Study Of Fatigue Crack Expansion Behavior Of Sintered Silver Interconnect Structures

Power modules in service due to changes in ambient temperature and frequent switching of devices will be subject to cyclic thermal and mechanical stress,prone to Ⅰ-Ⅱ composite fatigue cracking and expansion in the package interconnect layer,leading to failure of the interconnect layer,which in turn affects the service life of the power module.Nanosilver sintering technology,as a new type of high-temperature resistant packaging interconnect technology,is gradually being promoted and applied in the field of high-temperature high-power packaging interconnects.However,the fatigue failure mechanism of sintered silver interconnect structure is not clear.Therefore,it is significant to study the type Ⅰ-Ⅱ composite fatigue crack expansion process and failure mechanism of sintered silver interconnect structure.First,the sintered silver compact tension and shear（CTS）structure is modeled using ABAQUS software,and numerical simulations are performed based on the interaction integral method to solve the stress intensity factor K of the sintered silver layer of the CTS structure with different initial cracks and different loading angles,and to analyze the stress intensity factor K at the crack tip center point z/B=0 and The trends of K values along the thickness direction at the crack tip center point z/B=0 and along the leading edge of the crack were analyzed.The results show that the normalized K values of the specimens with different initial cracks at z/B=0 have the same trend,both the normalized K_Ⅰvalues are enhanced with the increase of the loading angle,and the K_Ⅱvalues are exactly the opposite.The normalized K values are more influenced by the changes of crack length and loading angle.At the leading edge of the crack,the distribution curve of normalized K_Ⅰshifts downward and K_Ⅱshifts upward,and the values of normalized K_Ⅰand K_Ⅱremain constant at 2z/B=0.9.The magnitude of K_Ⅲis directly related to the change of KⅡ at the leading edge of the crack.The normalized K values at z/B=0 were fitted by least squares method for specimens with different crack sizes and loading angles,and a normalized formula for normalized K values was developed.The normalized K values were found to be significantly higher for the all-material specimens than for the structural specimens by comparing the all-material specimens with the structural specimens.Then,static fracture tests of the sintered silver CTS structure were carried out to study the effects of sintering pressure,initial crack length and loading angle on the static fracture behavior of the structure and to analyze the static fracture failure mechanism in combination with the fracture morphology.The results show that with the increase of sintering pressure,the porosity of the sintered silver layer decreases and the ultimate load value of the structure increases.The length of the initial crack increases the structural bonding area,which in turn increases the structural ultimate load value.The ultimate load value of structure type Ⅱ loading（shear,loading angle of 0°）is significantly higher than that of type Ⅰ loading（tensile,loading angle of90°）.The sintered silver structure exhibits brittle fracture during type Ⅰ loading;during type Ⅱ loading,it gradually changes to ductile fracture with the increase of sintering pressure.Finally,sintered silver CTS structure type Ⅰ-Ⅱ fatigue crack extension tests were carried out to obtain the crack extension length value a versus fatigue life N curves and calculate the fatigue crack extension rate da/d N.The effects of sintering pressure,peak load,load retention time and loading angle on the fatigue crack extension behavior of the structure were studied,and the fatigue crack extension failure mechanism was analyzed in combination with the fracture morphology.The results show that the crack extension rate of the sintered silver structure decreases with the increase of sintering pressure and the fatigue extension life increases with the increase of sintering pressure.The increase of peak load increases the stresses inside the sintered silver particles and therefore decreases the fatigue crack extension life of the structure.An increase in load retention time promotes crack extension and decreases the fatigue crack extension life of the structure.At the same stress ratio,the fatigue crack extension life decreases with the increase of loading angle.For the sintered silver structure,the fatigue crack extension behavior of the sintered silver structure is effectively predicted by combining experiments and simulations,using the equivalent force strength factor amplitude K_effas the fracture parameter based on the Pairs formula,and establishing a unified equation for the loading angle and load-dependent fatigue crack extension rate.The fatigue fracture analysis shows that the specimens with large loading angles are more likely to initiate brittle fracture when the loading loads are uniform.In summary,this paper investigates the expansion of type Ⅰ-Ⅱ composite cracks in the sintered silver interconnected structure specimens and derives the corresponding law,which is expected to be a reference basis for judging the reliability of the nano-silver interconnected structure in terms of composite crack expansion.