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August 2008

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Subject:
From:
Mike Fenner <[log in to unmask]>
Reply To:
Date:
Thu, 7 Aug 2008 11:51:09 +0100
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Sn62 has a solidus of 179C [Sn63 is 183C]. Liquidus will depend on the
actual composition supplied as small within spec changes  in the Ag content
can elevate it slightly. So for all practical purposes in SMT assembly none
of this is really significant and the alloys are pretty well
interchangeable. There are some differences in physical properties which
usually aren't significant in service for most people nearly all of the
time.

On larger joints and/or processes with slow ramp down, the Ag content can
lead to growth of intermetallics which cause surface roughness seen as
dulling. The opposite of intuition which leads people to think that silver
in the solder will make it brighter and shinier. Probably people reporting
better joint formation are actually seeing improvements brought about by
change in flux with new supplier or better setup coincident with the
evaluation/change of alloy, something like that. Whatever the explanation it
dimensions the "benefit" of changing the alloy: if there is any its smaller
than other line variables.

At current metal prices the 2% Ag increases the metal cost considerably, so
it is hard to justify the alloy.





Regards 

Mike Fenner 
Indium Corporation 

T: + 44 1908 580 400 
M: + 44 7810 526 317 
F: + 44 1908 580 411 
E: [log in to unmask] 
W: www.indium.com 
Pb-free: www.Pb-Free.com 

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Indium Corporation
Incorporated in New York, USA
Registered office: (Not for correspondence) 
77-97 Harpur Street, Bedford , UK
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-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Joyce Koo
Sent: Wednesday, August 06, 2008 4:36 PM
To: [log in to unmask]
Subject: Re: [TN] SOLDER FOR PCB  REFLOW PRO CESS

As far as I can recall there is a difference of slightly longere time above
liqudus, and surface tension , which produce better soldering results.
However, without knowing the specific application requirement, any selection
is shot in the dark.  
--------------------------
Sent using BlackBerry


----- Original Message -----
From: TechNet <[log in to unmask]>
To: [log in to unmask] <[log in to unmask]>
Sent: Wed Aug 06 10:25:53 2008
Subject: Re: [TN] SOLDER FOR PCB  REFLOW PRO CESS

Hi Ben,
Actually, it has never been proven that Sn62/Pb36/Ag2 prevents leaching on
components finished with silver—it appears to sound reasonable, but is an
urban myth.
Sn62/Pb36/Ag2 is somewhat stronger [which is totally without practical
value— and the still stronger SAC-solders cause pad cratering] than Sn63Pb
and has a slightly lower Solidus/Liquidus temperature. Creep-fatigue-wise
there is no significant difference.

Werner
Future workshops:
Reliability Issues with Lead-Free Soldering Processes, September 22,
Schaumburg Failure Mode and Root Cause Analyses Reliability (Fatigue,
Brittle Fracture, ENIG), September 22, Schaumburg Solder Joint Reliability:
Parts 1 & 4, Oct. 9, Moscow, Russia Solder Joint Reliability: Part 4, Oct.
17, Timisoara, Rumania Solder Joint Reliability: Parts 1 & 4, Oct. 22,
Tallinn, Estonia



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