See, it just starts ( I like arguing with Werner ).
I made some investigation on leadless components after 8000 cycles -20
/ 100deg.C; 2degr./min; dwell time30 min. where we polished the solder
so that we where able to see the grains. What we saw was the breakdown
of the coarse solder structure after soldering into smaller grains.
This refinement occurred between pad and component as well as along the
caps in the meniscus. In my understanding the first due to the CTE
mismatch of component and board the latter due to the local mismatch of
component and solder. We observed the refinement in the meniscus also
in kind of a slip band. It had the shape of a wedge with the tip
heading to the PCB. The cracks we saw where always growing within that
zone. The slip bands that are approx. 15um thick occurred in shear test
specimen with a solder layer that was 50um thick. On some occasions I
was able to see a similar effect in SMT solder joints when they where
around 50um thick. The refined bands measured between 10 and 20 plus
something um.
Due to these findings I believe that the enhancing of the solder
thickness for better reliability has a limit of roughly 30um. Thicker
joints will not contribute to the joint reliability. Well, that's what
I think but I would never dare to contradict Werner in public (HeHe).
Regarding ETH Werner, I must say that every student does some
reliability investigations on an appropriate level. No research is done
in that field in ETH. We transferred it all to EMPA and also the
lecture at ETH is held by us. Well, anyway EMPA is part of ETH.
My job at EMPA is not very different from before exempt one thing. We
are now allowed to perform failure analysis for money and we drown in
work. Luckily I still have research projects ( reliability of lead less
solder, Reuse Repair Recycling of electronic waste ) which enable me to
escape the everyday hassle and to do work which is intellectually
satisfying and gives room for scientific thoughts. All in all I like my
job.
Best regards
Guenter
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