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Reliability of novel ceramic encapsulation materials for electronic packaging
Keywords: cement-based encapsulation, miniaturization, power density
A major trend of development in power electronic systems is the miniaturization and the corresponding increase in power density targeting at optimum efficiency of power electronics in all sectors of industry. In this context, wide band gap (WBG) power semiconductors, such as silicon carbide (SiC) and gallium nitride (GaN), play an important role as they have the capability to process electric power at higher voltages and temperatures with smaller die areas and less power losses. These characteristics do not only result in superior power conversion but enable significantly reduced volumes also at system level, due to decreased cooling requirements and smaller passive components, also contributing to overall lower costs. However, reduced device sizes and higher switching frequencies are attended by an increased power loss density of WBG-semiconductors, and furthermore lead to higher operation temperatures of up to 300 °C. Thus, several material innovations in WBG power module packaging are under development to realize the high temperature operation potential and to promote successful application of WBG power semiconductors. Especially the improvement of the high temperature withstanding capability of encapsulation materials for power semiconductors and passive components is of major interest. Current encapsulation materials such as epoxy compounds and silicon gels were selected to withstand a maximum operation temperature of about 175 °C. However, at higher temperatures of above 200 °C the lifetime of these materials will be reduced due to thermal degradation effects. Thus, a further increase in electronics efficiency and reliability can only be achieved by novel encapsulation materials with higher temperature stability, improved heat conductance and adapted thermomechanical properties. Therefore, novel encapsulation materials based on cements with improved thermal properties such as calcium aluminate cement (CAC) based systems and phosphate cement (PC) based systems are currently under industrial development. These new systems are intended to tolerate operation temperatures of up to 300 °C and to reach a thermal conductivity of 5 to 10 W / (m*K) (prior < 2 W / (m*K)) while minimizing thermomechanical stresses on the semiconductor devices due to suitable coefficients of thermal expansion. Within the present work, the CAC and PC material systems with their basic material properties will be introduced. The main focus consists in a thorough material characterization (including microstructural, mechanical and thermal analysis of the new encapsulants) and interactions with the embedded electronic components, e.g. with the contact interfaces of the semiconductors, the wire bonds and the substrate metallizations. The aim of these investigations is to identify and to exclude potential defect risks at an early development stage, thus providing guidance for technology optimization with regard to high quality and reliability during future application. For this goal, the applied non-destructive and destructive analysis methods had to be adapted to the boundary conditions of cement-based materials. This allowed for reliable crack and void detection within the encapsulants. The ability to control and quantify these micro defects is important because they are expected to cause reliability problems during long-term operation, especially in atmospheres with high humidity. A second task consisted in the analysis of defect reactions and corrosion effects that were observed on test specimens after thermal and electrical testing. These results were correlated to the ionic content of the cement materials. The investigations carried out in this study form the basis for further material developments in the field of cement-based encapsulating materials for improved WBG electronic systems and passive components.
Stefan Kaessner, Doctoral Researcher
Robert Bosch GmbH
Renningen, BW

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