Multi-physics Simulation And Experimental Study Of Electromagnetic Compaction Of Silver-based Brazing Powders

With the development of super-large-scale integrated circuit and miniaturized chip components,the lead-free and cadmium-free environmental-friendly brazing material with high reliability has been a research focus nowadays.The relative density and its distribution within the brazing powder compact can be effectively improved by electromagnetic compaction.The brazing compact by electromagnetic compaction has showed its advantages on brazing performance and preparation cost,which has great application potential in the microelectronic packaging industry.Therefore,the research on the densification mechanism of multi-metallic mixed brazing powders by electromagnetic compaction is important for the theoretical exploration and practical application.The electromagnetic pulsed compaction tests of brazing powders were carried out with WG-III forming machine.An electromagnetic-mechanical-thermal coupled model of brazing powder electromagnetic compaction was established.The force and energy parameters,the macroscopic and microscopic densification behaviors of multi-metallic mixed brazing powders,and the temperature rise within brazing powder compact were numerically and experimentally investigated.Furthermore,the sintering densification performance related to the relative density of brazing powder compact was theoretically studied and predicted,and then evaluated by sintering densification tests.The effects of the discharge capacitance parameters,spiral coil structure,driving board structure and its assembly dimension on the discharge current were analyzed by the multi-physics simulation and experimental tests.It was revealed that the amplitude of discharge current is nearly linear with the discharge voltage.The amplitude and duration of discharge current are improved by increasing discharge capacitance.The discharge current is changed by the electrical resistance and inductance of flat spiral coil.The mutual inductance between the pulsed magnetic field of discharge flat spiral coil and the induced magnetic field of driving board is determined by the distance between the spiral coil and driving board.The magnetic distribution of the pulsed magnetic field,the characteristic of magnetic driving force and mechanical motion of driving board,the heat generation and the temperature rise of the discharge coil and driving board were studied in the force and energy analysis.The magnetic density between the spiral coil and driving board firstly increases and then decreases along the radial direction,with the maximum magnetic density at the half radius and the zero magnetic density at the central part of driving board.The eddy current in driving board is mainly generated in the top surface,and it firstly increases and then decreases,with a wave-changing under each coil section.The eddy current rapidly decreases along the depth direction,and nearly decreases to zero at about 1.3mm depth.The dynamic forces,the heat generation and the temperature rise in the driving board are determined by the eddy current distribution,which are importance concerns for the energy converting mechanism of pulsed magnetic field.The discrete flow of powders particle and the plastic deformation of metallic powder particle were combined to study the densification evolution process in brazing powder electromagnetic pulsed compaction.The densification process of multi-metallic mixed brazing powder compaction consists of two stages: the particle rearrangements,the frictional sliding and local contact deformation in the early stage,the large-strain high strain-rate plastic deformation of powder particle and the general coordination plastic deformation in the later stage.The majority of compacting energy is consumed by the large-strain high strain-rate plastic deformation of powder particles.The stress gradient within brazing powders is the driving force of particle flow,and the evolution mechanism of relative density gradient was revealed by the stress distribution within powder compact during powder densification.On the basis of multi-particle finite element simulation,the effects of the die wall/particle friction,inter-particle friction,particle size and the compacting energy on the densification behaviors were comprehensively analyzed.By investigating the effects of friction on particle flow,the die wall/ particle friction makes the powder particles near the die walls delayed in axial flow,which results in the relative density gradients existing in the cylindrical surface of powder compacts.The inter-particle friction can reduce the general particle flow and suppress the coordination plastic deformations,which leads to the relative density gradients within brazing powder compact.By the response surface analysis and experimental tests of the particle size,the reasonable size ratio of brazing powder particle can improve the particle flow and enhance the compaction densification.It is revealed by theoretical and experimental analysis that the compacting energy per powder mass can more precisely predict the compaction densification quality.The mechanism of heat generation and temperature rise in the brazing powder compacting process were studied,and the effect of temperature rise on powder densification was evaluated.The plastic deformation of powder particles,die wall/particle friction,and inter-particle friction are main heating sources for powder compact.Temperature rise is beneficial for promoting the relative density,but the temperature gradient within powder compact is bad for uniform densification.The inter-particle friction blocks the particle flow to make the heat concentrated near top punch,which results in the axial temperature gradient within powder compact.Meanwhile,the die wall/particle friction causes the radial temperature gradient within the powder compact.The sintering model related to relative density of powder compact was established.The mechanism of the volume expansion at lower sintering temperature and volume shrink at higher sintering temperature during the sintering densification of powder compact were revealed by the sinterin densification model.The effects of relative density,relative density gradient,and shape of powder compact on the sintering deformation were analyzed.The sintering densification tests and the brazing tests were completed by the optimized densification design,which can provide theoretical references for dimension design of powder compact and sintering process optimization.