Preparation And Interfacial Reaction Of High Density Indium Bumps For Micro-LED

With the development trend of Micro-LED packaging towards large size and narrow pitch,using metal indium as a low-temperature interconnect material has become a method to alleviate the thermal mismatch problem during chip packaging.At this stage,the size of interconnect bumps in Micro-LEDs has been reduced to less than 50μm,and the fabrication of small-sized indium bump arrays has become difficult.The reliability of interconnection is a huge challenge,and there is no low-cost,high-quality balling method,and the research on the interfacial reliability of the small-scale In-rich Au/In system is insufficient.This paper focuses on the high-quality fabrication process of indium bumps in Micro-LED packaging and the interfacial reaction and mechanical properties changes of small-sized Au/In systems under thermal load.The main work is as follows:（1）Based on the ANAND viscoplastic model and the modified Coffin-Manson fatigue life prediction model,the effect of solder joint materials on the reliability of Micro-LED flip-chip bonded modules under thermal cyclic loading was studied.The study found that,compared with Sn63Pb37 and SAC305 solders,In solder has the smallest equivalent stress in thermal cycling,which is 6.7%of SAC305 solder and 4.9%of Sn63Pb37 solder,and has the smallest accumulation of strain energy and the highest fatigue life.It can be seen that In is used as a solder joint.Materials can improve the thermal reliability of devices to a certain extent.（2）A high-density patterned indium bump array with a diameter of20μm and a pitch of 80μm was prepared by the method of reflow.The metal Ti with strong binding force to Au and good corrosion resistance was used as the solder mask layer.Au As the wetting layer,an indium film shrink-ball pad structure with internal wetting and external non-wetting is formed;the parameters in the previous processes such as development and stripping are optimized;the removal of oxide layer,oxygen isolation,and reflow reagents in the reflow process are explored and summarized.The main points of the reflow process without destroying the surface tension of the indium film.Finally,liquid flux is used as the reflow reagent to successfully obtain a uniform indium ball array with uniform size,smooth surface and a ball formation rate greater than 99.8%in the atmosphere.（3）The interfacial reaction and mechanical properties of small-sized indium bump samples with Au wetting layers of 100 nm and 1μm were investigated after aging at 70°C for 50 h,100 h,200 h and 400 h.The study found that the kinetics of the Au/In reaction followed a parabolic law,the growth of the reactive compound was controlled by diffusion and was significantly affected by temperature,and the reduction in the size of the solder joint resulted in an increase in the proportion of IMC in the interconnect solder joint.After reflow and aging The thicker intermetallic compound gradually closes the diffusion channels of atoms and blocks the diffusion and migration between atoms.Therefore,the IMC generated by the small-sized In-rich Au/In system will appear delamination,and the position close to the indium solder mainly generates In-rich Phase Au In₂,Au-rich phase is formed near the wetting layer;with the increase of aging time,the thickness of IMC at the interconnection interface increases,and there is a phenomenon of lateral diffusion,and Au In₂ grains are coarsened under the effect of aging,The fracture surface morphology of the interface changes from ductile to brittle,resulting in a decrease in the shear strength of the solder joints.After 400h aging treatment,the shear strength of the indium bump with a wetting layer of 100 nm and the indium bump with a wetting layer of 1μm decreased.The strength is greater,but the shear strength decreases more rapidly after the aging treatment due to the thicker Au layer and the rapid growth of the IMC.In general,the indium bumps with Au as the wetting layer still have larger shear strength after thermal aging.In this study,the key process factors in the reflow shrinkage ball were identified,the process feasibility of flux reflow under atmospheric atmosphere was preliminarily confirmed,and the interfacial reaction under the 20μm Au/In system was studied.It provides a reference for the low-cost fabrication and solder joint reliability evaluation of Micro-LED module flip-chip solder joints.