Interaction Effect of Voids and Standoff Height on Thermomechanical Durability of BGA Solder Joints

This paper documents simulation studies on the interactive effect of standoff height and void volume on the thermomechanical durability of ball-grid-array solder joints using a 3-D viscoplastic finite element analysis. Surface Evolver software was used to find the optimized shape of the solder joints and standoff height by minimizing the surface energy as voids with different sizes were placed in solder balls. A global-local modeling approach was then utilized to model the thermomechanical durability of voided solder joints. Void-area-fraction ranges of 14%-40% were analyzed. A nonmonotonic behavior of durability versus void area fraction was observed. The results showed that, if the void is completely inside the solder ball and has no interface with the boundaries of the joint, it does not have a detrimental effect and even improves the durability as the void size increases. However, voids located at the interface of the solder joint and copper pads were found very detrimental to durability. Factors such as load bearing area, stress concentration factor, and overall compliance of the structure were found responsible for the nonmonotonic behavior of the joints. An analytical micromechanics approach was used to calculate the compliance of the structure, and a nonmonotonic trend in phase with the durability trend was observed. The stress concentration factor also showed the same nonmonotonic trend. The rise of these two factors for the void interfacing with copper pads in addition to the decreased-load-bearing-area effect resulted in a drastic decrease in durability.