Werner and all interested,

Too catagorical? It must be something I picked up in the jungle. I?ll seek
help. However, in my only post on this subject, I was reporting MY
observations over the years concerning thousands of boards I have worked on
and with.

Werner, as always, makes good points and better explanations in his latest
retort concerning what causes hole wall separation and why non-functional
pads help prevent it?s occurrence though not magnitude or propagation, or
some such thing. However, If magnitude is synonomous with amount or degree,
I would argue this point later based on what you say about resin amount. As
for propagation, something has to be there in the first place for it to
propagate.

In this post, I would point out that resin recession and hole wall
separation can be, and usually is, minimized in most higher layer count
MLB?s compared with some time ago. I don?t know if Werner is old enough to
remember the days when defense board suppliers were required to build MLB?s
with two plies of prepreg minimum in each dielectric thickness. If so, I?m
sure he had something to do with the practice?s demise.  I?m equally sure
everyone knows why but, if not, it was required to ?prevent? shorting
between two conductive layers via a glass strand that might somehow become
conductive.

In more recent times, the two ply requirement gave way to single ply
constructions. Therefore, there is less resin per ply, when boards are
?properly? constructed. This especially is true considering 12 layer and
over designs beyond a thickness of .062?. The primary reasons for this are
impedance and associated dielectric thickness requirements most often
requiring 50 ohms.

Again, in the good old days, to construct, say, a 10 layer type, because of
the two ply requirement, usually it was made up of two plies of 1080 core
and prepreg material (I used one ply 106 with one of 2113 to add more
dimensional stability though the 106 was about 70% resin ? very juicy) to
make its overall thickness .062?. 1080 glass style material is, as everyone
knows, very resin rich. Therefore, the phenomenon Werner describes,
concerning resin expanding, contracting, and expanding again was much more
common than now with single ply constructions that are not nearly as resin
rich. There?s a long story to all this but it should be unnecessary to
discuss it now.

So, in a 16 layer board, as one example, requiring 50 ohms impedance, the
whole structure can be made up of  single and two ply 2116 material with a
Dk of about 4.0 ? 4.3. 2116 material is very well balanced concerning its
resin to glass ratio at about 53%. This provides much less resin to expand
in any direction but for its CTE. However, the resin does not move so much
it can cause recession or separation. Of course, in any event, good plating
adhesion is required.

All this brings me to: You don?t need no dummy pads to prevent the
aforementioned problem. There?s obviously much more to this story and I
think folks have written extensively about it. Werner has written so much
stuff it will take me another life to read it all.

Earl Moon

---------------------------------------------------------------------------------
Technet Mail List provided as a free service by IPC using LISTSERV 1.8d
To unsubscribe, send a message to [log in to unmask] with following text in
the BODY (NOT the subject field): SIGNOFF Technet
To temporarily halt delivery of Technet send e-mail to [log in to unmask]: SET Technet NOMAIL
To receive ONE mailing per day of all the posts: send e-mail to [log in to unmask]: SET Technet Digest
Search previous postings at: www.ipc.org > On-Line Resources & Databases > E-mail Archives
Please visit IPC web site (http://www.ipc.org/html/forum.htm) for additional
information, or contact Keach Sasamori at [log in to unmask] or 847-509-9700 ext.5315
---------------------------------------------------------------------------------