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 -----------------------------------------------------