Study On Metallurgical Mechanism And Properties Of Sn Based Composite Solder Paste Based On Ultrasonic

With the rapid development of the semiconductor industry,the development of chips with high power,integration and miniaturization has been an overwhelming trend.Meanwhile,the working environment has become severer especially in the fields of aerospace,automotive electronics as well as artificial intelligence,which proposed the higher demand for the reliability in high temperature.Currently,the method of the transient liquid phase（TLP）with Sn-based solder to obtain the joint with high temperature resistance was an extremely time-consuming process.Thus,there is an urgent demand for a cost-effective solder and a compatible matching process to obtain a high-melting-point joint with high-efficiency process.In this study,a series of novel composite solder paste with different sizes of Cu and Sn particles were prepared.The results indicate that,for the solder containing 40wt.%Cu particles,the sole IMC of Cu₃Sn joints with a minority of residual Cu were obtained by ultrasonic-assisted TLP at 250℃in 10 s.The highest shear strength at ambient temperature was up to 49.96 MPa.For the solder joint containing 40 wt.%mixed Cu particles,it exhibits the shear strength of 46.64 MPa at high temperature of350℃and the thermal conductivity was 258.2 W/m·K,which exhibits the significant potential for the high temperature electronic devices.During the process of ultrasonic-assisted TLP,the mechanism of interface metallurgical reaction was as follows:to begin with,the composite solder exhibits the large solid-liquid contact area when the Sn particles melt,so the reaction rate is accelerated to some extent.Then,the cavitation effect accelerates the reaction rate effectively between with Sn and Cu when ultrasonic propagates into the liquid.Besides,stripping effect by ultrasonic on the surface of Cu particles and substrates increased atomic diffusion rate between Sn and Cu.The ultrasonic stripping effect of the IMC formed on the surface of the Cu particles and the Cu substrates,and thus the diffusion distance for further reaction of the Cu atoms with Sn atoms is reduced.In addition,the thermal behavior and regularity of a series of composite solders were further explored.The results of differential scanning calorimetry showed that for the composite solders,the larger the solid-liquid contact area,the lower the melting peak temperature of Sn and the smaller the enthalpy value.Meanwhile,the exothermic peak temperature of the formation of Cu₆Sn₅ IMC was lower and the enthalpy value was higher;the endothermic peak temperature of the melting of Cu₆Sn₅ IMC was higher,and its enthalpy value was larger.The in-situ temperature results in the joint showed that the larger the solid-liquid contact area during heating process,the more significant the cavitation effect during the exertion of ultrasonic,and therefore the higher the in-situ local temperature in the joint.The results of the proportion analysis of the phases in the joints showed that for the composite solder with the same Cu content and different specific surface area,the larger the solid-liquid contact area,the lower the content of residual Cu and Cu₃Sn IMC and the higher the content of Cu₆Sn₅ phase.For the composite solder with different Cu content,the larger the solid-liquid contact area,the higher the content of Cu₃Sn phase in the joint.