From: Bernie McEwen-LBM022 <[email protected]> Subject: RE: Coated Printed Boards
Any material declared as a conformal coating must meet the abrasion resistance tests in EN60950 2.9.5. A general purpose silk-screened solder-resist is not compliant. Internal layers of a multilayer PCB may have creepage distances reduced to the clearances because they are a pollution degree one environment. But spacings cannot be reduced further because although the laminate layer covering the tracks would meet the abrasion test, the material does not extend between the tracks; it sits on top, leaving a void (or a bonding material that may have voids) between tracks. Provided a single layer meets the relevant proof tests, then two layers of laminate are a compliant insulator. Tracks may cross or run together provided at least two layers of laminate separate them. Regards. Bernie McEwen. > ---------- > From: Duane J. Marcroft[SMTP:[email protected]] > Reply To: [email protected] > Sent: 11 May 1998 17:58 > To: [email protected] > Subject: Re: Coated Printed Boards > > Richard, > > The PCB coating is silkscreened. Because it is a silkscreen, it is > by its > very nature unreliable as an insulator. Silkscreening produces many > voids > in the surface (air bubbles) and too much variation in coating > thickness. > As a consequence it cannot be considered. > > To be an viable insulator (in calculating creepage and clearance) > coating must > be at least 2 layers of film (sheet or tape of Kapton.. etc). In the > opinion of many in the industry a silkscreen coating is too unreliable > to > be considered. > > Duane Marcroft > Telecom and Data Communications Consutant > > ___________________________________ > > >Doug, > > > >I accept the reasoning for a clearance when there are mounted > >components, but can we now consider areas on PCB's where there are NO > >components. > >See PCB cross sectional drawings below - > > > >Scenario A > >This is a representation of the 2nd paragraph of 2.9.5 lines 1 and 2. > >The coating will provide the necessary insulation so the separation > >distances can be reduced from those of tables 3, 4, 5 and 6. > > > >Scenario B > >This is a representation of the 2nd paragraph of 2.9.5 lines 2 and 3. > >Assume the following: > >Secondary Circuit > >Basic Insulation > >Working Voltage - 150 V r.m.s > >Transient rating - 1500V > >Pollution degree - 2 > >Material group - IIIa > > > >The clearance distance given by Table 5 for the above is - 1.0mm > > > >The creepage distance given in Table 6 for the above is - 1.6mm > > > >Is it not possible that if the distance between the conductors was > >specificed from Table 5 that the creepage distance would be > >insufficient, given that a charge is more likely to travel across a > >surface (creepage) than through air (clearance)? > > > >Note 2 to Table 6 states "If the creepage distance derived from table > 6 > >is less than the applicable clearance from tables 3 and 4 or from 5, > as > >appropriate, then the value for that clearance shall be applied as > the > >value for the minimum creepage distance. > >This only happens when the CTI of the material is above 600 and the > >voltage is below 200V (for a 1500V transient). > >The CTI is not specified in 2.9.5, is it assumed that the CTI will be > >above 600 otherwise it won't pass the electric strength tests? > > > >I suspect that line 3 of the second paragraph should read "the > distances > >in tables 3,4,5 and/or 6 apply". > > > > > >Regards > > > >Richard Steele > >Equipment Engineering Group > >Fujitsu Telecommunications Europe Limited > > > > > >Douglas Mckean wrote: > >> > >> When analyzing printed circuit boards, BOTH clearance AND creepage > >> are issues. The flat pcb fab (no parts installed or soldered) > would > >> most likely have nothing but creepage issues - distances along > >> the surface of the board ("as the crow walks"). > >> > >> An assembled pcb with parts that could compromise creepage > >> (parts laid down on the board for example or wires added > >> as a mod after assembly or maybe even a piggyback board) > >> would have clearance issues as well ("as the crow flies"). > >> > >> Thus, the reason why both clearance and creepage are included > >> in all the IEC-950 based standards that I've seen: UL-1950, > >> EN-60950, ... > >> > >> Regards, Doug > >> > > > >Attachment converted: Marcroft Biz:xx165pdeengsaf.002.082.gif > (GIFf/JVWR) > >(00003318) > > >
