There are UL Recognized conformal coatings (such as Chemtronics Inc., Kennesaw GA #AR-2000 & SR-2000) which can serve in lieu of spacings, and should not require additional testing if used as certified. The problem becomes one of application - who wants the mess and bottleneck on a production line, much less the problem of keeping the stuff out of where it does not belong? Another problem is getting the required thickness where it is needed, especially if it involves component lead ends - the stuff naturally does not want to build up on a sharp point.
Mike Harris/Teccom ----- Original Message ----- From: John Juhasz To: '[email protected]' ; [email protected] Cc: [email protected] Sent: Friday, February 01, 2002 11:30 AM Subject: RE: Pollution Degree vs. Creapage Distance Just some anecdotal info . . . There was an occassion at a previous employ where, due to a brief mental hiccup on the part of several folks, a printed circuit get fabricated with not enough spacing (damn close though to the required) between a mains trace and secondary in an internal layer for a card-cage backplane. The discussion that ensued with knowledgable folks at a well-known NRTL brought forth the following: a) Extra tests - thermal aging and thermal cycling tests need to be performed. Time consuming and expensive. b) Tight quality control on the part of the fabricator to ensure layer dimensions, adhesion, etc. Reduces flexbility to change vendors at a moment's notice. c) Routine electric strength testing by us. In short, they noted that while possible, it is difficult to maintain the pollution degree 1 in a printed circuit. Typically the pollution degree is applied to 'potted' items. Taking all into consideration it was easier for us (and less expensive) to respin the board. John Juhasz Fiber Options Bohemia, NY -----Original Message----- From: [email protected] [mailto:[email protected]] Sent: Friday, February 01, 2002 12:50 PM To: [email protected] Cc: [email protected] Subject: Re: Pollution Degree vs. Creapage Distance In a message dated 1/31/2002, Rick Busche writes: Does the application of a solder mask allow for a change from pollution degree 2 to pollution degree 1? I understand that conformal coating requires significant testing when used to reduce spacings per table 7, but in this case I am only asking if solder mask can be used to improve the pollution concern. Hi Rick: You do not mention which standard you are looking at, but if it is one of the IEC 950 derivatives there are some clauses that specifically address the questions you have. For example, in EN 60950, Third Edition, clause 2.10.5.3 addresses printed circuit boards, and clause 2.10.6 addresses solder mask. To the extent you can use inner layers, clause 2.10.5.3 should provide you some of the relief you seek. I have not ever tried to qualify a solder mask under clause 2.10.6 due to the burden of the additional tests, but you may want to consider it. Joe Randolph Telecom Design Consultant Randolph Telecom, Inc. 781-721-2848 http://www.randolph-telecom.com
