| Ball grid array(BGA)package is widely used in integrated circuit packaging owing to its advantages of high density,high thermal conductivity and low inductance pins.In the BGA packaging system,BGA solder joints play the role of electrical interconnection,mechanical support and heat dissipation channel,which are often the weakest link of the entire system.Among the electronic products with a BGA package,the failure of BGA solder joints is one of the main reasons for the failure of these electronic products.BGA solder joints in real service are often subjected to the simultaneous action of electrical,thermal and mechanical loads.However,the miniaturization and multi-function of electronic products make the size of solder joints decreases,and the current density,temperature and mechanical load in solder joints increase continuously,which will lead to more serious reliability problems.To evaluate the mechanical reliability of solder joints in near real service,the shear deformation,fatigue performance,fracture behavior,failure mechanism and their size-dependence of BGA solder joints under electro-mechanical coupled loads were studied in this paper.First,the shear deformation,fracture behavior and mechanical properties of BGA solder joints with different substrates under electro-mechanical coupled loads were studied,and the influence of the non-thermal effect of electric current on the shear properties of solder joints was preliminarily discussed.Then,the influence of the non-thermal effects and Joule heat on the shear strength of solder joints was studied,and the contribution of the non-thermal effect and Joule heat to the shear strength of solder joints was analyzed quantitatively.Furthermore,the evolution law and mechanism of shear performance and fracture behavior of solder joints with different sizes under electro-mechanical coupled loads were discussed.Next,the shear fatigue performance,fatigue fracture and failure mechanism of solder joints under different current densities were studied,and the feasibility of applying the classical solder constitutive model to the fatigue life prediction of solder joints was discussed.Finally,the size dependence of fatigue performance and fracture behavior of solder joints under electro-mechanical coupled loads was studied.The main findings obtained in this paper are as follows:BGA solder joints with different substrates(Cu/SAC305/Cu,Ni/SAC305/Ni,Cu/SAC305/Ni and Ni/SAC305/Cu solder joints)under electro-mechanical coupled loads exhibit similar deformation characteristics(i.e.,consisting of linear deformation stages and nonlinear deformation stages),fracture behavior and approximate shear strength.In the process of increasing current density(0-6.0×10~3A/cm~2),the linear deformation stage gradually becomes the dominant deformation,the shear strength of the solder joint increases first and then decreases,and the fracture position gradually transfers from the solder matrix to the solder/IMC layer interface,showing a significant ductile-brittle fracture mode change.In addition,the non-thermal effect of electric current induces the increase of dislocation density in the solder matrix is the main reason for the increase in shear strength under electro-mechanical coupled loads.While current crowding at the IMC grooves and strain mismatch at the solder/IMC layer interface are the driving forces that trigger the formation of interfacial cracks and the propagation of cracks along the solder/IMC layer interface,respectively.The change in shear strength of the BGA structure Cu/SAC305/Cu solder joints under electro-mechanical coupled loads is the result of neutralization between the non-thermal effect and Joule heat.The mathematical relationship between shear strength and temperature was established based on the force-thermal energy density equivalence principle.The trend of an almost linear decrease in shear strength with increasing temperature indicates that the effect of Joule heat on the shear performance of solder joints only shows a deterioration effect.Through accurate temperature measurement of solder joints(Joule heat characterization),the non-thermal effects of the electric current is successfully decoupled and its contribution to the shear strength is quantified.Compared to Joule heat,the non-thermal effects of the electric current is highly dependent on the current density and the test temperature.With the increase of current density,the influence of the non-thermal effects on the shear strength gradually changes from an enhancement effect to a deterioration effect.When the test temperature increases(25 to 175℃),the enhancement effect of the non-thermal effect is gradually weakened and eventually disappears.Meanwhile,the critical current density of the transition from enhancement effect to deterioration effect decreases.In addition,the increase in test temperature will lead to a decrease in the critical current density for the transition from solder matrix fracture to solder/IMC layer fracture,and an increase in the interfacial strain mismatch.The shear strength of BGA structure Cu/SAC305/Cu solder joints with different heights(100μm,300μm and 500μm)decreases with increasing current density(6.0×10~3to 1.1×10~4A/cm~2).A smaller solder joint has a higher shear strength at the same current density,exhibiting a size effect of“smaller and stronger”.The difference in shear strength caused by changes in solder joint height depends on the Joule heat in the solder joint,but is hardly affected by the non-thermal effect of the electric current.Compared with the electro-mechanical coupling test,solder joints with smaller height under no current stressing also exhibit a higher shear strength,which is related to the stress-strain state in the solder joints:As the height of the solder joints increases,the stress triaxiality decreases,the Von Mises stress and plastic strain energy increase,and their distribution in solder joints is more uneven.In addition,the current crowding and interfacial strain mismatch in the solder joint under electro-mechanical coupled loads increase with the increase of the solder joint height,resulting in a decrease in the critical current density for the transition from solder matrix fracture to the solder/IMC layer interface fracture.The fatigue life of BGA structure Cu/SAC305/Cu solder joints under electro-mechanical coupled loads is lower than that of without current stressing,and decreases monotonically with increasing current density.The decrease in fatigue life can be attributed to more severe recrystallization in solder joint and current crowding at the tip in the process of the crack propagation under current stressing.When predicting the fatigue life of the solder joint,it is found that the cumulative plastic strain energy density and plastic strain amplitude calculated by the classic Anand solder constitutive model show a monotonic decrease,and a changing trend of first increase and then decrease with increasing current density,respectively.Since the calculated results are inconsistent with the actual fatigue life of the solder joint,which indicates that the classic Anand solder constitutive model is not suitable for the prediction of solder joint fatigue life under electro-mechanical coupled loads.In addition,the fracture behavior of the solder joint shows that under no current stressing or a relatively low current density,fatigue cracks tend to propagate along the path where the strain energy is concentrated,that is,showing an arc shape consistent with the strain energy distribution.At a high current density,the concentration of strain energy in an arc distribution is reduced,and fatigue cracks are driven by the solder/IMC layer interface strain mismatch and propagate along the interface.The fatigue life of BGA structures Cu/SAC305/Cu solder joints with different heights(100μm,300μm and 500μm)shows a monotonic decrease with increasing current density under electro-mechanical coupled loads.The influence of change in solder joint height on fatigue life can be divided into two cases:(1)When the displacement amplitude applied to the solder joint is the same(d_a=15μm),the fatigue life increases with increasing the solder joint height;(2)When the strain amplitude applied to the solder joint is the same(γ_a=0.05),the fatigue life decreases with increasing the solder joint height.In addition,for the solder joints with a height of 100μm,the fracture occurred in the solder matrix,the fracture position gradually approached the substrate side with the increasing current density,and the fracture path was along the substrate plane,which is parallel to the loading direction.For the solder joints with a height of 300μm or 500μm,as the increase of current density,the fracture position gradually shifts from the solder matrix to the solder/IMC layer interface.The main reason for the change of fracture behavior is that the stress and strain energy concentration area of solder joint transfers to interface and solder matrix respectively with increasing the solder joint height. |
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