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1995

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Subject:
From:
Jon Holmen <[log in to unmask]>
Date:
Fri, 1 Dec 1995 09:43:02 -0600 (CST)
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Alain,

I talked to Floyd Gentry, current Chairman of the IPC-A-600 and Arny 
Andrade who was the chairman of the A-600E. You can email Arny if you 
would like mor information on the tests performed by Sandia. His address 
is [log in to unmask] Their replies are as follows:
-------------------------------------

Multilayer boards without etchback exhibit higher electrical resistance during
thermal shock and cycling. This has been substantiated by controlled 
environmental testing conducted here at Sandia National Laboratories on boards
specifically designed for this purpose.
     
     It's not stated whether these boards are epoxy or polyimide construction.
Polyimides which are typically used for high temperature applications can 
generally be fabricated with only a smear removal process due to their higher 
glass transition temperature (Tg) which results in less thermal expansion.  
However, drill smear can occur in any of these materials which must be removed
before plating.  

     Consequently, I would personally insist on at least a smear removal 
process at the very minimum.
     
     I would also question the validity of the 10-15% cost savings. If a smear
removal is substituted for deep etchback, a shorter cycle might be possible 
and glass etch might be eliminated.  However, I feel that a small reduction in
cycle time would not justify a potential reduction in long term reliability.
     
     I would recommend that either a etchback or smear removal be continued 
depending upon the material construction and/or application requirements. I 
don't know of any reputable fabricator who couldn't accommodate one of these 
requirements in a cost effective manor.
     
     Floyd Gentry
     Sandia National Labs
__________________________________________________     

Etchback enhances the innerlayer interconnects. One ounce inner layers measure
approximately 1.1 to 1.3 mils in thickness but with 1-mil of etchback the 
contact area increases to 3.1 to 3.3 mils establishing a "c" type 
interconnect. Inner layer connections are stressed during assembly 
soldering operations and can result in interconnection cracks and/or opens but
filled with solder. This connection can result in latent interconnection 
failure.

Automotive environments are very severe and should be designed to high-rel 
requirements.

Lab tests at Sandia verified the need of etchback. Test PWBs with both 
chemically cleaned and etchback interconnections were fabricated.

Thermal cycles at 500F were performed using hot oil and IR as the heat medium.
Resistance measurements were taken after each cycle.

Results: Chemically cleaned interconnects increased resistance ending in open 
circuits. Etchback interconnects maintained their initial resistance. 
40-cycles were performed with each heat medium with the same results.

Therefore 10% to 15% savings may be false economy.

Arny Andrade
Sandia National Labs


****************************************************
Jon Holmen
Technical Project Manager
IPC
2215 Sanders Road
Northbrook IL  60062-6135
Phone (708) 509-9700 ex329
Fax   (708) 509-9798
e-mail  [log in to unmask]
*****************************************************


On Tue, 21 Nov 1995, Fouquet, Alain wrote:

> 
> To all,
> 
> In an effort to reduce cost, our boards manufacturers are proposing to 
> remove the requirements for etchback.  This is reported to be a saving of 
> 10-15% of the bare board.  For an equipment subject to be used in an 
> environment which may be similar to a car and possibly marine types, 
> questions are:
> 
> 1- Do we put ourselves in a position where reliability will drop 
> significantly if etchback is not specified?
> 
> 2- Is etchback normally used in the automotive industry for multi-layer 
> board if used?  If not what alternative to use?
> 
> 3- Is the 10-15% cost reduction realistic?
> 
> 4- General consumables like computers, are they using this process?
> 
> 
> Any other comments would be appreciated.
> 
> 
> Alain Fouquet
> Canadian Marconi Company
> Ville Saint-Laurent, QC, Canada
> email: [log in to unmask]
> 
> 



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