Phil:
Of more concern than what is stated here is with the carbon compositions if 
the negative etchback gets too much the mechanism for cleaning the interface 
by lifting the carbon material off the surface does not seem to work and the 
entire interface can have it across it not just at the corners.
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From: [log in to unmask]
To: [log in to unmask]
Subject: Fab: Re:Negative etchback
Date: Thursday, May 23, 1996 11:19AM

Again we have probably set up a phony limit in negative etchback:
i.e., 0.0005 inch for the MIL Spec and Class 3 of RB-276.  Class 1& 2 of
RB-276 does allow 0.001 inch.

I have done several tests on negative etchback and have not found any
functional or later defect occurring as a result of negative etchback up to 
2
mils.  Two defects that can occur with negative etch back is like the one
shown in IPC-A-600E page 69 (nonconforming) and for direct plate an
incomplete removal of the carbon/graphite deposit at the corners of the
etchbacked foil; this causes the plating to not plate completely across the
interface and leave little black spots in the corners. With the black spots
no electrical failure or separation occurred during multple solder floats.

In my tests negative etchback decreased the incidence of first-buried- layer
peel-off and foil-cracks. ( You change the position of greates stress to the
electroplate structure and don't need Class 3 foil).  In my experience 
unless
some plating anomaly such as is seen in IPC-A-600E is created, the integrity
of the holes are slightly improved by using negative etchback.

Phil Hinton Hinton "PWB" Engineering.
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