Meaning
Material failure often results from the internal stress caused by differing expansion rates. Engineering designs must account for thermal expansion mismatch when bonding materials with different physical properties. Temperature changes cause one substance to grow or shrink more than the other, creating tension at the interface.
Interface Stress
Internal tension develops at the boundary between two substances as the ambient temperature changes. If one material expands more rapidly than its neighbor, the bond between them experiences shear forces. These forces can lead to delamination or cracking in the weaker material.
Electronic components are particularly vulnerable because they combine metals, ceramics, composites, and polymers in a small area. The magnitude of the stress is proportional to the temperature delta and the difference in expansion coefficients. Cracks often start at the corners of a component where the stress concentration is highest.
Design Constraint
Engineering teams must select materials with similar coefficients of thermal expansion to ensure long term stability. Flexible adhesives can sometimes absorb the movement and prevent the buildup of stress. This selection process is a primary step in the development of printed circuit boards and semiconductor packages.
High power applications generate heat that exacerbates the problem.
Reliability Metric
Fatigue life is calculated by subjecting the assembly to rapid temperature cycles. A failure usually appears after a specific number of cycles depending on the severity of the expansion difference. This data allows manufacturers to estimate the service life of a product in the field.
