Mr. Peter My experience with agency engineers doing NRTL evaluations has been a bit different.
When CC is used to meet spacing requirements, it can be considered a component of an insulation system. And UL may require, for some boards, a Type Test based on 1446. Although it seems to be more typical for the application and testing of CC to be specified in the end-item std (e.g., 60950-1, cl 1.5.2). If the CC application is not required to meet the requirements of the end-item std, then its use must verify that it does not affect the electrical or mechanical properties of the end-use box. Some Section Generals of FUS reports also require that the CC process be specified, monitored and subject to recurring tests. I am not certain if the process control applys only to usage required to meet spacing and di-electric withstand. According to UL engineers that I've talked to, applicable UL stds for PWBs are 94, 746, 1446, and of course 796. But the only thing that UL796 seems to say about CC is that (cl 10.1) it shall be evaluated for flammability and any mechanical effect on other PWB materials. While 796 has specific electrical and mechanical performance requirements for PWBs, the target seems to be the board house, not the end-use item mfr; which is where the CC is probably applied. The basic std for CCN QMJU is UL94; part of the scope says "The methods described in this Standard involve standard size specimens and are intended to be used solely to measure and describe the flammability properties of materials...". So UL94 cannot be used by itself for end-item application of CC. I would be very interested in yours and others' experience with direct NRTL engineering opinions on this subject. luck, Brian -----Original Message----- From: Peter L. Tarver [ mailto:[email protected]] Sent: Tuesday, December 16, 2003 10:28 AM To: [email protected] Subject: RE: question concerning conformal coating of PWB Brian - I don't know what your experience with conformal coatings are and I do not intend to contradict that experience, but UL1446 does not apply to conformal coatings on printed wiring boards. Refer to the scope of UL1446 at: http://ulstandardsinfonet.ul.com/scopes/1446.html which applies primarily to wound part (transformer, motors and the like). The applicable standard would be UL796 or UL746C (my memory is a little fuzzy on this, but Greg Galluccio can certainly let us know). The UL product category for conformal coatings is: Coatings for Use on Recognized Printed Wiring Boards - Component (QMJU2) I recommend interested parties read the UL Guide Card for QMJU2. The primary consideration, other than suitability for use in lieu of some fraction of the otherwise required creepage distance, is flammability of the combined board/coating. Even when used for customer reasons and not otherwise, the flammability consideration should be taken into account. Regards, Peter L. Tarver, PE [email protected] -----Original Message----- From: Brian O'Connell Sent: Tuesday, December 16, 2003 8:04 AM UL tends to view conformal coating as part of an insulation system, but this is not a requirement according to UL60950-1. Even if not required for spacing, you still must perform all Type Tests using conformal-coated PWBs, and the material should be described in the FUS report. If you do not want to always CC the PWB, carefully select which Type Tests are to be repeated using PWBs both with and without CC. This will demonstrate that CC should not affect the system safety. The UL specialist for insulation systems is Mr. A.J. Baig. He will be able to determine if UL1446 even scopes this issue...
