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Jack inquired about differences in high-voltage (electrical spacing) isolation
requirements between components and printed boards.
Date: 5/30/96 10:47 AM
From: Jack Olson
>I'm not sure if this is the best place to ask electronics engineering
>type questions; so if anyone knows of an engineering-related forum,
>please let me know...
>
>I am in the process of designing a power board to military specs, and
>one of the requirements is "2500V isolation between primary and
>secondary". Using the formula that says 0.12 mils/volt, I am looking
>at a 300mil clearance wherever appropriate, right?
You're correct for a type "B4" permanent polymeric coated printed board and
for a type "A7" conformally coated printed board assembly per Table 3-1 in
IPC's-D-275.
>One of the components in the design is an opto-isolator in a DIP8
>package, and the data sheet guarantees 3000V insulation between
>primary and secondary. Since it is packaged in a DIP, and I am
>required to use 65 mil pads, I am only left with 235 mils between
>primary and secondary, which works out to (235/0.12) 1958.3 volts.
>
>Am I misinterpreting the requirements?
IMO - No
>Is their 3000V claim based on a different dielectric or something
>internal to the component? If so, how is that meaningful knowing the
>DIP package must be mounted to something with a lower value?
IMO, it's the differences in the requirements and ratings between component
manufacturers and printed boards.
Your component is "guaranteed 3 kV" but I suspect it's not "rated" for
continuous operation at that voltage. I'm not sure about you MIL SPEC
requirement for 2500 V, but I suspect it is for some limited time duration, I
suspect, typically in the range of 3-10 seconds.
To my knowledge, the component manufacturer's requirements use essentially
pristine "new" parts, not remembering for sure, but off-hand, the components
may??? be tested for isolation after some humidity aging, then again I suspect
maybe not.
A quick check of one of the optical isolator application manuals gets out of
very easily, their "rated/guaranteed" is 2.5 kV for up to 4 seconds, then in
the application notes, they cop-out and state the continuous voltage depends
on the cleanliness, humidity, etc of the application and you should
test/qualify to your environment.
The D-275 requirements are based on of long-term environmental testing in all
kinds of environments, salt air, desert, altitude, dust, dirt and atmospheric
pollutants.
>Is the 0.12 mils/volt spec over-estimated to be on the "safe" side?
Yes it's very conservative to ensure a very low risk.
>Are the terms "insulation" and "isolation" interchangeable?
IMO no, without going to the dictionary, insulation is generally used with a
term like resistance, meaning that ites not perfect, there are some
losses/conduction. Isolation generally means that something on one side of
the isolator has no effect on something on the other side of the isolator. In
some ways, an isolator would be a perfect insulator for something.
>Can I use this component (and/or) Am I splitting hairs?
IMO, yes, I don't think you're splitting hairs, but:
* You just need to keep as much separation (isolation) as you can between the
land patterns, then use the increased spacing for the remaining condutor
spacing.
* Make sure the printed board manufacturer "really cleans" the surface of the
printed board prior to the application of solder resist (applicable)
* Make sure the printed board doesn't get dirtied up during assembly due to
handling
* Make sure assembly residues (low residue fluxes or cleaning residues) are
suitable for the application and don't create future paths for electrical
leakage
* I have no experience with low residue fluxes and high voltages, so from the
old school, clean it really good, then conformally coat with a good adherent
coating.
>thanks in advance, Jack
Hope this helps
Ralph Hersey
e-mail: [log in to unmask]
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