Bob,

It has been my experience that etchback significantly enhances the reliability of the product. Especially if you are dealing with products over 6 layers. We have an internal standard that required a positive etchback of at least .0002" for all products over 6 layers specifically to avoid the possibility of post separation. I have found that due to the inherent construction of high layer count products which typically requires high resin content prepregs, ( 1080 and 106 glass) the expansion properties are higher than that of 4 and 6 layers which typically use the larger 7628 glass style.   

If you specify a positive etchback in accordance with 55110 or RB 276, the etchback limitations are in fact specified.  In addition, IPC-A-600E has a requirement for class 2 boards of .0002 - .003". 

Hope this helps,

Ed Cosper
Director of Quality Assurance and Engineering
Graphic Electronics Inc.
Tulsa, OK


----------
From:  BOB HAYNES[SMTP:[log in to unmask]]
Sent:  Tuesday, April 08, 1997 12:15 PM
To:  [log in to unmask]
Subject:  Etchback 

The purpose of this request is to solicited some opinions as to the 
merits of etchback.


My company has recently had some field returns which analysis has
disclosed that the failures are a result of post separation. The PWB
supplier ,when contacted, suggested that if our drawing had required
etchback there would have been a less of a chance for a failure of this
nature to occur.


My company uses both IPC-RB-276 (IPC-6011&IPC-6012) and
MIL-STD-55110E as procurement specifications and it so happens we 
have experienced failures in PWB's procured using both 
specifications. The guidance furnished by either is not definitive 
just if the Master Drawing say do it the etchback has to fall within 
certain limits.


We have always gone with only a chemical desmear (permanganate) with 
no apparent epoxy removal. We have never specified a chemical 
etchback for fear of latent defects from residual chemicals. Our PWB 
supplier tells us there is now available plasma etchback/chemical
desmear (permanganate) which we are told eliminates the potential
for the latent defect problem.


Does anyone have data which would provide information as to which 
process would make the more reliable product. Any and all replies and 
opinions welcome.  

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