Hi Jack,
I have not worked with FR-4 designs that have this high of voltage on
them. Thus, I can only guess at this point. But I think that a minimum,
two separate FR-4 layers and a total thickness range of somewhere in the
~5-8 mil range is probably where you may need to be. There should be
some folks out there who have designed boards with very high voltage on
them though who can offer better advice than my guess above.
Regards, Joel
Joel S. Peiffer
3M Electronic Materials Solutions Division
3M Center, Building 201-1E-17
St. Paul, MN 55144
Tel: (651) 575-1464
Cell: (612) 327-1983
[log in to unmask]
From: Jack Olson <[log in to unmask]>
To: <[log in to unmask]>
Date: 06/20/2014 10:56 AM
Subject: Re: [DC] z-axis separation for high voltage
Sent by: DesignerCouncil <[log in to unmask]>
Thanks for taking the time to respond (and to everyone else who responded
too),
and I understand your point about derating,
but I am more focused on the STARTING clearance for 1700V,
IPC Table 6-1 gives me something slightly over 125 mils,
and my board is only 62 mil thickness, about half!
I wasn't initially worried
because if you look at the slash sheets for dielectric material, its more
in the KV range PER MIL
but now I'm starting to wonder...
Where can I learn more about this?
On Fri, Jun 20, 2014 at 10:45 AM, Joel S. Peiffer <[log in to unmask]>
wrote:
> William provides some good advice to derate the materials insulation
> resistance significantly as well as providing some of the reasons for
> doing this (copper surface roughness, dielectric defects).
>
> I would like to go into a little more detail on derating. The boards
> operating conditions (temp, humidity) and expected service life need to
be
> taken into account as well. High temp/humidity operating environments
may
> call for additional derating. The overlap area of high voltage should
> also be taken into consideration. If the area is very small, the
> likelihood of a material defect is very small. However, if the area is
> very large, the likelihood of a material defect will increase in
> proportion to the area and the material may have to be derated
> appropriately. Finally, the derating should be dependent on the
> dielectric thickness. Thus, thicker materials need to be derated less
> than thinner materials. For example, a 0.5 mil defect may not have much
> impact on a 5 mil thick material but it will have severe consequences on
a
> 1 mil thick material.
>
> If you need to utilize very thin dielectrics to ensure you meet you z
axis
> thickness requirements, you may want to look at materials other than
FR-4
> to provide the required insulation resistance requirements. Some of the
> polyimide film materials have excellent insulation resistance and may be
a
> better choice in this situation.
>
> Regards, Joel
>
>
> Joel S. Peiffer
> 3M Electronic Materials Solutions Division
> 3M Center, Building 201-1E-17
> St. Paul, MN 55144
> Tel: (651) 575-1464
> Cell: (612) 327-1983
> [log in to unmask]
>
>
>
> From: "Brooks, William" <[log in to unmask]>
> To: <[log in to unmask]>
> Date: 06/20/2014 10:11 AM
> Subject: Re: [DC] z-axis separation for high voltage
> Sent by: DesignerCouncil <[log in to unmask]>
>
>
>
> The epoxy/glass dielectric actually is spec'd at 750V per mil insulation
> resistance perpendicular to the layers according to Mil-P-13949/4C for
FR4
> glass epoxy laminated materials. So 1.3 mils minimum of material
> perpendicular to the layers would net you 1KV insulation resistance top
to
> bottom or internal layer to layer.
>
> Always give yourself some margin... The back side of the copper foil is
> rough and can be 'spiked' on the laminated side for foil adhesion...
those
> spikes can cause the spacing to be less than the minimum target spacing
> you require when laminated. Also there can be bubbles or voids in the
> materials that could occur right between your two high voltage
> conductors... Unless you are trying to mimic a capacitance plane pair,
> there is little or no reason to have the high voltage lines that close.
> You don't want a short to occur internally in the board. Give yourself a
> buffer of at least a couple of additional layers of 2 mil prepreg on
> internal layers. Just saying... :) Good design practice is to de-rate by
> 50%.
>
> I don't warranty sharing my observations... or advice... but if you feel
> the need to compensate me... I like microbrewery beers... the Darker
> Belgian style... I'm not a big hops lover... though I like the hoppy
beers
> too just not as much... :)
>
>
> William Brooks, CID+
> Senior MTS (Contract)
> 2747 Loker Ave West
> Carlsbad, CA 92010-6603
> 760-930-7212
> Fax: 760.918.8332
> Mobile: 760.216.0170
> E-mail: [log in to unmask]
>
>
>
> -----Original Message-----
> From: DesignerCouncil [mailto:[log in to unmask]] On Behalf Of Jack
> Olson
> Sent: Friday, June 20, 2014 7:35 AM
> To: [log in to unmask]
> Subject: [DC] z-axis separation for high voltage
>
> maybe I'm having a "not enough coffee yet" morning, but I was asked how
> much separation I need between layers for high voltage.
>
> 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 a 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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