Mail*Link(r) SMTP FWD>DES:ISOLATION 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] ------------------ RFC822 Header Follows ------------------ snip-sniped it all