Marko
The main problem with solder joint reliability is the strain induced in the
joint and how fast this strain will deform the solder. Solder deforms
somewhat like a well chown chewing gum. The more load you apply the faster
is deforms. Also with constatd load. Fast deformation causes a different,
more destructive deformation mechanism in tin/lead solder than slow
deformation. If a IC package has copper leads, that are easy to bend, just
a small  force due to the differences in the coefficiants of thermal
expansion is applied to the solder joints  inducing a slow deformation in
the solder. Stiff Alloy 42 leads do cause higher loads in the joints
deforming the solder at a higher rate. This leads to a faster crack growth
in solder joints due to thermal cycling.

Best regards
Guenter