Bi-material assembly with a low-modulus-and/or-low-fabrication-temperature bonding material at its ends: optimized stress relief

摘要

Simple and physically meaningful predictive analytical models are developed for the assessment of thermal stresses in a ball-grid-array or in a column-grid-array with a low modulus solder material at the peripheral portions of the assembly. It is shown that a significant thermal stress relief can be achieved, if the bonding system is designed in such a way that the interfacial shearing stress at the ends of the high-modulus-and-high-bonding-tem-perature mid-portion of the assembly at its boundary with the low-modulus-and-low-bonding-temperature peripheral portion is made equal to the stress at the assembly ends. Stress relief as high as 34.3 % can be obtained in the optimized design. If such a design is employed, there is a possibility that no inelastic stresses and strains in the solder joints will occur, and the fatigue life of the vulnerable solder material will be increased dramatically.