Cu-diamond Composites Prepared Via Electrodeposition And Its Thermal Properties

In recent years,thermal management has become significant important for enhancedreliability and performance in microelectronics,energy harvesting systems and lighting components.The development of high thermal conductivity(TC)materials with low coefficient of thermal expansion(CTE)is urgent.In particular,copper matrix diamond composites(Cu-diamond)have became one of the most important candidates for thermal management materials due to their attractive TC and low CTE.The common methods to improve the TC of copper matrix composites are spark plasma sintering,hot-pressing sintering,powder metallurgy method,high pressure sintering technology,and infiltration method.These methods are unavoidably based on high temperature(?1000 K)and high pressure(?60 MPa).Such rigorous condition not only increases energy consumption and price,but also harms to microstructure.Optimizing high-temperature methods is very challenging.In addition,synthesis at high temperature is incompatible with micromachining technology.Thus,it is urgent to develop an alternative method to obtain Cu-diamond composites with high TC.This paper utilizes low-temperature electrodeposition for synthesizing void-free Cu-diamond composites,which have compactly combined diamond/Cu interface and good thermal properties of high TC and low CTE.Firstly,conventional electroplating process is utilized to study Cu-diamond composites,finding that microstructure deteriorations(voids/gaps and nodules)are unavailable during the process and badly combined interface occurs,which leads to high interfacial thermal resistance(ITC).Such microstructure deteriorations result from the huge difference of copper deposition rate in the varisized heterotypic intervals,which are formed by the randomly distributed diamond particles.As a result,the TC of Cu-diamond composites(171.67 W/m K)is lower than pure copper(400 W/m K).Thus,harmoning the copper deposition rate in varisized heterotypic intervals is the key factor for synthesizing void-free Cu-diamond composites.Secondly,by imitating the latest technology of TSV,the potential additives are optimized and void defects of traditional electroplating Cu-diamond composites are overcomed,leading to the preparation of void-free composites.Novel electroplating process with two specific additives(accelerator DVF-B and suppresser DVF-C)is utilized to synthesize Cu-diamond composites,which finds that microstructure deteriorations are eliminated during the whole electroplating process.With the uniformly distributed diamond particles and compactly combined diamond/Cu interface,Cu-diamond composites achieve the high TC of 614.87 W/m K.The effect and mechanism of DVF-B and DVF-C on microstructure,crystallization,interfacial combination and thermal properties of Cu-diamond composites are deeply investigated.Based on the specific nature(diffusion,adsorption,desorption,coordination),the two competitive additives work selectively and cooperatively to eliminate deterioration of voids/gaps and nodules for Cu-diamond composites,leading to the well-combined diamond/copper interface and high TC.DVF-B tends to promote copper fully filling in small and micro intervals formed by diamond particles,while DVF-C prefers to restrain copper deposition in large intervals and leveling copper nodules.Thirdly,the influences of process parameters and properties on TC and CTE of the composites are studied.The detailed electroplating conditions are investigated,including current density,temperature and forced convection:Due to the nucleation-growth mechanism,the current density need to be less than 30 m A/cm² to limit the formation of copper nodules and level copper matrix.Because the additives are sensitive to temperature,the electroplating temperature should be lower than 30°C.Owing to the difference of adsorption of additives under strong agitation,the stirring speed need to be smaller than 300 rpm,to ensure the selective-cooperative mechanism of DVF-B and DVF-C.The detailed effects of diamond content(ranged from 12.8 to 42.0%)on Cu-diamond composites are investigated:The TC of the Cu-diamond composite increases from 446.82 W/m K to 614.87 W/m K.The average ITC values of diamond/Cu interfaces are 4.65-4.88×10⁷ W/m² K.The CTE of Cu-diamond composite is remarkably reduced with the diamond particles addition.These TC,CTE and ITC results are in good agreement with the theoretical values,which are predicted by H-J model and DEM model for TC,Additive model and Kerner model for CTE,AMM model for ITC.The impacts of diamond particle sizes(ranged from 10 to 400?m)on Cu-diamond composites are investigated:The TC of Cu-diamond composites is improved with the increase of diamond particle sizes.A critical size of diamond particles for improved the TC of Cu-diamond composites is clearly observed and the critical value(22?m)is derived from Kipitza theory.Based on the critical size analysis,the Cu-diamond composite reinforced with large diamond particles(400?m)is synthesized,which possesses the TC of 846.52 W/m K and the CTE of 7.2×10^-6/K.Using the optimal conditions,the composites with bimodal diamond particles are produced by electroplating,reaching a higher diamond volume fraction(20%higher)than the composites reinforced with single diamond particles.Lastly,the novel electroplating technology is integrated into UV-LIGA technology system.For the development of high heat flux microchannel heat sink,a novel micro heat sink structure(pin fins)integrated with high thermal conductive Cu-diamond composites via micromachining process is produced.The whole spoiler column is complete without defects.Diamond particles are compactly combined with copper matrix and evenly dispersed in the spoiler column.The novel electroplating technology is compatible with micromachining technology.