Effect of polyimide baking on bump resistance in flip-chip solder joints |
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| Authors: | Hsi-Kuei Cheng Shien-Ping Feng Yi-Jen Lai Kuo-Chio Liu Ying-Lang Wang Tzeng-Feng Liu Chih-Ming Chen |
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| Affiliation: | 1. Department of Materials Science and Engineering, National Chiao Tung University, Hsinchu 300, Taiwan;2. Department of Mechanical Engineering, The University of Hong Kong, Pokfulam, Hong Kong;3. Backend Operations Division, Taiwan Semiconductor Manufacturing Company (TSMC), Ltd., Taiwan;4. Institute of Lighting and Energy Photonics, National Chiao Tung University, Hsinchu 300, Taiwan;5. Department of Chemical Engineering, National Chung Hsing University, Taichung 402, Taiwan |
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| Abstract: | 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 mΩ to 225 mΩ. 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 mΩ to 10.8 mΩ. |
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