OUSD (R&E) CRITICAL TECHNOLOGY AREA(S): Microelectronics OBJECTIVE: Development and characterization of new innovative encapsulation materials that are compatible with existing manufacturing methods, materials, and commercially available packages. Materials investigated shall be compared to the performance of established encapsulation materials used for high voltage power device packaging encapsulation. Investigation into the long-term stability of material performance when subjected to high temperatures (HT), high voltages (HV), wide frequency ranges, and high-pressure and humidity environments for packages intended for aircraft and spacecraft applications. DESCRIPTION: The long-term stability of encapsulation material properties (electrical, morphological, chemical, and mechanical) is a key factor for whole system reliability under operational and environmental constraints [7]. Silicones and epoxies are typically considered for embedding, potting, and/or encapsulating HV/HT electronic assemblies [4]. Soft dielectric materials, such as silicone gel, are used to encapsulate modules and prevent electrical discharges in air, as well as, to protect semiconductors, substrates, and connections against humidity, dirt, and vibration [4]. Embedding materials must be characterized for use in these types of HV/HT electronic assemblies. A focus in the characterization is the dielectric strength, which is influenced by the following factors: environmental exposure, electrode effects, temperature, voltage application, frequency, and specimen width [3]. The mechanical stresses experienced by packages can also significantly influence the dielectric properties of polymeric dielectrics [5]. In more compact packaging technology for high-power density wide band gap (WBG) devices, the local electric field can be enhanced, which may become large enough to raise partial discharges (PD); localized gaseous breakdown within the modules insulation system [1]. High activity of PDs damages the insulating silicone gel, leading to electrical insulation failure and reduction in the reliability of the module [1]. Partial discharge that occurs in micro-voids will cause accelerated aging and early failure. Voids inside the silicone gel significantly accelerate the aging of the materials even under normal operating electrical stress [1]. For these reasons, emphasis has been placed on the partial discharge, aging, and electrical treeing of semiconductors' packaging material [6]. Partial discharge has been recognized as a suitable technique to assess polymeric materials for insulation applications along with High-Current Arc Resistance to Ignition (HAI); a method that studies and assesses the electrical insulation flammability [3]. Soft encapsulation materials play a significant role in improving both semiconductor die and module package voltage ratings, especially under enhanced electrical and thermal constraints, by isolating the circuits from the effects of impurities and avoided fractures from thermomechanical stresses [7][2]. A variety of material innovations have been explored thus far, but further characterization and development is required before new materials can be used in practice. Some material solutions that have been explored are composite materials that offer the opportunity to provide a suitable product with the final application's required performance, thereby optimizing the price-performance ratio [3]. The emergence of micro and nano-based inorganic oxide fillers with optimal filler-loadings further enhances the required insulation characteristics of neat epoxy [5]. Another route investigated was applying functional materials on the highly stressed regions to reduce the electric field and the use of dielectric liquids which are incompressible, to fill voids and exhibit a self-healing effect [1][2]. While methods for achieving long-term stability of package encapsulate material have been explored through several means, full performance characterization of any proposed material advancement with consideration of extreme service conditions (HV, HT, and high moisture) is required prior to fielded application in aircraft or spacecraft. Electronic devices in aircraft are expected to meet operating temperature on order of 250C-300C and spacecraft and nuclear power systems requirements are on order of 200C-400C [4]. Consideration of the material and material application to device compatibility with existing manufacturing techniques is critical to reduce imposed cost of implementing material solutions. PHASE I: Perform a feasibility study of innovative or advanced materials that can be used for HT and HV applications in the field of aircraft and spacecraft, with consideration for characterizing the long-term stability of the material while exposed to HT, HV, various frequency ranges, and humid environments. Develop a testing plan and methodology that considers to operation conditions of interest (i.e. temperatures above 250C up to 400C and high humidity conditions), as well as HAI and PDs. Materials of interest shall target the following performance specifications: 1. Material(s) must be capable of operating at a temperature of 250C minimum and targeting operational temperatures as high as 400C 2. Coefficient of thermal expansion (CTE) values shall target values similar to those of typical substrate materials that would interface with the encapsulation materials (such as Al2O3, Copper, BeO, etc.), which typically have CTE values around 10 to 7 10^-6/C 3. Pass MIL-STD-202 Humidity (Steady State) condition A 4. Pass UL 746A High Amp Arc Ignition test PLC 0 5. Adhesion to a wide variety of substrates including metals, composites, glass, ceramics, and plastics 6. Show compatibility with existing manufacturing techniques PHASE II: Using the methods developed in Phase I, materials identified to be representative of current encapsulation materials and materials that could be applied to higher temperature (200C-400C) and higher operational voltages (5kV-20kV) shall undergo material characterization. Material performance characterization shall report performance in the following areas: 1. Electrical testing of material volume resistivity(ASTM D257), dielectric strength (ASTM D149), HAI and PD 2. Thermal testing to determine the coefficient of thermal expansion (CTE) (ASTM D696), conductivity (ASTMC177), and relative thermal index (UL 746). 3. Physical testing of heat deflection temperature and max service temperature 4. Material reliability in moisture or humid environments 5. The above tests shall also test for influence of voltage, frequency, environmental temperature and humidity have on material performance, stability and aging: a. Testing voltage ranges: 700V-1.7kV, 1.7kV-3.3kV, 3.3kV-10kV, 10kV-20kV, with a focus on the 10kV to 20kV range b. Stability under isothermal aging from: -40 up to 400C c. Frequencies up to 100kHz The performer is expected to test to the above value ranges or conditions. If unable to do so, justification for excluding the data set must be demonstrated. The performer is expected to show repeatability in the data collected as well as deliver the testing data and samples for which the experiments were performed, as applicable. PHASE III DUAL USE APPLICATIONS: The encapsulation materials developed can be marketed towards manufacturing and packaging industries and materials distributers for commercial application. Materials developed could be marketable toward DoD for use in DoD applications that are fielded in demanding environments/conditions. The material testing and characterization capability could also be marketed as a service to material design experts in industry. REFERENCES: 1. Ghassemi, Mona. (2018). Electrical Insulation Weaknesses in Wide Bandgap Devices. 10.5772/intechopen.77657. 2. Abdelmalik, Abdelghaffar & Liland, K.B.. (2020). Electric field enhancement control in active junction of IGBT power module. Journal of Physical Science. 31. 1-15. 10.21315/jps2020.31.3.1. 3. Haque, S. K. Manirul et al. Application and Suitability of Polymeric Materials as Insulators in Electrical Equipment. Energies 14 (2021): 2758. 4. Hopkins, D.C. & Bowers, J.S.. (2001). Characterization of advanced materials for high voltage/high temperature power electronics packaging. Conference Proceedings - IEEE Applied Power Electronics Conference and Exposition - APEC. 2. 1062 - 1067 vol.2. 10.1109/APEC.2001.912498. 5. Iqbal, Muhammad & Khattak, Abraiz & Ali, Asghar & Ullah, Nasim & Alahmadi, Ahmad & Khan, Adam. (2021). Influence of Ramped Compression on the Dielectric Behavior of the High-Voltage Epoxy Composites. Polymers. 13. 3150. 10.3390/polym13183150. 6. Chen, Chi, et al. "Transport Characteristics of Interfacial Charge in SiC Semiconductor Epoxy Resin Packaging Materials." Frontiers in Chemistry (2022): 316. 7. Locatelli, Marie-Laure & Khazaka, Rabih & Diaham, Sombel & Pham, Cong Duc & Bechara, Mireille & Dinculescu, S. & Bidan, Pierre. (2014). Evaluation of Encapsulation Materials for High-Temperature Power Device Packaging. Power Electronics, IEEE Transactions on. 29. 2281-2288. 10.1109/TPEL.2013.2279997 KEYWORDS: High voltage, package encapsulation, high temperature, partial discharge, electronics
