The Challenge of Attaching Dissimilar Materials - Structural Interfaces and Attachments in Biology - page 4

Fig. 1.1 Failure of a metal-
composite joint. The cracks
are clearly visible. The onset
of failure at the edge of the
joint is identified by a white
arrow (adapted, with
permission, from [ 3 ])
1.2.1.1 Metal-to-Composite Components
A classical problem in mechanics involves the strength of adhesively bonded joints.
In particular, this problem is relevant to numerous applications found in hybrid
metal-composite structures in aerospace engineering, naval architecture, and other
fields. The properties of metal and composite adherends are typically different,
magnifying the complexity of the analysis, as the joints involve three vastly
different materials (i.e., metal, composite, and adhesive). The extensive literature
on adhesive joints and methods of their analysis is summarized in several reviews,
such as [ 1 ] and [ 2 ]. Failure is usually initiated by a crack in the adhesive layer
between the metal and the composite, originating from the edge of the joint as a
result of a local concentration of peeling and transverse shear stresses (Fig. 1.1 ).
These high local stresses result in the onset of a Mode I fracture, which subse-
quently propagates along the interface and unzips the joint.
1.2.1.2 Electronic Packaging Problems
A typical interface between dissimilar materials in electronics is the die (chip)-substrate
assembly. Failure often occurs due to an elevated temperature, in which case a
mismatch between the coefficients of thermal expansion leads to delamination
cracking along the interface between the heat sink (substrate) and the chip. An
example of such crack between a copper heat sink and a laser diode is shown in
Fig. 1.2 . One of the materials in such bimaterial assemblies exhibits low thermal
expansion and is brittle, increasing the vulnerability of the joint [ 4 ]. The underfill