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April 1997

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Subject:
From:
Ralph Hersey <[log in to unmask]>
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Date:
Wed, 16 Apr 1997 08:14:54 -0700
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David--

I agree with Susan.

For the last three years, in the Design-through-Acceptance of printed
boards workshop, we cover the various levels of wicking acceptance for
the three classes of products.  [Comment: IMO, the existing dimensional
requirements are not based on technical requirements, rather the
acceptance criteria is graduated to "look good" for very little, some,
and a-lot; respectively for Class 3, 2, and 1.  There may have been some
consideration for a combination of minimum annular ring and dielectric
spacing (conductor spacing).] 

During in the workshop discussion, I mention the real requirements are
to maintain a minimum electrical (dielectric) spacing between
"electrically different" conductive patterns (conductors, plated-through
holes, or lands).  

>From a design point of view, by requiring lands on all layers
(functional or not) coupled with internal land-to-conductor spacing
ensures/controls the electrical spacing because designers will not be
able to create a design with a minimum plated-through hole -to-
conductor spacing, which could be compromised by manufacturing
registration.

>From an inspection/quality/MRB action, for most applications, wicking is
almost a "don't care", from a practical point of view, as long as the
minimum dielectric spacing is maintained.  Wicking is some what a
"process indicator" that the drilling process should be investigated or
there may be a material problem with the bonding of the resin to the
reinforcement fibers.

A couple of thoughts:

1) A caution is the direction (angle) of wicking with respect to a
nearby conductor.  If the wicking is angled (along the fibers in the
yarn) toward the conductors on the planar surface of the fabric, then
this is a "high risk" condiction because there may be "microvoids" along
the fibers that could reduce the electrical spacing below minimum, or
create such an electrical stress on the dielectric to stimulate
cathodic/anodic filament growths.

2) Conversely, if the wicking is angled away from the conductor, then it
is a reasonable (very low) risk, and should be acceptable for most low
voltage applications (<50 Vdc or ac peak).

I also mention that a good design, where long-term reliability is a
serious design requirement, does not have closely spaced (~1mm
edge-to-edge) of plated-through holes on the same grid lines to where
the reinforcement yarns bridge the spacing between holes.  It is better
to off-set the plated-through holes either diagonally or such that
plated-through hole centers are on a grid that is greater than half of
the sum of their hole diameters.

Hope these thoughts add to your info. pile.

Ralph Hersey

Ralph Hersey & Associates
Phn:  510.454.9805
FAX:  510.454.8905
[log in to unmask]

------------------------------------
Subject: FAB: Wicking in PTH
Resent-Date:  Tue, 15 Apr 1997 12:43:58 -0700
Resent-From:  [log in to unmask]
Date: Tue, 15 Apr 1997 10:44:17 -0500 (CDT)
From: David Bergman <[log in to unmask]>
To: [log in to unmask]


I received a call from a member inquiring whether there is any IPC 
activity taking place on wicking.  With PWB designs becoming
increasingly 
dense, spacings on internal layers can be at risk with minor 
misregistration and solder wicking.

I know that ITRI is working the registration issue.  Is anyone actively 
working to study wicking. Looking at:

o  Material differences
o  Alternate metallization techniques
o  Different glass styles
o  Other?

Anything will help.

Best Regards

__________________________________________________

David W. Bergman, V.P. of Technical Programs
IPC
2215 Sanders Road
Northbrook, IL  60062-6135
847-509-9700 x340 Phone
847-509-9798 Fax
email  [log in to unmask]
www  http://www.ipc.org
faxback support 800-646-0089
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