I und J

Thermo-elasto-plastic phase-field modelling of mechanical behaviours of sintered nano-silver with randomly distributed micro-pores

Characterization and reconstruction for stochastically distributed void morphology in nano‑silver sintered joints

The massive development of Hybrid and Electrical Vehicles (HEV) is strongly impacting the semiconductor industry demanding for highly reliable Power Electronic components. Within the engine compartment installation space is of major concern, therefore small size and high integration level of the modules are needed. Conventionally devices are typically soldered to ceramics substrates that are vacuum soldered to water-cooled base plates. The known reliability limitations of traditional solder joints are significantly limiting the power density increase, limiting the maximum operative temperature and representing a strong constrain for using high performances devices such as wide bandgap compound like Silicon Carbide (SiC). Silver sintering today has started to replace the solder joint from chips to carrier substrates, leaving one major reliability bottleneck. Combining properly temperature, time and pressure, a strong, highly electrically and thermally conductive bond is formed. The aim of this work is to develop an integrated methodology, numerical and experimental, to assess the Ag sintering die attach process for a SiC power MOSFET. Different process parameters have been benchmarked by means of physical analyses, performed at time zero and also after liquid-to-liquid thermal shock aging test. The sintering flakes densification process has been reproduced by Finite Element Analysis and the obtained morphological texture has been used for extracting the mechanical properties of the layer as a function of the thermo-compression process itself. A simulation method, based on the evaluation of the inelastic strain accounted per cycle has been used for matching the experimental results according to an aging model.

Design and Process Optimization of a Sintered Joint for Power Electronics Automotive Applications

Sintered silver (SAG) is a widely-used die attach (DA) technology in automotive industry, especially for power modules in electric vehicles (EV) defined by higher power densities. High operation temperatures (> 175°C) are particularly resulting from the application of dies based on wide bandgap semiconductors materials like SiC and GaN. The thermo-mechanical stress resulting from large temperature swings in EV applications fatigues the porous layer of SAG. In order to estimate the reliability of die attach layers physics-of-failure (PoF) based lifetime models are needed. In this study, comparative parametric simulations are used to estimate the influence of geometric factors on the lifetime of the die attach layer. In addition, the state of the art of accelerated fatigue experiments is presented.

A Parametric Simulative Study for Lifetime Prediction of Sintered Silver Die Attach Under Different Accelerated Testing Conditions

Sintered silver is a very promising option in the field of automotive power electronics for die-attach and substrate-attach material as it offers many benefits compared to conventional solders. One main advantage is the higher and therefore better thermal and electrical conductivity, which is necessary due to the usage of SiC and GaN as a basic material for semiconductors. Another one is the higher melting point of sintered silver in comparison to solder, making it possible to use power electronics at higher temperatures. Especially for higher temperatures, it is assumed that this results in an improved reliability. Whereas a lot of information about the behavior of sinter layers is known, e.g., thermal properties, material characterization of mechanical properties are still not understood in detail as the results are highly scattered. A reliable test method has to be developed, and this has to be done for cyclic testing, too, which is essential for determining reliability. As mechanical properties are strongly dependent on process parameters, their influence has to be analyzed as well. For example, the influence of the porosity distribution of a sintered layer in between the semiconductor and the upper side of the DBC on the mechanical properties and their reliability is hardly investigated by now. It can be observed that the porosity varies, and that it often increases from the middle of the plane towards the edges. The influence on the stresses and strains and on reliability has to be determined and if required, improvements have to be developed. In the field of FEM simulation, too, a lot of work has to be done. Therefore, the right input material data, which has to be characterized as already described, is absolutely essential. It is also necessary to implement a material model which describes all of the occurring phenomena like primary and secondary creep. This paper will give an overview on the current status and state of the art of silver sintering, focusing on the issues of material characterization and reliability testing.

Silver Sintering in Power Electronics: The State of the Art in Material Characterization and Reliability Testing

The increased attention for sintered silver as die attach attracts also interest in its reliability assessment. Since the porous joint material shows rate-dependent behaviour at elevated temperature, its visco-plastic behaviour needs to be quantified. Though tensile tests are frequently used for that task is still a challenge to manufacture homogeneous specimens large enough to be safely handled and tensile tested. In this paper the we have presented a comprehensive technology for manufacturing sintered silver specimen for micro bending test. The sample length is about 10 mm but can still be tested with nanoindenter equipment. These specimens were bent at 25 °C at different strain rates. The corresponding stress-strain curves were obtained with transformation functions computed by a FE model. Tensile specimens of sintered silver were manufactured and tensile tested at 25, 100 and 175 °C for investigation of rate dependency and relaxation behaviour. For 25 °C no rate dependency was observed. At 100 °C and 175 °C significant reduction of tensile strength and increase of tensile strain was observed at smaller strain rates. The creep was caused by thermal activation of inner surface diffusion.

Micro Bending Test on Double Cantilever Beams: A specimen-centred approach to accurate determination of the visco-plastic properties of Sintered Silver for Power Electronics applications

Sintered Ag (SAG) is nowadays used for die attach in field of automotive power electronics. Nevertheless, failure mechanisms in this porous layer under different loading conditions are not completely understood yet. Different joined materials like SiC (die) with Au finish can lead to diffusion processes and therefore different lifetime. Additionally, a non-homogenous layer of porosity can lead to a new failure mechanism. Typically, the crack propagates from the edges of the layer to the center. In this paper, a crack initiation that is locally dependent on the porosity will be shown. Passive temperature cycling and high temperature aging test are performed using a SiC MOSFET and micro sized Ag paste. First simulation results indicate the highest plastic strain regimes at the edges of the sinter layer. The need for other approaches is shown and suggestions therefore introduced. In addition, the need of material data for different states of the microstructure (e.g. porosity) is explained.

Marco Schaal

Papers

Silver Sintering in Power Electronics: The State of the Art in Material Characterization and Reliability Testing

Micro Bending Test on Double Cantilever Beams: A specimen-centred approach to accurate determination of the visco-plastic properties of Sintered Silver for Power Electronics applications

Reliability Assessment of Ag Sintered Joints Using a SiC Semiconductor and Determination of Failure Mechanism in the Field of Power Electronics