Experimental Research And Reliability Analysis On Lead-free Solder Joints In Microelectronic Packaging

With the advanced packaging technologies appearing constantly, the reliability of the solder joints is also completely increased. The most commonly used method for solder joint reliability analysis is to select a large number of actual electronic components and put them in actual working environment until the solder joint become failure. But this method is time-consuming and costly. At present, the most commonly used method is based on experiment in light of a computing simulation of finite element analysis. It is necessary to have an accurate understanding of mechanical properties of solder joints. In the actual simulation, most methods are used to take tests on mechanical properties of bulk materials, if solder joint model uses the relations among elastic, bilinear or trilinear elastic-plastic, strain rate effect and temperature effect of solder joints shown in droping or impact are neglected. With the rapid development of microelectronics industry, the size of actual solder joints has reached the micron or even nanometer scale, to understand the true mechanical properties of solder joints, it need to test the actual solder joints.In this paper, the mechanical properties of micro-solder joints placed in plane array package in real working conditions and the solder joint reliability under drop/impact are studied systematically. The important results are summarized from the following several aspects:1,Lead-free solder Sn3.0Ag0.5Cu and Pb-containing solder Sn63Pb37, which is before and after reflow process, were measured by nanoindentation in microelectronics packaging, while the load-displacement curves, displacement- elastic modulus curve and displacement-hardness curves were recorded. Using reverse analysis method and numerical simulation, we have established the link between experimental curves measured indentation experiments and elastic-plastic material parameters of solders, put forward the simple calculation method for representative plastic strain and the correspondent stress, given the value of representative stress for lead-free and Pb-containing solders and determine representative plastic strain of such materials asεr=0.0252. Further the elastic-plastic constitutive equation for lead-free solder joint in microelectronics packaging is achieved.2,In this paper, power hardening creep constitutive equations,ε= Cσn, is used to describe the creep mechanical properties of lead-free solder joints. From the indentation creep testing, the creep strain rate sensitivity exponent for lead-free and Pb-containing solders is derived from dimensionless analysis. Then, combining numerical simulation with dimensional analysis, the value of C in creep constitutive equations are obtained using search method. So power hardening creep constitutive equations for micro-solder joints can be obtained. The creep time-creep displacement curves calculated from numerical simulation match well with those measured from nanoindentation experiment in room temperature, the results indicating that the steady-state creep model is valid in describing the creep mechanical properties of solder joints.3,A series of dynamic compressive experiments of lead-free solders Sn3.5Ag, Sn3.0Ag0.5Cu and 63Sn37Pb were conducted at high strain rates (1000s-1 - 4000s-1) using a split Hopkinson pressure bar. Measurements were conducted at temperature ranging from -40℃to 125℃. The results indicate that the dynamic yield strength of the three solders is greater than their static yield strength and all the three solders are rate sensitive and temperature dependent, with the increase of strain rate, it was found that yield strength and flow stress increasing remarkably. When the ambient temperature is higher than room temperature, the material becomes softening and its strength weakens, vice verse, the material hardens and the intensity of it increases. The material constants in Johnson-Cook model were determined from experiments. So it give dynamic constitutive relation including strain-rate effect and temperature effects for solder joints, in order to further study the reliability of electronic products to provide a research foundation.4,According to the condition B in the drop test standard of JEDEC, create 3D finite element modal of VFBGA packages in drop and impact, the material model of solder joints can be indicated by Cowper-Symonds model obtained from experiment data, taking into the consideration of strain rate and temperature effect. With conducting reliable analysis of VFBGA packaging in board-level drop condition, impact acceleration curve is applied to numerical simulation in Input-G method. Our conclusion is as follows: The mechanical impact and bending of PCB caused by mechanical shock are the main reasons which induce the drop failure of solder joint in the drop impact conditions; it is not neglect to think over the strain rate and thermal effect of solders in numerical simulation; the peeling stress of solder joints which is located in the components/solder joint side is maximum at the outermost corner of BGA package, it indicates that the solder joint closed to components is easy to failure; An impact life prediction model, which is formulated using power law, based on maximum peeling stress of critical solder joints is proposed for board level drop test to estimate the number of drops to failure, the average impact life for two lead-free solders Sn3.0Ag0.5Cu and Sn3.5Ag is 132.84 and 59.02 respectively, Sn3.0Ag0.5Cu in the drop test showed a better ability to resist deformation.