Study On Failure Mechanism And Reliability For Micro-bump Interconnection Under Thermo-electric Stresses

The market demand for the integration and miniaturization of electronic products has enabled the rapid development of high-density packaging technologies such as flip-chip micro-interconnect bumps.Flip-chip bumps are widely used in automotive,aerospace and other high-reliability electronic products due to their fine pitch,micro size and high I/O density.However,as the bump size shrinks and the power consumption increases,the increasing of the loaded current density and the volume fraction of the IMC layer can induce new thermal,electrical,mechanical and other reliability issues.Therefore,the research on the failure mechanism and rapid reliability evaluation technology of the bump interconnect structure under thermoelectric stress is an urgent problem to be solved in the process of promoting the high reliability of electronic products.The copper pillar bumps interconnect in the first level package and the mixed solder in the secondary level package are studied as objects to conduct research on microstructure,electrical properties,failure mechanism analysis,and physical model construction and reliability evaluation technology.For typical products,a mixed solder bump optimized design scheme based on Pb content and structure is proposed to achieve reliability evaluation based on failure physics,providing a basis for the reliability design and evaluation of domestic micro-interconnected electronic products for high reliability applications.The main work of the thesis and the major innovations achieved are as follows:1.Cu/solder/Cu interconnect structures are studied as research object.Considering the effects of three kinds of physical mechanisms on the migration of Cu atoms and the growth of IMC layer,such as electron-wind-force,Cu atoms diffusion and dissolution.The kinetic model of the growth of inter-metallic compounds under thermoelectric stress is deduced using the theorem of conservation of Cu atom diffusion flux.The model shows that the growth of the IMC layer at the interconnection interface presents a significant polarity difference under the conditions of thermoelectric stress.2.The interface behavior,lifetime distribution,failure mechanism and influencing factors of flip-chip copper pillar bumps composed of Cu/Ni/SnAg1.8/Cu under 9 groups of thermoelectric stress such as 125?and 3×10⁴A/cm²are studied based on thermoelectric reliability test,infrared thermal imaging and microstructure analysis methods.It is found that there are four major kinds of failure mechanisms of Cu pillar bumps under thermoelectric stress,including Cu bonding pad dissolution,lamellar voids,erosion of the cathode nickel coating and Cu₃Sn alloying.Furthermore,it is found that copper pillar bumps crack failure can be divided into three phases,including Cu₆Sn₅ growth and Sn solder consumption,Cu₃Sn transforming growth,and void formation and crack growth.Based on the Black equation and infrared thermal image analysis,the effect of Joule heating effect was quantified,and a thermoelectric reliability model of the copper pillar bumps interconnection considering the Joule heating effect is constructed.The results show that,under the thermoelectric stress,the diffusion of atoms is dominated by the electron-wind-force,but under a single high temperature stress,the diffusion of atoms is dominated by the temperature gradient.Compared with the single high temperature stress,the comprehensive thermoelectric stress significantly accelerates and changes the growth behavior and failure mechanisms of IMC layer in the copper pillar interconnection.3.Breakthroughs in mixed solder bump preparation technology with different lead content,solidification behavior,mechanical properties,microstructure analysis and evaluation technology.The main failure modes of mixed solder bumps under thermoelectric stress are obtained,and ratio of lead content in mixed solder with better overall performance is proposed.The microscopic mechanism of the crack failure of mixed solder bumps is analyzed,including the polarity growth of inter-metallic compounds,Sn/Pb segregation,and void expansion.Considering the effects of current accumulation and Joule heating,the Black electro-migration reliability model is modified to obtain a thermoelectric reliability model of mixed solder bumps.Based on the thermal fatigue failure mechanism of mixed solder bumps and Engelmaier equation,a thermal fatigue life prediction model for mixed solder bumps is constructed,which provides a scientific theoretical basis for the high reliability design and evaluation of mixed solder bumps,and helps solving the problem of lacking of basic data support in reliability design of mixed solder bumps for highly reliable applications.4.A comprehensive evaluation method for reliability of micro-interconnected electronic components that fuses failure physical models and reliability mathematics methods is proposed.The localization of reliability evaluation tool software based on failure physics is realized,and the comprehensive reliability evaluation of micro-interconnected electronic components is solved.The reliability evaluation method based on failure physics is pushed from the level of basic research to engineering application.5.The technical methods proposed in this dissertation and the research results obtained have been applied in major national projects.For a certain type of micro-interconnected electronic components,the material optimization results of an optimized Pb content of4.67%wt and the results of solder joint size optimization are obtained,which reduce the maximum equivalent stress of critical solder joints by 18.85%.The reliability evaluation tool software is used to carry out reliability prediction under the mission profile.The average failure time obtained by the prediction is 18890 hours,and meets the requirements of product reliability index with an average failure time greater than 10000 hours.A comprehensive reliability evaluation based on engineering application is achieved.