Study On Heat Transport And Multi-scale Coupling Of Laser Ball-spraying Micro Interconnection Process

As a new type of micro bump manufacturing technology for electronic packaging,the laser ball-spraying micro interconnection technology generally accurately matches the process parameters of laser equipment through reasonable design.The solder ball is heated and melted under the action of laser,so that it can form the required solder ball bumps completely and at an appropriate temperature to realize micro interconnection.The whole process is completed in milliseconds.Compared with the traditional reflow soldering process,this technology has the characteristics of local heating,fast cooling and fast heating.It is not only applicable to the fabrication of micro solder joints for system level large-area,high-density array packaging,but also adapted the process requirements of more sensitive devices in the 5G communication.However,its process development and reliability application still face many problems to be solved:（1）under the action of laser and nitrogen,the dynamic evolution process and its precise control technology have not been fully mastered;（2）the dynamic mechanism of interfacial reaction of micron scale solder balls under the condition of rapid heating and rapid cooling is not clear.Therefore,this paper takes the dynamic evolution process of laser ball-spraying micro interconnection as the research object,and analyzes it through the combination of simulation model and experimental results.On the model research,a multi-scale coupling study is established through the macro temperature and flow field coupling model and the micro interface intermetallic compound growth model.The main research contents and conclusions are as follows:（1）Modeling and analysis of heat transport multi physical field coupling in laser ball-spraying micro interconnection processCombined with the characteristics of laser spraying micro interconnection process,considering the melting flow in the nozzle and the dynamic heat transfer process of collision with the pad and spreading solidification after spraying,the temperature-flow field coupling model is established.Through the coaxial temperature measurement experiment,the results show that the overall trend is consistent,the accuracy of the model is high,and the dynamic heat transport process in the whole physical field can be obtained;The process problems in the experiment are simulated and analyzed.It is found that the excessive nitrogen pressure leads to the oscillation of the ball;Nozzle solder ball plugging caused by insufficient laser energy input;Welding ball deviation caused by uneven wettability of nozzle hole and inner wall.（2）Analysis on the influence of laser spray ball micro interconnection process factors on solder bump position deviationThe temperature field verification experiment found that the solder bump deviation seriously affects the welding quality and consistency.The causes of solder joint position deviation are analyzed through the experimental design.The results show that the process parameters mainly affect the fluid properties of the solder ball in the melting process,while environmental factors such as nozzle surface and nozzle height affect the occurrence of deviation and the size of deviation rate respectively.The relationship between each factor and the result is analyzed by orthogonal experimental design,The results show that the most influential factor is the pad diameter,then the laser power,and then the nozzle height,and finally the nitrogen pressure and heating time.The reduction of pad diameter can significantly reduce the offset rate,but it will increase the probability of solder resist ablation.The best combination of process factors is inferred through range analysis.Finally,it is concluded that the best combination is laser power 85%,laser action time 80ms,nitrogen pressure 800pa,welding height 1.6mm and pad diameter 0.5mm.（3）Analysis of the influence of laser ball-spraying micro interconnection process factors on interface microstructure and morphologyThe distribution of interface temperature field under high undercooling is obtained through the simulation model.In order to study the formation of interface IMC under high undercooling solidification,the results of scanning electron lens and energy spectrum analysis show that the intermetallic compounds with uniform thickness of 0.3～3um at the interface are mainly（Ni,Cu）₆Sn₅ and（Ni,Cu）₃Sn₄.These two components exist near the Ni plating layer and solder side respectively,and the morphology of intermetallic compounds at the interface mainly includes tiny scallop-shaped,small planar-shaped and large scallop-shaped.The increase of laser energy can change the shape of intermetallic compound at the interface from tiny scallop-shaped to small planar-shaped and large scallop-shaped in turn.The thickness is related to the energy input and the overall shape of solder bump.The shape of solder bump will affect the local cooling rate at the interface.Therefore,high energy input will improve the holding time of liquidus,make the molten solder ball wet the pad,and the solidification cooling rate is high under large spreading area,resulting in the thinning of intermetallic compound thickness.（4）Modeling and simulation analysis of IMC growth at micro interface of laser ball-spraying under multi-scale couplingCombined with the above analysis of macro temperature field and micro morphology,through the establishment of cellular automata model and unidirectional coupling with macro coupling model,the growth of interfacial intermetallic compounds under multi-scale coupling is analyzed.Based on the experimental data,the growth of IMC thickness under different cooling rates is analyzed,the thickness difference between the middle position of the solder bump and the edge IMC caused by the inconsistency of local cooling rate caused by the difference of spreading area is verified.Through the integration of multi-scale coupling model,a multi-scale prediction and process design system of laser ball-spraying micro interconnection is built based on COMSOL and MATLAB.In this paper,through the above research,the dynamic evolution law of the solder ball under the action of laser is obtained,the temperature-flow field coupling model considering the solder ball melting and spray-spreading process is established,and the effects of environmental parameters and process parameters on the solder bump deviation are studied.Through the establishment of cellular automata and macro coupling model,the multi-scale coupling research is carried out,and the experimental verification is carried out,and the growth law of interface IMC under high undercooling solidification is obtained,It has certain engineering significance for the design and precision control application of laser ball-spraying micro interconnection process.