Eric,
I don't know for sure, but I believe it is a combination of solid solution
strengthening of the tin phase and a more macro mechanism involving pinning
of grain boundaries by the Ag3Sn intermetallic phase. I did not resolve the
deformation mechanisms as part of my research. I was more interested in
empirical constitutive relationships of Ag (& Cu) concentration versus creep
rate because that data was not available at the time to support finite
element modeling of solder joints.
Best regards.
Ed
-----Original Message-----
From: TechNet [mailto:[log in to unmask]]On Behalf Of Eric Christison
Sent: Thursday, July 26, 2001 10:02 AM
To: [log in to unmask]
Subject: Re: [TN] solder paste with 2% silver
Ed,
As a matter of interest. Are the improved mechanical properties a result of
the silver atoms limiting dislocations in the Xtal structure?
Regards,
>
> I did my PhD thesis on the creep behavior of SN62 versus SN63 (UW -
> 1995).
> SN62 (62Sn-36Pb-2Ag) was significantly more creep resistant and had a
> higher
> elastic modulus than SN63. This might make it more resistant to creep
> rupture failure and thermal fatigue (due to creep-fatigue interactions).
> Much of this research is published as cited below.
>
> Best regards,
> Ed Hare
>
> 1995 "Stress Relaxation Behavior of Eutectic Tin-Lead Solder", Journal
> of
> Electronic Materials, October 1995.
>
> 1994 "Stress Relaxation Behavior of Eutectic Tin-Lead Solder with
> Silver
> and
> Copper Additions", Ph.D. Dissertation, University of Washington, 1994.
>
> 1993 "The Effect of Ag Additions on the Stress Relaxation Behavior of
> Eutectic Tin-Lead Solder", TMS Annual Meeting, Denver, CO.
>
> -----Original Message-----
> From: TechNet [mailto:[log in to unmask]]On Behalf Of Peter Barton
> Sent: Thursday, July 26, 2001 7:35 AM
> To: [log in to unmask]
> Subject: Re: [TN] solder paste with 2% silver
>
>
> One other reason given for preference of 62Sn/36Pb/2Ag is that it is a
> ternary alloy with a 'pasty' range between 177deg C and 189 deg. C as
> opposed to 63Sn/37Pb which is a eutectic. This slower transition from
> solidus to liquidous is helpful when soldering low mass parts that are
> prone
> to tombstoning such as 0402 and 0201.
>
> Also according to the data published by the International Tin Research
> Institute the 2% silver version gives a stronger soldered joint.
>
> Pete Barton
> ACW Technology Ltd
>
> ===== Original Message from "TechNet E-Mail Forum." <[log in to unmask]> at
> 26/07/01 14:31
> >From "Soldering in Electronics" by Klein Wassink:
> >
> >"Note on solder alloy composition:
> >Solder paste in hybrid circuit technology (for thick-film circuits)
> usually
> has a metal
> composition of tin62-lead36-silver2. It should be realised that the
> addition
> of silver is not at all necessary for the silver (-palladium) metallisatio
> n
> of the
> >components, but for the much thinner silver-palladium conductors on the
> tick-film substrates.
> The use of the more expensive silver containing alloy, instead of the
> common
> tin60-lead40 alloy, for (relow) soldering on printed boards with copper
> >solder lands is not based on technological necessity, but sometimes on
> >better availability of this alloy (in the form of paste) and in most
> cases
> merely on habit.
> For the rest: with silver loaded alloy no harm is done !"
> >
> >Kind regards,
> >
> >Daan Terstegge
> >SMT Centre
> >Thales Communications
> >Unclassified mail
> >Personal Website: http://www.smtinfo.net
> >
> >
> >
> >
> >>>> "Kasprzak, Bill (sys) USX" <[log in to unmask]> 07/26 1:54 pm >>>
> >To all:
> >
> >I've been reading, with great interest, the responses so far to the use
> of
> >2% silver paste. I am not being critical of the folks who responded so
> far
> >but, each one cites a "reported" or "understood" advantage to using 2%
> >paste.
> >
> >I have also heard some of same reasons for considering a switch to a 2%
> >silver paste. Can any of the metallurgists on this forum confirm some
> of
> >these reported advantages and perhaps recommend that given x,y, and z,
> one
> >should consider using 2% silver solder alloys?
> >
> >Good topic.
> >
> >Bill Kasprzak
> >Moog Inc., Electronic Assembly Engineering
> >
> >
> >> -----Original Message-----
> >> From: Eric Christison [SMTP:[log in to unmask]]
> >> Sent: Thursday, July 26, 2001 4:20 AM
> >> To: [log in to unmask]
> >> Subject: Re: [TN] solder paste with 2% silver
> >>
> >> > I always understood that you had to use a paste with 2% silver if
> you
> >> > were using components
> >> > with terminations containing Palladium. The 2% silver is also
> supposed
> >> to
> >> > five you a shinier
> >> > finished solder joint.
> >> >
> >> > Steve.
> >> >
> >> > Larry Koens wrote:
> >> >
> >> > > Got a question for everyone,
> >> > >
> >> > > I just switched companies and the new company that I'm with now
> uses
> a
> >> > > 62/36/2 solder paste. I've always used the 63/37 formula. The
> guys
> who
> >> > > decided to use the 2% silver are no longer with the company. My
> >> > question
> >> > > to you is, why would they want the 2% silver in the paste? They
> know
> >> > > something I don't?!
> >> > >
> >>
> >> The silver should give the alloy higher strength, better creep
> resistance
> >> and a higher melting point. Perhaps your application benefits from one
> of
> >> these differences?
> >>
> >> Regards,
> >>
> >>
> >>
> >> Eric Christison
Eric Christison
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