Ceramic Substrate
Ceramic substrate is a substrate embed with material like alumina, aluminum nitride, boron nitride, and silicon nitride. Ceramic substrates have been applied in electronic components for decades from electronic integrated circuits to power modules. The ceramic substrate has multiple functions. Combining properties of mechanical strength and thermal management, and electrical isolation. The superior structural integrity, insulation electric properties, and heat dissipation performance support reliable operation in demanding environments. While heat dissipate can conduction heat loss in the chip and improve system performance. Thermal management, heterogeneous systems and temperature resistance decide ceramic substrate technology. The thermal expansion of substrate must consistent with other parts materials. It also remain chemical stability with increasing temperature. In terms of electrical properties , ceramic substrate should high resistance to prevent current loss. Strong dielectric insulation enables the substrate to handle elevated voltages, whereas a low dielectric constant contributes to lower parasitic capacitance and improved signal integrity.
Types of ceramic substrate
Alumina
Alumina oxide is cheapest material which used in high power RF electronic circuits and multi chip module. Owing to its electrical insulation (volume resistivity >10¹⁴ Ω·cm), relatively high thermal conductivity (~28 W/m·K), high dielectric strength (~10 kV/mm), and outstanding thermal stability. The physical properties of alumina is dependent on chemical purity and microstructual density. Alumina is valued for its diverse set of physical, thermal, electrical, and mechanical characteristics. The melting point up to 2054℃, and the chemical properties are stable. The excellent properties like strength, hardness and abrasion resistance, making it suit for applications such as abrasives, bearings, cutting tools. Due to the high electric resistance, alumina is often used in pure form as part of electrical components.
Aluminum nitride
Aln is regarded as one of the most advanced ceramic substrate material, providing high thermal conductivity with excellent electrical insulation, low dielectric constant, silicon matched thermal expansion, dependable mechanical properties, mechanical strength, and a low dielectric constant. Although the theoretical thermal conductivity of AlN at 300 K is approximately 320 W/m·K, commercially densified AlN ceramic typically exhibit values ranging from 80 to 260 W/m·K, depending on material purity and processing conditions[1]. The aluminium conductivity decided by presence of impurities, especially oxygen. The thermal expansion of AIN is 4- 5 x 10- 6 K – 1, which similar to silicon. The thermal compatibility enables aluminium nitride substrate to support direct die attachment in advanced VLSI application packaging. Compared with alumina and beryllia, AlN exhibits slightly lower electrical resistivity; however, its value of around 10¹³ Ω·cm is still sufficiently high for most electronic substrate and packaging applications. With a dielectric constant near 8.8 and a breakdown strength of approximately 14 kV/mm. The dielectric constant of AIN increases with temperature at high frequency. Other property of aluminum is high strain in elevated temperature. In addition, aluminium oxide lighter than other metal material.
Si₃N₄
Silicon nitride is a ceramic material composed of silicon and nitrogen atom linked by strong covalent bonds. The density of only 3.18 g/cm³, high hardness (3000–5000 HV), low thermal expansion (2.75 × 10⁻⁶ K⁻¹), and thermal conductivity of approximately 72 W/m·K. Its superior oxidation resistance, corrosion resistance, and fracture toughness enable reliable performance in harsh operating environments and high reliability applications. Hence, it is suit for advanced applications in gas turbine hubs and blades as engineering material. The high temperature strength and excellent creep resistance of Si₃N₄ and SiC support them reliable performance between 1400°C and 1500°C[2]. All the properties conclude thermal crack resistance and chemical stability match with ceramics, so silicon nitride also can be metal cutting tools.
SiC
Silicon carbide exhibits excellent physical and thermal characteristics. However, it has pros and cons. Nevertheless, it remains an excellent choice for extreme temperature condition and high power modules. On the other hand, it is impractical to grow SiC crystals from a molten phase, leading to small size of substrate. Silicon carbide (SiC) possesses a range of outstanding physical properties, including a wide bandgap of approximately 2.3–3.3 eV, a high critical breakdown electric field of (0.8–3) × 10⁶ V/cm[3], a high saturation drift velocity of about 2 × 10⁷ cm/s, and excellent thermal conductivity of around 4.9 W/(cm·K).
Ceramic Substrate Manufacturing Technologies
DBC
DBC technology is a way to bond copper to alumina and aluminum nitride. Meanwhile, Advanced copper to copper bonding technologies produced highly efficient liquid cooling systems with complex internal microchannel structures, enhancing thermal management for laser diodes and other high power electronics. DBC structures provide lower parasitic capacitance. A typical DBC substrate has a capacitance of only ~18 pF/cm², whereas IM substrates reach around 80 pF/cm² due to their thin polymer based insulation layer. DBC can undertake high current. For example, The bending strength of DBC is decided by ceramic strength and metallic thickness. A 1 mm wide copper conductor with a thickness of 0.3 mm, fabricated on a 0.63 mm thick Al₂O₃ ceramic substrate, experiencing only a 17°C temperature increase when operating at a continuous current of 100 A with conventional cooling. The flexural strength of DBC substrate increases with metal thickness.
AMB
Active material brazing is an evolution of Direct Bonded Copper (DBC) technology. AMB substrate are fabricated by active brazing filler alloys, such as Ag Cu Ti. Which form titanium based reaction layers at the ceramic nitride interface to achieve strong metallurgical bonding. The strong reaction between metal and ceramic making AMB substrate performance well in durability and thermal resistance than conventional DBC substrate. The maximize merit is decrease component distortion, low material dissolution, it is not easy for welding. However, the limitation is obvious. The poor interaction between liquid filler metals and ceramic surfaces, leading to limited spreading behavior and weak interfacial bonding. Furthermore, as result of low melting point of metal, the high temperature reliability and strength of brazed joints can be reduced.
DPC
Direct plating copper is a metal process in which copper is deposited onto a substrate without the use of a conventional copper seed layer. Conductive barrier materials such as ruthenium (Ru) or Ru based alloys are used to provide both adhesion and diffusion barrier functions. Compared with DBC and AMB technology, DPC substrates can achieve finer circuit pattern through photolithography processes, which meet the requirement of high precision packaging for micro scale devices. Meanwhile, the bonding strength between copper and ceramic in DPC substrates is stable. The process can be completed at relatively low temperature, which avoids thermal damage to ceramic materials.
Thermal Properties
For electronic packaging, the main target is transfer heat from semiconductor devices to external environment in thermal management. To be specific, the thermal path of an electronic systems includes heat conduction inside the package. Then, heat convey to the heat sink, through heat sink dissipate to the environment[4]. Thermal management rely on two kinds of materials. Substrate material is the foundation for mechanical support and electrical insulation. While the thermal interface material as a bridge to improve cooling performance. In addition, In order to decrease thermal stress from components or solder failure, the match of thermal expansion between semiconductor materials and ceramic substrate need to be attention.
Mechanical Properties
Mechanical stress can significantly affect device production, functional performance, and service lifetime in electronics. Parameters such as elastic modulus, fracture toughness, extensibility, and cohesive strength influence ability of a material to withstand mechanical deformation, interfacial stress, and thermal cycling. Moreover, sufficient extensibility and fracture resistance improve structural durability. In multilayer electronic systems, different thermal expansion coefficients may generate different stresses during temperature fluctuations[5]. Which produce toughness and energy absorption capacity to prevent crack formation, delamination, and mechanical failure.