Cavitation Dynamics And Its Application During Ultrasonic-assisted Soldering

Ultrasonic-assisted soldering can realize the connection between solder and non-wetting substrate without flux in air,such a process mainly depends on the cavitation inside the liquid solder.The cavitation during ultrasonic-assisted soldering usually occurs in thin solder layer with width of millimeter or even micron.The special space and boundary conditions cause the cavitation in the thin solder layer to exhibit a series of special properties which are quite different from that in bulk liquid.In this work,the cavitation characteristics of thin solder layer during ultrasonic soldering are the main research objective.The morphology,behavior,dynamics and erosion mechanism of cavitation bubbles are studied using finite element simulation and experimental observation.Acoustic pressure presents an alternately positive and negative change within one ultrasonic period during ultrasonic-assisted spreading.Cavitation bubbles nucleate and grow in the negative-pressure stage,and contract and collapse in the positive-pressure stage.Cavitation intensity of the thin layer shows a decreasing trend with time increasing,which is divided into violent and stable stages.The bubble density is high during the violent cavitation stage,and cavitation clouds with long lives can be observed.The bubble density is low during the stable cavitation stage,and most bubbles are tiny with life of one ultrasonic period.The cavitation of the thin solder layer presents strong/weak change with a period of dozens of ultrasonic periods.High ultrasonication power and substrate characteristic impedance result into high bubble density.Erosion is pronounced at the spreading center due to longer cavitation time after ultrasonic-assisted spreading.Erosion effect gradually decreases towards the spreading front.A ―cavitation blind area‖ always exists at the spreading front,where the oxide layer is difficult to remove.Such a region can be eliminated by preventing the spreading front from being a free state.The acoustic pressure in the thin solder layer increases over 3 times inside the narrow clearance between substrate during ultrasonic-assisted soldering.Thus,much higher cavitation intensity could be obtained and the distribution of the cavitation bubbles is more uniform.The cavitation during ultrasonic-assisted soldering also shows a decreasing trend with time increasing.Higher acoustic intensity can obviously accelerate the degassing process,significantly shortening the violent cavitation stage.Cavitation of thin solder layer inside the narrow clearance also presents strong/weak change with shorter variation period.Acoustic pressure also presents alternately positive/negative change within one ultrasonic period.Cavitation bubbles nucleate and grow in the negative-pressure stage,and contract and collapse in the positive-pressure stage.High ultrasonic power,large substrate characteristic impedance and narrow clearance result into high acoustic pressure and thus high bubble density.Pronounced erosion occurs inside the narrow clearance during ultrasonic-assisted soldering,and it further increases with decreasing the clearance width.The thin solder inside the narrow clearance has rather short cavitation incubation period.The incubation period is shorter than 0.5 s inside a 0.2 mm-wide clearance.A portion of acoustic energy converts into kinetic/potential energy during spreading,causing the aoucstic pressure to be lower than the cavitation threshold of and subsequent non-cavitation.Based on the high-speed photography and the finite element simulation,the cavitation thresholds of eutectic Ga In alloy at room temperature and pure Sn at 250 °C are estimated to be 0.65 and 1.26 MPa,respectively.The bubble dynamics calculation results show that the maxim um growth velocity of a cavitation bubble with a nucleation radius of 10 μm could reach 40 m/s when the thin solder layer with a free surface experiences sinusoidal sound pressure.After nucleation,the cavitation bubble inside the narrow clearance during ultrasonic-assisted soldering grows faster and owns larger final radius.Narrow clearance,high ultrasonic power and large substrate characteristic impedance cause high growth velocity and large final radius of the cavitation bubble.The force produced by a single cavitation bubble is approximately 0.054-0.067 N and a microjet generated by the collapse of a cavitation bubble in the thin layer of Ga In alloy is approximately 542.87-673.56 MPa.The grains of the solder can be obviously refined when ultrasonic-assisted soldered pure aluminum with pure Sn.The Sn grains are refined from 2.62 to 1.04 ?μm,the Al content in the joint seam increases from 0.93 to 4.89%,and the shear strength and hardness increases by 65.4 and 52.7% when the clearance width is reduced from0.8 to 0.2 mm.The theoretical calculation and experimental results show that the strong cavitation in the narrow clearance can lead to instant nucleation of Sn,which is mainly responsible for the grain refinement during soldering.7075 Al alloys are soldered using Sn-9Zn solder.The Sn grains are refined from 2.83 to 2.56 ?μm,the Zn grains were refined from 0.36 to 0.35?μm,the Al content in the joint seam increases from 3.35 to 10.84%,and the shear strength and hardness increases by 34.6 and 35.3%when the clearance width is reduced from 0.8 to 0.2 mm.Also,the softness of the7075 Al alloys can be successfully avoided because the soldering temperature is only200 °C.The porous ceramics are successfully soldered with ultrasonic assisted capillary and strong cavitation inside the narrow clearance.The solder penetrates into the gap of porous ceramics through capillary under ultrasonication,and the infiltration distance increases at prolonged time.Ceramic particles are eroded and flow into the joint due to the strong cavitation inside the gap of the porous ceramics.