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
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[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
----- Original Message -----
From: "<Erik de Kluizenaar>" <[log in to unmask]>
To: <[log in to unmask]>
Sent: Tuesday, October 24, 2000 1:39 PM
Subject: Re: [LF] Timelines for conversion to Lead-Free
Dear Mr. Han Jiangbo, dear all,
Europe as such does not make such a choice. There is no official body here
that would prescribe this. However, it is becoming common opinion throughout
the world, including Europe, that the SnAgCu eutectic alloy is the general
purpose lead-free solution.
WEEE has been adopted by the European Commission only. That is not the same
as adopted as a European law. It still is a draft. It has to go to the EU
parliament, and to the Board of Ministers of the member countries for
amendments and voting. This is a
trajectory that can easily take two years, and the contents can still be
changed.
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]@[log in to unmask] on 2000/10/24 06:00:06 AM
Please respond to [log in to unmask]@SMTP
Sent by: [log in to unmask]
To: [log in to unmask]@SMTP
cc:
Subject: Re: [LF] Timelines for conversion to Lead-Free
Classification:
Hi,
I would like to confirm two pieces of information.
(1) Europe has recommended NEMI's selection (SnAgCu) as the 'standard'
eutectic tin-lead solder replacement.
(2) EC WEEE proposal has been officially adopted.
Appreciate if anyone can help to clarify whether the above is true.
Best regards,
Han Jiangbo, Agilent Technologies
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