Hi TechNet!

Just an added note to Jerry Cupples's reply:


>>"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."


*We are using 63/37 solder paste with typical reflow profiles and not having
any problems with the TI palladium finished ICs. The Pd diffuses into the 
solder joint and SEM analysis can't find it. Keep in mind there is very little 
Pd on those components which makes the solderability sometimes a problem. The 
Pd finishes are very flux sensitive (or at least our testing has shown 
that!)*.


Dave Hillman
Rockwell Collins
[log in to unmask]


______________________________ Reply Separator 
_________________________________
Subject: RE: Re[2]: Solder Paste Plastic or Eutectic (#975031)
Author:  [log in to unmask] at ccmgw1
Date:    11/25/96 4:46 PM


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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