Reliability Study Of 3D PoP Stacking Assembly And Fatigue Life Analysis Under Vibraition Loading

With the rapid development trend and the application of complex environment of the portable and space military electronic products,people pay more and more attentions to the vibration reliability of electronic products.As a widespread 3D packaging,the use of the PoP(Package on Package)is widespread in recent years,but raises a number of issues at the same time,especially with the widespread use of PoP in portable electronic devices.The PoP affected by vibration loading shows a trend of gradually increase,and the vibration reliability problems of the PoP also gradually increasing.Therefore,a further research to the reliability mechanism and the optimization design of the PoP under vibration loading is significant to the widespread use of the PoP.In this thesis,the PoP was selected as the research object,and a series of vibration fatigue tests combining with the finite element simulation and the interface micro-analysis has been carried out.The vibration reliability of the PoP was first step by step studied,which includes the analysis of the fatigue failure characteristic,the fatigue-life prediction,the optimization design of the fatigue structure,and the strengthening technologies.The main results and conclusions are as follows:The routing design of the test PCB board and the design of the fixture were carried out are to build the modal and the vibration test platform,respectively.Based on the meshing theory,the three-dimensional finite element model of the test vehicle was established and verified by experimental tests.The establishment of the finite element model provides a base for correctly evaluating the vibration reliability of the PoP.The failure analysis of the PoP under vibration was investigated by using the four-points fixed harmonic vibration tests and FEM analysis.The results indicate that the reliability of top package is much better than the bottom package,and the outermost corner solder joint of the bottom package is critical for the PoP assembly.The important failure mechanism of the PoP solder joints is that the inertial bearing mass of the bottom solder joints is greater than the top,which causes the larger alternating pressure stress acting on the bottom solder joints,and leads to the failure happened at the bottom solder joints firstly.The thickening of the IMC layer caused by the multi-reflow in the process of PoP assembly and the initial packaging warpage caused by the special stack structure of the PoP accelerate the failure of the solder joints under vibration loading.These three aspects all have a direct impact to the failure of the PoP.The joint crack could usually originate in the bottleneck position of solder body,and might extend within bulk solder and then along the interface between the IMC layer and bulk solder.It was confirmed that the failure mode of the PoP solder joint under vibration is the mixed failure mode containing the brittle fracture and ductile fracture.Combining with the vibration fatigue test results and the two parameter Weibull distribution,the PoP S-N fatigue characteristic curve was established by using the FE simulation analysis.The comparison of the S-N curves of different structures indicates the S-N curve depends on the package structure.The PoP is less durable than the 2-D packaging under vibration owes to the initial package warpage that leads to the residual stress in the PoP solder joints.Based on the random vibration fatigue test and finite element simulation analysis,a fatigue life prediction method for the PoP under random vibration was established by introducing the correction factor ? on the basis of the narrow-band random vibration fatigue life model,and was verified by the results of random vibration fatigue test.Based on two cases,three interval method and other several kinds of frequency domain life prediction models were analyzed,and the correctness of current method was validated again.From the perspective of the structural optimization design for the vibration reliability,the Taguchi optimization method was applied to improve the vibration reliability of the PoP by FE method.The results show that the bottom solder joint standoff is the critical control factor on the vibration reliability of the PoP,closely followed by the thickness of bottom substrate and the top solder joint standoff,and the body size of the PoP has little effect on the vibration reliability.The higher the bottom solder standoff within the specified control factor level,the higher the reliability of the Po P,which indicates that enhancing bottom solder joint standoff is an effective approach to improve the vibration reliability of the PoP.The optimal set of control factors clearly exceeds all the experiment cells,could reduce the ?? by 36.6% compared to the original design,and obviously improves the vibration reliability of the PoP.Based on the facts that the bottom solder joints are the weak link of the PoP under vibration,the vibration reliability of the PoP structure with different underfill types was investigated by finite element method.Results show that the critical solder joint is different with different underfill methods,and both underfill methods can improve the reliability significantly.Considering the cost of production and assembly times in underfill manufacturing,the edgebonded is the most appropriate method to improve the vibration reliability of PoP structure.Interestingly,the study also found that the failure sequence of the bottom and the top is affected by underfill.The bottom solder balls have gotten enough protection from the full-filled underfill,and the failure of the top package will probably be prior to the bottom package under vibration loading.The study for the vibration reliability of the PoP structure with different underfill types provides an important reference for the PoP reliability design.