Influence Of Ultrasonic Application Method On The Morphology And Properties Of Joints In Ultrasonic Assisted Soldering

In order to adapt to the wide application of semiconductor chips in 5G communications,artificial intelligence,new energy vehicles,rail transit and other fields,integrated circuit technology continues to develop,and microelectronic devices have become increasingly miniaturized and highly integrated,which brings new challenges to chip heat dissipation.Thus,the high-temperature reliability of the chip becomes critical,and the low melting point of the traditional Sn-based lead-free solder cannot meet the requirements.Ultrasonic-assisted chip bonding technology is a reliable technology to obtain high temperature resistant joints at low processing temperature at low cost.Commonly used ultrasound includes transverse wave ultrasound and longitudinal wave ultrasound,but no one has performed comparative study on two different types of ultrasound on the joint morphology and properties.There is an urgent need to find a more suitable ultrasonic type to provide a reference for obtaining excellent performance joints.Three composite solders,pure Sn,Sn-10Cu(wt.%),and Sn-20Cu(wt.%),are designed.Under the conditions of an ultrasonic power of 130 W and an action time of 5 s,the morphology and properties of the three types solder joints under transverse and longitudinal ultrasound were compared.When the content of Cu particles is increased from 10 wt.%to 20 wt.%,the increase in IMC content of joints under longitudinal waves is greater than that of transverse waves,which increases from 9.61%to 15.25%.The increase rate reaches 58.69%.The shear strength of pure Sn solder joints under transverse wave ultrasound and longitudinal wave ultrasound is similar.Increasing the content of Cu particles,the increase in shear strength gradually expands.When the Cu particles are 10 wt.%,the shear strength of the transverse wave joint is 33MPa,and the shear strength of the longitudinal wave joint is 38MPa.When Cu particles are 20 wt.%,the shear strength of the transverse wave joint is 41MPa,and the shear strength of the longitudinal wave joint is 57MPa.The increase of Cu content makes the joint change from plastic fracture of Sn to brittle fracture of compound Cu₆Sn₅.The shear strength of the joint after high temperature aging increases first and subsequently decreases.The IMC with higher strength is generated during aging,which is the transition from plastic fracture of Sn to brittle fracture of Cu₆Sn₅.However,under the same aging time,the fracture modes of transverse wave ultrasonic and longitudinal wave ultrasonic joints are different.When aging for 360h,degraded pores appear and the shear strength decreases.The shear strength of the joint after 700 cycles of thermal cycle basically unchanged,and the joint has good thermal stress resistance.Compared with the reflow process,compounds on the Cu surface are peeled off by ultrasonic,allowing the reaction to proceed quickly and generate a large amount of compounds in a short time.The time exponents n under transverse wave and longitudinal wave are 0.8779 and 0.9987,respectively.The growth of the compound in the joint under transverse wave ultrasound is controlled by both the diffusion process and the chemical reaction rate,and the growth of the compound in the joint under longitudinal wave ultrasound is only controlled by the chemical reaction rate.Under the same energy input,the two ultrasound effects are different.Compared with transverse wave ultrasound,the time exponents n of longitudinal wave ultrasound is closer to 1,which means that under the premise of the same energy input,longitudinal wave has a stronger ability to peel compounds than transverse wave.