Boundary Displacement
Relative displacement between two bonded materials of differing thermal expansion rates creates mechanical shear forces along their shared boundary. In electronic assemblies, interfacial shear strain develops at the joint between the copper pad on the circuit board and the alloy solder bump. This strain concentrates at the outermost edges of the joints, where the thermal expansion difference between the rigid component and the laminate substrate is most pronounced.
It drives the mechanical degradation of the joint over multiple temperature cycles. This behavior is particularly aggressive in lead-free solder alloys that possess higher stiffness and lower ductility than leaded solders.
Crack Initiation
Repetitive shear deformation at the material junction leads to the accumulation of microstructural damage and microcracking. Under thermal cycling, high interfacial shear strain causes plastic deformation in the bulk solder near the intermetallic compound layer. This localized stress eventually initiates cracks that propagate across the joint, resulting in an open circuit.
Choosing solder alloys with high fatigue resistance or applying underfill resins helps distribute the stress across a broader surface area, delaying crack formation.
Mitigating Design
Modifying pad geometry and substrate thickness can reduce the shear forces at the junction. Lowering the overall thickness of the circuit board increases its flexibility, allowing the board to bend slightly to absorb thermal mismatches. This flexure reduces the interfacial shear strain, protecting the solder joints of large silicon dies.
