sure I can use the "internal" column B1 of Table 6-1 in IPC-2221 (using Table 6-1 for z-axis was discussed in an IPC committee meeting and no one disagreed)

but the number I get for 1700V is =
(.25 mm for the first 500V) plus (.0025 mm for each of the other 1200V, 3
mm)
equals 3.25 mm

For one thing, it already seems like I'm off-track because .25 for 500V doesn't correspond very well with 3 mm for 1200V, but if you can't trust IPC..... well, let's not go there.

My REAL question is that, although I'm safe using 3.25 mm, my board is not that thick!
Is there a smaller z-axis clearance that can be used for 1700V? across typical FR4 material?
(we are using a RoHS compatible 170Tg /126)

What's the MINIMUM layer spacing I can use for 1700V?

thanks,
Jack


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