Microstructural Evolution And Service Reliability Of Sn-58Bi-Ag Solder

With the rapid development of information technology,the demand for portable electronic devices has greatly increased.In order to reduce costs and minimize the damage of electronic components during packaging,there is an urgent need to promote the use of low-temperature solders in consumer electronics.Sn-Bi eutectic alloy has a low melting point,exhibits high mechanical strength and creep resistance,which makes it an ideal choice for low temperature soldering.However,the Bi-rich phase in Sn-Bi alloy will coarsen during service,resulting in brittle failure of the solder joints.The addition of silver can overcome this shortcoming to a certain extent.Therefore,Sn-58Bi-Ag alloy is regarded as a low-temperature solder alloy with the most potential for industrial application,which attracts much attention in both academia and industry.To characterize the microstructure evolution of Sn-58Bi-Ag solder alloy during service quantitatively and estimate its reliability,we designed thermal cycle experiment and aging experiment for Sn-58Bi-Ag solder joint,as well as stress aging experiment for pure Sn solder joint.Accompanying systematic and precise characterization of the solder joints,the service behaviors of Sn-58Bi-Ag and Sn alloy during reliability tests were clarified.According to the thermal cycle test of Sn-58Bi-Ag solder joints,the microstructure of the as-reflowed solder joint is composed of the interfacial IMC,the irregular lamellar bismuth-rich phase,the irregular lamellar tin-rich phase,as well as the short needle-like Bi-rich phase and the ellipsoidal Bi-rich phase precipitated inside the tin-rich phase.The thickness of the IMC and the average intercept of the irregular lamellar Bi-rich phase are gradually increased with the thermal cycles.The relationship between IMC thickness and cycles exhibits power functions with an exponent less than 1,so does the relationship between the intercept of lamellar Bi-rich phase and thermal cycles.During thermal cycling,the higher the top temperature is,the bigger the final thickness of the IMC and the final value of the Bi-rich intercept.For samples at a top cycle temperature of 110?,the ellipsoidal Bi-rich phase disappears quickly with the increase of the temperature cycle,and only short needle-like Bi-rich phase was left at last.For the sample at a top cycle temperature of 85?,the short needle-like Bi-rich phase disappeared quickly,leaving a group of ellipsoidal Bi-rich phase in the matrix.During the aging experiment of Sn-58Bi-Ag solder joints,the thickness of IMC has a linear relationship with the aging time,which proves that the IMC growth at this temperature is reaction-controlled rather than diffusion-controlled.The intercept of Bi-rich phase obeys similar power function law as in the thermal cycle experiment.Both ellipsoidal and needle-like small Bi-rich phases quickly disappeared during the aging process.Eventually,only the coarsened bulky Bi-rich phase remained at last.The failure analysis of the drop test samples shows that with the prolonging of the aging time,the failure mechanism of the solder joint gradually changes from quasi-cleavage fracture to cleavage fracture,and the failure location gradually changes from the inside of the solder to the Bi-rich phase combine with Sn-rich phase/IMC interface.When different stress states were applied within Cu/Sn/Cu solder joints during aging,the growth morphology of IMC will be affected obviously.For the cross-sectional morphology,the compression makes the IMC/Sn interface smoother and reduces the degree of undulation of IMC,while the tensile stress makes the IMC/Sn interface rougher and the degree of undulation becomes bigger.From the top view point,the top surface of IMC under compression is smoother and there are fewer protruding grains,while for tension sample the top surface of IMC has many large protruding grains with obvious interface fluctuations.The stress state also affects the growth rate of the IMC.The compressive stress makes the IMC grow faster,while IMC under tension has slower growth rate.The growth kinetic analysis shows that no matter which stress state is,the growth factor of IMC is all about 0.4,that is,the growth is controlled by both grain boundary diffusion and volume diffusion.Moreover,the average grain size,raver,and it's variance,w,of the IMC in stress-free sample is 3.37?m and 1.83 respectively,the average grain size of the IMC under compressive stress is smaller and more uniformly distributed(raver=2.74?m,w=1.69)than those under tensile stress(raver=3.91?m,w=1.89).Combining the above experimental results and analyses,it can be deduced that different stress states affect the Oswald ripening process in IMC growth.Compressive stress delays the Oswald ripening process resulting in limited growth of advantage grains,while tensile stress accelerates the Oswald ripening process and expands the growth advantages of the advantage grains.