This is a typical question people raise when they hear silicon is a strong
material.

Silicon is a "strong" material since the "stress at its fracture" is high.
However, silicon is a lousy material in terms of "toughness", since its "strain
at fracture" is considerably lower than metals, which can deform plastically.
As any element made from brittle materials, mechanical failure of silicon parts
is generally initiated from defects, and, IC-grade silicon being an almost
perfect crystalline material, the "surface defects" determines the failure. In
your case, with the help of the stress concentration (i.e., V-groove), the area
near the v-groove was stretched beyond the "fracture strain", which the
surroundings (plastic package) creates.

Overall, silicon is a strong material when the driving factor is stress, but
not a tough material when the driving factor is strain. The force from the
package is so large, compared with the resistance from the chip, that the
situation is close to the strain-driven deformation. I would use silicon for a
spring that should not age even after long use under heavy load (ie., high
yield stress required), but I will never use silicon for my bullet-proof vest
(ie., toughness required).

Prof. CJ Kim
Mechanical and Aerospace Engineering
UCLA