Right, that is what happening with solders (specifically with Sn-Pb), UNLESS there is a grain boundary segregation of a specific third element(s), which can prevent recrystallization. SEM would be useless for that and much more sophisticated analysis would require to prove the point.
SAC doesn't behave the same way as Sn-Pb alloys do (we've never managed to see recrystallizatioin even at elevated temperatures and applied load), which made (and still does) me think that the GB are either pinned by particles of intermetallics or by segregation.
Regards,
Vladimir
Vladimir
SENTEC Testing Laboratory Inc.
11 Canadian Road, Unit 7.
Scarborough, ON M1R 5G1
Tel: (416) 899-1882
Fax: (905) 882-8812
www.sentec.ca
-----Original Message-----
From: "David D. Hillman" <[log in to unmask]>
Date: Tue, 27 Apr 2010 17:04:43
To: <[log in to unmask]>
Subject: Re: [TN] Failure fatigue photos
Hi Inge - the really neat thing about solder is that we are using it at
its homogenous temperature - that is the point where a metal's grain
structure can recrystralize/transform. We use solders at a 0.7-0.9
homologous temperature in comparison to structural steel which we use at a
0.16 homologous temperature. Most solder joints have an homogeneous
microstructure within 2 days of being formed. There have also been a
number of experiments where solder joints are "quenched" to produce a very
very fine microstructure which two days later recrystalized into a
uniform, coarser microstructure. The conclusions you made may have had
their validity due to the cleanliness of the structure (aka the
imperfections and trace elements) and some solidification kinetics but
having a grain size root cause is suspect. Sounds like you got to do some
very cool SEM analysis on a bunch of solder joints which is always
interesting.
Dave
Inge <[log in to unmask]>
Sent by: TechNet <[log in to unmask]>
04/27/2010 04:22 PM
Please respond to
TechNet E-Mail Forum <[log in to unmask]>; Please respond to
Inge <[log in to unmask]>
To
[log in to unmask]
cc
Subject
Re: [TN] Failure fatigue photos
With all the respect for your oustanding knowledge here, but we found that
the fine grain zone closest to the lead was much cleaner while the coarser
structure had lots of inclusions (e.g. dross), traces of iron and copper
between the grain borders and other imperfections. We draw the conclusion
that the difference between these two regions caused a somewhat weaker
zone
(more sensitive to stress) We shifted to variations in the cooling rate,
and
were very satified to see that the time to failue increased a lot with
decreasing the cooling time significantly! But Siemens did not say Thank
you
for the help....but that's another story.
Inge
----- Original Message -----
From: "David D. Hillman" <[log in to unmask]>
To: <[log in to unmask]>
Sent: Tuesday, April 27, 2010 10:43 PM
Subject: Re: [TN] Failure fatigue photos
Hi Inge - you don't necessarily need to have a microstructure
solidification gradient grain size. Thermal and vibrational induced
stresses can change a completely uniform microstructure to a banded
microstructure. The IPC will be soon publishing the IPC -AJ-820 Assembly &
Joining handbook which has several good photos plus verbiage of banded
microstructure.
Dave
"Inge" <[log in to unmask]>
04/27/2010 03:22 PM
Please respond to
"Inge" <[log in to unmask]>
To
<[log in to unmask]>, <[log in to unmask]>
cc
Subject
Re: [TN] Failure fatigue photos
Hi both of you,
I made a job for Siemens once, about fatigue. The board behind the
instrument panel of a truck vibrated on the roads in South America,
resulting in lifted leads and the trucks became inoperable. I simulated
the
situation in the vibrator lab, and could see clearly what happened. Just
as
Dave told you, the crack will finally occur in a band which is represented
by fine structure on the one side and a coarse and elongated on the other.
It has partly to do with the solidification to do, fine grains closest to
the legs and coarser the more you are from that site. I'll send one very
illustrative photo. I've missed how to fix that instantly. How do you
bring
the photos to that fast presentation URL?
Inge
--------------------------------------------------
From: "David D. Hillman" <[log in to unmask]>
Sent: den 27 April 2010 06:33
To: <[log in to unmask]>
Subject: Re: [TN] Failure fatigue photos
> Hi Ioan - Do you have any optical photos of the fracture? Thermal cycle
> fatigue failures typically develop "banding (aka microstructure
> coarsening)" in the solder joint shear zone. The solder joint
> microstructure evolves into distinct grains of tin and lead which shows
up
> better in optical photos. The fracture in your photos appears to be
> fatigue but its hard to see the microstructure in your photos. I added a
> photo of what typical thermal cycle induced SnPb coarsening looks like
on
> the IPC Photo Gallery.
>
> http://ipc-technet.groupsite.com/gallery/13118
>
> Dave Hillman
> Rockwell Collins
> [log in to unmask]
>
>
>
> Ioan Tempea <[log in to unmask]>
> Sent by: TechNet <[log in to unmask]>
> 04/27/2010 07:59 AM
> Please respond to
> TechNet E-Mail Forum <[log in to unmask]>; Please respond to
> Ioan Tempea <[log in to unmask]>
>
>
> To
> [log in to unmask]
> cc
>
> Subject
> [TN] Failure fatigue photos
>
>
>
>
>
>
> Dear Technos,
>
>
>
> It looks like, for the first time, I have the occasion to look at
fatigue
> failed joints. The lab analysis we've got states this, but does not
> abounds in details.
>
>
>
> One of the conclusions is "the crack propagates around harder Sn
shearing
> the softer Pb, but the Sn and Pb zones seem fairly large for regular
> solder structure". Citing from Klein Wassink "During cyclic plastic
> deformation the microstructure inside the material changes in the zones
> where the deformation is concentrated. This leads to local grain
> growth..."
>
>
>
> I have deposited the micro-sections on the IPCs website
> http://ipc-technet.groupsite.com/gallery/13116
>
>
>
> I would need your expert opinion regarding the failure. Is this fatigue?
> The big pure Sn and pure Pb areas, are they normal size for fatigued
> joints?
>
>
>
> Thanks,
>
>
>
> Ioan Tempea, ing.
> Ingénieur Principal de Fabrication / Senior Manufacturing Engineer
> T | 450.967.7100 ext.244
> E | [log in to unmask] <mailto:[log in to unmask]>
> W | www.digico.cc <http://www.digico.cc/>
>
>
> N'imprimer que si nécessaire - Print only if you must
>
>
>
>
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