Analytical Shear Lag Modeling for Strain Transmission into Bottom Terminated Packages
Analytical shear lag models quantify strain transfer from boards to bottom-terminated packages, showing compliant die attach drops offset drift by 98 percent.
Shear Lag
Stress Mechanism
Tension transfer between bonded structural layers occurs through shear stress at the interface, causing the axial load to vary across the length of the connection. The concept of shear lag describes this unequal stress distribution, where strain is highest at the edges of the bond and lowest in the center. Analyzing this mechanism is critical for designing reliable microelectronic assemblies that use adhesives or solder joints.
Structural engineers utilize numerical models to predict the stress distribution.
Joint Design
Bonded joints experience concentrated strain at their outer boundaries, making the edges the most vulnerable points for fatigue crack initiation. In microelectronics, shear lag affects how thermal stresses are transferred between the silicon die and the substrate through the die-attach adhesive. Designers must choose adhesives with appropriate shear moduli to distribute the stresses more evenly.
This design choice prevents localized joint failure under temperature cycles.
Mathematical Modeling
Stress analysis models calculate the distribution of shear forces along the bonded interface as a function of the joint geometry. The shear lag model predicts that increasing the bond length does not infinitely increase the load capacity.
Experimental Validation
Verifying these stress profiles requires using digital image correlation or high-precision strain gauges to capture the local deformation of the assembly. The strain measurements demonstrate that shear lag causes a peak in deformation at the extreme boundaries of the bonded region. These findings are used to calibrate finite element models and adjust adhesive thickness specifications.
Ensuring the accuracy of these models allows manufacturers to optimize the structural design of high-stress packages.