Research On Gold Bump Thermosonic Flip-chip Bonding Quality Control

As an area-array interconnection process,flip-chip bonding has been widely used in high-density VLSI and high-frequency IC package area with the advantages of short interconnect length and high I/O density.Compared with the traditional flip chip process by soldering,thermosonic flip chip bonding has shown better performance on short process time and low bonding temperature,successfully applied in packaging LED array,SAW filter and MEMS devices.Besides as thermosonic flip chip bonding uses metal bumps(like gold and copper)which have better electrical properties to achieve interconnection,it has better application prospect in packaging rf and microwave devices working at higher frequency.At present,most applications of thermosonic flip chip technology are peripheral array devices with relatively low I/Os.However in area-array device thermosonic flip-chip packaging practice,it is obviously difficult to guarantee the bonding quality of all bumps in the array.The previous method of matching the process parameters for single bump bonding has obvious limitation,and the bonding process reliability cannot be guaranteed.This paper focuses on the bonding quality issues of thermosonic flip-chip bonding process for area-array application based on electroplating gold bump.Experimental and modeling studies are carried out on the relationship between die shear fracture morphology and bonding quality,process parameter control of single bump quality,bonding quality variation of bump array and bonding quality control of the whole array.The main contents and contributions are as follows:(1)Thermosonic flip-chip bonding of area-array flip chip is realized by utilizing flip-chip device with an ultrasonic bonding tool.Die shear test and fracture mode analysis are performed after flip-chip bonding.Four typical shear fracture morphologies are analyzed,and the failure modes of open/short circuit failure joints are analyzed combined with the electrical connection test results;3-D finite element modeling approach of bump thermosonic flip chip bonding is discussed.Based on the dynamic finite element simulation software ANSYS Workbench LS-dyna,bonding formation process of a single bump is simulated to capture the stress and plastic strain evolution rules of the bump,UBM and bonding interface.Combined with several fracture modes for failure joints,the internal mechanism of these failure modes are obtained,as well as the influence factors of single joint bonding quality.(2)Thermosonic flip-chip bonding dynamic finite element simulations with a single bump are carried out under different bonding time and ultrasonic amplitude respectively.From which the trend of the transient characteristics variation of the bonding system is obtained with the variation of process parameters.And the work mechanism of ultrasonic and bonding pressure in the bonding formation process is discussed.Several bonding experiments are carried out with different process parameter sets,corresponding.The variations of shear strength and the fracture mode distribution are observed,from which the effects of ultrasonic amplitude,bonding pressure and time on the bonding quality of a joint were obtained.(3)A 3-D finite element simulation model of thermosonic flip-chip bonding structure with 3×3 bump array is developed to capture the stress/strain distribution differences between bumps at different locations in bump array.Analysis shows that the uneven bonding pressure distribution on bump array and the offset of bump vibration are system causes of the bonding diversity between the center and edge bumps,meanwhile random factors such as the coplanarity of the bump array and flip chip pads increase the unevenness of the bonding pressure distribution.Then,the approach of judging the joint bonding quality by non-destructive detection is discussed.Finally,focused on the bonding structure used in this paper,a bonding quality control criterion based on the joint diameter after bonding is presented.