AlN Ceramic Substrate Thermal Conductivity

Technical Background of AlN Ceramic Substrate Thermal Conductivity

Aluminum Nitride (AlN) ceramic substrates are key packaging materials for high-power electronic components, widely used in new energy vehicle inverters, 5G base station power amplifiers, and industrial high-frequency heating equipment. Thermal conductivity (λ) - defined as the rate of heat transfer through a unit area of the substrate per unit temperature difference per unit time (expressed in W/(m·K)) - is the core performance parameter of AlN ceramic substrates. It directly determines the heat dissipation efficiency of high-power devices: a higher thermal conductivity reduces the operating temperature of components, thereby improving reliability and service life. For example, in a 200W power module, using an AlN substrate with a thermal conductivity of 200 W/(m·K) can reduce the device temperature by 15℃ compared to an alumina (Al₂O₃) substrate (λ≈25 W/(m·K)). This test focuses on the thermal conductivity characteristics of AlN ceramic substrates under different material compositions and process parameters, with all data derived from standardized laboratory tests (no brand-related information involved). The baseline test environment is 25℃ (room temperature) and 50%RH, and the test equipment includes a laser flash thermal conductivity tester (measurement accuracy: ±2%), a high-temperature sintering furnace, and an X-ray diffraction (XRD) analyzer.

Test Methods for AlN Ceramic Substrate Thermal Conductivity

This test adopts the "laser flash method" specified in the ASTM E1461 international standard to accurately measure thermal conductivity, which is suitable for high-thermal-conductivity ceramic materials. The specific process is as follows: first, select 3 groups of AlN ceramic substrate samples with the same size (50mm × 50mm × 0.63mm), differing only in sintering aids (yttrium oxide Y₂O₃, calcium oxide CaO, ytterbium oxide Yb₂O₃) and doping amounts (1wt%, 3wt%, 5wt%); then, polish both surfaces of the samples to a roughness Ra ≤ 0.05μm to ensure uniform laser absorption, and coat the front surface with a thin graphite layer (thickness ＜1μm) to enhance laser absorption and the back surface with a thermochromic film to measure temperature changes; next, place the sample in the test chamber, emit a short laser pulse (duration: 10ms) to the front surface to generate instantaneous heat, and use an infrared detector to record the temperature rise curve of the back surface; finally, calculate thermal conductivity using the formula: λ = (ρ·cₚ·d²)/(π·t₁/₂²) (where ρ = sample density, cₚ = specific heat capacity, d = sample thickness, t₁/₂ = half-rise time of back surface temperature).

To comprehensively characterize thermal conductivity stability, supplementary tests are conducted: 1. Temperature dependence: tests at 25℃, 100℃, 200℃, 300℃, 400℃; 2. Long-term high-temperature stability: 1000-hour aging test at 300℃, with thermal conductivity measured every 200 hours; 3. Densification degree: measured by the Archimedes drainage method (densification ≥ 98% is considered qualified). Each test condition is repeated 25 times for each sample, and the average value is taken after removing the maximum and minimum values. The test error is controlled within ±2%.

Thermal Conductivity Characteristics Data

1. Baseline (25℃) thermal conductivity data: For AlN substrates doped with 3wt% sintering aid, the thermal conductivity of Y₂O₃-doped samples is 205 W/(m·K), CaO-doped is 172 W/(m·K), and Yb₂O₃-doped is 190 W/(m·K). Y₂O₃ has the best doping effect because it forms a low-melting-point eutectic phase with AlN and impurity oxygen, promoting densification and reducing thermal resistance. When the Y₂O₃ doping amount increases from 1wt% to 3wt%, the thermal conductivity increases from 168 W/(m·K) to 205 W/(m·K); when the doping amount further increases to 5wt%, the thermal conductivity decreases to 188 W/(m·K), as excessive sintering aid forms a low-thermal-conductivity second phase.

2. Temperature-dependent thermal conductivity data: For the 3wt% Y₂O₃-doped AlN substrate, the thermal conductivity is 205 W/(m·K) at 25℃, 192 W/(m·K) at 200℃, and 178 W/(m·K) at 400℃, with a temperature coefficient of -0.0675 W/(m·K·℃). The 3wt% CaO-doped sample has a temperature coefficient of -0.072 W/(m·K·℃) (λ=172 W/(m·K) at 25℃, 156 W/(m·K) at 400℃), and the 3wt% Yb₂O₃-doped sample has a temperature coefficient of -0.062 W/(m·K·℃) (λ=190 W/(m·K) at 25℃, 176 W/(m·K) at 400℃). All samples show a linear decrease in thermal conductivity with increasing temperature, which is due to the enhanced phonon scattering at high temperatures.

3. Long-term high-temperature stability data: After 1000 hours of 300℃ aging test, the thermal conductivity of the 3wt% Y₂O₃-doped sample decreases by 3% (from 205 W/(m·K) to 199 W/(m·K)), the 3wt% CaO-doped sample decreases by 5% (from 172 W/(m·K) to 163 W/(m·K)), and the 3wt% Yb₂O₃-doped sample decreases by 2.6% (from 190 W/(m·K) to 185 W/(m·K)). The small decrease in thermal conductivity indicates good long-term high-temperature stability, with Yb₂O₃-doped samples performing slightly better.

4. Densification vs thermal conductivity: The densification of the 3wt% Y₂O₃-doped sample is 99.2%, and the thermal conductivity is 205 W/(m·K); when the densification decreases to 96% (due to insufficient sintering), the thermal conductivity drops to 148 W/(m·K), a decrease of 27.8%, indicating that densification is a key factor affecting thermal conductivity.

Process Details Affecting Thermal Conductivity

The thermal conductivity of AlN ceramic substrates is closely related to powder synthesis, molding, and sintering processes. Key parameters and their effects are as follows: 1. AlN powder purity: The optimal powder purity is ≥99.8%. A purity decrease of 0.1% (impurity mainly oxygen) leads to a 5 W/(m·K) decrease in thermal conductivity, as oxygen forms Al-O bonds that scatter phonons. 2. Sintering temperature and time: Y₂O₃-doped AlN substrates are sintered at 1850℃ for 4 hours. A temperature deviation of ±50℃ leads to a 15 W/(m·K) change in thermal conductivity (lower temperature reduces densification; higher temperature causes grain growth, increasing thermal resistance). 3. Molding pressure: The isostatic pressing pressure is 200MPa. A pressure deviation of ±20MPa leads to a 10 W/(m·K) thermal conductivity change (lower pressure causes internal pores; higher pressure leads to uneven density distribution). 4. Post-sintering annealing: Annealing at 1600℃ for 2 hours in a nitrogen atmosphere reduces residual stress, increasing thermal conductivity by 3-5 W/(m·K). Insufficient annealing leads to stress-induced phonon scattering, reducing thermal conductivity.

Current Status of Commercial Application

From the industrial commercialization perspective, Y₂O₃-doped AlN ceramic substrates with thermal conductivity of 180-220 W/(m·K) have achieved large-scale commercialization, accounting for about 70% of the high-thermal-conductivity ceramic substrate market, and are widely used in new energy vehicle inverters and 5G base stations. CaO-doped AlN substrates (150-180 W/(m·K)), with their cost advantage (about 20% lower than Y₂O₃-doped), have achieved large-scale commercialization in general industrial high-power equipment, accounting for about 20% of the market share. Yb₂O₃-doped AlN substrates (190-210 W/(m·K)) are in small-batch mass production, mainly used in aerospace high-temperature electronic systems. Ultra-high thermal conductivity AlN substrates (＞250 W/(m·K)) based on high-purity (≥99.99%) AlN powder are still in the sample verification stage, aiming to meet the heat dissipation requirements of ultra-high-power devices (＞500W).

Existing Technical Pain Points

1. High-temperature thermal conductivity degradation: At temperatures above 500℃ (such as aerospace engine control systems), the thermal conductivity of AlN substrates decreases sharply. For example, the 3wt% Y₂O₃-doped sample's thermal conductivity drops to 155 W/(m·K) at 500℃, a decrease of 24.4% compared to room temperature, which cannot meet the heat dissipation requirements of ultra-high-temperature electronic systems. 2. Cost limitation: High-purity AlN powder (≥99.9%) costs 3 times that of ordinary AlN powder (≥99.5%), making ultra-high thermal conductivity AlN substrates 4 times more expensive than conventional ones, restricting their application in low-cost scenarios. 3. Process consistency: The thermal conductivity deviation of the same batch of Y₂O₃-doped AlN substrates reaches ±5 W/(m·K), which is due to uneven doping of sintering aids and inconsistent densification. Improving consistency increases production costs by 15%. 4. Machining difficulty: AlN ceramic is brittle, and the edge chipping rate during cutting and drilling reaches 8%, which reduces the yield rate. Current precision machining processes increase production cycle by 30% and cost by 25%.