Study of thermomigration on lead-free micro bump in three-dimensional integrated circuits packaging

• Main Authors: Jhu, Wei-Cheng, 朱偉誠
• Other Authors: Ouyang, Fan-Yi
• Format: Others
• Language: zh-TW
• Published: 2013
• Online Access: http://ndltd.ncl.edu.tw/handle/86369048520593365554

碩士 === 國立清華大學 === 工程與系統科學系 === 101 === In order to deal with the requirements of consumer electronic products, the package technology is currently transition from flip chip technology to three dimensional integrated circuits (3D ICs). Compared to flip chip technology, the bump height of microbumps in 3D ICs is shrunk by an order of magnitude smaller than flip chip package. In this thesis, we studied the microstructural evolution of Pb-free solders in 3D ICs packaged under the influence of a temperature gradient. The behaviors of thermomigration of Pb-free solder between flip chip and 3D ICs packaged were compared and discussed. When the bump height is about 100 μm in the flip chip samples, Sn atoms moved to the hot end under the temperature gradient, and it induced the Sn mass protrusion at the hot end and Sn mass depletion at cold end. Additionally, we found voids form at the cold end. However, when the bump height is about 5 μm in the 3D IC samples, no significant microstructural evolution of Sn was found; instead, the serious dissolution of Ni under-bump metallization (UBM) at hot end was observed. We suggest that this discrepancy between flip chip solder joints and 3D IC microbumps is mainly attributed to the effect of stronger back stress in 3D IC microbumps and the presence of thicker Ni UBM in the 3D IC package. In the flip chip solder joints, the thermomigration driving force of Sn is larger than back stress, resulting in the Sn mass protrusion on the hot end and Sn mass depletion on the cold end. On the other hand, for 3D IC samples, the thermomigration driving force is counteracted by back stress. Moreover, the critical temperature gradient in terms of different bump heights is also discussed, showing below which there will be no net effect of thermomigration of Sn. It is noted that the critical temperature gradient to trigger thermomigration of the Sn atom decreases with increasing the solder bump heights. Furthermore, we also found the asymmetrical growth of Ni3Sn4 intermetallic compounds at both interfaces along with serious consumption of Ni UBM on hot end. We proposed this phenomenon mainly attributed to the thermal gradient driven Ni moving toward the cold end. The molar heat of transport of Ni in Sn was calculated to be + 0.578 kJ/mole.