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January 2007

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Subject:
From:
"valerie St. Cyr" <[log in to unmask]>
Reply To:
TechNet E-Mail Forum <[log in to unmask]>, [log in to unmask]
Date:
Tue, 30 Jan 2007 13:37:17 -0500
Content-Type:
text/plain
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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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