Enhanced Breakdown Voltage for All-SiC Modules HINATA, Yuichiro * TANIGUCHI, Katsumi * HORI, Motohito * A B S T R A C T In recent years, SiC devices have been widespread mainly in fields that require a breakdown voltage of approximately 1 kv. They are expected to be used in the high voltage fields that require a breakdown voltage from 3 to 1 kv such as railways, as well as the automotive field that require high reliability such as hybrid vehicles and electric vehicles. Fuji Electric has developed a newly structured package featuring copper pin connections and resin molding to achieve SiC modules with high breakdown voltage. Based on the results of electric field simulations and thermal analysis, the electric field strength relaxation and high heat radiation are achieved by the optimization of the positioning and thickness of electrodes on the insulation. 1. Introduction As interest in environmental issues including global warming is increasing, reduction of emissions of greenhouse gases such as CO 2 is called for, and it is expected that high efficient power conversion technologies realize energy saving. Power semiconductors play a major role in power conversion equipment. Silicon (Si) semiconductor devices, which have been the mainstream, have improved over many years and their performance is approaching the theoretical limits based on their physical properties. Accordingly, wide band gap semiconductor devices such as silicon carbide (SiC) and gallium nitride (GaN) are being developed vigorously. In particular, SiC devices are capable of dramatically reducing the loss and expected to contribute toe energy saving by decreasing the losses of power electronics products. At present, they are becoming widespread in fields that require a breakdown voltage of approximately 1 kv, such as power conditioning subsystems (PCSs) for photovoltaic power generation and power supplies for data servers. In the future, it is expected that SiC devices will be employed in fields that require high reliability such as hybrid electric vehicles and electric vehicles and high-voltage fields from 3 to 1 kv such as railways. Fuji Electric has developed a newly structured package consisting of copper pin connections and resin molding for All-SiC modules in place of conventional structures consisting of wire bonding and silicone gel molding. By applying these technologies, enhanced breakdown voltage for All-SiC modules are realized. 2. Basic Module Structure and Issues to be Resolved for Increasing Breakdown Voltage As shown in Fig. 1, the structure of an All-SiC module is significantly different from that of a conventional silicon insulated gate bipolar transistor (Si-IGBT) module (1),(2). For developed All-SiC module, copper pins formed on the power are used as joint technology instead of conventional aluminum wire. This structure enables loading high current and high-density packaging of SiC devices. As the ceramic insulating to mount semiconductor chips, silicon nitride (Si 3N 4) insulating with a Silicone gel Copper pin Front copper plate Back copper plate insulating (a) Developed structure (All-SiC module) Aluminum wiring Semiconductor chip Power insulating Terminal Resin case Copper base issue: Power Semiconductors Contributing in Energy Management Solder (b) Conventional structure (Si-IGBT module) * Electronic Devices Business Group, Fuji Electric Co., Ltd. Fig.1 Module structure 227
thicker copper plate compared with conventional has been used to reduce the thermal resistance. In addition, application of epoxy resin instead of the conventional silicone gel as a molding resin prevents degradation of the solder layer and deterioration of the insulation performance in high-temperature operation, achieving high reliability. For long-term usage of power semiconductor modules, it is necessary to ensure stable insulation performance against thermal stress and voltage variations depending on the usage condition and environment. For the insulation design of power semiconductor modules, the breakdown electric field is one of the important factor. Electric field strength is greatly affected by the voltage applied to the materials, the shapes of the constituent materials and dielectric constant. In addition, electric field strength generally increases at the defects of the molding material, such as voids and peeled parts, and the edge of copper electrodes on ceramics insulating s. For silicone gel that is used as the molding resin in conventional structure, voids or cracks tend to be generated in operation at a high temperature of 175 C or higher, and that possibly causes breakdown. For that reason, determination of an appropriate molding resin and a ceramic is important in order to develop All-SiC packages with high breakdown voltage capability of operation at high temperatures. Furthermore, it is necessary to develop the structure that enables the electric field mitigation of the boundary region of power s and ceramic insulating s. 3. Package Design Technology for high Breakdown Voltage 3.1 Package design relating to insulation performance Regions with high electric field strength in a semiconductor module tend to be located in the insulators, such as epoxy resin and ceramics, at the edge of a copper plate or at the edge of a semiconductor chip surface. Breakdown modes of power semiconductor modules are classified into ceramic penetration breakdown originating from high electric field strength point and creeping breakdown along joint region between the epoxy resin and the surface of the insulators, such as copper plate and ceramics. We focused on the triple points between the copper plates, epoxy resin, and ceramics because of their high electric field and performed electric field simulation. Figure 2 shows the electric field strength distributions of a power module with same length from the edge of ceramics to both copper plate [see Fig. 2 (a)], and with different length from the edge of ceramics to both copper plates [see Fig. 2 (b)]. In both cases, the electric field simulation are performed under the same condition for the thickness and type of ceramic, the thickness of the copper plate, and the type of epoxy resin. The results of the simulation indicate that the Cross-section view of analysis model (a) Equal distances from edge of ceramic to edges of front and back copper plates Front copper plate Back copper plate insulating Electric field Strong Weak Triple point (b) Different distances from edge of ceramic to edges of front and back copper plates Fig.2 Results of electric field simulation (electric field strength distributions) highest electric field strength point is located at the triple point between front copper plate, ceramics and epoxy resin. Figure 3 shows the maximum electric field strength change in both cases of power modules: to change the ceramic thickness from that in Fig. 2 (a) and to change the position of the surface copper plate from that in Fig. 2 (b). From the simulation results, increasing the thickness of the ceramic and equalizing the distances between the edge of the ceramic and both side of copper plates lead to the mitigation of the electric field strength. However, thick ceramics degrades the heat dissipation performance of the module. In addition, Electric field strength (a.u.) Electric field mitigation.5 thickness (a.u.) (a) Effect of ceramic thickness Position difference Front copper plate Back copper plate Electric field mitigation.5 Position difference (a.u.) (b) Effect of positions of surface and back copper plates Fig.3 Results of electric field simulation (changes in electric field strength) Electric field strength (a.u.) 228 FUJI ELECTRIC REVIEW vol.62 no.4 216
the change of the thickness or position of the copper plates may leads large thermal stress due to the difference in the coefficient of thermal expansion of materials, causing the thermal deformation of the ceramic insulating. This possibly causes cracks in the ceramic, leading to degrade the insulation performance. The thermal resistance of ceramics generally accounts for 2% to 3% of the thermal resistance of power module. As shown in Fig. 3, electric field strength varies more greatly in a region where the ceramic is thinner, and increasing the thickness of ceramic can reduce the electric field strength to less than half of the maximum value. However, the thermal resistance of the ceramic increases nearly in proportion to the thickness and the heat dissipation performance is significantly deteriorated. Accordingly, the structural design that optimizes insulation and heat dissipation performance is required. Solder insulating Thermal grease Cooling fin (a) Developed structure (without copper base) Temperature High Solder insulating Copper base Thermal grease Cooling fin (b) Conventional structure (with copper base) Low Fig.4 Results of thermal analysis (temperature distributions) Thermal resistance Rth (j-c) (a.u.) Solder under chip Front copper plate Back copper plate Solder under insulating Copper base insulating 5.5 5.5 Developed structure Conventional structure thickness (a.u.) Fig.5 Relationship between insulating thickness and thermal resistance 3.2 Package structure with high heat dissipation performance We carried out thermal analysis for the conventional and developed structures. Figure 4 shows the temperature distributions. In the developed structure, the thickness of the front copper plate under the chip decrease the thermal resistance of the module because heat diffusion in the in-plate direction within the front copper plate lead to a reduction in the thermal resistance of ceramic with low heat conductivity (3)-(5). Figure 5 shows the relationship between the ceramic thickness and thermal resistance. The developed structure allows thermal resistance to be significantly reduced compared with that of a conventional structure. This achieves both high insulation and high heat dissipation performance even if the ceramic thickness is increased to improve insulation performance. On the other hands, the effect of the reduction of the thermal resistance depends on the semiconductor chip size and heat conductivity of ceramics. Accordingly, we maximize the reduction of the thermal by optimizing the module structure depending on the current and voltage ratings. 4. Evaluation of Molding Resin for Enhanced Breakdown Voltage Initial breakdown voltage testing and high-temperature and voltage application testing at humidity environment for a long time were conducted to evaluate the insulation performance of modules. In particular, assuming operating conditions at high-temperature and high-voltage environment, the breakdown voltage of silicone gel used for conventional structures decreases as the temperature increases. Meanwhile, the deterioration of the insulation performance of epoxy resin at high-temperature condition is smaller than those of silicone gel. Therefore, epoxy resin is superior to use in a high-temperature and high-breakdown-voltage environment. 4.1 Insulation evaluation of molding resin We compared the insulation performance of silicone gel used for the conventional structures and epoxy resin molding used for the developed structure. We prepared test samples that have the same shape of ceramic and copper plate and different molding materials (see Fig. 6), applied a voltage across the terminals bonded with the surface electrode and the back electrode, and measured the breakdown voltage. Figure 7 and Fig. 8 show the relationship between the breakdown voltage and cumulative breakdown rate and the breakdown points respectively. When the cumulative breakdown rate is 1%, the breakdown voltage of epoxy resin is 16.3 kv, which is approximately 1.9 times as high as that of silicone gel, 8.8 kv. The breakdown for silicone gel molding proceed in the silicone gel from the triple points between the front copper plate, ceramic and silicone gel to the back copper plate. On the other issue: Power Semiconductors Contributing in Energy Management Enhanced Breakdown Voltage for All-SiC Modules 229
Front electrode Fig.6 Test sample shape Cumulative breakdown rate (%) 99.9 95. 8. 5. 1. 5..5.1.5.1 Silicone gel Molding resin (silicone gel or epoxy resin) Case insulating Back electrode If partial discharge is generated, degradation of encapsulation material originated from the discharge point is propagated, and that is likely to result in a breakdown after the long term operation. Defective products can be identified and eliminated by verifying the generation of a partial discharge, and that prevent a breakdown of the products. Figure 9 shows the results of partial discharge testing of test samples using silicone gel molding and epoxy resin molding. The voltage at which electric charges start to increase as the voltage rises is defined as the partial discharge inception voltage (PDIV), and the voltage at which electric charges decrease to zero as the voltage drops is defined as the partial discharge extinction voltage (PDEV). For silicone gel molding, the PDIV was 7 kv. Meanwhile, With epoxy resin molding, no partial discharge occurred even at 1 kv, indicating it is less likely to generate a partial discharge compared with silicone gel molding. Figure 1 shows the PDIV and PDEV observed in the repeated partial discharge testing. For the sample with epoxy resin molding, partial discharge was generated not in the molding resin but along the outside of the case at approximately 15 kv. The graph uses the 1 2 5 1 2 3 5 8 Breakdown voltage (kv) 1, Voltage rise Voltage drop 1, Voltage rise Voltage drop Fig.7 Relationship between breakdown voltage and cumulative breakdown rate Copper base, silicone gel (transparent) Electric charge q(pc) 1 1 Electric charge q(pc) 1 1 No discharge up to 1 kv Breakdown point 1 5 1 Applied voltage (RMS value) (kv) (a) Silicone gel molding 1 5 1 Applied voltage (RMS value) (kv) (b) molding (a) Silicone gel molding (enlarged photo of top surface) Fig.8 Breakdown points Front copper plate (b) molding (photo after polishing surface copper plate) hands, for the epoxy resin, the breakdown is due to ceramic penetration. This indicates that the insulation performance of the epoxy resin molding is determined by the breakdown capability of the ceramic insulating itself, and improving the thickness and breakdown voltage of the ceramic allows the breakdown voltage to be further enhanced. 4.2 Life expectancy evaluation of molding resin As a method of evaluating the long-term product lifetime based on an initial product evaluation, it is effective to investigate the existence of partial discharge. Fig.9 Results of partial discharge testing on test samples Voltage (a.u.) Partial discharge inception voltage Partial discharge extinction voltage molding Silicone gel molding 1 2 3 4 5 6 Number of repetitions Fig.1 Partial discharge inception voltage and partial discharge extinction voltage 23 FUJI ELECTRIC REVIEW vol.62 no.4 216
values observed in the test. The PDIV of epoxy resin exhibits twice higher than that of silicone gel. In silicone gel molding, once partial discharge is generated, the PDIV gradually decreases as the number of repetitions increases. It is assumed that voids resulting from cracks in the silicone gel originating from the discharge points or bubbles due to the generation of cracked gas are generated, and that lead to degradation propagating in the silicone gel or along the boundary between the silicone gel and ceramic. Meanwhile, in the epoxy resin, partial discharge at the same testing voltage does not generate. Therefore, we conclude that degradation due to partial discharge is not likely to occur in long time operation, and the molding resin is a promising technology to enhance the breakdown voltage of SiC devices. 5. Postscript This paper has described the methodologies to enhance the breakdown voltage for All-SiC modules. The effect of the structure of the power module on the mitigation of electric field strength and heat dissipation performance has been studied based on simulation. Furthermore, we investigated the difference in insula- tion performance depending on encapsulation material. In the future, by expanding the application area of All-SiC modules with enhanced breakdown voltage by further improving their reliability, we will contribute to the development of power electronics technology and the realization of a low-carbon society. References (1) Nakamura, H. et al. All-SiC Module Packaging Technology. FUJI ELECTRIC REVIEW. 215, vol.61, no.4, p.224-227. (2) Nashida, N. et al. All-SiC Module for Mega-Solar Power Conditioner. FUJI ELECTRIC REVIEW. 214, vol.6, no.4, p.214-218. (3) Horio, M. et al. New Power Module Structure with Low Thermal Impedance and High Reliability for SiC Devices, Proceedings of PCIM, 211, p.229-234. (4) Ikeda, Y. et al. Investigation on Wirebond-less Power Module Structure with High-density Packaging and High Reliability, Proceedings of ISPSD, 211, p.272-275. (5) Horio, M. et al. Ultra Compact and High Reliable SiC MOSFET Power Module with 2 ºC Operating Capability, Proceedings of ISPSD, 212, p.81-84. issue: Power Semiconductors Contributing in Energy Management Enhanced Breakdown Voltage for All-SiC Modules 231
*