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February 2005

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Subject:
From:
Brian Ellis <[log in to unmask]>
Reply To:
TechNet E-Mail Forum <[log in to unmask]>, Brian Ellis <[log in to unmask]>
Date:
Thu, 3 Feb 2005 16:13:42 +0200
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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.
>>
>>
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