Dear Eric,
This is somewhat different than our experience. The coating on leads is
very thin and in wave soldering a mechanical removal of the finish occurs,
like sugar-into-water removal, so that it is the base metal that is really
being soldered to. The IBM paper that I referenced earlier claims that the
SnAgCu eutectic is low and from my interpretation recommends alloys closer
2.7Ag. The paper states that "for compositions equal to or less than 2.7 wt.
% Ag , the Ag3Sn phase cannot nucleate or grow within the liquid in a solder
joint without undercoolings in excess of 20 °C. Since the typical
undercooling required for nucleation found in the present study was
approximately 20 °C, Ag3Sn plate formation is surpressed, independent of
cooling rate. This hypothesis was tested, as shown in Fig. 2, with an alloy
of 2.5 wt. % Ag. No plates formed even at the lowest cooling rates in a
sample size of more than 150 solder joints".
Also in our experiece the structure of Sn and SnCu are actually quite
different and therefore are difficult to compare.
In terms of wetting, because the high silver alloys wet slower, the dwell
time in the wave with these alloys is longer, resulting in more heat
exposure to the component. Using a better-wetting alloy may minimize
component damage.
Regards,
David
----- Original Message -----
From: [log in to unmask]
To: (Leadfree Electronics Assembly Forum) ; David Suraski [AIM]
Sent: Tuesday, November 26, 2002 9:27 AM
Subject: Re: [LF] Lead-Free Wave Soldering
David, all,
Particularly with small components and very fine pitch components the amount
of metal from the finish can be in the same order of magnitude as that of
the applied solder paste deposits. In such cases, the dilution may be as
much as about 50%. This will produce a melting range roughly half of the
temperature difference between pure tin and eutectic SnAgCu, so halfway
between 232 and 217C. Resulting range about 217-225C.
Regarding the vulnerability of many component types to overheating (MSL
problems with semiconductor devices; blow-up of film-capacitors; deformation
of thermo-plastic bodies; etc. etc.) every degree C is one too much.
Therefore, I prefer compositions that bring me the smallest melting range on
top of 217C.
Regarding fatigue life:
Our test results suggest the following ratio's:
SnPb40 = 1
SnAgCu = 1 (at moderate temperature cycling range; -20/+100) and 0.8 (at
large cycling range -40/+125)
SnCu0.7 = 0.5 to 0.6
Sn has a very similar microstructure as SnCu0.7 so, I expect a similar
fatigue life. Now if we shift considerably from eutectic SnAgCu into the
direction of SnCu and Sn compositions, I expect a similar decrease in
fatigue life.
Of course, it will be very interesting to assess the influence of silver
content on fatigue life experimentally. However, this will be a large
experiment and I have no budget that allows me to do so.
Regards, Erik
----------------------------------------------------------------------------
-----------------------------------------------------------------
Erik E. de Kluizenaar
PHILIPS CFT - Electronic Packaging & Assembly (EP&A)
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 homepage: http://www.philips.com; PHILIPS CFT homepage:
http://www.cft.philips.com
Internal PHILIPS only:
http://pww.cft.philips.com/cfteurope/electronics/elpajo/index.htm
----------------------------------------------------------------------------
------------------------------------------------------------------
"David Suraski [AIM]" <[log in to unmask]>
Sent by: Leadfree <[log in to unmask]>
2002-11-26 14:15
Please respond to "(Leadfree Electronics Assembly Forum)"; Please respond to
"David Suraski [AIM]"
To: [log in to unmask]
cc: (bcc: E.E. de Kluizenaar/EHV/CFT/PHILIPS)
Subject: Re: [LF] Lead-Free Wave Soldering
Classification:
Dear Erik,
Thanks for your explanation. You mention that the tin-diluted SACS (and
SAC, I presume) alloy "will be off-eutectic having a relatively large
melting range and a shorter fatigue life." At which alloy composition are
you seeing this large melting range? I was under the impression that alloys
within this family, even with wide elemental swings (within reason, of
course), maintain a narrow pasty range with a m.p. of ~ 217-218C. Do you
know if there exists any comparative fatigue data on the variations of these
alloys?
Sorry for all the questions, but with much of the world focusing on varying
SnAgCuX alloys (2.5, 3.0, 3.5, 4.0Ag, etc.) I think it makes for an
interesting and worthwhile discussion.
Of course, comments from other forum members would be excellent. IBM
recently had an interesting paper on this family of alloys called "Ag3Sn
PLATE FORMATION IN THE SOLIDIFICATION OF NEAR TERNARY EUTECTIC Sn-Ag-Cu
ALLOYS".
Regards,
David
----- Original Message -----
From: [log in to unmask]
To: (Leadfree Electronics Assembly Forum) ; David Suraski [AIM]
Sent: Tuesday, November 26, 2002 3:15 AM
Subject: Re: [LF] Lead-Free Wave Soldering
David, all,
I prefer a relatively high silver content for metallurgical reasons, as
follows.
Most lead-free component finishes are or will be high tin content, probably
pure matt tin. This will dilute the solder, making that the mixture during
reflow will be off-eutectic having a relatively large melting range and a
shorter fatigue life. Therefore I personally prefer a slight over-saturation
with silver in order to compensate for this effect. Rather 4.0% of silver
than, say, 3.0%.
Regarding the wetting capability of lead-free alloys; that is substantially
lower than that of eutectic tin-lead. The small differences between the
various lead-free alloys are nothing compared to that difference so, I payed
little attention to this after completion of the IDEALS project.
Regards, Erik
----------------------------------------------------------------------------
-----------------------------------------------------------------
Erik E. de Kluizenaar
PHILIPS CFT - Electronic Packaging & Assembly (EP&A)
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 homepage: http://www.philips.com; PHILIPS CFT homepage:
http://www.cft.philips.com
Internal PHILIPS only:
http://pww.cft.philips.com/cfteurope/electronics/elpajo/index.htm
----------------------------------------------------------------------------
------------------------------------------------------------------
"David Suraski [AIM]" <[log in to unmask]>
Sent by: Leadfree <[log in to unmask]>
2002-11-25 21:12
Please respond to "(Leadfree Electronics Assembly Forum)"; Please respond to
"David Suraski [AIM]"
To: [log in to unmask]
cc: (bcc: E.E. de Kluizenaar/EHV/CFT/PHILIPS)
Subject: Re: [LF] Lead-Free Wave Soldering
Classification:
Hi Erik,
I really enjoyed the lead-free wave soldering papers that you made available
to the forum. Thanks for the contribution.
1 quick question, and I can't believe I'm asking this, but... is there any
reason why the SnAg3.8Cu0.7Sb0.25 alloy was reviewed instead of a SACS alloy
with a lower silver content? From our experience the alloys with a lower
silver content (< 3%) wet better.
Regards,
David Suraski
AIM
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