Flexible Bumping Technology And Applications In MEMS Flip Chip Packaging

Micro Electro Mechanical Systems (MEMS) are the latest technology wave behindthe information revolution.Flip Chip Technology could be a good choice for MEMSpackaging. Whereas the standardized Flip Chip process is not suitable for MEMSpackaging. This thesis focuses on the flexible bumping technology for MEMS die. As the Al pad of MEMS die is not solderable, the special under bump metal (UBM)layer needs to be deposited on Al pad before bumping. The process of depositingNi-P/Cu UBM on Al pad by electroless plating is disused. Morphologies andsolderability of the UBM are investigated. Experimental investigations show that themorphology of UBM is determined by the time of zincation, phosphorus-content ofNi-P layer, and the roughness of the Al pad, the Cu layer enhances the solderability ofUBM. Two types of processes are selected for MEMS die bumping. The first type is aso-called indirect process. The solder balls are fabricated on glass first, and then aretransferred to the pads on die. The second type is a direct process. The solder balls arefabricated on pads directly by solder paste dispensing and reflowing. Screen-printingprocess is employed to fabricate the solder balls during the first type of bumpingprocess. Design of the printing stencil and the particle size of the solder paste arediscussed. Experimental investigations show that through hole diameter of stencilshould be 7 times or more bigger than the average particle size (APS) of the solder paste,thickness of the stencil should be 4 times of APS or more. It's the base requirement formaking the uniform bumps. A new technology for bumps batch transfer is presented in this thesis. In thistechnology, the bumps are fixed and transferred by a Si substrate with tiny pits that arefabricated by anisotropic wet etching technology. In the last part of the thesis, Bumping on pressure sensor die and glass chip arediscussed as the applications of the flexible bumping technology. Shear test has beenemployed to verify the strength of the bump structure. Experimental investigationsshow that crack took place on IMC rich layer of the interface between UBM and solderbump, and the fracture surface showed the tough fracture failure mechanism. No flakingaway of UBM from Al surface has been found. These results show that the shearstrength of this bump structure is acceptable.