Thanks, Keith. This is very informative, and goes hand-in-hand with my
recent experiences with Sn100.
I had an opportunity to work with Kester K100LD Sn100 wire solder and
EasySpheres, and also Nihon Sn100CL "real RMA" cored flux wire and the
SnCuNi +Ge eBalls while performing some laser solder process development
and testing for a government customer three weeks ago. Last week I again
used the Kester K100LD while performing rework/repair during IPC
recertification at STI, during which I performed lots and lots of hand
soldering and hot gas rework, so I became intimately familiar with Sn100
wire. I confirmed the instructor's observation that Sn100 alloy performs
completely different from SAC305, SAC307, SAC407, etc, and is closest to
63/37. It proved to be much easier to solder in hot gas reflow, laser
soldering during the development work, and when hand soldering it
produced far superior solder joints from a wetting and appearance
perspective than did the SAC305/405 alloy solders
I found these two wire solders and solder ball products to be very
similar to standard 63/37 while soldering and performing hand rework and
BGA ball attach using laser and convection. Both Kester and Nihon Sn100
solders performed very well in vibration and shear testing, but not
quite as good as Sn63/Pb37. I will get temperature cycling data in
about 4-5 weeks.
However, the laser-solder process with Sn100 from either Kester or Nihon
had much better test results than any other lead-free alloy that was
convection soldered, and also when the other Pb-free alloys were
laser-soldered. This proved to be a big factor in the vibration and
shear testing.
After having fun performing rework on some very difficult-to-remove
components that were soldered with SAC305 a few weeks ago, I can tell
you it is much easier to rework the same components when they are
soldered with Sn100. One reason is the lower temperature requirement for
the Sn100 alloy, and the other is that a certain amount of ductility
helps in removing the clinched leads without any pad or barrel damage.
My observation is that the ductility of the Sn100 is much greater than
SAC305, and for most purposes this would therefore be a better lead-free
solution in terms of reliability than SAC305. The ease of rework says a
lot for the use of Sn100, it presents less of a temperature shock than
is required with SAC30X.
But Sn63Pb37 is still the best choice in terms of reliability, if one is
allowed to use it.
How much more (or less) of a tin-whisker risk does Sn100CL present than
the SAC alloys?
________________________________
From: [log in to unmask] [mailto:[log in to unmask]]
Sent: Tuesday, August 12, 2008 9:12 AM
To: Stadem, Richard D.; [log in to unmask]
Cc: [log in to unmask]
Subject: Re: FW: [LF] [TN] SN100 for Reflow Application
Hi Richard,
Sorry that it has taken a month to find your e-mail.
In regard to your very pertinent questions:
1. Does the elimination of such a small amount of silver from the alloy
contribute to any significant increase in the solder's ductility after
soldering is completed?
One can infer from the steady reduction in Ag contents as the industry
addresses the issue of the drop-test vulnerability of SAC alloy that any
reduction in Ag seems to be helpful. Because of the low solubility of
Ag in the tin matrix any Ag results in the appearance of some Ag3Sn as a
constituent of the eutectic if not as a primary phase so that going from
low Ag to no Ag has an effect. But in regard to your next question:
2.Specifically, can you provide data as to exactly what the ductility
delta is between all three alloys? I would expect a significant
difference between 63/37 and SN100 or SAC305, due to the percentage of
lead.
I do not have an "apples/apples" comparison I can quote at the moment.
I think that there is a misunderstanding in regard to your third
question:
3. Also, with the understanding that VPS would probably provide better
wetting, how would it otherwise affect the ductility of the alloy after
soldering is completed? The profile used is essentially the same.
I was not suggesting that the properties of the solder joint would be
any different if it had been reflowed in VPS with a similar profile to
oven reflow. All I was saying was that because of the lower Delta T of
that process it has been found that SN100C can be reflowed with a lower
peak temperature than in oven reflow i.e. down to 235C. The only
reason that I can think of for better wetting with VPS would be that the
vapour provides a low oxygen environment that reduces the load on the
flux and the solderable finish.
In regard to your fourth question
4. For a given assembly that has inherently high CTE stresses due to
design issues as Amol's assembly presumably has, are you saying that
using SN100 and VPS rather than SAC305 and convection reflow will cure
his problems?
Reliability is a complex issue with many factors affecting the final
outcome so I could not make the categorical statement that you suggest
in your question. In some situations an alloy like SAC305 with a high
modulus and high flow stress would yield a longer service life than a
softer alloy like SN100C. In other situations the reverse would apply.
All I would say is that using a more compliant alloy could make a
difference to the outcome particularly if the CTE-induced strains are
large.
5. How do the long-term creep fatigue rates of Sn100 compare with SAC305
and SN63Pb37?
Nihon Superior now has stress/strain data on bulk SN100C at strain rates
of 1.4x10-1s-1,10-4s-1 and 10-5s-1 and temperatures of -40, -25, 0, 25,
75 and 125C which they are prepared to consider sharing with people who
can make serious use of such data.
Keith Sweatman
Nihon Superior Co., Ltd.
In a message dated 16/07/2008 11:34:30 PM E. Australia Standard Time,
[log in to unmask] writes:
Subj:FW: [LF] [TN] SN100 for Reflow Application
Date:16/07/2008 11:34:30 PM E. Australia Standard Time
From:[log in to unmask]
To:[log in to unmask], [log in to unmask]
CC:[log in to unmask]
Sent from the Internet ()
Hi, Keith
Thank you for your input on this.
It raises five questions:
1. Does the elimination of such a small amount of silver from
the alloy
contribute to any significant increase in the solder's ductility
after
soldering is completed?
2.Specifically, can you provide data as to exactly what the
ductility
delta is between all three alloys? I would expect a significant
difference between 63/37 and SN100 or SAC305, due to the
percentage of
lead.
3. Also, with the understanding that VPS would probably provide
better
wetting, how would it otherwise affect the ductility of the
alloy after
soldering is completed? The profile used is essentially the
same.
4. For a given assembly that has inherently high CTE stresses
due to
design issues as Amol's assembly presumably has, are you saying
that
using SN100 and VPS rather than SAC305 and convection reflow
will cure
his problems?
5. How do the long-term creep fatigue rates of Sn100 compare
with SAC305
and SN63Pb37?
I am not a metallurgist, and I am at a loss as to where to find
this
information.
-----Original Message-----
From: Leadfree [mailto:[log in to unmask]] On Behalf Of
[log in to unmask]
Sent: Wednesday, July 16, 2008 5:03 AM
To: [log in to unmask]
Subject: Re: [LF] [TN] SN100 for Reflow Application
Although the correspondence under this title has been very
interesting
and informative this string drifted somewhat from the original
question,
which was:
From: "Kane, Amol (349)" <[log in to unmask]>
Subject: SN100 for Reflow Application
X-To: TechNet E-Mail Forum <[log in to unmask]>, Dear technetters,
Is
anybody using SN100 (or other high Sn alloys) for SMT reflow
soldering
applications? what have your experiences been with this alloy?
I have a PCB with a BGA, that is cracking due to mechanical
stress (that
is not SMT process related), we have a combination of solder
cracking,
and pad cratering. The PCB currently uses SAC305. We also build
a SnPb
variant of the same exact board with no issues (the bare board
material
is different for SnPb and LF versions). I was therefore
wondering
whether SCA305 alloy properties could be a contributing factor
this
defect. How does the ductility and shear strength of SN100C
compare to
SAC305 and 67-37SNPb alloy?
To return to the original question, since it has not been clear
from the
correspondence exactly what the nature of the crack was I cannot
say
with any
certainty whether the result would be any better with SN100C
than with
SAC305.
However, to answer the specific question on the ductility and
shear
strength of SN100C, it is a more ductile alloy than SAC305 and
has a
lower flow stress
and in that regard is more like 63/37 SnPb. If the cracking
occurred
in a
situation where the joint was subjected to substantial strain
then the
greater compliance of the SN100C could mean that the solder
would
accommodate that strain without cracking and without
transmitting the
stress to the substrate or the component. Cratering can be a
symptom of
unrelieved stress being transmitted into the laminate instead of
being
absorbed by deformation of the solder itself, e.g. the cratering
failure
with area array devices in dropped cell phones
with SAC305 joints. Cracking of chip capacitors can be another
consequence of
strain being transmitted rather than accommodated by the solder.
In regard to a later comment on the poor results obtained in
reflow with
the 99C alloy (Sn-0.7Cu), although SN100C is based on that same
alloy
microalloying with Ni and Ge substantially changes the behaviour
of the
alloy, as has been well documented in several peer-reviewed
scientific
papers. Reflowed SN100C joints are smooth, bright and generally
free of
shrinkage defects and can be difficult to distinguish from
63/37SnPb
joints.
The higher melting point of SN100C (227C/440F compared with the
215-220C/419-428F of SAC305) has been raised as a concern but
the
experience of the increasing number of lines running with SN100C
paste
indicates that because of the modifying effect of the
microalloying
additions the alloy needs less superheat
(excess of the peak temperature over the melting point) than
SAC305.
For
reflow lines reflowing SAC305 with a peak temperature around
240-245C/464-473F, which seems to be a profile very commonly
used with
that alloy, SN100C has
proved to a "drop-in-replacement". However, depending on the
Delta T,
we would be
more cautious about suggesting SN100C would be a
drop-in-replacement for
SAC305 for lines running with a peak temperature
230-235C/446-455F.
However,
results obtained with the new generation of vapour phase reflow
systems
indicate that good results can be achieved with SN100C with a
liquid
with a boiling point of 235C/455F.
Keith Sweatman
Nihon Superior Co., Ltd
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