and Fracture of Engineering Materials
concern of semiconductor packaging engineers is the proper design
and assembly of IC packages so that these will not just meet their
quality standards, but their reliability requirements as well.
More often than not, the failure of an IC package failure involves the
deformation or fracture of one or more of its parts or features.
It is therefore important for assembly engineers to have a basic
knowledge of why and how engineering materials deform and, in extreme
cases, even fracture.
is the phenomenon in which a material undergoes changes in dimensions in
response to mechanical forces. The deformation is said to be
if the material returns to its original size and shape upon removal of
the applied load. On the other hand, it is referred to as
deformation if the application of the mechanical load results in a
permanent change in the dimensions of the material, i.e., it doesn't
return to its original size and shape even if the mechanical load is no
longer being applied to it.
or rupture, is the phenomenon wherein a structural component or feature
breaks into two or more pieces. In the semiconductor industry, we all
know that a package or any of its parts doesn't have to fracture before
it becomes a failure.
which is defined as
to a specification, can occur once a deformation causes the package to
not meet any of its specifications, with or without fracture. This
is why bent leads and warped packages are both considered as assembly
is defined as the force applied per unit area. Engineering strain,
on the other hand, is the ratio of the change in a dimension to its
original value. These definitions may be mathematically expressed
= F / Ao
ε = Δl /
where Ao and lo are the original cross-sectional area and length of the
specimen subjected to a uniaxial force F, respectively, while l is the
instantaneous length of the specimen.
values of strain during elastic deformation, the strain experienced by a
specimen is linearly related to the stress applied on it. This
linear relationship between stress and strain is known as
The ratio of the stress to strain in the linear elastic region is known
which is also known as the elastic modulus. Thus,
Young's modulus is a measure of the
material, i.e., the higher the Young's modulus of a material, the
stiffer it is, and the less strain it exhibits for a given stress.
of a material is the critical value that the applied stress needs to
exceed for the deformation to become permanent. If a material is
loaded beyond its elastic limit, it can no longer go back to its
original shape and size upon removal of the force. Such a
permanent deformation is also known as plastic deformation. Once
the plastic deformation region is reached, the stress-strain
relationship ceases to be linear and no longer obeys Hooke's Law.
increase in hardness upon experiencing plastic straining, a phenomenon
In metals, strain hardening is due to interactions between dislocations
in the metallic crystal, which significantly decrease the mobility of
the dislocations. In polymers, strain hardening results from chain
alignment in the stress direction. Unlike metals and polymers,
ceramics do not exhibit plastic deformation due to the restricted
mobility of their dislocations, making ceramics brittle.
difference between elastic and plastic deformation is in how they change
the shape and volume of a specimen. A specimen that's under
elastic deformation will change more in volume than its shape, since
only the separation distance between atoms change, while retaining the
atoms' nearest neighbors. A specimen that's under plastic
deformation, however, will change more in shape than in volume, since
this type of deformation does not alter the bond lengths, but results in
slip processes within its microstructure.
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