Browsing by Author "Liu, Kuo-Chio"

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Effect of polyimide baking on bump resistance in flip-chip solder joints
(2014-03-01)
Cheng, Hsi-Kuei
;
Feng, Shien-Ping
;
Lai, Yi-Jen
;
Liu, Kuo-Chio
;
Wang, Ying-Lang
The effect of polyimide (PI) thermal process on the bump resistance of flip-chip solder joint is investigated for 28 nm technology device with aggressive extreme low-k (ELK) dielectric film scheme and lead-free solder. Kelvin structure is designed in the bump array to measure the resistance of single solder bump. An additional low-temperature pre-baking before standard PI curing increases the bump resistance from 9.3 m Omega to 225 m Omega. The bump resistance increment is well explained by a PI outgassing model established based on the results of Gas Chromatography-Mass Spectrophotometer (GC-MS) analysis. The PI outgassing substances re-deposit on the Al bump pad, increasing the resistance of interface between under-bump metallurgy (UBM) and underneath Al pad. The resistance of interface is twenty-times higher than pure solder bump, which dominates the measured value of bump resistance. Low-temperature plasma etching prior to UBM deposition is proposed to retard the PI outgassing, and it effectively reduces the bump resistance from 225 m Omega to 10.8 m Omega. (C) 2013 Published by Elsevier Ltd.
Microstructural Evolution of Cu/Solder/Cu Pillar-Type Structures with Different Diffusion Barriers
(2016-08)
Cheng, Hsi-Kuei
;
Lin, Yu-Jie
;
Chen, Chih-Ming
;
Liu, Kuo-Chio
;
Wang, Ying-Lang
Microstructural evolution of the Cu/solder/Cu pillar-type bonding structures with a reduced solder volume subjected to thermal aging at 423 K to 473 K(150 A degrees C to 200 A degrees C) was investigated. In a bonding structure employing a Ni single layer as the diffusion barrier, solder was consumed with formation of the Ni3Sn4 phase at the bonding interfaces due to an usual Sn/Ni interfacial reaction. However, an unusual Sn/Cu reaction occurred with formation of the Cu6Sn5 (and Cu3Sn) phase on the periphery of the Cu pillar due to out-diffusion of Sn toward the pillar periphery. Consumption of solder was accelerated by the above two reactions which led to the formation of a continuous gap in the bonding structure. Employment of a thicker Ni layer plus a Cu cap layer as the diffusion barrier in the bonding structure effectively blocked out-diffusion of Sn toward the periphery of the Cu pillar and therefore retarded the gap formation. The retardation effect was attributed to an increment of diffusion distance on the pillar periphery due to an effective diffusion barrier composed by Ni and thicker Cu-Sn (Cu6Sn5 + Cu3Sn) phase layers. Detailed phase identification and microstructural evolution in the bonding structures were also investigated using scanning electron microscopy and transmission electron microscopy.
Unusual Morphological Evolution of the Cu Pillar/Solder Micro-joints During High-Temperature Annealing
(2015-05-01)
Cheng, Hsi-Kuei
;
Lin, Yu-Jie
;
Chang, Hou-Chien
;
Liu, Kuo-Chio
;
Wang, Ying-Lang
The morphological evolution of Cu pillars capped with Sn-1.8 wt pct Ag solder during high-temperature annealing was investigated. Two intermetallic compounds (Cu6Sn5 + Cu3Sn) formed at the Cu/solder interface which changed the pillar\'s morphology. Surface diffusion enhanced the growth of Cu6Sn5 + Cu3Sn around the pillar brim and transformed the pillar top from a plane into a truncated cone. Bulk diffusion dominated after long-term annealing and the pillar top became hemispherical shaped. A mechanism was proposed to explain the unusual morphological evolution. (C) The Minerals, Metals & Materials Society and ASM International 2015