Stella
What do you mean with side concave fillet?
If you have no solder on the sides of the terminations of your
component don't bother.
If you have no fillet at the ends you might run into a problem with
your customer. These fillets do contribute to a certain extend to the
reliability of the solder joint. Thermal cycling tests show, that
cracks are growing simultaneously in the layer between the component as
well as in the fillet due to the local CTE mismatch component/solder.
The crack under the component keeps on growing more or less
horizontally through the fillet once it exits the gap between component
and pad, the other grows vertically along the cap. The failure occurs
in pure thermal cycling if the two cracks meet or, in conjunction with
vibrations, if the load bearing area is to small to hold the component
in place.To decide whether or not the boards can be used, it is
important to know whether the extra number of cycles (as an estimate
I'd say without vibrations max. 1000 cycles with a temperature-swing of
120 deg C when the average lifetime of "normal" solder-joints is
approx. 9000 cycles) is essential for the reliability of the assembly.
Remember, that the number of cycles to failure goes approx. with the
square of the temperature swing. This means, if you half the swing you
have four times more cycles to failure.
On the other hand it is very likely that these solder-joints are more
reliable than hand-soldered repairs and lets face it, if the board
relies on the additional cycles to failure given by a filet at the end
it is anyway on the edge to cause reliability problems.
Best regards
Guenter
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