Interaction Effect of Voids and Standoff Height on Thermo-Mechanical Durability of BGA Solder Joints

This paper documents simulation studies on interactive effect of standoff height and void volume on thermo-mechanical durability of Ball Grid Array (BGA) solder joints using a 3D visco-plastic finite element analysis. SE 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 thermo-mechanical durability of voided solder joints. Void area fraction ranges of 14% to 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 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 non-monotonic behavior of the joints. An analytical micro-mechanics 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 non monotonic trend. Rise of these two factors for the void interfacing with copper pads in addition to the decreased load bearing area effect resulted in drastic decrease in durability.