Application Study Of Chemical Coating And Liquid Phase Diffusion Connection For Power Device Packaging

Electronic devices are developing in the direction of high power density,miniaturization and high reliability,among which high-power devices based on thirdgeneration semiconductors put forward higher requirements on the connection form of packaging,and the traditional packaging interconnection materials can no longer meet the requirements of their use.In this paper,we study the chemical plating surface treatment and particle coating process,and adopt the transient liquid-phase diffusion connection technology to meet the demand of "low temperature connection,high temperature service" in power device packaging.In order to obtain coated particle solder suitable for transient liquid-phase diffusion connection,the copper substrate chemical nickel plating process is studied,including acidic nickel plating method with sodium hypophosphite as reducing agent and alkaline nickel plating method with hydrazine hydrate as reducing agent.The copper substrate chemical tin plating process was studied,including the acidic replacement method of tin plating with thiourea as a complex and alkaline disproportionation reaction tin plating.At the same time,electrochemical tests were done on the chemical plating solution,mainly open-circuit potential as well as polarization tests were performed on the used plating solution to analyze the deposition mechanism of both metals,and the data obtained in the polarization curves were used to build a chemical plating deposition model in Comsol software to simulate the chemical deposition reaction process.The XRD and SEM results of each coating were analyzed and compared,and finally,a transient liquid-phase diffusion connection experiment was conducted using copper powder tin plating as the solder to characterize the microscopic morphology of the joint cross-section and to test the shear strength of the sintered joints with each particle size of tin-plated copper powder.The acidic chemical nickel plating with sodium hypophosphite as the reducing agent reacted smoothly and the layer was dense and uniform,but the XRD results showed that the crystallinity was poor and the layer contained phosphorus;the alkaline chemical nickel plating solution with hydrazine hydrate as the reducing agent reacted violently and the result was a thin pure nickel layer,both of which were difficult to be applied to the transient liquidphase diffusion connection.The electrochemical test results of the solution with sodium hypophosphite as the reducing agent showed that the temperature rise is necessary for the reaction to occur.The higher the thiourea concentration in the acidic pre-tinning solution,the lower the open circuit potential of the plating solution.In the plating solution without thiourea addition,the open-circuit potential is positive and does not allow the reaction to occur.The acid replacement method of tin plating reacts faster,and the resulting layer is less than 1 μm,and it is used as an activation layer to start the alkaline disproportionation reaction to obtain a rough tin plating layer of about 50 μm.The process was applied to copper powder tin plating experiments,and three particle sizes of 200nm～550nm,1μm and 5μm were obtained,and agglomeration was observed in the SEM images.The shear strengths of the sintered joints from the three particle sizes were 12.06 MPa,27.15 MPa and 15.48 MPa,respectively.