The reliability enhancement of solderless flip chip interconnection for copper (cu) stud bump using Thermosonic Bonding (TSB) process

The reliability of the package is important because it can ensure the device can be used for a longer time. Flip chip (FC) is the first interconnection in semiconductor packaging process that has more advantages than wire bond process. Lead free solder (SnAg or SAC) and Cu pillar capped with lead free solder are the common chip materials used in flip chip. However, Kirkendall void will commonly occur when the FC samples undergo reliability stress test and this might affect the lifetime of the devices. The appearance of Kirkendall void is because of two different metal with different diffusion rate formed the intermetallic compound (IMC) and consumes all Sn. FC technique also has the effect on the joint. Thermosonic flip chip bonding (TSB FC) has advantages over the thermocompression flip chip bonding (TCB FC) due to the lower bonding time and temperature. This is because the introduction of ultrasonic. Acoustic plastic effect which produced by ultrasonic energy is widely applied in wire or flip chip bonder for metals bonding. In this project, TSB FC and Cu stud bump have been demonstrated to resolve the Kirkendall void issue as there is no one using this method so far. Cu stud bump has been optimized in terms of free air ball (FAB) formation in order to get consistent round shape balls. Design of experiment (DOE) screening and full factorial were used to define the suitable parameter for bump cutting and ball bond. Other than that, optimization of smooth distance and separation height of bump cutting were also found to be significant to minimize short tail and long tail occurrence during bump cutting process. Furthermore, ultrasonic generator (USG), USG prebleed, and interaction between them in ball bond optimization were essential to be studied in order to have higher bump shear strength, intermetallic compound coverage, and reduction of Al splash. Moreover, studies on factors that affecting the die shear strength of TSB FC has been also investigated in this research. Two factors that influenced the die shear strength were namely ultrasonic power and bonding duration. Medium ultrasonic power of 6.2 W and high bonding duration of 0.5 s tend to yield higher die shear strength. Since TCB FC was the closest competitor to the TSB FC, performances of both samples were compared in this research. It was found that both samples show no delamination in the molding compound area. Besides, they have passed the open short test and no void occurrence in the joints. However, as for the electrical resistance TCB FC samples is 5.2% higher than TSB FC samples. Another finding was discovered when reliability of both samples were compared after temperature cycling (TC) 500. TCB FC sample was found to have broken line at the top joint. Whereas, TSB FC sample were still in a good condition and went through extreme temperature cycling of TC 2000. High temperature storage from time zero until 1008 h at 150 °C has also been performed on TSB FC samples in the reliability stress test. Reliability stress results for TSB FC samples show slower rate of electrical resistance increment in comparison with Lee and Orii works. Therefore, based on the results obtained in this research, it can be concluded that TSB FC and Cu stud bump have the potential to replace solder chip bumping in FC ball grid array (BGA) package.