In-situ Plasticity And Creep Behavior Of Cu/SAC/Cu Micro Solder Joints Under Variable Temperature

With the widely application of the surface mount technology,the size of electronic components is getting smaller and smaller,making the size of the solder joints become smaller and smaller.However,the solder joints are subjecting to higher thermal,electrical and mechanical load,which will resulted in reliabilities.The reliability of the solder joints will affect the whole performance of the electronic products.This requires the solder alloys not only to have a good brazing process and performance,but also to have better mechanical properties,especially the resistance to fatigue and creep,to meet the demand of electronic products for the increasing reliability of micro solder joints,and make them reliable in the course of service.In this paper,the BGA solder joints formed by patented the low silver lead-free solder SAC0705 Bi Ni and the widely used high silver solder SAC305 are used.The plastic and creep behavior of the solder joints are studied by means of nanoindentation test under variable temperature(25 ?,50 ?,75 ?,100 ? and 125 ?)conditions.In order to obtain the influence of the trace elements Bi,Ni and Ag content on the microcrystalline high temperature creep performance and plasticity,the low silver lead-free solder SAC0705 was used as the contrast.During the test,the temperature of the sample was measured in real time by temperature logging and Fluke VT02 visual infrared thermometer.The indentation morphology under different temperature was taken by the nanoindentation instrument.The indentation cross section was obtained by FIB microfabrication technique,and the indentation morphology was observed and analyzed by SEM.The plastic deformation behavior of three kinds of micro solder joints is studied at the room temperature.The indentation load-depth curve obtained by nanoindentation method is analyzed by physical inversion.The elastic modulus and strain hardening exponent of the micro solder joints are obtained.The stressstrain constitutive model of micro solder joints is established.The resistance to plastic deformation at room temperature of three kinds of the bulks of micro solder joints was compared.The effect of strain rates on the plastic deformation behavior of the bulks of micro solder joints was analyzed.The results show that among the three kinds of micro solder joints,Cu/SAC0705 Bi Ni/Cu has the strongest resistance to plastic deformation.With the increase of strain rate,the indentation depth and indentation size decrease gradually.The hardness value of the bulks of micro solder joints increases with the increase of strain rate.By means of nanoindentation method and self-made micro heating auxiliary device,the law of the indentation hardness,elastic modulus and plastic factor with the changed temperatures of three kinds of micro solder joints is obtained,respectively,as well as their constitutive model with the change of temperatures are established,respectively.The results prove that the indentation hardness exponentially decaying with the temperature.The indentation elastic modulus is negatively correlated with the temperature.The plasticity factor is positively correlated with the temperature.The KH(the indentation hardness temperature effect factor)and KE(the indentation elastic modulus temperature effect factor)of the bulks of Cu/SAC0705/Cu are the most sensitive factors to temperature.The ?H(the plastic factor)of the three kinds of the bulks of micro solder joints tend to be consistent with the temperature rise.The creep stress exponent and the creep activation energy of the three kinds of the bulks of micro solder joints at different temperatures(25?~125?)are obtained,respectively.The steady state creep constitutive model of the three kinds of the bulks of micro solder joints is established,respectively.The mechanism of creep deformation of three kinds of the bulks of micro solder joints is revealed.The “indentation thermal-force coupling circular area” is found around the surface of indentation and the bottom tip of the indentation at 100 ? and 125 ?.The “indentation thermal-force coupling circular area” is caused by the combination of a certain time,high enough temperature and external force.The creep behavior of three kinds of micro solder joints in different loading modes are compared and analyzed.The creep behaviors of the three kinds of micro solder joints are obtained by using the step load-unload,load-unload,preset indentation depth load-unload test modes.The indentation creep rate and indentation relaxation rate are compared.The multi-step load-unload method causes the strain hardening,time hardening and creep hardening of the bulks of solder joints,and finally resulted in significantly increase of the creep hardness.The influence of the indentation size effect and the geometric dimension effect on the creep performance of the bulks of Cu/SAC305/Cu micro solder joints was studied.The smaller indentation depth,the greater indentation hardness value is,showing a significant indentation size effect.The characteristic length of the material h*,which is used to characterize the effect of the penetration depth on the hardness value,decreases with the increase of the temperature.h* decreases with the increase of temperature.Under the same soldering process parameters,the smaller the geometrical size of Cu/SAC305/Cu micro solder joints,the finer the grain in the bulks of the micro solder joints.Under the same load conditions,the bulks of the smaller size micro solder joints exhibit the better creep resistance.