Reliability of silver sintered large area joints : temperature dependent mechanical properties under static and cyclic loading

Abstract

In der Leistungselektronik werden seit Jahrzehnten bleihaltige Lote als Verbindungsmaterial verwendet. Die Erhöhung der Packungsdichte bei immer geringerer Chipgröße hat jedoch zu steigenden Anforderungen an das Wärmemanagement geführt. Bei erhöhter Betriebstemperatur können Unterschiede von Wärmeausdehnungskoeffizienten hohe Spannungen in benachbarten Bauteilen verursachen. Daher führen plastische Verformung und Kriechen zu einer ernsthaften Beeinträchtigung der Zuverlässigkeit von Materialverbindungen. Silbersintern ist eine mit Kupfer verträgliche, bleifreie Alternative hinsichtlich der Verbindungstechnik für Module der Leistungselektronik. Silber hat nicht nur eine hohe elektrische und thermische Leitfähigkeit, sondern auch eine hohe Schmelztemperatur (ca. 960 °C) und ist deshalb ein vielversprechendes Material für Anwendungen bei erhöhter Temperatur. Obwohl die grundlegenden Eigenschaften von Sintermaterialien wie Kupfer und Silber gut bekannt sind, ist es erforderlich Prozessparameter an Materialien und deren Geometrie anzupassen, um die Lebensdauererwartung der Verbindungen zu steigern. In dieser Arbeit wurden die mechanischen Eigenschaften des gesinterten, porösen Silbers sowohl unter statischer als auch unter zyklischer Last systematisch untersucht. Dafür wurden hauptsächlich zwei Arten von Proben produziert: gesintertes Ag als freistehende Streifenschicht, sowie als Cu-Ag-Cu Überlappungsverbindung. Charakteristische Materialparameter wie Dichte, elektrischer Widerstand, Elastizitätsmodul, oder Zug- und Scherfestigkeit wurden für die drucklos und mit Druck gesinterten Probenserien ermittelt. Die thermomechanischen Eigenschaften, die Lebensdauer und das Verhalten der Spannungsrelaxation von definierten Cu-Ag-Cu Verbindungen wurden bei verschiedenen Temperaturen analysiert. Der Vergleich des Ermüdungsverhaltens unterschiedlicher Probentypen wurde mittels Wöhler-Kurve und Weibull-Wahrscheinlichkeit-Plot dargestellt. Die Parameter eines Materialmodells, welches eine Kombination aus Norton-Kriechen und Gurson-Plastizität darstellt, konnten an die Ergebnisse der durchgeführten Tests und Analysen angepasst werden. Dieses Materialmodell konnte in die kommerzielle Software ABAQUS implementiert werden und dient nun als numerisches Hilfsmittel zur Zuverlässigkeitsbewertung von Ag-gesinterten Kupferverbindungen.

In power electronics Pb-containing solders have been used over decades as bonding material. However, increase of packaging density at lower chip weight and size has led to increasing requirements regarding thermal management during operation. Especially at elevated operation temperature, the mismatch of the coefficients of thermal expansion can cause high stresses in constrained materials, and therefore plastic deformation and creep failure become a serious issue of joint reliability. Silver sintering is a copper compatible and lead-free alternative interconnection technology for power modules. In addition to high electrical and thermal conductivity, silver has a high melting point (960 °C) in comparison to conventional solder alloys. Hence, silver is a promising material for applications at elevated temperature. Although the basic characteristics of sintering materials such as copper and silver are well understood, adjustments of process parameters, material combinations and geometry are required to enhance lifetime expectations based on reliability prognosis of the interconnects. In this work, characteristic features such as density, electrical resistivity, tensile and shear strength of sintered Ag were analyzed, which are influenced by the porous structure arising in consequence of sample preparation and sintering conditions (pressure, temperature, and time). Two main types of samples were produced: either as sintered Ag-film or as Cu-Ag-Cu lap joint. Further, series of pressureless and pressure-assisted sintered specimen were defined. The thermo-mechanical properties, fatigue lifetime and stress relaxation behaviour of copper joints were investigated at various testing temperatures. In comparison to the as-sintered, so-called 0-hour samples, some sample series were heat treated for 250 hours at 250 °C prior to testing, either in air or in sealed glass protection with restricted oxygen content. The correlating microstructure was investigated. In the presence of oxygen, a growth in the grain size of the sintered silver was observed, and in case of the Cu-Ag-Cu lap-joints, copper-oxide formation occurred.The Young’s modulus of the dumbbell shaped samples prepared without the heat treatment was measured by a setup including a laser speckle extensometer, which enabled contactless strain measurement of the sintered silver. In conclusion, samples aged for 250 hours at elevated temperature showed a reduced elastic modulus compared to samples, which were not heat treated. A decrease of the Young’s modulus was also observed for the elevated test temperature of 200 °C.For the density measurement of the porous silver two methods were used: the mass/volume method and the optical method of digital image processing. For the latter, to reveal the cross-sectional surface, the samples were either fractured (also in liquid nitrogen) or embedded in resin and grinded. A local cut with focused ion beam (FIB) enabled the insight into the porous structure below the surface showing pitch holes of the Ag-layer. With this optical method, no significant change in density arising from heat treatment at 250°C could be measured.In comparison, in general lower density values were measured with use of the mass/volume method. For the pressureless sintered specimens, after ageing at 250 °C for 250 hours, a change in the porosities was observed, the samples got denser.The electrical measurements delivered increasing resistivity with increasing porosity, and with increasing testing temperature (200 °C), an increase of the electrical resistivity was observed, as expected. The mean ultimate tensile strength of dumbbell shaped specimens of sintered silver as well as the mean shear strength of the copper lap-joints showed in general a decrease for testing temperatures of 130 °C and 200 °C, compared to 25 °C. With the increase of temperature, the brittle fracture characteristics of low temperatures changed into a ductile behavior.The mechanical cyclic lifetime was determined for various specimen of the Cu-Ag-Cu lap-joints sintered under different parameters. Finally, the samples of which the copper substrate was pre-sputtered with a thin Ag-layer, then stencil printed with Ag-paste and sintered under pressure were selected for the main test series. The behaviour of cyclic fatigue of these samples at room and elevated temperatures were compared for the 0-hour and 250-hours aged types. The number of cycles to failure Nf were analyzed with Weibull probability plots. In general, the cyclic fatigue life was significantly reduced, when samples were tested at high temperature. However, heat treatment prior to mechanical testing improved the cyclic lifetime of the lap-joints significantly.The creep properties of the pressureless and pressure assisted sintered specimens were examined with experiments, where samples were stretched to a defined stress level, depending on the ultimate strength of the specimens, and the relaxation was observed over one hour. Tests were carried out at 25 °C, 130 °C and 200 °C. The results of the performed tests and analyses could be fitted by the parameters of a theoretical material model based on a combination of Norton creep and Gurson plasticity. The constitutive equations of the model describe the creep properties of sintered silver in dependence of its porosity. Thereby, the temperature dependence was described by an Arrhenius term using the activation energy Q. The material model was implemented in the commercial software of ABAQUS through user subroutine UMAT and can now be used as helpful tool for reliability assessment of Ag-sintered copper joints.