Hallo to all,
the discussion on the contents of the different metals in alloys is
interesting and (for me) a little bit amusing at the same time (see
different emails attached): there is a discussion whether 0.1% (or even
0.01% !!!!) more or less is the exact composition. On the other hand looking
on a datasheet of a well known supplier of solders the tolerance range of
the contents is about 1%. Additionally I learned on a meeting yesterday that
the exact composion seems not to be as critical as mentioned in the
discussion on the forum and that for a SnAg3.5 alloy up to 4% (!!!!!!!!!!!)
of Pb (leached from components leads) seems not to be critical. The
colleagues who reported this came from different companies resp. university:
a car electronic supplier with experience for > 3 years on continous
production, from a TV manufacturer who had run his devices under stress
conditions for several years and from the technical university of munich
where a lot of scientific investigations have been done. Most important: the
results of all have been quite the same !
So what's to do ?? For me personally the experience of those who have worked
with the 'new' material for a long time and a large number of parts is more
interesting than a discussion based on pure theory.
Best regards
W.-D. Schmidt
(Product-Engineer)
SEL Defense Systems
PO-Box 1760
D-75117 Pforzheim
Tel.: +49 7231 15 3386
Fax: +49 7231 15 3390
eMail: [log in to unmask]
> -----Ursprüngliche Nachricht-----
> Von: Kay Nimmo [SMTP:[log in to unmask]]
> Gesendet am: Mittwoch, 25. Oktober 2000 11:08
> An: [log in to unmask]
> Betreff: Re: [LF] Timelines for conversion to Lead-Free
>
> The exact ternary eutectic is still not fully agreed. I think on average
> opinion that it is close to around 3.4%Ag 0.6%Cu. Of course in practice it
> may not be that important - Erik's comments on microstructure describe
> well the technical reasons for higher Ag content (and also the cost
> disadvantage).
>
> Kay
>
> -----Message d'origine-----
> De: <Erik de Kluizenaar> [mailto:[log in to unmask]]
> Date: mercredi 25 octobre 2000 09:54
> @: [log in to unmask]
> Objet: Re: [LF] Timelines for conversion to Lead-Free
>
>
> Dear David, dear all,
>
> For me tin-lead has its eutectic composition at Sn61.9Pb38.1, and that of
> tin-silver-copper lies at SnAg3.8Cu0.7.
>
> Yes, it has a small pasty range, speaking in temperature range. However,
> the solidification structure contains a lot of primary tin-rich dendrites
> (50 vol.% and more) and a eutectic fraction filling in the space between
> these dendrites. Apparently, it solidifies not at all according to the
> equilibrium phase diagram.
> The eutectic structure has the benificial fatigue ductility. Therefore, to
> my opinion, we should aim at a maximum content of eutectic phase. That can
> be achieved by adding some more silver, say, up to 4.0 mass%. In reality,
> the composition varies +/- 0.1, maybe even 0.2 % anyway, so, 4.0% is
> still within the technical specification of eutectic composition. This
> will not increase the melting range significantly either.
> Moreover, reflow soldered joints consist of a mixture of component plating
> and solder from the paste. Tin-based lead-free finishes will consist of
> almost pure tin. This will dilute the silver content of the mixture,
> sometimes by about 50%, and cause a pasty range of (possibly 217-224
> deg.C) and an unfavourable microstructure with a very large primary tin
> fraction. All in all good reasons to put in 3.8-4.0% of solver instead of
> significantly lower concentrations of 3.0% and below.
> I realise that this is expensive, will take even more silver out of the
> world market, etc., but I think that is is the right choice for the
> general-purpose lead-free alloy.
>
> PS:
> I am still amazed about the narrow pasty ranges that are reported in
> literature. For SnBi5Ag1Sb2 we found a very large range: from below 180
> deg.C to about 225 deg.C. We published the DSC curve in the previously
> cited papers in Berlin and Miami.
>
> Best regards, Erik
> --------------------------------------------------------------------------
> --------------------------------------------------------
> Erik E. de Kluizenaar
> Philips CFT - Electronic Packaging & Joining (EP&J)
> Building SAQ-p, p/o box 218, 5600 MD Eindhoven - The Netherlands
> Tel/Fax: (+31 40 27) 36679/36815; E:mail [log in to unmask]
> PHILIPS worldwide homepage: http://www.philips.com
> Internal PHILIPS only:
> http://pww.cft.philips.com/cfteurope/electronics/elpajo/index.htm
> --------------------------------------------------------------------------
> ---------------------------------------------------------
>
>
>
>
>
> [log in to unmask] on 2000/10/24 08:35:20 PM
> To: E.E. de Kluizenaar/EHV/CFT/PHILIPS@EMEA3
> [log in to unmask]@SMTP
> cc:
> Subject: Re: Re: [LF] Timelines for conversion to Lead-Free
> Classification:
>
> What is the SnAgCu eutectic? I have heard 3 or 4 different parties claim
> a particular alloy to be the eutectic formulation while contradicting the
> work of others. For my money, it seems like we'll never know with 100%
> confidence, simply b/c a few parties are interested in touting their own
> alloys. Furthermore, what does it really matter, as most SnAgCu alloys
> seem
> to have only a 1-2C pasty range? After all, the tin/lead alloy we all
> refer to as eutectic (Sn63/Pb37) is not the true eutectic, as any phase
> diagram will prove.
>
> David
>
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