Ofer,
From what I've read, (short answer) there is the
potential for problems but they may not actually show
up.
Longer answer: Bismuth does form a very
low-temperature-melting point alloy when combined with
tin AND lead. The one document I glanced at before
writing this e-mail said 138 C, but I am pretty sure I
have heard figures like 92 C also.
However, the counter-argument I have seen is that
when SnBi is a component finish, the Bismuth is only
present in pretty low percentages (like 4-5%, I think
??). There is a not a huge volume of solder involved
in the surface finish anyway, especially not in
comparison with the volume of solder to make a joint
between the PCB pad and the component. Therefore,
even with an SnBi finish, the actual percentage of
Bismuth in the end solder joint will not be high
enough to cause the low temperature melting point
concerns.
I haven't heard anything about Bismuth affecting the
reliability of the joint, so I would think the best
test would be a worst-case temperature test - maximum
rated ambient, unit powered on and configured such
that SnBi-finished components get maximum heating.
This may take multiple configurations depending on how
many components there are in the product and how many
operating modes the product has.
Hopefully there will be replies from people on
the list who have had actual experience with this
issue. I think all the literature I have seen to date
said that it could be an issue, but I don't remember
seeing any case studies where it actually WAS an
issue.
-Camille Good
Portland, Oregon
--- Ofer Cohen <[log in to unmask]>
wrote:
> Hello all,
> Some of our vendors suggest their components with
> SnBi coating. Some of them suggest only this finish.
> Although this finish is lead-free, it is being
> assembled (currently) using SnPb paste. Are there
> any known problems? Is there any effect on short
> time or long time reliability?
>
> Regards
> Ofer Cohen
> Manager
> Quality Assurance, Reliability and Production
> Technologies
> Seabridge Ltd. - A Siemens Company
>
>
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