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May 1997

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From:
"ALAN COCHRANE" <[log in to unmask]>
Date:
Thu, 01 May 97 18:28:10 PST
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     Andy,
     
     You may want to look at the time the drill is in the hole.  Excessive 
     time in the hole may cause the FEP to become very plyable and cause 
     smear.  I don't know how much FEP(thickness) is in the stack-up in 
     typical applications the amount is very thin(<.003).  You will likely 
     have to go through a sodium napthalyene etch if you are not able to 
     prevent the smear initially.
     
     Regards,
     
     Alan B. Cochrane
     Director of New Technology
     Multek Inc.
     


______________________________ Reply Separator _________________________________
Subject: FAB: IPC-HF-318 and Teflon Smear
Author:  [log in to unmask] at INTERNET_GATEWAY
Date:    5/1/97 2:38 PM


     I have a multilayer RF PWB application, using Rogers' Duroid 6002, 
     procured to IPC-HF-318.  FEP film is being used as the bond film, 1/2 
     ounce copper is used on the innerlayers.  The part is used in a 
     high-reliability application.
     
     The problem is that a large percentage of product is being failed at 
     microsection due to excessive smear.  IPC-HF-318 allows up to 25% 
     smear along the vertical interconnect at the T-Joint.  We regularly 
     see T-Joints with up to 50% and 75% smear.  Not all holes in all 
     sections have this much smear, but if it happens in one hole, the 
     entire panel is rejected.
     
     RF tests show that, regardless of the amount of smear seen in the 
     cross-section, the boards are electrically (RF) acceptable.  Also, 
     we've performed thermal cycle testing which indicated no degradation 
     in RF performance.
     
     SEM micrographs of "as drilled" holes confirm that the smear is due to 
     the drill operation.  Two sets of drill studies have been performed, 
     with only marginal improvement.  The smear is not removed when 
     subjected to plasma cleaning.  
     
     My questions include:
     - Is this amount of smear unusual?  If it's not, should IPC-HF-318 (or 
     its successor) be changed?
     - Have others seen it before?
     - Is this another quirk of 6002?
     - Was IPC-HF-318 written for 1 ounce copper, therefore implying that 
     .00035 inches of smear (25% of 1 ounce copper) is really acceptable?
     - Is there any embraced reliability data that may indicate it is not a 
     reliability problem?
     
     I would appreciate any help in either reducing the problem (smear) or 
     in determining that IPC-HF-318 is not the appropriate criteria to be 
     used in accepting/rejecting this condition.
     
     
     Andy Pedersen
     Harris Corp.
     [log in to unmask]
     407-729-2143
     
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