The Challenge of Attaching Dissimilar Materials - Structural Interfaces and Attachments in Biology - page 5

Fig. 1.2 Crack propagating along the diode-to-heat sink attachment (adapted, with permission,
from [ 5 ])

(i.e., epoxy adhesive between the chip and substrate) may somewhat relieve the
effect of thermal loading, but the problem of fracture along the interface of dissimilar
materials remains a critical issue in electronic packaging. The highest stress singu-
larity occurs at the corner of electronic packages, as is evidenced by experimental
results (Fig. 1.3 ).

1.2.1.3 Sandwich Structures: Facing-to-Core Bond

Sandwich structures represent an extrapolation of the concept of an I-beam or a
truss, combining two predominantly load-carrying flanges with one relatively
underloaded web to maximize the bending strength and stiffness. In a sandwich
panel, the flanges are replaced with facings that are typically fiber-reinforced
composites or metallic, while the core is solid polymeric, foam, or honeycomb.
In the latter case, the honeycomb core is either composite or metallic. The vast
majority of sandwich structures utilize heavier and stiffer materials in the facings
and lighter materials with a lower strength and stiffness in the core (in case of
honeycomb cores, “lighter” typically refers to the weight density, rather than the
weight of a bulk core material). In these cases, debonding of facings from the core
represents a major failure mode that is also difficult to detect by inspection.
An example, of a sandwich panel with facings and a solid polymeric core
experiencing interfacial debonding as a result of a low-velocity (5 m/s) impact
with a 25 mm hemispherical

indenter is shown in Fig. 1.4 . The facings