Sorry, no dendrites are a surface electrochemical thingie. What I'm talking about is a kind of electrophoresis. :) Brian Johnson, Joseph wrote: > Brian, You are right in all aspects of your explanation! > > Also the name of this migration that you are talking about I believe is > called "Dendrites" (I probably have it spelled wrong...). I have seen > photo's of traces actually growing conductive whiskers. This is another > reason to space your low voltage traces as specified. This way you don't > start to get a lot of field failures after 5-7 years. > > Joe J. > > -----Original Message----- > From: TechNet [mailto:[log in to unmask]] On Behalf Of Brian Ellis > Sent: Thursday, February 03, 2005 4:32 AM > To: [log in to unmask] > Subject: Re: [TN] Technical basis for Table 6-1 of IPC-2221 > > Your surmise may be partially true if you had FR-4 between the conductors, > but you don't, you have air, and often humid air, at that. > When I was a student, I was given a rule of thumb that humid air broke down > (i.e. ionised) at a gradient of 15,000 V/inch (600 V/mm). Does that tell you > anything? Of course, this rule of thumb is not a scientific absolute as > there are many more variables, such as air pressure (altitude), temperature, > relative humidity and, in the case of semi-planar devices such as PCBs, > surface contamination. > > However, it doesn't even end there. FR-4 contains molecules of sodium > chloride which "attract" humidity and can therefore ionise under the > influence of a voltage gradient, and migrate within the resin matrix. > This is measurable at gradients as low as 5 V/mm and can cause all sorts of > funny effects at very high impedances, and it can "remember" its history > over hours, because the fields between the positive and negative ions (after > the causal gradient is removed) are so low that the reverse migration is > very slow. I described this phenomenon in papers published in the second > half of the 1980s and I also adjudicated the dissertation of a student at > the Swiss Federal Institute of Technology who studied this phenomenon. > Because of this, I recommended a max gradient of 100 V/mm for critical > tracks on circuits working up to 10 megohms impedance, down to 5 V/mm at the > order of a teraohm. > > Brian > > - Bogert wrote: > >>February 2, 2005 >> >>Table 6-1 defines minimum electrical spacing requirements between traces > > on a PWB. What is the technical basis for the Table? Is there a formula > associated with the values in the table? For example, if I have a PWB > operating at 30 volts DC, the minimum spacing required between internal > traces is 0.05 mm. What if a PWB operates at 30 VDC but only has 0.04 mm > spacing, is there a technical concern? Based on what formula or technical > consideration? > >>The minimum spacing in the Table are much less than the electric strength > > requirements for FR4. For example, my understanding is that FR4 is about > 1250 volts/mil. Therefore, at 1 mil of spacing the PWB should be able to > withstand 1250 volts between conductors. > >>The values in the table are also different than those included in UL 840. > > Why? > >>Any help you can offer on this point would be appreciated. >> >> >>--------------------------------------------------- >>Technet Mail List provided as a service by IPC using LISTSERV 1.8e To >>unsubscribe, send a message to [log in to unmask] with following text in >>the BODY (NOT the subject field): SIGNOFF Technet To temporarily halt >>or (re-start) delivery of Technet send e-mail to [log in to unmask]: SET >>Technet NOMAIL or (MAIL) To receive ONE mailing per day of all the >>posts: send e-mail to [log in to unmask]: SET Technet Digest Search the >>archives of previous posts at: http://listserv.ipc.org/archives Please >>visit IPC web site http://www.ipc.org/contentpage.asp?Pageid=4.3.16 >>for additional information, or contact Keach Sasamori at >>[log in to unmask] or 847-615-7100 ext.2815 >>----------------------------------------------------- >> > > > --------------------------------------------------- > Technet Mail List provided as a service by IPC using LISTSERV 1.8e To > unsubscribe, send a message to [log in to unmask] with following text in the > BODY (NOT the subject field): SIGNOFF Technet To temporarily halt or > (re-start) delivery of Technet send e-mail to [log in to unmask]: SET Technet > NOMAIL or (MAIL) To receive ONE mailing per day of all the posts: send > e-mail to [log in to unmask]: SET Technet Digest Search the archives of > previous posts at: http://listserv.ipc.org/archives Please visit IPC web > site http://www.ipc.org/contentpage.asp?Pageid=4.3.16 for additional > information, or contact Keach Sasamori at [log in to unmask] or 847-615-7100 > ext.2815 > ----------------------------------------------------- > > --------------------------------------------------- > Technet Mail List provided as a service by IPC using LISTSERV 1.8e > To unsubscribe, send a message to [log in to unmask] with following text in > the BODY (NOT the subject field): SIGNOFF Technet > To temporarily halt or (re-start) delivery of Technet send e-mail to [log in to unmask]: SET Technet NOMAIL or (MAIL) > To receive ONE mailing per day of all the posts: send e-mail to [log in to unmask]: SET Technet Digest > Search the archives of previous posts at: http://listserv.ipc.org/archives > Please visit IPC web site http://www.ipc.org/contentpage.asp?Pageid=4.3.16 for additional information, or contact Keach Sasamori at [log in to unmask] or 847-615-7100 ext.2815 > ----------------------------------------------------- > --------------------------------------------------- Technet Mail List provided as a service by IPC using LISTSERV 1.8e To unsubscribe, send a message to [log in to unmask] with following text in the BODY (NOT the subject field): SIGNOFF Technet To temporarily halt or (re-start) delivery of Technet send e-mail to [log in to unmask]: SET Technet NOMAIL or (MAIL) To receive ONE mailing per day of all the posts: send e-mail to [log in to unmask]: SET Technet Digest Search the archives of previous posts at: http://listserv.ipc.org/archives Please visit IPC web site http://www.ipc.org/contentpage.asp?Pageid=4.3.16 for additional information, or contact Keach Sasamori at [log in to unmask] or 847-615-7100 ext.2815 -----------------------------------------------------