Meaning
Mechanical tension or compression acting across the boundary between two dissimilar materials arises from mismatches in their physical properties or thermal expansion rates. This interfacial stress concentrates along the joint plane when the assembly experiences temperature fluctuations, creating a shearing force that threatens the adhesion of the joined layers. It governs the mechanical stability of multilayers, thin films, and soldered assemblies in electronic devices.
The magnitude of the force depends on the temperature range, the stiffness of the materials, and the geometry of the interface.
Thermal Expansion
Dissimilar coefficients of thermal expansion generate differential expansion and contraction during heat cycling. When a printed circuit board expands faster than a silicon die, the solder joint between them bears the resulting displacement. This displacement translates directly into high shear forces concentrated at the outer edges of the contact pads.
Deformation Response
Elastic and plastic deformation within the solder alloy redistributes the load to prevent immediate catastrophic failure. Over time, however, the continuous cycle of stress accumulation leads to localized damage along the boundary. This damage weakens the bond line and prepares the path for cracks to propagate.
Geometric Influence
Sharp corners and sudden transitions in joint profile increase the local stress concentration factor by several times. Rounded fillets and gradual transitions distribute the mechanical loads more evenly across the entire surface area. Modifying the joint geometry thus extends the operational life of the assembly under cyclic thermal conditions.
