HV breakdown is a current topic here as well. In a basic double sided application that has a continuos 7KVAC with frequencies up to 30 MHz, is there any advantage to using the more expensive polyimide laminate in place of FR4? Is there any truth to the rumor that polyimide is more 'arc tolerant' because of it's higher Tg? Norm Dill [log in to unmask] ---------- From: TechNet-request To: technet Subject: Re: DES:FAB:ASSY: Breakdown Date: Wednesday, August 07, 1996 8:46AM Dom Bragge asked a question on high voltage design rules: 12 mil space 1500 V First the caveats: The voltage you quote is definitely in the "marginal" area. I believe that it violate IEC guidelines. I would be less concerned if the voltage represented a fault condition or was applied for a short time. A continuous application of 1500 V is very risky. That said, let me address your question. There are at least four possible failure modes. Ranked in order of risk they are the following: dust buildup, surface flash-over, dielectric failure, and air breakdown. Let me comment on the risk from each of these Dust build up is the most serious problem. I assume the voltage is DC (if it is AC there are other problems). The fields associated with a DC voltage attract dust. (Think of the high voltage supply in a TV set). Over time the dust will build up and span conductors. Under high humidity conditions, dust absorbs moisture and creates a leakage path that can carbonize leading to failure. We have seen this failure under field conditions. The only solution is to this is to maximize the separation of uncoated features and to assure there are absolutely no pinholes in the soldermask. Pinholes are the real risk so a double soldermask coating should be used. A conformal coating of the assembled board would also be a good idea. Surface flash-over is the second serious risk. Any dielectric surface provides a breakdown path for high voltage. The breakdown follows the surface, so a smooth surface is riskier than a rough surface (Think of the shape of the insulators they use on high voltage transmission lines). Unfortunately soldermask is very smooth and 1500 volts can arc across gaps up to 30 mils wide. This phenomena is initiated at any "triple point" where air, dielectric and metal meet. The solution is to assure that the conductor is totally encapsulated. Any soldermask pinhole is a source of an arc that will flash to the nearest uncoated land. Therefore you must assure pinhole free soldermask. I have little information on the long term reliability of either FR-4 or soldermask under high voltage conditions. Voltage is an acceleration factor in dielectric failure, but my sense is that most modern materials provide margin and 1200 volts is not a problem. I have seen application where standing voltages of 300 to 500 volts gave no long term problem. I would be willing to gamble that you could get away with 1500 volts. However there is a risk of long term dielectric failure, particularly if the temperature or humidity is high. Air breakdown should not be a problem. The Paschen curve says that the maximum spark gap for 1500 volts is about 8.5 mils. If you assure a 12 mil space air gap breakdown will not be an issue. To summarize. The two biggest issues with 1500 V are dust buildup and surface flash-over. The solution to both problems is to absolutely exclude soldermask pinholes. (The electric field easily finds even the most minute hole on a line edge). If you can exclude pinholes and guarantee a 12 mil space I think you may a reasonable shot at a good system life. Good Luck Bob Holmes Lucent Technologies [log in to unmask] ------------- Begin Original Message ------------- Using 1.6mm FR4 with LPISM, we need information on how to prevent breakdown between tracks on the outside layer separated by 12thou, when subjected to 1500V continuously. What sort of coatings/processes aid this? -- Regards, (A4fonHPUX9.0onHP715,710) JNA Telecommunications Limited ___ _____ 16 Smith Street Dom Bragge (VK2YAK) __ / / |/ / _ | Chatswood NSW 2067 PWB Designer, (R&Ddiv) / // / / __ | AUSTRALIA Tel: (+61 2) 9935 5792 \___/_/|_/_/ |_| email [log in to unmask] Fax: (+61 2) 9417 3862 http://www.jna.com.au *************************************************************************** * TechNet mail list is provided as a service by IPC using SmartList v3.05 * *************************************************************************** * To unsubscribe from this list at any time, send a message to: * * [log in to unmask] with <subject: unsubscribe> and no text. * *************************************************************************** *************************************************************************** * TechNet mail list is provided as a service by IPC using SmartList v3.05 * *************************************************************************** * To unsubscribe from this list at any time, send a message to: * * [log in to unmask] with <subject: unsubscribe> and no text. * *************************************************************************** *************************************************************************** * TechNet mail list is provided as a service by IPC using SmartList v3.05 * *************************************************************************** * To unsubscribe from this list at any time, send a message to: * * [log in to unmask] with <subject: unsubscribe> and no text. * ***************************************************************************