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February 2007

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Subject:
From:
Ahne Oosterhof <[log in to unmask]>
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Date:
Tue, 6 Feb 2007 08:28:51 -0800
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See also note from Bill Brooks: 
http://listserv.ipc.org/scripts/wa.exe?A2=ind0107&L=DesignerCouncil&D=0&T=0&
P=5539

Air, DC: 75-100V/mil 
AC: use 3x rms value
UL-61010A-1

There are rules and there are rules! Most of them have grown out of painful
experiences and some have come from testing. The first ones are good, the
second ones can be questioned.

There are many variables when it comes to voltages and spacings. The
Engineering Handbook gives some give numbers, but they apply to sealevel
usage and clean environments. So you have to know under what conditions
rules were established.

If the product is going to be used at altitudes over 10,000 ft, the spacings
have to increase. If the environment is dirty (e.g. full of dust, rich in
sulfur fumes as near a refinery) the spacings have to increase. Not at 70V,
but at higher voltages when you have sharp points the spacings have to
increase. I would not trust the solder mask to provide any protection, just
consider it a bonus in the margin of safety.

But these are all things the design engineer has to know and account for. If
he does not, I would send him an e-mail (written documentation!) and point
out the discrepancies in the design. Or at review meetings bring the subject
up. You don't weant to be holding the bag when the product fails in the
customer's hands.

One thing you can do to improve your situation is to cut a space between the
conductors. Air is a better insulator than circuit board material. However,
it does weaken the board. 

Compromises, compromises; life is full of them.

Have fun anyway,
Ahne.




-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of valerie St. Cyr
Sent: Tuesday, 30 January, 2007 10:37
To: [log in to unmask]
Subject: [TN] Spacing Requirement for Adjacent PFP pads carrying 70V

Dear Technetters,

I have a design issue to resolve that involves spacing and voltage. I have
read IPC-2221 6.3 Electrical Clearance and all the sub-sections, including
Table 6-1, electrical Conductor Spacing, as has the Engineer and we cannot
agree on the application. So, if someone could address this specific
problem, especially if you have a design like what I will describe, that
would be most helpful.

We have a 1.5 mm (.059") pitch Press-Fit connector; the surface pads are
1.19 mm (.047") with pad to pad spaces of .30 mm (.012"). Adjacent pins
carry 70 Volts. Do we or do we not need to put soldermask up onto the pads?
Depending on how one reads the spec, the spacing conductor to conductor
needs to be either .13 mm (.005") with mask or .60 mm (.023"). 
It's clear reading the spec that "conductor" means trace because 6.3.4
excludes soldering lands (I don't know why; they are conductive). (ps: 
this is not a harsh environment product; it is installed in either a clean
factory and/or office-like environment). He discounts the Bare Board column
B4 since the pads are uncoated (soldermask clearances) and hence he argues
there is effectively no mask even though we have mask between pads. 
If we discount the mask between pads, then we need to use column B2, which
is .60 mm, which we don't have.  So, to have .6 mm "spacing", the Engineer
wants to encroach mask onto the pads. 

When I look at the Assembly columns, the section titles say for component
leads ...  the leads are roughly .50 mm (.020") and so on 1.5 mm pitch, are
1.0 mm apart (.025"); if 6.3.6 and 6.3.7 do apply to "leads" and not the
pads they terminate in, then the spacing is fine at either the .13 or the
.50 mm spacing. But the Engineer is reading the Assembly columns as
terminations, and we can't meet .50 mm spacing (uncoated); so 

We are at a stalemate.  While we don't need to make solder filets because
the connector is press-fit, the Engineer wants soldermask encroachment. We
can't agree, first, if they need to be encroached; and then, second, if they
do, how much over we need to go.

The problem is that with registration issues thrown in, the fabricator is
getting lots of false opens at shorts-and-opens electrical test because the
probes are hitting the mask more often that not. (He already designed it
with mask opening +4 mils over FHS.)

How do we interpret Table 6-1 (if at all) for pads carrying 70 volts?

Thank you, thank you, thank you in advance.

Regards, Valerie

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