Recently we have solderability problems with Ni/Au kapton flex boards
(0,1 µm Au, 3 µm Ni, 35m Cu).
We have produced and assembled large number of boards.
On 10 % of the boards we observe interconnection failures after final
assembly of the pcbs. If  the suspected solder joints are lightly
stressed , the connections are easily broken, leaving an open circuits.
 It is  easy to peel  these parts with solder material  from the boards.
The pads below the removed parts are discolored dark gray and are sleek .
Some times the handsoldering of the broken joins was successful but
after small strain the joints  broke again.
Additional vapor phase soldering  did not improved the the strength of
the joints.
This is new problem for us. Does anyone have any experience with this?
How to rework the boards? How to determine when a board should be
reworked or
scrapped? What about reliability issues? Latent failures?
I would appreciate your help very much
  regards

--
Waclaw Karpinski
I. Physikalisches Institut
RWTH Aachen
D - 52074  Aachen

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