A Study On Interface Failure Analysis And Reliability For High Temperature Electronic Packaging

Nano-silver paste and DBC (direct bond copper) substrate with alumina layer were selected for building up a die-attached element for high temperature electronic packaging. The packaging elements failed under high temperature thermal cycling. Delamination between the silver paste and DBC substrate, silver paste failure and DBC substrate failure were three kinds of failure modes for the die-attached element. The failure mechanisms of packaging elements were studied focusing on the three kinds of failure modes to provide a method to evaluate the reliability of high temperature electronic SMT packaging. The mainly research contents of this dissertation were following.Thermal fatigue characterics of sintered nano-silver paste were studied using thermal cycling test, die shear test and microstructure observation. The results showed that the sintered silver paste was a porous metal with cavity ratio more than 10%. The cavity ratio increased with the numbers of thermal cycling, corresponding to the die shear strength decreased. The evolution of cavity ratio in the sintered nano-silver paste was observed under thermal cycling and a damage variable was defined related to cavity ratio. A non linear continuous damage mechanics model integrated with the defined damage variable and the interface shear strength requirement of nano-silver paste at high temperature was used to study the thermal fatigue lifetime prediction of die-attached samples. Some mechanical fatigue tests were conducted using LED chips attached nano-silver paste samples instead of thermal cycling tests for saving test time. The Chaboche model could be used well for predicting the fatigue lifetime of those samples.The bond strength of three different types of copper coating substrates (gold-plated, silver-plated and non-coating) and nano-silver paste sintering in the same conditions were studied through energy spectrum analysis and X-ray diffraction (XRD) to explore the sintering effect on the composition of the interface material. Molecular Dynamics (MD) simulation was employed to study bonding energy and bonding strength. The simulation results were verified using die shear test of nano-silver paste attached samples. The calculated bond strength is the same trend with the die shear experimental results of nanosilver paste-attached samples. The silver-plated substrate has the largest die shear strength and interface bonding energy, followed by gold-plated substrate. Copper substrate without coating was the weakest. Molecular dynamics simulation could be a simple and rapid method for the evaluation of interconnection strength of nano-silver paste with different coating materials.Thermal stresses were analyzed in the DBC substrate using FEM with a group of parameters of the referenced Chaboche model which described the copper plasticity behavior under high temperature. The relationship between the fillet of copper layer in the DBC substrate and the thermal fatigue lifetime of DBC was discussed based on the thermal stresses and thermal plastic strain distribution. One group of DBC substrate samples with edge tail length of 1mm and the other group of DBC substrate samples without edge tail length were designed for thermal cycling test at -55oC~250oC temperature range for further investigating the relationship between the copper fillet and thermal fatigue lifetime of DBC substrate. The results showed that the fillet of copper layer changed the maximum thermal stress site and reduced the plastic strains in the substrate. The thermal fatigue lifetime could be improved by the fillet of copper layer in the substrate.Several groups of thermal cycling tests at different temperature range were conducted for further evaluating the initiation lifetime of DBC substrate under high thermal cycling. The results showed that the model for predicting extreme low fatigue lifetime prediction could be used to study the thermal fatigue lifetime prediction of DBC substrate well. Four groups of samples with different lengths and widths and ratios of length and width were made for thermal cycling test at -55oC~250oC temperature range for further investigating the characterics of crack growth of DBC substrate under high temperature thermal cycling. Thermal stress intensity factor was used to evaluate the cracking behavior of DBC substrate. The results showed that the propagation of the crack in DBC substrate could be divided into three stages. The crack grew quickly at the first stage and slowly developed at the second stage. It stopped at the last stage. The crack propagation rate decreased with the crack length increased. The ratio of length and width had no obvious influence on the crack propagation rate in the substrate. The thermal stress intensity factor increased with the thick of ceramic increased, but decreased with the crack length increased. The crack growth stopped if the thermal stress intensity factor was smaller than its critical value.