Markus Klingler

Solder fatigue failures in a new designed power module under Power Cycling

Abstract Today a point has been reached where lifetimes of power modules are limited by the standard packaging technologies, such as wire bonding. To surpass these limits, a new power module was designed using Cu clips as interconnects instead of Al wire bonds. With this new design the structure robustness should be improved and lead to a reliability gain but in counterpart it requires an additional solder layer in order to fix the clip onto the die. This paper studies the failure mechanisms occurring in these two solder layers under power cycling. The behavior of solder layers is precisely analyzed by performing power cycling tests and by taking advantage of Finite Elements simulations. Furthermore an experimental and numerical sensitivity study on test parameters is conducted. Results obtained enable the definition of solder lifetime prediction models.

Reliability investigation of large area solder joints in power electronics modules and its simulative representation

Abstract Power electronics (PE) modules for inverter units in hybrid/electric vehicles (H/EV) generate a large amount of heat which needs to be dissipated. This is often done via a liquid cooled metallic baseplate which acts as a heat sink. The interconnect between the PE module and the baseplate is realized using a large area lead-free solder joint which under passive temperature cycling (pTC) tests, develops adhesive cracks (delamination) at the solder-intermetallic compound (IMC) interface. Such cracks reduce the capability of the solder joint to effectively transfer heat to the baseplate and potentially lead to device failure due to overheating. Considering the large number of potential designs for various application of such PE modules, an understanding of the influence of the mechanical behaviour of individual assembly components such as the power substrate, baseplate or the solder joint itself on reliability is of great interest and utility. This study therefore provides an in-depth reliability assessment of multiple physical variants aged under three different pTC profiles. The investigation reveals certain clear trends with respect to warpage, stiffness and size of the joining partners. A detailed Finite Element Method (FEM) simulation methodology was also developed that represents the delamination behaviour for lifetime assessment.

Study on the interdependence of soldering profile, ageing conditions and intermetallic layer thickness in large area solder joints and its influence on reliability

This study was aimed at quantifying critically relevant topics about the influence of intermetallic compounds (IMC) within power electronics reliability such as the influence of soldering profile on the growth rate of IMC in large area solder joints, estimation of IMC thickness during non-isothermal ageing and the impact of IMC thickness on reliability. To this end, test samples were soldered using four different soldering profiles by varying hold time at peak temperature and cooling rate. Isothermal ageing was performed at 125°C, 150°C and 175°C. IMC thickness measurements were taken at ageing durations of 200h, 500h and 1000h respectively. Using linear regression on experimental data and the finite differences method, a differential equation was derived and applied to predict IMC thickness for any arbitrary thermal ageing profile, for example, active or passive temperature cycling. Additionally, finite element method (FEM) simulations were carried out to evaluate the influence of IMC thickness under three potential failure modes. These failure modes result from either substrate war page or in-plane shear deformation. The study has been able to show the effects of soldering profiles on the IMC growth rate and also the influence of IMC thickness on stress state near crack locations using FEM simulations.

Novel concepts for modelling and enhancing the DBC-Al 2 O 3 lifetime

Direct bonded copper (DBC) substrates are widely used in electronic power devices to enable high electric currents and to electrically isolate silicon dies from the heat sinks. DBC substrates are copper/ceramic/copper composites. They exhibit residual stresses after their bonding process caused by the CTE mismatch between copper and ceramics. In automotive applications, electronic power devices experience high temperature variations, which can lead to cracking in the DBC ceramic. As a consequence, the resulting diminished heat dissipation inevitably will lead to failure in the power device. Thus a lifetime model to predict the rate of cracking is very helpful.In this paper, we show the fundamental work based on finite element simulation. This is the key method for a planned lifetime model of DBC-A1z03. The DBC substrates are subjected to passive temperature cycles. By means of linear elastic fracture mechanics, we model the stress at the most heavily strained regions in the DBCA1203. With a parameter study, we were able to model the dependency of stresses on the DBC substrate symmetry and the A1203 thickness efficiently. This enables a proper planning of the DBC substrate layout. In the experimental part, we could show that stresses induced by thermal-mechanical loads are accompanied by chemical reactions at the crack tip. Lifetime studies of DBC substrates under varying oxygen contents and humidities are shown in detail. This finding provides the chance to increase the DBC lifetime significantly in a very efficient way.