Late again,
too little time for mailreading, however,
a)I would like one or two images on these fatigue sliding zones.
b)6,000cycles, that's a lot. Danielsson talked about even 20,000 cycles for automotive boards. What do they say at Delphi?
thanks in advance/Ingemar
Dear Werner
You are absolutely right when you state that the bands occur in the regions
with the highest loading. I know the work of B. Morris and I am aware that
we do not fully agree with someone who is not a nobody. Anyway. As much as
I understand, Morris refers do the size of the lead phases when he speaks
of grain coarsening bands. Yes, the lead phases do coarsen. But no, the tin
phases do not. About 5 years ago we developed a metallographic preparation
procedure do make the tin grains visible. This is possible since the grains
are slightly optical active which means that they polarize light depending
on the orientation of their cristallographic axes. With this method we saw,
that within these grain coarsening bands the lead phases are coarsening
while the tin phases are refining or, as I wrote, they are breaking up into
smaller grains when plastic strain is induced.
I agree with you, that the bands are barely visible after slow testing or
after field failures just like that since the lead phases have uniformly
coarsened due to diffusion. However, looking at the tin phases makes the
bands clearly visible this will explain why the band are so pronounced
after accelerated testing. I think, the coarsening of lead in the slip
bands is faster because the diffusion path is along the grain boundaries
and since the tin phases in the bands are smaller than in the surrounding
material there are more grain boundaries thus speeding up the diffusion.
But naturally with time and temperature, also in the surrounding solder
lead will coarsen.
That the bands ( as Morris says coarsened and I call refined ) are the
precursors of the microvoiding makes sense for me, since due to my findings
this is the place where most of the deformation happens, once the bands are
formed because they are easier deformable than the not recristallized
solder. Naturally, a solder layer of 50um is totally recristallized it
might be even more ( the 15um on either side are not a fixed value there
are variations of course ).
However, I think I should go into your data and learn more of you ( as I
already did ).
I attach some pictures (PICT- Format ) (not to TechNet since the server
refuses too large postings )
- One picture of a solder joint of a 1210 after soldering,
- one after 6000 cycles -20 / 100 degr.C; 2Degr.C/min.; 30 minutes dwell time
- one of a shear test specimen which had 50% engineering strain before we
applied a straight scratch and induced another 30% engineering strain.
However, it must be noted, that since the strain is concentrated within the
slip bands the actual strain was much bigger.
Best regards Werner
Guenter
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