Stress Distribution
Structural stresses in multi-layered electronic assemblies are transferred through adhesive and solder interfaces rather than uniformly throughout the bulk materials. This phenomenon of shear lag describes the non-uniform distribution of shear strain along the bonded interface between a component and the substrate. It dictates that the highest stresses concentrate at the outer edges of the joint while the center remains relatively unstressed.
The effect decreases as the bond line thickness increases.
Interfacial Load
Stiff components mounted on flexible circuit boards can cause severe strain localization under mechanical bending. Because of shear lag, the load transfer does not happen instantaneously but builds up over a transition length. This localized strain concentration can exceed the yield strength of the bonding agent or the laminate pad.
Designers optimize joint geometry to ensure that the transition length fits within the physical contact area of the component. When the adhesive layer is extremely thin, the transition length is short, which leads to higher stress spikes at the component boundary.
Reliability Impact
Thermomechanical failure in surface mount joints is often accelerated by stress concentrations that occur during thermal cycling. By analyzing shear lag, engineers calculate the optimum length of joint lines to reduce interfacial peeling stresses. This evaluation dictates the choice of compliant solder alloys or underfill resins.
