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 28nm 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 9.3mΩ to 225mΩ. 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 225mΩ to 10.8mΩ.