Mechanical Response
Permanent deformation occurs when a metal exceeds its elastic limit under thermal or mechanical loading during board fabrication and assembly. Measuring copper plastic strain helps engineers predict the fatigue life of plated through holes and surface traces subjected to repeated temperature cycling. Metallic structures that enter this state do not return to their original dimensions once the stress is removed.
Thermal Stress
Differences in the coefficient of thermal expansion between the epoxy resin and the conductive metal generate internal forces during soldering. Excess copper plastic strain leads to work hardening and eventual cracking at the barrel of a via or the shoulder of a land. Simulation tools model these stresses to determine if the ductility of the plated metal is sufficient for the intended environment.
Engineers use the resulting data to modify the heating rates or to select materials with better matched expansion properties to improve the longevity of the interconnect.
Fatigue Boundary
Material properties dictate the number of cycles a connection can withstand before a total fracture occurs. Reducing copper plastic strain involves choosing laminates with lower expansion rates or increasing the thickness of the plating to distribute the load more evenly. Reliability is compromised when the cumulative movement exceeds the fracture toughness of the metal layer.
