Wafer applied and no flow underfill screening for 3D stacks

As the demand for 3D packaging increases, selecting reliable and cost effective materials to be used to build these complex packages has gained a lot of importance. As current IC technology nodes are becoming “Moore-than-Moore” challenging, thus industry and research institutes alike are trying to find ways of addressing this challenge. The integration of new types of underfill materials in 3D stacking is one very important part of the package material set that will determine its reliability and cost effectiveness. With the introduction of 3D technology, bump sizes and pitches have been scaled down significantly which in turn has also shrank underfill gaps between dies which complicates the assembly of 3D stacks. The need of new underfill materials and underfilling concepts becomes inevitable. It is quite difficult to make traditional capillary type underfills and underfilling methods to work due to the very narrow gaps and fine bump pitches that 3D stacks have. Pre-applied underfills (Wafer Applied or No Flow) with or without fillers (submicron or Nano-fillers) may prove to be a suitable solution for this concern. Using a 2 die-stack test vehicle with a bump pitch of 40 μm (with Cu and Cu/Sn bumps) and an underfill gap of 13.5 μm, four (4) different underfill materials (2 NUFs and 2 WAUFs) were screened. This paper will report on the assessment done for both wafer applied and no flow underfill materials, the differences in the application process, the material´s filling and stacking process capabilities and finally the reliability of the 3D stacks. The materials were initially screened based on the test vehicle geometry then processed thru the different phases of the screening process. The changes in thermo-compression bonding parameters used in the experiment to improve the electrical yields will also be discussed. It will also be shown how underfill materials with and without fillers differ in the thermo-compression bonding force- required to be able to get good bump-to-bump connection.