Hi Ingemar,
Actually, a slightly hour-glass shaped solder joint is best from a
reliability point-of-view; the more barrel-shaped (squashed) the joint, the
more uneven the strain-distribution. That means the smallest cross-sections
have the highest strains and they are near the SJ-pad interfaces where the
local expansion mismatch acts and is added to the higher global strains.

Werner Engelmaier

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