by Vijay Bolloju, Director R&D, iVP Semiconductor
Widespread electrification of everything is pushing the boundaries of Power Electronics systems. The demand for high power densities and lower weight in systems necessitates the use of novel materials.
The newer generation power semiconductor devices can operate at higher temperatures. Operating at higher temperatures can increase the power densities and reduce the Power device costs of the system. At the same time, it poses reliability concerns due to dielectric breakdown, deformation, and increased leakage currents due to ionic contamination of the moulding compounds. Packaging materials capable of reliably operating at higher temperatures are needed to exploit their capabilities to the fullest.
It is also evident from the recent trends that the operating voltages of systems like EVs, Data Centres, telecom, etc, are on the rise. Higher operating voltages warrant a higher degree of compliance for the safety of the users.
The cost breakdown of the high-power electronic systems shows that more than half of the cost comes from non-semiconductor materials. Materials such as plastics used for packaging, thermal interface materials (TIM), sealing compounds, heat dissipaters such as heat sinks, cooling liquids, substrates, connectors, etc.
Substrates play a major role in the thermal performance, structural stability, and reliability of the systems. FR4 PCBs have very poor thermal conductivity (0.24 W/m-K) and are commonly used for low-power systems. FR4 also has low Tg (~ 130 °C) and limits the operating range for the power semiconductors. These substrates are not recommended for high-power applications.
Aluminium Metal core PCBs (MCPCBs)are also widely used for building electronic circuits. These substrates have relatively higher thermal conductivity (2 W/m-K) and higher Tg. MCPCBs offer better mechanical stability and thermal performance. Though multi-layer MCPCBs are available, the most common MCPCBs are single-layer due to cost considerations. This will limit the ability to make the systems compact.
Ceramic substrates such as alumina (Al2O3), aluminium nitride (AlN) have excellent thermal conductivity and mechanical stability. Alumina has 100X higher thermal conductivity (24 W/m-K) and aluminium nitride has 1000X higher thermal conductivity (240 W/m-K) than FR4 material. They also render superior reliability and high-temperature operation capability. They are perfectly suited for high-power systems. They are also single-layer due to cost considerations.
The selection of the substrate materials should be based on the cost-performance criteria. Cost of the substrates increases in this order: FR4 PCBs, MCPCBs, and ceramic substrates. But the power semiconductor costs will reduce in the reverse order due to the improvement in the thermal conductivity. The reliability of the system also depends on the substrate choice – Ceramics offering the best, and FR4 the least. So, a sensible trade-off should be considered to make a suitable choice.
Thermal interface materials (TIM) also have a profound effect on the system performance, reliability, and cost. They are often neglected and not paid due attention. But they can really help in enhancing the thermal performance of the system and even reducing the number of power devices needed to implement the designs. TIMs also help in providing dielectric insulation to the system. So, an ideal TIM has high thermal conductivity and high dielectric strength. Choosing a proper TIM that meets the system requirements can help in reducing overall system cost and size.
Choosing proper substrate materials, TIM, and heat dissipator can reduce the system cost and size considerably and lead to frugal designs.
A holistic approach to design from the selection of power device technologies, substrates, TIM, and power dissipators may result in high-performance, reliable, and lower-cost systems.
Currently, the Indian materials ecosystem is poor and needs to be revamped to serve the power electronics industry to achieve higher performance metrics. The plastics, substrates, TIM, and other materials can be locally developed using advances in materials such as nano-materials, carbon compounds, engineering plastics, composite materials, etc. India has a mature ceramics industry serving the energy sector, the medical industry, etc. The technologies can be used to make substrate materials for power electronics applications. Metallization of the ceramic substrates to print the circuits is also an essential skill set to be developed.
High thermal conductivity composite materials, metal foam forming, and phase change materials can elevate the thermal performance of the systems. If the system can be cooled using advanced materials without the need for a liquid cooling system, it can considerably reduce the system cost and improve the reliability of the system.
All the materials described above that can improve system performance and reliability while reducing cost (Frugal innovations) can be developed and manufactured locally. A concerted and collaborative effort is all it needs.
