Good afternoon,

At 20.49 2013.05.26, alan.b.pearce@stfc.ac.uk wrote:
>> Using less soldering will help, as said, and also
>> keeping the capacitor well down on the PCB (not lifted by the solder on =
its
>> pads).=20
>
>Umm, that contradicts practice in the space industry. ESA guidelines=20
>specify 0 to 0.4mm elevation of the capacitor off the PCB so that the=20
>solder can flex and provide some mechanical relief.

Thanks for pointing that out, I've read it somewhere but the source didn't
seem excellent (I remember this because I didn't save that webpage, but I
then recalled the detail which you are now correcting).


>Also the PCB is=20
>kept over a heater that maintains it at around 75C, and the component=20
>heated to anywhere between 100C and 150C before getting a soldering=20
>iron near them.

This is not too practical unfortunately, so in the end I decided to keep
the multilayer ceramic technology, but through hole instead of SMD, as it
seems much more mechanically robust and I had enough PCB space for that.


>This all minimises the thermal shock during soldering,=20
>and in reflow soldering this is all done by the temperature profile.

Of course. But for hand soldering it starts to become overkill.


>> Lead-free soldering causes less stress than Sn-Pb soldering, because it'=
s
>> significantly stiffer than the "older" soldering [ see
>> http://www.dfrsolutions.com/pdfs/2006_Cracking_Pb-free.pdf ]. My
>> application is Sn-Pb (automotive, RoHS exempt) and thus is "weaker" (for
>> once!).
>
>Haven't looked at that document yet, but cannot understand why a=20
>solder that gives less flexure is better when the component and PCB=20
>have significantly different temp coefficients.

That surprised me too.


>Thanks for all the links anyway, I will be having a look at them to=20
>see what I can specifically learn.

You are welcome. On a side note about this history and lessons I learnt,
weirdly there was no mention of MLCC "thermal and mechanical fragility"
in a book by Tim Williams that I've almost learnt by heart, "The Circuit
Designer's Companion", a book that is on the practical side of things, but
weirdly didn't mention cracks at all. I've inherited a lot of experience
(without actually needing to produce the magic smoke) from Dr. Williams
thanks to his book, maybe I should advice him to add a note about MLCC
cracking problems in the next edition of his book. ;)

Wonderful thing the PIClist!

With kind regards,
Mario


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