Underwriters Labs, (U.L.) also qualifies PWB manufacturers to a maximum unbroken 
conductive area, of a 1" circle, 2" circle, 3" circle, or any size you think you need. 
For board designs with unbroken areas larger than the qualified size, I presume, the 
manufacturers product is not U.L. recognized.  (My qualification used a 3" circle.)

The point is that UL still tests and qualifies for this attribute, and you pointed out 
that MIL/IPC-D-275 talks about it.  That just about covers the market.

The UL contact, unofficially, was not aware of any recent problems with this design 
feature, and he said he sees a lot of them.  So, based on my (and his) experience, I 
would agree with your fabricator that the specs are testing for problems that the 
industry has corrected.  

Your design question really is this:  What is the largest conductive area that can 
consistently be built by LMCO's least capable PWB supplier of that product type?  A few 
phone calls should answer that.  Be interested in what size conductive area they used in 
their UL testing.

George Franck
Raytheon E-Systems
Falls Church Va

+++++++++++++++++++++++++++++++++++++++++
[log in to unmask] wrote:
Recently I have had some discusions regarding breaking up large (greater than
1" circle) conductive areas.  Both Mil-Std-275 and IPC-D-275 recommend doing
this to prevent blistering.  During prototyping of several military, RF PWAs we
found that the 'breaking up' of the ground planes adversely affected perfor-
mance.  In discussing this with one of our fab suppliers to determine what
compromises we could make to improve electrical performance, I learned that
'breaking up' the ground plane was no longer necessary due to improvements in
processes and materials.  I was told that there were problems in the late 70s
and early 80s but not any more.

Are the specs cited out of date with respect to this issue and current processes?  
Should I be concerned about not breaking up large conductive areas?  If yes,
how large is too large?  That is, is a 1" circle still a valid threshold?

This is more of a general question but the design specifics are - 6 layer MLB,
11"x 9"x .063", SMOBC, layers 1, 4 and 6 are ground planes with layer 5 a
controlled impedence layer.  Largest area not broken up by holes is approx. 8
square inches.

Thanks to all for your comments and opinions.

Jeff

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