Etude du vieillissement de polymères isolants utilisés dans le packaging des modules de puissance haute température

La recherche permanente de l'intégration et/ou du fonctionnement dans des régions chaudes des dispositifs électroniques de puissance se traduit par une augmentation du niveau des contraintes électriques et thermiques imposées à tous leurs constituants. Cela concerne en particulier les constituants des modules de puissance. Comme suite à une étude bibliographique qui a permis d'analyser les différentes structures de packaging pouvant être adaptées à un fonctionnement à haute température, il ressort en particulier un besoin en couches diélectriques minces afin d'isoler les différentes parties du module. Dans ce contexte, les travaux ont porté sur la détermination de la limite d'utilisation en température de deux matériaux diélectriques polymères (un polyimide BPDA/PDA et un parylène fluoré PA-HT), pouvant être aptes à constituer la couche de passivation des puces de carbure de silicium, ou la couche intermétallique ou de protection de surface au sein des modules de puissance. Afin de parvenir à ce but, des caractérisations électriques à l'instant initial (t0) ont été menées sous hautes températures, jusqu'à 400 °C. Ensuite, l'évolution des propriétés (en particulier électriques) des matériaux durant le vieillissement thermique et thermo-oxydatif, à des températures supérieures ou égales à 250 °C, pour des milliers d'heures, a été mesurée et analysée. A t0, le champ de rupture moyen des matériaux reste élevé et supérieur à 2 MV/cm à 300 °C, pour les films les plus épais testés (8 µm). La conductivité DC, dans une gamme de température entre 300 °C et 400 °C, montre un comportement semi-résistif pour le BPDA/PDA et un comportement qui passe d'isolant à semi-résistif pour le PA-HT. Durant le vieillissement sous N2, aucune dégradation du BPDA/PDA n'est observée jusqu'à 360 °C. A 300 °C sous air, une stabilité de la tension de rupture lorsque ce dernier est vieilli sur substrat en silicium (Si), et une dégradation lente dépendante de l'épaisseur initiale lors du vieillissement sur substrat en acier inoxydable (A.I.) sont observées. La dégradation se révèle surfacique liée à la présence de l'oxygène ambiant. Elle est d'autant plus prononcée que la température du vieillissement augmente, et apparaît alors également sur les substrats en Si. Le PA-HT déposé sur un substrat en A.I. a été vieilli sous air entre 300 °C et 360 °C. L'étude montre que ces films paraissent prometteurs pour les applications à 300 °C, avec une cristallisation isotherme qui affecte favorablement les propriétés diélectriques du matériau. Pour les températures plus élevées, une dégradation activée thermiquement apparaît et les films inférieurs à 5 µm d'épaisseur, ne peuvent pas dépasser 1000 heures de vieillissement sous air à 360 °C. Par conséquent, en se basant sur les propriétés électriques intrinsèques ainsi que sur leur évolution en vieillissement isotherme, les films de BPDA/PDA et de PA-HT semblent appropriés pour fonctionner pendant de longues durées à 300 °C sous air. Pour les températures plus élevées (360 °C), la stabilité sous air pour de longues durées reste problématique en particulier sur A.I. Par ailleurs, des solutions permettant de limiter la dégradation thermo-oxydative ou paraissant plus prometteuses, ainsi que des traitements thermiques permettant l'amélioration de la résistivité électrique à haute température à t0 sont proposés.The trend for integration and/or high ambient temperature operation of power electronics modules induces higher electrical and thermal stresses on their components. Based on a bibliographic study that allows evaluating different structures of packaging able to operate at high temperatures, thin dielectric layers are needed in order to insulate the different parts of the module. Therefore, the aim of this work was to define the potentiality of two dielectric polymers to operate at high temperatures (the first one is a polyimide BPDA-PDA and the second one is a fluorinated parylene PA-HT), and to be used as passivation layer for silicon carbide semiconductors or as dielectric layer between and on the metal frames. In order to reach the objective, characterizations of the dielectric properties up to 400 °C at the initial time (noted as t0) were performed. Then, the properties evolution (especially electrical ones) during the thermo-oxidative aging for temperature higher than 250 °C and long periods (several thousands of hours) were controlled periodically. At t0, the films show a good dielectric strength and the breakdown field remain higher than 2 MV/cm for the thicker tested films (8 µm). The DC conductivity show semi-resistive values for the BPDA-PDA between 300 °C and 400 °C and the values vary between resistive and semi-resistive ones for the PA-HT in the same temperature range. During the aging under N2, no degradation is observed up to 360 °C for BPDA-PDA polyimide. At 300 °C in air, stability of the breakdown voltage is observed when the BPDA-PDA is aged on Si substrate, while a slow degradation depending on the initial thicknesses is observed for films deposited on stainless steel substrate (S.S.). This degradation, related to the oxygen presence in air, affect the surface layer and is thermally activated. The degradation appears also for BPDA-PDA on Si substrate at 360 °C in air. The PA-HT films were deposited on S.S. substrates and aged in air at 300 °C, 340 °C and 360 °C. Results show the potentiality of the material for 300 °C application, with the occurring of cold crystallization that improves the low field dielectric properties. For the higher tested temperatures, thin films (5 µm) seem to be unsuitable for long periods applications and cannot pass 1000 hours at 360 °C. Hence, based on the initial dielectric properties and their evolution during the aging, the two polymers seems to be suitable for 300 °C applications. However, for higher temperatures (360 °C), the stability in air of the two materials, especially on the S.S. substrate is not insured. Otherwise, solutions against the thermo-oxydative aging seem promising, and thermal treatments allowing the improvement of the electrical resistivity at the initial time are proposed

Real-time crystallization in fluorinated parylene probed by conductivity spectra

Dielectric relaxation spectroscopy experiments were performed at high temperature on fluorinated parylene films during the occurrence of the isothermal crystalline phase transition. For this polymer, since the difference between the glass transition temperature (Tg ) and the phase transition temperature (Tc ) is very strong (Tc  ≥ 4Tg ), segmental and dipolar relaxation usually used to probe the crystallization are not shown in the experiment frequency window (10−1 to 106 Hz) during the crystallization. The charge diffusion becomes the only electrical marker that allows probing the phase transition. During the transition phase, a continuous decrease of about two orders of magnitude is observed in the conductivity values below an offset frequency (fc ) with a tendency to stabilization after 600 min. Below the offset frequency, the decrease of the normalized conductivity to the initial value as function of time is frequency independent. The same behavior is also observed for the fc values that decrease from 160 Hz to about 20 Hz. Above the offset frequency, the electronic hopping mechanism is also affected by the phase transition and the power law exponent (n) of the AC conductivity shows a variation from 0.7 to 0.95 during the first 600 min that tend to stabilize thereafter. Accordingly, three parameters (n, fc , and AC conductivity values for frequencies below f c) extracted from the AC conductivity spectra in different frequency windows seem suitable to probe the crystalline phase transition

Dielectric strength of parylene HT

The dielectric strength of parylene HT (PA-HT) films was studied at room temperature in a wide thickness range from 500 nm to 50 μm and was correlated with nano- and microstructure analyses. X-ray diffraction and polarized optical microscopy have revealed an enhancement of crystallization and spherulites development, respectively, with increasing the material thickness (d). Moreover, a critical thickness dC (between 5 and 10 μm) is identified corresponding to the beginning of spherulite developments in the films. Two distinct behaviors of the dielectric strength (FB ) appear in the thickness range. For d ≥ dC , PA-HT films exhibit a decrease in the breakdown field following a negative slope (FB  ∼ d −0.4), while for d < dC , it increases with increasing the thickness (FB  ∼ d 0.3). An optimal thickness doptim  ∼ 5 μm corresponding to a maximum dielectric strength (FB  ∼ 10 MV/cm) is obtained. A model of spherulite development in PA-HT films with increasing the thickness is proposed. The decrease in FB above dC is explained by the spherulites development, whereas its increase below dC is induced by the crystallites growth. An annealing of the material shows both an enhancement of FB and an increase of the crystallites and spherulites dimensions, whatever the thickness. The breakdown field becomes thickness-independent below dC showing a strong influence of the nano-scale structural parameters. On the contrary, both nano- and micro-scale structural parameters appear as influent on FB for d ≥ dC

Evaluation of direct printed heat sinks on metallized ceramic substrate for high-performance power modules

In this article, we propose a new packaging technology enabling the development of a high-performance power module for harsh environments. This approach is based on the use of the selective laser melting (SLM) technique in order to directly print metal heat sinks on the backside of the metallized substrate. In order to explore the viability of this method, the assembled parts were evaluated thoroughly after the manufacturing process. Moreover, their robustness was assessed during aging under harsh conditions. Results show that the ultimate tensile strength and yield strength of the printed alloy are higher than the casted AlSi7Mg0.6 counterpart. The interfaces between the printed alloy and the substrate Al metal layer do not show any weaknesses, and shear stress values are higher than 100 MPa. For all heat sink patterns, the substrate warpage is reduced during thermal cycling due to the Al alloy creeping, while the highly curved substrates show cracks in the ceramic after 400 cycles. Accordingly, direct printing of heat sink with patterns based on fins array reveals a promising path for highreliability, high-performance power module packaging

Direct heat sink printing on metallized ceramic substrate for power electronics applications: heat treatment identification and its impacts

The aim of this paper is to evaluate a new packaging technology developed for high power density and harsh environment power module applications. The assembly was achieved using Selective Laser Melting (SLM) technique in order to directly print AlSi7Mg0.6 alloy heat sinks on the back side of direct bonded aluminum metallized substrate. Thermal simulations were conducted in order to evaluate the potential benefit of this technology. Results show a reduction of more than 22% of the junction to ambient thermal resistance compared to conventional structure. Experiments were conducted on both aluminum and nickel finish metallization of the Direct Bonded Aluminum (DBA) substrates. The assemblies were studied under several scales from the metallurgical and mechanical study of the interfaces to the measurement of the macroscopic strains of the substrates. The heat treatment temperature of 250°C has been identified based on the hardness versus temperature curve of the AlSi7Mg0.6 alloy. After a stress relieve thermal treatment of 2 hours at 250°C, the warpage of highly deformed substrates with printed heat sink is reduced significantly (more than 30% in some cases) without altering the metallurgy and the mechanical properties of the interfaces for both Ni and Al finish layers. The thermal conductivity of the printed alloy is improved by about 20% at temperatures below 100°C after the heat treatmen

Étude du vieillissement de polymères isolants utilisés dans le packaging des modules de puissance haute température

TOULOUSE3-BU Sciences (315552104) / SudocSudocFranceF

Evaluation of the thermal contact resistance between bulk copper and metal foam using transient measurements

International audienc

Partial Discharges and Electroluminescence Measurements on Power Electronic Substrates Embeded in Liquids

This paper presents an experimental study of partial discharges (PD) and electroluminescence properties of dielectric liquids (dibenzyltoluene & silicone oil) embedding a Silicon Nitride (Si3N4) ceramic substrate. With silicone oil partial discharges appear above 3 kV whereas no PD activity is recorded with dibenzyltoluene up to 6 kV. This indicates that with this Si3N4 substrate, partial discharges actually originate from the liquid, contrary to previous similar measurements carried out with other ceramics (Al2O3, AlN). Optical light detection and visualization also shows that DBT is highly electroluminescent. Silicone oil and Si3N4 show no electroluminescence, making it possible to study PD activity with a high sensitivity via measurement of emitted light. Since light measurements can be done whatever the shape of voltage wave, contrary to electrical PD recording, the detection of PDs with fast rise voltage becomes possible

Assessment of contribution of EHD to the cooling of power semiconductor devices immersed in dielectric liquids

International audienceThis paper presents an experimental study of EHD heat transfer enhancement in dielectric liquids, and an evaluation of its contribution to cooling of power electronic components. Heat transfer measurements are carried out in a vertical coaxial electrode system filled with benzyltoluene liquids. The influence of voltage wave shape (AC, DC, impulses), frequency, and liquid viscosity on heat transfer enhancement are measured. The optimal heat transfer occurs for a specific frequency with ac and impulse voltages, and liquids of lower viscosity. Unfortunately, in a typical power electronics module geometry (power diode on a ceramic substrate), almost no improvement of the diode cooling is recorded when this structure is filled with a liquid

Etude des potentialités du Parylene HT comme isolant au sein des modules de puissance Haute Température

Cette présentation a pour objectif d'évaluer le parylène AF4 en tant que candidat à l'isolation électrique 'haute température' au sein des nouvelles structures de modules de puissance. La propriété de rigidité diélectrique et la stabilité thermique du Parylene HT (forme commerciale du parylène AF4) sont plus particulièrement détaillées ici, en fonction de l'épaisseur du film déposé, de sa température, et du temps de stockage dans l'air. Un lien avec l'état de la structure semi-cristalline du matériau est proposé. La durabilité des propriétés physico-chimiques et diélectriques du Parylene HT, au-delà de 2000 heures dans l'air à 300 °C, est présentée pour la première fois. L'étude des propriétés électriques du parylène AF4 dans une gamme de température étendue jusqu'à 400 °C, constitue plus généralement l'originalité des travaux menés