Constantino
I cannot give you actual studies that have been done, only the mentality
that went into the need for desear processing instead of etchback.  Some 20+
years ago when etchback was used the laminate systems also had low Tg and
therefore significant Z-axis expansion.  The inner layers, if not made of
copper foil that was High Temperature Elongation, cracked at the point where
they were not supported by resin.  So a board shop could either use higher
Tg material (not available at that time) HTE foil (not really controlled at
that time and expensive to buy) OR they could minimize the amount of copper
hanging within the hole wall copper plating unsupported in any way by
minimizing the amount of etchback.

The desmear process provided a good connection across the inner layer face
AND was not subject to cracking of the inner layer.

Even today's specification recommend no more than 0.5 mil measureable
etchback.

Polyimide has little z-axis expansion and therefore, if the inner connect
has shown to be good after thermal Stress testing and assembly should sreve
the life of the board.

Susan Mansilla
Robisan Laboratory, Inc


-----Original Message-----
From: IPC-600-6012 [mailto:[log in to unmask]]On Behalf Of Constantino
J. Gonzalez
Sent: Monday, September 13, 2004 11:59 AM
To: [log in to unmask]
Subject: [IPC-600-6012] Etchback Question


Hello help....

My question is, do you have any knowledge of any studies having been
performed regarding reliability of ethchback vs. non, or low etchback on
innerconnect joints of polyimide PWBs?

My customer is asupplier for an aerospace company, and the issue is that
they recently have found through microsection analysis that some boards they
have already installed in a vehicle are suspect, with regards to etchback.
I am looking for supporting documentation to disposition the boards.  Our
requirement is for etchback per 6012 CL III, but I don't believe they are
necessarily un-usable. The sections are otherwise structurally sound, no
post or plating sep, 1oz and greater conductors, etc.

As always, I appreciate any info you might be able to pass on.

Regards,

Tino Gonzalez