Effect of polyimide baking on bump resistance in flip-chip solder joints

• Effect of PI pre-baking on bump resistance of flip-chip solder joint is investigated.
• Pre-baking of PI improves chip-to-substrate bonding but increases bump resistance.
• A PI outgassing model is proposed to explain bump resistance increment.
• Lowering plasma etching temperature reduces PI outgassing and bump resistance.

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.