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April 1997

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Fri, 11 Apr 1997 15:46:08 -0400 (EDT)
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    I do not agree that using "etchback" will eliminate Post Separation (the
new term for IPC-T-50E is "Internal layer separation" rather than post
separation).  The use of chemicals such as permangate or dry plasma etching
to remove the resin adjacent to the internal layer connection will certainly
remove epoxy smear.  This is what "etchback" was designed to do.  In my
experience the use of smear removal which cleans the resin smear off  the
internal layer without overetching to make the internal layer protrude in to
the hole is just as effective as etchback and is much easier to control.
 There are many opinions about the reliability of the PTH with etchback
versus hole smear but I do not have data to support one position or the
other. 
    Internal layer separation is a result of leaving a deposit on the face of
the internal layer which is subsequently metallized and plated and it is NOT
epoy resin.  The cause of innerlayer separation is associated with the
chemistry used with hole metallization and will differ with each type
chemistry (deposits from the metallization bath stabilizer and inadequate
surface copper cleaning are examples) .  I have seen internal layer
separation with regular formaldehyde chemistry and with the direct plate
process from Atototech.  A lot of work has been done to look for the
contaminate on the inner layer but the end result is that a metallurgical
bond is not formed with the plated copper and the joint will fail under
thermal stress from a solder float at 550 F for 10 sec.  In my opinion the
elimination of internal layer separation will occur with good control of the
metallization process.  It is definetly difficult to detect but I would
suggest that all the Testing Labs associated with the IPC Post Separation
Task Group can find the problem on purchased PWBs.  I believe it is prudent
to use them as a "referee" to verify hole quality on an ongoing basis.  
    There are definitely different levels of severity of the poor internal
layer connection and it can be seen in vertical crossections as a line with
some good connection and some bad.  These are the most difficult to detect by
typical microsection in the horizontal direction.  If you would like to learn
more about this problem I would invite you to participate in the Post
Separation TG.
Foster Gray
Chairman, IPC Post Separation Task Group
PC Interconnects
Austin, Tx
e-mail: [log in to unmask]

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