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February 1998

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Subject:
From:
Fred Johnson <[log in to unmask]>
Reply To:
TechNet E-Mail Forum.
Date:
Tue, 24 Feb 1998 22:52:00 +0000
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Richard,
-perhaps you could elaborate further - are you sure you mean a ring
void? or are you seeing a barrel crack in the center of the hole?  Is there
anything else "unusual" about the plated barrel of the hole -  is the copper
thin in the problem area, etc.?  How about on the surface of the part - are
the pads lifting?

Usually the greatest stress is near the surface of the part, and you might
expect to see some innerconnect problems or cracks in the plated copper
at the knees of the holes if you have a lot of Z-axis expansion trouble
from undercured or low Tg material.   It's pretty unusual for a copper bath
to be so screwed up that the ductility goes completely away (leading to
barrel cracks), but it does happen - but frequently the knees crack first
(unless the plating is thin in the center of the hole, i.e. dogboning).

Speaking as a fabricator, I don't know why you'd put thermals on some
planes and not on others; maybe you can explain why!

Regards,
Fred J.

>>> Richard G Smith <[log in to unmask]> 02/24/98 05:13pm >>>
     Fellow Techies,

     I need your help!!

     Here's the situation. We have an eight layer PCB with four internal
     planes. Layers 3 and 4 of the planes have thermally relieved vias,
     layers 5 and 6 do not. The via size is .018 +.000 -.018 finished and
     are tented with soldermask.

     The problem is that ring voids occur sporadiclly on layers 5 and 6
     (non-thermal ties) after multiple thermal excursions during assembly.
     A microsection of the board has confirmed this.

     I contend that the board was fabricated improperly using lower grade
     FR-4 and in conjunction with other issues such as low ductility copper
     and no thermal ties caused this problem. I am a PCB Designer so I may
     be biased.

     The assembler is saying that this is purely a design issue based upon
     the fact that no thermal ties were provided.

     In all my resarch no one (including fab houses) has indicated that
     this problem is related to the lack of a thermal.


     Any insight would be greatly appreciated.

     Rick

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