25 November 2016

iNEMI Research Webinar: Developing Damage Models for Solder Joints Exposed to Complex Stress States: Influence of Potting, Coating, Mirroring, and Housing



Solder joints are subjected to increasingly higher thermal and mechanical loads as component size decreases and interconnect density increases. Encapsulants are used to protect components from moisture, corrosion, vibration and thermal exposure in high-reliability environments. These materials are available in a large variety of chemistries such as silicones, acrylics, urethanes and epoxies. The fatigue life of solder joints in ball grid array (BGA) components can be adversely effected by the mechanical properties of the various materials used to protect them. The combination of an encapsulant’s coefficient of thermal expansion (CTE), elastic modulus (E) and glass transition temperature (Tg), along with component configuration, places solder joints in a mixed mode stress state.

During thermo-mechanical fatigue (TMF) loading conditions, the encapsulant’s expansion could generate large axial stresses that, combined with shear strains, place solder joints in cyclic multiaxial non-proportional TMF conditions. Defining mechanistic damage parameters for such complex loading conditions is essential to better predict reliability of solder interconnects in field environments. Experimental data of axially dominated TMF loading of solder joints is necessary due to the empirical nature of fatigue life models.

This work presents the development of a novel specimen designed to create axial loads in solder joints. Encapsulation materials are used in the specimen as the main axial deformation driver. Materials with specific mechanical properties are selected along with certain temperature ranges to control the stress range in solder joints. The specimen’s configuration allows for accurately controlling the magnitude of the axial stress and subjects solder joints to TMF under varying temperatures and dwell times. Experimental results are used to evaluate the applicability of energy-based fatigue life models of solder joints in axially dominated TMF loading conditions. A modeling approach based on cracking energy density is tested in an effort to fit a damage parameter to TMF life of solder joints in axial loading.




Craig Hillman and Maxim Serebreni



Thursday the 8th of December 2016

05:00 PM CET