SiP Thermal Stress Simulation And Structural Optimization Design Research

SiP technology is one of the mainstream technologies in the post Moore law era.However,the complex process structure of SiP also brings enormous challenges to reliability,in which SiP failure caused by thermal stress is more prominent.In this paper,the thermal stress analysis model of the typical SiP structure SWIFT process is established by using the finite element analysis software COMSOL.The warpage of SWIFT structure and the thermal stress distribution of the key structure bumps are studied,and the effects of different materials and structural parameters on the warpage and bump thermal stress of SWIFT structure are analyzed.Thel design of experiment is used to deal with the multi factor statistical analysis and get the corresponding structural optimization design plan.The two step optimization method of Taguchi response surface is put forward with emphasis on the thermal stress reliability problem of bumps,which significantly improves the reliability of SWIFT structure bump,and is expected to effectively reduce the SiP development cycle and cost.The main research results are as follows:(1)The warpage failure of SWIFT structure is studied by finite element analysis.The finite element simulation software COMSOL is used to simulate the relationship between the overmold thickness of SWIFT structure,the thickness of the substrate and the warpage of the structure.The results show that the warpage of the SWIFT structure increases approximately linearly with the increase of the overmold thickness.When the overmold thickness ranges from 0mm to 0.4mm,the warpage of the SWIFT structure increases by 0.8 ?m.However,the warpage of the SWIFT structure warpage decreases with the increase of the substrate thickness.When the thickness of the substrate ranges from 0.3mm to 0.7mm,the warpage of the structure decreases by 1.1 ?m per 0.1mm,so when thinking the SWIFT structure at the single factor level,we should choose thinner overmold and thicker substrate structure model.The simulation results are compared with the theoretical analysis of the double plate model warpage.Agreement.At the same time,different epoxy mold compound materials also have obvious changes in the warpage of SWIFT structure.The influence of different epoxy molding materials on the warpage of SWIFT structure is compared by using the finite element analysis software COMSOL.It is shown that the warpage of SWIFT structure will increase with the increase of thermal expansion coefficient in the same environment.(2)The thermal stress and reliability of SWIFT bumps are studied.According to the strain rate relationship of bumps,the extreme point of the bump thermal stress distribution area is often the most prone to failure of the bump.The thermal stress of bumps in SWIFT structure is simulated by using finite element analysis software COMSOL under the condition of temperature cycle.The experimental results show that the extreme points of the bump thermal stress are mainly distributed in the periphery,which indicates that the bumps at the periphery are the most likely to fail,which is consistent with the most likely failure of the bumps in the actual application process.For the SWIFT upper and lower layer structure,comparative analysis of thermal stresses in the upper and lower layers of SWIFT structures,when the overmold thickness is selected as the variable,the results show that the thermal stress of the bottom bump is always bigger than that of the top bump,which also indicates that the SWIFT structure bump failure mainly concentrates on the bottom periphery bump.Therefore,the bump at the bottom and periphery of SWIFT structure is selected as the research object and the creep effect is found to be obvious by analysing the thermal stress distribution of the bumps around the bottom layer under the temperature cycle,The life of bump is predicted by the plastic strain-based Coffin Manson model and the energy-based Morrow model.The predicted lifetime of the Coffin Manson model is 1364 cycles,and the predicted lifetime of the Morrow model is 1524 cycles.The failure position is consistent,which is located at the interface of the bump.Finally,the comparison between dry film material as redistribution layer material and the traditional PI and PBO material shows that the bump stress of PI material is the lowest.(3)The Taguchi response surface two step optimization method is proposed to optimize the SWIFT structure.In the SWIFT structural design stage,based on the thermal stress simulation of SWIFT structure,the SWIFT structure is optimized by using the design of experiment.Taguchi method is used to analyze the key factors affecting the warpage of SWIFT structure.When the thickness of substrate is 0.3 mm,the thickness of overmold is 0 mm,the thickness of redistribution layer is 0.15 mm,the epoxy molding compound is G700 L,and the thickness of chip is 0.2 mm,the warpage of SWIFT structure is the smallest,which is 18.6 um.Based on the experimental flow technology of structural optimization,two-step optimization method called Taguchi response surface is established to optimize the thermal stress of bump.Firstly,Taguchi method is used to analyze the key factors affecting the thermal stress of bumps in SWIFT structure.It is found that when the thickness of redistribution layer is 0.15 mm,the thickness of chip layer is 0.1mm,the width of bumps is 0.3mm,the height of bumps is 0.2mm,the material of redistribution layer is PBO,the epoxy molding compound is CEL400 ZHF and the thickness of overmold is 0 mm,the minimum thermal stress of bump is 160.1MPa.Then,the thermal stress of bumps is further optimized by response surface methodology.The results show that the thermal stress of bump is 154 MPa when the width of bump is 0.3 mm,the height of bump is 0.2 mm,the thickness of redistribution layer is 0.13 mm and the thickness of chip layer is 0.15 mm,which is 3.75% higher than that of the first step.Based on the discussion results of Taguchi method and considering the warpage of SWIFT structure and thermal stress of bumps,a relatively optimized result is obtained.At this time,CEL4002 HF is chosen as epoxy molding compound,PBO is chosen as redistribution layer material,0.2mm is selected as die layer thickness,0.5mm is selected as substrate thickness.According to the plan,warpage of SWIFT structure is 25.5um and 159.7MPa is Von Mises thermal stress of bump.In addition,aiming at the comprehensive failure problem,a set of optimization process is proposed.The final structure warpage and thermal stress of micro-bumps are 21.9 ?m and 160 MPa,which are more balanced than single failure mode.