Design and Process for Solder Joint Compensation and Optimization for Flip-Chip Assemblies
Original Publication Date: 2003-May-28
Included in the Prior Art Database: 2003-May-28
The joining of organic substrates and silicon dice that have very different coefficients of thermal expansion (CTE) generates stress on the solder joint during assembly cooling from the solder solidification temperature. The stress in the joints is a function of the distance of the joint from the neutral point (DNP). In the flip-chip assembly, the neutral point is at the center of the die. Therefore, the outermost bumps are subject to highest stress. For some bump systems, the stress concentrations at given locations will be high enough to cause either delamination to the interface or cracking the device underneath. The preferred condition is that the solder will deform under the stress while the integrity of the interfaces is maintained. Typically, with same pad size for both device and substrate, the reflowed solder bumps will have a symmetrical geometry and the stress concentration point is at the interface between solder and UBM. Changing the shape of the joints can have the effect of decreasing the stress occurring at the interface and forcing deformation in the solder. This addresses the problems of premature solder cracking, UBM cracking, intermetallic cracking, delamination, while also meeting the needs of continual trends in technologies where pitches are increasingly fine, reflow temperatures are increasingly large, and dies have greater functionality and size. The modified solder joints are achieved through reduction of solder volume, shifting the pad locations, and change in the ratio of pad sizes between the device and substrate - this is done for the outer bumps. The inner bumps, left unchanged, are utilized to provide standoff for the modified bumps. A stress decrease of almost 50% is realized in the modified solder configuration as compared to the typical solder joint shape.