Assessment of high temperature reliability of molded smart power modules

Within the last years a constant increase of application temperatures for electronic packaging is observable, this is true for sensor applications, for power electronics packages and also for Smart Power Modules, integrating control logic and power ICs in one compact package. Drivers for HT use are on the one hand the introduction of GaN/SiC power electronic devices; these need modules to operate at temperatures higher than 200 °C, forming internal heat sources. On the other hand electronic devices will be implemented considerably closer to external heat sources as engines, gear boxes etc. with ambient temperatures > 175 °C. Today encapsulation of common power modules is mainly realized by silicone potting over DCB. But realization of Smart Power Modules, being truly heterogeneous as they integrate a wide variety of components, call for another encapsulation method. Transfer molding as a widespread and cost effective encapsulation process is selected for such modules, providing protection of semiconductors against temperature load, humidity, media, mechanical shock etc. Achieving high temperature reliability of smart power modules requires high temperature applicability of all components and materials involved. Special focus lies upon encapsulants, as polymers are the materials most influenced by elevated temperature. State of the art molded packages typically provide robustness up to 175 °C; but that temperature limit needs to be increased for encapsulation of advanced power electronics. Previous work evaluated state-of-the-art high temperature capable molding compounds [MC] with special focus on changes of thermo-mechanical material properties caused by thermal ageing [1]. As a result two compounds with the highest potential for high temperature use have been identified, which were chosen as the encapsulation material for reliability evaluation using a smart power module test vehicle. This test vehicle is a leadframe based mold-package, containing both power (represented by IGBTs) and logic (represented by daisy chains on HT capable organic or ceramic substrate); all encapsulated in a transfer molding process. FEM analysis of stress and strain has been done with initial and aged material properties, pointing out the reliability potential of molded modules. Ageing behavior of the smart power module is analyzed after applying temperature load; investigation was done with three different temperature loads: 200 °C, 220 °C, and 250 °C. Subsequently the packages were analyzed in initial state and after ageing, taking into account initial package quality and changes as delamination growth, warpage behaviour at RT and under temperature load, and loss of electrical integrity [2] In summary this paper describes the assessment of state-of-the-art MCs for encapsulation of advanced power electronics, all resulting in a recommended procedure for identifying and qualifying high temperature stable encapsulants. Additionally an outlook on future application for molded smart power modules will be provided.