Numerical Simulation And Experimental Study Of Temperature Rise In Ultrasonic Soldering

Ultrasonic soldering technique has many advantages, such as lowtemperature, short welding time, energy inputted to interconnect systemincluding ultrasonic vibration and pressure only, prevention of thermaldamage to the device, less heat deformation of the device and the substrate.This thesis combined experimental method with FEM (Finite Element Method)to analyse the mechanism of ultrasonic soldering connection, morphology ofinterface of ultrasonic soldering connection, morphology of solder joints, andreliability of solder joint.By using FEM software ANSYS and real-time monitoring usingthermocouples, we find that there is significant temperature gradient insidethe solder joints. The temperature close to the connecting interface can exceedthe melting point of the solder, so that the molten solder in the vicinity of theinterface forms a liquid film. By using the pad attached with Au foil, and setAu as the tracer element, we identify the thickness of the liquid film is about10μm, which illustrates the partial melting mechanism of ultrasonicthebrazing connection.By observing the grain morphology inside the ultrasonic brazing solderjoints, we find that there are significant differences between themicrostructure of solder near the connection interface and the orifinal soldermatrix. Buckle bowl region appears near the connection interface, and therewas significant grain refinement phenomenon in this region. The temperatureof the deformation region exceeds the recrystallization temperature of thesolder, so that deformation is thermal deformation. Since there will be muchfrictional heat being produced during the process of ultrasonic soldering,those thermal plastic deformations of solder joint are heated. There existed arelatively high temperature, large deformation, stress concentration, as aresult, all of these will lead to recovery and recrystallization during theprocess of plastic deformation, a large amount of refined and recrystallizedgrains inside the solder joints evevtually. By using the method of pre-agingtreatment after cold working, the effect of preload process on themicrostructure of solder joints can be exclused. It can be found that bowl-typeregion don’t occur, phenomenon of grain refinement only occur in thecicinity of the connection interface. It is produced by the process of re-crystallization nucleation during condensation in the region of moltensolder near the connection interface.connection interfaceThrough the experiment of micro shear, we can find that the size ofintermetallic compounds (IMC) can impact the shear strength of solder jointsand the position of cracking.The size of intermetallic compounds at the interface increases with theextension of time of ultrasonic soldering, the location of shear fractureoccurred is shifted from solder matrix to the interface of IMC and solder basegradually, as a result, the shear strength of solder joints decreases.