Flip-chip bonding using laser induced ultrasonic vibration

Abstract

Current thermosonic flip chip bonding technologies are adversely affected by chip-to-substrate co-planarity errors and bump/pad height variations which can lead to uneven bonding strength and, in extreme cases, chip cratering. This has limited the industrial uptake of thermosonic flip chip assembly. The aim of the research reported here was to explore the use of laser-generated ultrasound as an alternative ultrasound source in flip chip bonding. The research was motivated by the idea that, with greater control over the distribution of ultrasonic energy applied over the bonding interface, it should be possible to compensate for and mitigate the above effects. The main objective of this research work was to establish a working flip-chip bonding process using laser induced ultrasonic vibration. Initially, a literature review on current flip-chip bonding methods and laser ultrasonic methods was carried out. This suggested that confined laser ablation would be the most appropriate technique for generating strong ultrasonic vibration. Next, through modelling and simulation, an investigation was carried out to determine the suitable parameters and methods to be implemented during the experimental stage, including the pressure pulse amplitude, cavity width (irradiance spot size) and type of sacrificial material. Additional investigations were also carried out to explore the effect of applying different materials in generating ultrasonic vibration and also to show the effect of applying multiple pressure pulses simultaneously. In the experimental phase, a custom bonding rig was developed and used to explore the parameter space for thermosonic bonding on polymer substrates using ultrasound generated by a diode-pumped solid-state laser (355 nm wavelength). Initial experiments showed unstable bond strength due to the accumulation of heat which resulted in the appearance of an unwanted glue-like substance at the bonding interface. However, this issue was overcome through careful choice of process parameters combined with the introduction of off-axis laser irradiation. A process for bonding dummy test chips to flexible substrates was successfully established, and in the best case a die shear strength of 9.3 gf/bump was achieved.