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| Date: | Tue, 25 Apr 2006 11:03:19 +0300 |
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Under the circumstances you describe, it would appear that the
post-surface treatment cleaning is inadequate as residual contamination
must be remaining in the holes. You do not mention the PTH diameter and
small holes are capillary traps. Wash water will flood into the holes
but if clean rinse water cannot displace the contaminated wash water,
your problem becomes logical.
Rinsing is always a difficult problem and I recommend that the cleaning
equipment has rinse bars that form a fine spray, with droplet sizes
smaller than the hole diameter, so that many droplets impinging on a
hole will displace the capillary-held water. There is an optimum spray
velocity, as well. Too small and the displacement will be inadequate;
too great and you will get break-up of droplets rather than
displacement. I would suggest that velocities of 3-5 m/s at the board
surface would be a good starting point.
Another important point is that the cleaning should happen as soon as
possible after the contaminating process is completed.
Brian
Lin Leo wrote:
> Hi,
>
> I met ionic contamination problem at HASL and Immersion Tin surface finish board.(SEC values were measured from instrument Omega 600) I found significant difference at different patteren. The more through holes, the higher ionic contamination.
>
> Did anyone experience in this phenomena? Any literature/report has discussed the factors resulted in the ionic contamination? How to clean the boards to decrease this defect?
>
> Thanks
>
> Leo
>
>
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