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Thu, 30 Oct 1997 22:24:08 +0000 |
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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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