I was recently emailed an article worth reading. It can be found at:
http://www.ipcprintedcircuitexpo.org/

Effects of Surface Finish on High Frequency Signal Loss
Using Various Substrate Materials

It doesn't answer my specific questions but is interesting. As for why I'm
looking at Silver it is because we are moving to higher speed, BGA designs now.
I have been informed through a soldering and rework course instruction that
Gold
immersion is not the best choice for BGA designs considering longevity with Tin
and Gold migration, and proper wetting during assembly.





At 11:21 AM 12/07/2001 +0800, you wrote:
>Thanks for your comment, James. I have heard of reports not recommending
>ENIG for BGA applications, but haven't found any to read them myself. Can
>anyone help?  Recommendations vary depending on application, I find, but
>what thickness of Gold do you think would be appropriate? Our assembly
>house was more concerned about the thickness of the Nickel than they were
>about the Gold before we conducted solderability tests and found it was OK.
>
>Our application is Military Avionics, subject to the usual thermal cycling
>for such equipment and also some substantial, (relatively) low-frequency
>vibration. Board finishes like HASL, even with very good tolerences on the
>leveled solder height, do not seem to be as good as the Gold over Nickel in
>terms of flatness. Flatness for BGA's equals most even joint heights, which
>in turn means most even distribution of stresses (or minimising stress
>focal points) and this in turn leads to greater joint reliability under
>conditions of temperature extremes and vibration.
>
>If not Gold over Nickel as a board finish for this application (recommended
>by Cadence), then what do my learned TNagers suggest instead?
>
>Pete Duncan


Regards,

Colin Weber

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