Thermal Stress and Mechanical Strain Real Time Mapping in Intelligent Power Switches Device S. Panarello, C. Triolo, A. Testa and S. Patanè University of Messina, Messina, Italy Email {spanarello, trioloc, atesta, patanes}@unime.it D. Patti and S. Russo STMicroelectronics, Catania, Italy Email {davide.patti, sebastiano.russo}@st.com Abstract— Thermal stress and mechanical deformations are the principal causes of power devices premature failures, especially in those critical conditions where they work under repetitive high current pulses or during overloads. Currently, a large effort has been devoted, both in experimental and computer modeling techniques, to predict the lifetime of those power devices used in automotive applications where high reliability is mandatory. Moreover, the knowledge of the thermal and mechanical stress, during the operations, could allow an effective refining of the design rules to improve devices reliability. In this paper, a novel technique to experimentally measure both thermal and mechanical fatigue on IPSs (Intelligent Power Switches) is presented. I. INTRODUCTION Mechanical deformations are the principal causes of power devices premature failures. These deformations take place when the devices work in critical conditions, such as repetitive high current pulses or during overloads. Under these conditions, in a very short time, the power devices undergo high temperature variations that give rise to mechanical deformations and micro-structural damage at the interfaces and at the solder joints [1], [2]. In previous works [3], [4], we experimentally demonstrated that the temperature distribution of Intelligent Power Switches (IPSs) or discrete devices changes unevenly in few milliseconds with variations up to 90–100°C. Generally, the devices under test integrate advanced protective functions that operate by means of onboard current monitoring system and temperature sensors. Unfortunately high speed switching can lead to hot spots in unexpected areas and to a premature failure of the device. In these areas, the thermal stress can produce localized cracks and deformations. This fast gradient and the reached values of temperature influence the work parameters and the devices lifetime. In order to identify the overheated areas (hot spots), an experimental setup equipped with an IR optical detector has been built as described in [3]. This instrument is able to make a temporal and spatial scan of the device surface and it allows to evaluate the temporal evolution of the heating. In this context, the study of deformation mechanisms and their causes is important in order to guarantee high reliability and very long lifetime for IPSs. Consequently in the last years many experimental techniques and theoretical simulations have been proposed in order to measure micrometric mechanical deformations and cracks, such as: Digital Image Correlation (DIC) method [5], Finite Element Analysis [6], Shearography [7] and other solutions. In this paper, we propose a novel experimental technique to evaluate the mechanical deformations based on the optical lever principle. This method is similar to that used in the Atomic Force Microscopy (AFM) to investigate the surface morphology [8]. While in the AFM, the optical lever is used to detect the cantilever deflections and the collected signal, with an appropriate feedback circuit, allow to reconstruct the surface morphology, in the proposed technique the reflected beam allows to directly reconstruct surfaces deflections, without using a cantilever. In order to detect the mechanical deformations, two laser beams are directly focused on the device surface and the optical signals are collected by two orthogonally placed four quadrant photo-detectors. During the operations of the devices under test (DUT), the Joule heating deforms the surface which reflects the laser beams in different directions, according to the Snell’s law. Analyzing the orientation of reflected beams before and during the current pulse, the “breathing mode”, torsions and the temporal evolution of the surface distortions can be detected and measured. A thermo-mechanical analysis has been performed on double channel high-side drivers for automotive grounded loads. Comparing the results of both thermal and mechanical stress, we propose a novel methodology to test IPSs and other electronic devices in order to provide two key parameters, such as thermal stress and mechanical expansion, to be applied in the most common lifetime predicting models. II. MECHANICAL DEFORMATION MEASURING SYSTEM The realized instrument for the measurement of mechanical deformations is based on the optical lever principle. This method is sensible at very small (up to nanometer dimensions) deformations and displacements and therefore it is suitable for detecting micrometric distortions that take place in microelectronics devices during the operations. The experimental setup (see Fig. 1) consists of: Two laser diodes; 978-1-4799-2918-4/14/$31.00 ©2014 IEEE 321 Proceedings of the 26th International Symposium on Power Semiconductor Devices & IC's June 15-19, 2014 Waikoloa, Hawaii