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This paper presents a systematic underfill selection and characterization methods for 21 ×21 mm2 Cu/low-K flip chip packages (65 nm technology) with 150 μm bump pitch. This paper has also correlated the underfill characterization methods with the reliability results of 15×15 mm2 and 21×21 mm2 Cu/low-K flip chip packages. From the validations of underfill selection and characterization approach with the reliability of 21×21 mm2 Cu/low-K flip chip package, it was found that the reliability results correlated well with the adhesion test results. Underfill/flux compatibility and underfill flow performance are found to be important factors during underfill selection. Underfill should not be sieved out at the initial stage without actual reliability tests.