In the typical package, it includes the chip, modeling compounds, under fill, solder ball, solder bump, and the most important role is the substrate, which consists of the ABF epoxy, stackup via, traces, and copper planes. Assume the crack occurs in the cross-section between the inner ABF extended to via and caused the via crack. What’s the appropriate deformation with warpage that causes the crack? In and the most important role is the substrate, which consists of the ABF epoxy, stackup via, traces, and copper planes. Assume the crack occurs in the cross-section between the inner ABF extended to via and caused the via crack. What’s the appropriate deformation with warpage that causes the crack? In terms of quantitative mechanic buzzwords, the shear stress and shear strain play major roles in the thermal mech reliability analysis.

Shear Stress and Strain: Definitions
Shear Stress (τ tau): This is defined as the force per unit area acting parallel to the surface of a material. It is crucial in understanding how materials deform under applied forces that cause layers to slide past one another.
Shear Strain (γ gamma): This represents the deformation of a material in response to shear stress, calculated as the change in shape (angle) relative to the original shape. Stresses can develop due to thermal contraction. These stresses can create conditions conducive to shear failure at the interfaces, leading to cracks. Mechanical Loading: Under operational conditions, devices experience mechanical loads that can further exacerbate shear stresses, leading to potential failure.

Relevance in Packaging Failure In the scenario described, where a crack occurs at the interface between the inner ABF (Ajinomoto Build-up Film) and a via, the failure is primarily due to the mechanical stresses induced during thermal cycling or mechanical loading. The mismatch in thermal expansion coefficients among different materials (like the ABF epoxy and copper in the vias) can lead to significant shear stresses at these interfaces, especially when the materials contract or expand at different rates.
Thermal CTE mismatch: As the temperature changes, the different materials expand or contract differently, generating shear stresses that can lead to cracking, particularly at weak interfaces like those between the ABF and the via.

Interfacial Cracking: The cracks often initiate due to excessive shear stress at the interface, which can be exacerbated by factors such as the geometry of the via, the thickness of the ABF layer, and the overall design of the substrate.
Mechanisms Leading to Cracking Residual Stress: During the curing process of the epoxy, residual stresses can develop due to thermal contraction. These stresses can create conditions conducive to shear failure at the interfaces, leading to cracks.
Mechanical Loading: Under operational conditions, devices experience mechanical loads that can further exacerbate shear stresses, leading to potential failure.