Steve Abrahamson asked:

>I have a few questions about an issue that started this whole eutectic
>conversation.  What are the advantages of moving to a 62Sn/36Pb/2Ag solder
>paste.

Increased sales for you paste supplier?

No, it used to be mainly that the ceramic discrete parts had metallization
which was not protected by nickel barriers, and the additional silver in
the alloy reduced silver metallization leaching during reflow. This could
actually open the connection by pulling all the metallization off the
device. Now, most ceramic parts will have barrier plating.

The SN62 alloy (with 2% silver) will melt at a slightly lower temp than
SN63. It might make some difference if you have an assembly very senistive
to your peak reflow exposure.

> From the information that I have received from some solder paste
>reps is that the popularity of the silver alloy paste has dwindled from about
>50% of the volume to about 35%(according to Alpha, Kester, Indiun and Heraeus
>reps).  Our supplier demands an extra half a cent a gram for a 2% silver
>formula.

That sounds pretty reasonable, sliver is a lot more expensive than tin.

>Our 63/37 solder joints are 3-4 times the strength requirements set by IPC,
>thus an increase in strength is not cost effective with the high volumes of
>paste we use.  And even though we have seen a very low number of palladium
>silver components, I am not convinced that a 2% silver is going to buy us
>much since palladium is supposed have a 100% dissolution rate with tin provided
>your reflow profiles are set up correctly(with sufficient time over reflow).

I don't follow your logical procession there, but maybe I get lost easily.
But I think that most of the palladium you see will be on those d--n TI IC
leads, and the silver probably won't help you wet them. I thought tin-lead
was as strong as SN62.

>Has anyone out there recently moved from a 63/37 to a 62/36/2 or visa versa,
>and why.

At my former employer we dropped SN62 in favor of SN63 for cost reduction
and improved wetting.


cheers,


Jerry Cupples
Interphase Corporation
Dallas, TX USA
http://www.iphase.com


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