Mechanical Deformation
A physical deformation occurs when two bonded materials with different thermal expansion rates undergo temperature changes. This localized CTE mismatch strain arises frequently in microelectronic assemblies where silicon dies are mounted on ceramic or organic substrates. The mismatch produces forces at the interface of the joint, which can lead to warping of the package.
Thermal Expansion
The degree of dimensional change per degree of temperature change varies widely between metals, ceramics, and polymers. When these materials are combined in a single sensor package, any heating or cooling forces one material to expand more than another. This unequal expansion is the underlying cause of the physical strain, which requires engineers to select materials with matched expansion rates for critical interfaces.
Package Warpage
Structural bending occurs during the cooling phase of the assembly process as the materials attempt to return to their original dimensions. This action causes a physical bend in the substrate, which can disrupt the alignment of sensitive optical or mechanical components. The resulting curvature degrades the measurement accuracy of the enclosed sensor.
Failure Mechanism
Delamination and solder joint cracking represent the primary risks associated with this mechanical strain. Repeated temperature cycling continuously flexes the bonded interface. This action eventually shears the electrical connections.
