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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