Process And Mechanism Of Ultrasonic-assisted Active Soldering Of Sapphire At Low Temperature

Possession of high transmittance,superior chemical stability and excellent mechanical properties,sapphire has been widely used in aerospace and electronics fields.Reliable joining at low temperature is always required when facing the bonding of sapphire as structure parts or functional device substrates.In this paper,sapphire low-temperature bonding successfully realized by the process of ultrasonic soldering with Al or Ti activated Sn based solders。Interfacial strengthening mechanism and the influence of heat effect induced by ultrasound were studied,which laid a foundation for the application of sapphire in the future.The solderability of Al and sapphire under ultrasound was analyzied through the study of interfacial reaction and bonding mechanism between pure Al and sapphire in oder to rule out the influence of other elements.The results showed that deposition reaction took place at the interface between pure Al and sapphire,and the reaction product was Al₂O₃.O required for interfacial deposition reactions originated from the oxygen in air,and oxygen dissovled through sapphire/solders/air triple line.With the increase of ultrasonic time,the evolution of Al₂O₃reaction product layer follows the pattern of island morphology→porous layer morphology→dense layer morphology→porous layer morphology→dense layer mophology.The crystal structure of Al₂O₃reaction product is the same as that of sapphire matrix;therefore,Al₂O₃reaction products formed at the interface can be considered as epitaxial growth of single crystalα-Al₂O₃layer.Between pure Al solders and epitaxial growth ofα-Al₂O₃,a layer of 2-3 nano thickness of Al metal atomic layer was identified,and a transition layer was formed by combining Al metal atomic layer andα-Al₂O₃layer.Based on the solderability with Al,sapphires were joined at 250℃by ultrasonic hot dipping+ultrasonic soldering processes with prepared Sn-3.5Ag-4Al or Sn-9Zn-2Al solers.For Sn-3.5Ag-4Al solders,shear strength of the joints increases from 15 MPa to 41 MPa with an increasing duration of ultrasonic hot dipping time from 10 s to 300 s.Interfacial microstructures of the joints were charicterized,and a continuous amorphous Al₂O₃deposition reaction products layer was identified to form at the interface.Subsequently,the Al₂O₃reaction products layer is either directly joined with solders or joined solders through Ag₃Al₂phases or a structure of Ag₃Al₂phases/Al-rich phases.For Sn-9Zn-2Al solders,shear strength of the joints increases from 13 MPa to 46MPa with a extension of ultrasonic hot dipping time from 20 s to 2000 s.Interfacial microstructures of the joints were charicterized,and a continuous amorphous Al₂O₃deposition reaction product layer with partially crystallized a-Al₂O₃was identifed formed at the interface.Subsequently,the Al₂O₃reaction product layer is either directly joined with Sn-9Zn-2Al solders or joined with Sn-9Zn-2Al solders through Zn-rich phases.Study of the interfacial bonding mechanism of Sn-based Al-activated solders shows that a nano-scale thick Al metal atom layer exsists at the interface,which plays an important role of bonding the Al₂O₃reaction product layer and solders.Interfacial bonding strength of the joints obtained with Sn-3.5Ag-4Al or Sn-9Zn-2Al solders increases with increasing ultrasonic hot dipping time due to the high bonding strength of the formed interfacial strengthening phases Ag₃Al₂phases or Zn-rich phases as a result of enrichment of Ag or Zn elements at the interface with Al metal atomic layer.Due to inactivation of Ti at low temperature,local high temperature was introduced by ultrasound to activate Ti and realize sapphire soldering.Sapphire were soldered at 250℃by ultrasonic hot dipping+ultrasonic soldering procedures with Sn-3.5Ag-4Ti.When ultrasonic hot dipping time increases from5 s to 100 s,the average shear strength of the joints obtained increases from 23MPa to 38 MPa.Based on the interfacial microstructures charecterization,two kinds of reactions were confirmed to occur at the interface.The first is the activation reaction of active element Ti with sapphire.The second is the deposition reaction of dissolved Ti and Al with O in solders during cooling.Thermodynamic calculation of the activation reaction between Ti and sapphire shows that the occurrence temperature of activation reaction at least needs to exceed 1109.482 K with Ti O activation reaction products.Numerical simulation results of the solder pool ultrasonic amplitude and subsequent liquid solders sound pressure field under ultrasound radiation show that the center point of liquid solders stabilizes between positive sound pressure of 5.623 MPa and negative sound pressure of-4.112 MPa.Numerical calculation of the motion equation and thermal effect of a single spherical cavitation bubble shows that when the minimum relative radius of the bubble is 0.011,temperature of the cavitation bubble can reach the maximum value of 4.848×10⁴K.After the collapse of the cavitation bubble,the instantaneous rising temperature of the interface can reach 3.134×10⁴K through heat transferred to the solder.Therefore,due to heat effect introduced by ultrasound,interfacial activation reaction between Ti and sapphire can take place at a temperature as low as 250℃.