Study Of Microstructural Evolution And Electrical Monitoring Of The Defects In Mixed Solder Joints

Currently,due to the inadequate reliability data support of lead-free,for reasons of performance and high reliability,there still adopts the form of lead-free electronics components assembling with tin-lead solder in medical equipment and aerospace areas of high reliable electronic products.Under the growing trend towards miniaturization,the internal component content increasing,the diffusion distance of atom decreasing sharply and the soaring difficulty in solder defects detection will inevitably produce a significant influence on the mixed solder joints reliability.So the failure analysis and reliability researches of mixed solder joint are of great significance for high reliability microelectronic products.This thesis systematically carried out the research on the evolutionary mechanism of internal microstructure and defects,as well as electrical monitoring of the defects in mixed solder joints under coupling multi-field effect.The study shows that the Pb content has a significant effect on the microstructural evolution of mixed solder joints.Firstly,after reflow,Pb-rich phase,Ag3 Sn phase and(Cu,Ni)6Sn5 phase distribute as thin rod-like within the solder and the grain size refines with the increase in Pb content.Secondly,after ageing at 125? for 1000 h,Pb-rich phase and(Cu,Ni)6Sn5 phase coarsen and distribute as block-shaped.Thirdly,after aging 100 minutes at 125? with the current density of 3.96 x 103 A/cm2,the microstructure of Lead-free and Sn37 Pb solder joints shows no significant changes.However,SnPb phase separation phenomenon is obvious when the Pb content increases from 4.67wt% to 9.22wt%.Furthermore,with the Pb content rising from 13.64 wt % to 29.42 wt %,the pad loss and the uneven enrichment of IMC happen.Finally,under temperature cycle,the Pb content of Ni side has a remarkable effect on the IMC morphology,this research shows that the anti-fatigue performance of acicular-like(Cu,Ni)6Sn5 is better than that of(Cu,Ni)3Sn4.Under multi-field coupling,there are different forms of defects emerging in microstructure evolution process of mixed solder joints.Firstly,only organization coarsens and no obvious defects forms in the mixed solder joints after thermal aging.However,the voids,crack,SnPb phase separation and IMC enrichment appear under thermoelectric coupling.What's more,crack and void defects form after temperature cycling.Studies show that the defect evolution of mixed solder joints has a marked difference under muti-field coupling.First,when the solder joints have no obvious defects,the defect is hard forming under thermal stress.Under thermoelectric coupling effect,the low-lead samples easily derive cavity defects,while high-lead samples arise crack defects.Second,when the solder joints have the cavity defects,the cavities show longitudinal movement and lateral extension growth under thermal stress and thermoelectric coupling.Fianlly,when the solder joints have the crack defects,the cracks derive from solder joints corner,then grow horizontally,eventually cause open circuit.The research shows that electrical monitoring means of internal defects in mixed solder joints have their own applicabilities.Wheatstone bridge is sensitive to open circuit and therefore suitable for life monitoring of solder joints,but in case of small resistance measurement,a seious error might be caused by the line resistance and contact resistance.However,Kelvin bridge not only could effectively eliminate the impact of line resistance and contact resistance but also is sensitivity to internal crack and hole defects of solder joints.So Kelvin bridge can be used for defects monitoring via small resistance measurement.It is worth noting that measured resistance value is slightly higher than the actual solder joints resistance because Kelvin bridge could not strip the line resistance of substrate copper.Radio frequency(RF)impedance method is suitable for early defects monitoring,such as organization uneven distribution.But the RF impedance test equipment has specific sample interface requirements,so it is inconvenient for real-time monitoring.