One of our technical service engineers asked me about a strange migration
phenomenon one of our customers observed under passives and QFN's that have
been soldered with one of our no-clean SAC305 solder pastes. I've attached
an X-ray picture of a QFN exhibiting this strange failure, but I don't know
how it can be posted so you can look at it.. Further analysis after
removing the component showed that the shadows in the X-ray picture is
caused by a black material containing tin. The boards were conditioned 85°C
85%RH for 500 hr, but no voltage was applied. This phenomenon was not
observed under QFPs (not surprising), and seems to be independent of reflow
profile used. Additionally, the customer has not seen anything like this
with tin lead solder paste, however, the tin-lead solder pastes would have
a different fluxes. The tin "migration" is coming from both adjacent
solder joints, and there is no voltage applied, so it seems to be a
corrosion problem. Certainly QFN and big passives are good at trapping and
retaining water in the flux residue underneath them.
I've never seen anything like this; has anyone else out in TechNet land
seen this or have any ideas what caused this?
Thanks
Karen Tellefsen - Electrical Testing
[log in to unmask]
908-791-3069
(See attached file: Strange tin migration.JPG)
Enthone GmbH Geschäftsführer: Knut Balzer, Luc de Croo, Jürgen Dietrich Amtsgericht Düsseldorf HRB 45276
USt-Id-Nr. DE 811 205 946
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