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October 1997

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Subject:
From:
Paul Gould <[log in to unmask]>
Reply To:
TechNet Mail Forum.
Date:
Thu, 30 Oct 1997 22:24:08 +0000
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text/plain
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text/plain (58 lines)
Hi George,

This may be barking up the wrong tree but it could be that there is some
stray resist adjacent to this feature which may come from a) too narrow
a gap, say less than 2 mils, or b) resist overhang on a landless hole
close to the defect.
a) If there is a narrow gap, the resist can sometimes be undercut and
lift to lay across adjacent traces during developing. I saw this
recently where a 2 mil gap existed in error between a pad and its
connecting trace and was not picked up by the CAM checks because it was
connected. The sliver of resist caused a fine cut across the connecting
trace on a few panels.
b) If a sliver of resist overhangs a landless hole, this can also break
away and lay across the trace during developing which can result in a
cut.
Both these can cause very fine cuts.
There must be a repeating cause so it is unlikely to be copper foil or
process. Re-check the panel carefully after resist develop.

Good luck.

George Kotecki wrote:-
>Technetters,
>        We have experienced a strange problem on a recent lot of assembled
>PWB's.  We are experiencing conductor cracks  between a surface mount land and
>its associated via.  The pad is  .030" wide with a .010" trace attached going
>about .30" to a standard via hole.  The mounting pad is a part of a quad flat
>pack (32 pins).  THe failure always occurs in the upper right hand pad.  There
>is dry film solder mask on the board.  Mask clearance around the pads is .003".
>Board substrate is GIN material with a constraining core of copper.  What makes
>this strange, is this pattern is used on other designs all fab'ed from the same
>lot of material and assembled with similair parts and identical processes.
>        The cracking occurs at the solder fillet/mask interface.  Solder
>fillets
>appear normal.  The breaks are clean.  Tooling artwork was checked and no
>problems detected.  The problem manifests itself when the PWB is put under
>vibration albiet low levels.
>        Any and all help would be greatly appreciated.
>
>                                Thanks,
>                                George Kotecki
--
Paul Gould
[log in to unmask]
Isle of Wight,UK

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