Highly Accelerated Stress Test
Basically, you cyclically "kick the tires" hard to try and precipitate latent
failure mechanisms which could take years to show up. The test profile and
equipment is custom designed to correspond to the greatest perceived risks in
your construct. For voltage stress, the test is typically biased temperature
and humidity.
-----Original Message-----
From: Robert Kondner [mailto:[log in to unmask]]
Sent: Monday, June 23, 2014 4:26 PM
To: 'TechNet E-Mail Forum'; Wayne Thayer
Subject: RE: [TN] z-axis voltage separation
Wayne,
You wrote: "qualified via HAST"
Any hints on what HAST means? Now HSAT might be Hind Sight A Testing? :-)
Bob K.
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Wayne Thayer
Sent: Monday, June 23, 2014 3:44 PM
To: [log in to unmask]
Subject: Re: [TN] z-axis voltage separation
Surface flash and arcing in air are pretty well understood, but there are
large fudge factors for shape and humidity and contamination.
Breakover inside a dielectric which isn't fundamentally designed as a high
voltage material and consists of a bunch of fibers with resin filler has a
bunch of interesting failure modes, some of which take a long time to happen.
As has been suggested, you could get insignificant tiny arcs from fiber to
fiber which would eventually result in a "tree-like" failure in the material.
If you need high voltage high reliability in a dielectric, the material has to
be qualified via HAST in the usage condition. Manufacturers do sell
pre-qualified materials--FR4 isn't one of them.
So starting with surface flash criteria is one way to look at it, but I think
that is pretty darn conservative on it's own. I wouldn't add extra factors,
and you could probably go higher stress than that safely.
But as I previously indicated, go with CAF resistant material and go extremely
conservative if there's a plated through-hole near by.
And that's all you get for a nickel!
Wayne
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Robert Kondner
Sent: Monday, June 23, 2014 1:49 PM
To: [log in to unmask]
Subject: Re: [TN] z-axis voltage separation
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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