Subject: | |
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Reply To: | (Leadfree Electronics Assembly Forum) |
Date: | Sat, 13 Aug 2005 22:00:11 +0200 |
Content-Type: | text/plain |
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Keith,
Do they see difference between various additives?
Regards
Ofer Cohen
Seabridge - a Siemens company
> -----Original Message-----
> From: Leadfree [mailto:[log in to unmask]] On Behalf Of
> [log in to unmask]
> Sent: Saturday, August 13, 2005 06:58
> To: [log in to unmask]
> Subject: Re: [LF] 96.5% Tin / 3.5% Silver vs. SAC305
>
> This is a very late response but Iver Anderson's team at Iowa
> State University are doing very interesting work that
> includes study of the effect of quaternary additions on the
> solidification and the subsequent stability in the solid
> state of the Sn-Ag-Cu eutectic. These modifications do affect the
> sensitivity of the microstructure to cooling rate. The
> non-equilibrium
> microstructures that develop during solidification largely
> arise because of the difficulty
> of nucleating one of the phases predicted by the
> thermodynamics. It is
> usually the intermetallics that are difficult to nucleate
> which is why a theoretically eutectic composition ends up
> with a microstructure dominated by primary
> tin dendrites (with consequent grainy finish and shrinkage
> cracks). Amongst
> other things the quaternary additions seem to lower the free
> energy of nucleation of those intermetallics so the alloy can
> freeze more like the eutectic it is supposed to be.
>
> Keith Sweatman
> Nihon Superior Co., Ltd.
>
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