Evaluation of die attach materials for high temperature power electronics appictions and analysis of the Ag particles sintering solution

Sintering of Ag particles is an interesting solution as die-attach material for power electronics packaging in high temperature applications because it offers the outstanding properties of Ag (including a 961 ºC melting point) using low process temperature ( 300 ºC). In this work we have studied the processing methods for sintering Ag particles and we performed a comparative analysis with other die-attach materials based on thermal cycling. We have first analysed the effect of the main sintering process parameters on the final characteristics of the Ag die-attach layers with the aim at obtaining information for the direct application of Ag pastes in power assemblies, and for understanding the main mechanisms governing the properties of this material. This task required the development of specific set-ups and tools allowing the settling of different process conditions. The sintering parameters analysed were pressure, temperature, time, Ag paste drying method and substrate surface condition. The analysis based on FIB microsections and die-shear tests revealed that the densification of the Ag layer and its adherence to the Cu substrate clearly improve with higher sintering pressure and temperature, and is practically not affected with time. In the centre of the Ag layer, the level of densification is lower than in the periphery, where, in addition, a thin Cu oxide layer appears. Those facts evidenced that the die-attach periphery is strongly influenced by the surrounding ambient (air) and that in this region the organic components of the original paste are better eliminated than in the centre during the Ag paste drying step. This process has been identified as perhaps the most critical one during sintering and a drying method without the die on top of the paste has shown the best results. An interesting result of the study is that sintering pressure and temperature can be adjusted complementarily to obtain a given die-attach adhesion (die-shear value). In this way, the sintering process can be adapted to different assembly conditions such as, for example, samples compatible with high pressures but not with high temperatures or vice versa. A comparative analysis of the thermo-mechanical performances with other die-attach materials (AuGe, and PbSnAg) under harsh thermal cycling tests (-65 ºC to 275 ºC) has been carried out using test vehicles formed by several dice (Si or SiC) and Cu substrates (bare Cu, Ag and Au plated surface). The test vehicles were cycled in a specifically developed set-up and evaluated with an experimental protocol based on die-shear tests, failure mode determination and SAM inspection. The best initial die-shear strength was obtained in all cases for sintered Ag particles, contrasting with the highest degradation rate observed for this material dried with chip on top of the paste. In contrast, for the drying method without die, sintered Ag shows even better cycling results than the solder alloys. Concerning the semiconductor material, the thermal cycling results show that the degradation rate is faster for SiC- than for Si-based test vehicles, while concerning strictly the solder alloys PbSnAg shows better results than AuGe. In this sense, a thermo-mechanical simulation work was performed to understand the degradation mechanisms during thermal cycling. The simulations demonstrated that the higher normal stresses associated with temperature swings were located at the die-attach/substrate interface, with the maximum values in the corners of the die and its periphery. These results qualitatively explain the SAM images showing concentric die-attach delamination patterns and the fact that "stiffer" materials (with higher Young modulus, such as SiC and AuGe) show faster degradation rates.La sinterización de partículas de Ag es una solución interesante como material para die-attach para los módulos de la electrónica de potencia principalmente en aplicaciones de alta temperatura, ya que ofrece las propiedades sobresalientes de la Ag (incluyendo punto de fusión de 961 ºC) usando una baja temperatura de proceso ( 300 ºC). En este trabajo se ha estudiado el proceso para la sinterización de partículas de Ag y se realizó un análisis comparativo con otros materiales utilizando ciclos térmicos. Primero, hemos analizado el efecto de los principales parámetros del proceso de sinterización de la capa sinterizada de Ag con el objetivo de obtener información para la aplicación directa de pastas de Ag en módulos de potencia, y además, comprender los principales mecanismos que regulan las propiedades de este material. Esta tarea requirió el desarrollo de montajes experimentales y herramientas específicas que permitieron establecer las diferentes condiciones del proceso. Los parámetros de sinterización analizados fueron la presión, temperatura, tiempo, método de secado de la pasta de Ag y las condiciones de la superficie del sustrato. El análisis basado en las microsecciones FIB y los test de die-shear revelaron que la densificación de la capa de Ag y su adherencia al sustrato de Cu mejora claramente con mayor presión y temperatura de sinterización, y que se ve poco afectado con el tiempo. En el centro de la capa de Ag, el nivel de densificación es menor que en la periferia, donde, además, aparece una fina capa de óxido de Cu. Estos hechos evidenciaron que la periferia del die-attach está fuertemente influenciada por el ambiente (aire) y que en esta región se eliminaron mejor que en el centro los componentes orgánicos de la pasta de Ag (durante la etapa de secado). Este proceso ha sido identificado como el más crítico durante la sinterización y el método de pre-secado sin el chip en la parte superior de la pasta, ha sido el que ha mostrado los mejores resultados. Un resultado interesante del estudio es que, la presión y la temperatura de sinterización se pueden ajustar de forma complementaria para obtener una adhesión requerida (valor de die-shear). De esta manera, el proceso de sinterización se puede adaptar a diferentes condiciones del ensamblado, tales como las muestras compatibles con altas presiones, pero no con altas temperaturas o viceversa. Se ha llevado a cabo un análisis comparativo del rendimiento termo-mecánico con otros materiales de die-attach (AuGe y PbSnAg) con pruebas de ciclos térmicos severos (-65 ºC a 275 ºC) utilizando vehículos de prueba formados por varios chips (Si o SiC) y sustratos (Cu sin metalizar y metalizados con Ag y Au). Las muestras fueron cicladas (en un montaje experimental desarrollado específicamente) y evaluados con un protocolo basado en test de die-shear e inspecciones SAM, para determinar los modos de fallo. La mejor fuerza se obtuvo en todos los casos para las partículas sinterizadas de Ag, aunque la tasa de degradación más alta fue para las muestras secadas con chip. En contraste, para el método de secado sin chip, el sinterizado de Ag muestra incluso mejores resultados en el ciclo térmico, que las aleaciones de soldadura. En cuanto a la material semiconductor, los resultados de los ciclos térmicos muestran que la tasa de degradación es más rápida para las muestras con SiC, que para las elaboradas con Si, mientras que para las soldaduras (aleaciones) el PbSnAg muestra mejores resultados que el Auge. En este sentido, se realizó un trabajo de simulación termo-mecánica para entender los mecanismos de degradación durante el ciclo térmico. Las simulaciones demostraron que los estreses normales más altos asociados con los cambios de temperatura se encuentran en la interfaz die-attach / sustrato, con los valores máximos en las esquinas del chip y su periferia. Estos resultados explican cualitativamente las imágenes SAM que muestran patrones de delaminación concéntricas en die-attach y además, que los materiales "más rígidos" (con mayor módulo de Young, como SiC y AuGe) muestran tasas de degradación más rápidas.Sintering of Ag particles is an interesting solution as die-attach material for power electronics packaging in high temperature applications because it offers the outstanding properties of Ag (including a 961 ºC melting point) using low process temperature ( 300 ºC). In this work we have studied the processing methods for sintering Ag particles and we performed a comparative analysis with other die-attach materials based on thermal cycling. We have first analysed the effect of the main sintering process parameters on the final characteristics of the Ag die-attach layers with the aim at obtaining information for the direct application of Ag pastes in power assemblies, and for understanding the main mechanisms governing the properties of this material. This task required the development of specific set-ups and tools allowing the settling of different process conditions. The sintering parameters analysed were pressure, temperature, time, Ag paste drying method and substrate surface condition. The analysis based on FIB microsections and die-shear tests revealed that the densification of the Ag layer and its adherence to the Cu substrate clearly improve with higher sintering pressure and temperature, and is practically not affected with time. In the centre of the Ag layer, the level of densification is lower than in the periphery, where, in addition, a thin Cu oxide layer appears. Those facts evidenced that the die-attach periphery is strongly influenced by the surrounding ambient (air) and that in this region the organic components of the original paste are better eliminated than in the centre during the Ag paste drying step. This process has been identified as perhaps the most critical one during sintering and a drying method without the die on top of the paste has shown the best results. An interesting result of the study is that sintering pressure and temperature can be adjusted complementarily to obtain a given die-attach adhesion (die-shear value). In this way, the sintering process can be adapted to different assembly conditions such as, for example, samples compatible with high pressures but not with high temperatures or vice versa. A comparative analysis of the thermo-mechanical performances with other die-attach materials (AuGe, and PbSnAg) under harsh thermal cycling tests (-65 ºC to 275 ºC) has been carried out using test vehicles formed by several dice (Si or SiC) and Cu substrates (bare Cu, Ag and Au plated surface). The test vehicles were cycled in a specifically developed set-up and evaluated with an experimental protocol based on die-shear tests, failure mode determination and SAM inspection. The best initial die-shear strength was obtained in all cases for sintered Ag particles, contrasting with the highest degradation rate observed for this material dried with chip on top of the paste. In contrast, for the drying method without die, sintered Ag shows even better cycling results than the solder alloys. Concerning the semiconductor material, the thermal cycling results show that the degradation rate is faster for SiC- than for Si-based test vehicles, while concerning strictly the solder alloys PbSnAg shows better results than AuGe. In this sense, a thermo-mechanical simulation work was performed to understand the degradation mechanisms during thermal cycling. The simulations demonstrated that the higher normal stresses associated with temperature swings were located at the die-attach/substrate interface, with the maximum values in the corners of the die and its periphery. These results qualitatively explain the SAM images showing concentric die-attach delamination patterns and the fact that "stiffer" materials (with higher Young modulus, such as SiC and AuGe) show faster degradation rates