Interfacial Hygroscopic Swelling Stress Analysis in Surface Mount Encapsulants
Interfacial hygroscopic swelling stress arises from differential moisture expansion, compounding thermal mismatch and driving package delamination during reflow.
Peel Stress Mechanics
Bond Tension
Force applied perpendicular to an adhesive interface creates high local concentrations of energy at the edge of the join. The study of peel stress mechanics focuses on the distribution of these loads as a flexible layer is pulled away from a substrate, requiring an analysis of the bending moment and the elastic modulus of the peeling arm. Force is concentrated at the crack tip.
This concentration makes the bond more susceptible to crack initiation at much lower total loads than uniform tension.
Energy Dissipation
Work done to deform the material during separation contributes to the total resistance of the bond. In peel stress mechanics the stiffness of the backing material determines how much the load is concentrated at the tip of the crack.
Fracture Mode
Separation can occur within the adhesive or at the interface between the glue and the part. Analyzing the peel stress mechanics allows engineers to predict whether a failure will be cohesive or adhesive based on the surface energy of the materials.
Test Geometry
Angle of pull and the rate of separation are the two variables that most influence the measured strength. Standardized peel stress mechanics tests often use a 90 degree or 180 degree angle to ensure that results are repeatable across different laboratories.