PCB Thermal Design Basics (1): Substrate Material Selection

Introduction

Thermal design is a first-order driver of product reliability, performance, and cost. Long before heat sinks, heat pipes, or airflow are considered, the substrate material sets the baseline for how heat is generated, spread, and constrained on the PCB. A sub-optimal laminate choice can amplify solder-joint fatigue, promote PTH cracking via high Z-axis expansion, and degrade signal integrity through temperature-dependent dielectric drift. Getting the substrate right is therefore one of the most leverageable decisions in the thermal stack.

This installment focuses on the material layer where thermal behavior is determined. It frames substrate selection around three coupled parameters: thermal conductivity, coefficient of thermal expansion (CTE), and glass transition temperature (Tg). It connects these to practical outcomes: CTE matching to the package, anisotropy management (X/Y versus Z), copper-thickness choices, and process-window alignment for reflow and temperature cycling.

For a concise primer on heat sources, heat-flow paths, and common mitigation tactics before diving into material trade-offs, see NextPCB's overview: Fundamentals of PCB Thermal Design. That article explains the system-level heat-flow picture; this installment complements it by going deeper into substrate selection and CTE-matching workflows, including screening tables, rule-of-thumb thresholds, and application-specific recommendations.

By the end, you will have a repeatable approach to shortlist materials (FR-4 or high-Tg epoxies, metal-core, copper-core, ceramics), evaluate their thermal and CTE trade-offs against your package and process window, and prepare for subsequent steps in the series such as vias and stackups, copper planning, TIM selection, and verification by simulation and test.

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2.  PCB Thermal Design Basics (2): Component Layout
3. PCB Thermal Design Basics (3): PCB Wiring Layout

1. Definition and Importance of PCB Substrates

A PCB substrate is the foundational material that carries electronic components and provides electrical signal transmission paths. Substrate properties directly affect a device's thermal stability, mechanical strength, and signal integrity. Substrate selection is the first step of PCB thermal design and must comprehensively consider key parameters such as thermal conductivity, coefficient of thermal expansion (CTE), and glass transition temperature (Tg). This article explains selection principles, CTE-matching methods, and engineering application points as a reference for PCB designers, engineers, and related professionals.

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2. PCB Substrate Types and Performance Parameters

PCB substrates must meet both soldering-process requirements and the substrate's intrinsic thermal resistance. Under the premise of manufacturability, give priority to copper-clad systems with good thermal resistance and low CTE, or those compatible or matched with the component CTE. By aligning thermal expansion behavior at the materials level, minimize the relative CTE difference between components and the PCB substrate to suppress thermal stress and reliability risks caused by dimensional mismatch during soldering and subsequent temperature cycling.

Tg is a core indicator of substrate thermal resistance: when Tg is low, the CTE is typically higher, and Z-axis (board thickness) expansion is especially pronounced, which tends to cause damage to metallized or plated-through holes. A higher Tg generally corresponds to a lower CTE and better thermal resistance. However, excessively high Tg can make the substrate brittle and harder to machine, for example drilling and profiling may be more prone to burrs or cracks. Therefore, selection should balance thermal resistance (high Tg and low CTE) and manufacturability (toughness and machinability).

Conductor current causes self-heating of PCB traces. After adding the specified ambient temperature, the allowable operating temperature should preferably not exceed 125 °C. This is a commonly used engineering control value, with the exact limit dependent on the grade and specification of the chosen laminate. Components mounted on the board also transfer heat into the PCB and raise local temperatures, affecting overall thermal balance and hot-spot distribution. Consequently, both material selection and PCB thermal design should account for these factors and keep hot-spot temperatures at or below 125 °C. For routing and stack-up, increase copper thickness where feasible, or optimize copper pours and heat-spreading paths, while meeting electrical and impedance requirements, to lower trace thermal resistance and improve thermal margin.

With the miniaturization of power electronics such as switch-mode power supplies, SMD components are widely used, and conventional external heatsinks can be difficult to install on some power devices or result in long thermal paths. In such scenarios, prioritize low thermal-resistance, high thermal-efficiency substrates and structures to shorten heat-flow paths and reduce hot-spot temperature rise:

• Metal-core PCBs (MCPCB), such as aluminum-core and iron-core copper-clad boards, offer better in-plane heat spreading and dissipation than conventional FR-4 and support SMD assembly, making them suitable as direct carriers for power devices.
• Copper-core PCBs introduce a copper plate as the central heat-spreading core, with high-thermal-conductivity epoxy-glass bonding sheets or high-thermal-conductivity epoxy resins as the dielectric. This enables double-sided SMD assembly and efficiently channels device heat into the copper core for outward diffusion.
• Ceramic substrates provide lower thermal resistance and low CTE matched to some ceramic packages, ideal for high heat-flux density and high-reliability applications, with strength, cost, and processability evaluated.
• For devices with bottom thermal pads, solder the pad directly to the metal layer of a metal-core or copper-core PCB to realize near-junction heat removal and in-plane spreading, which reduces junction temperature and thermal resistance.

When conventional heatsinks are impractical, metal-core and copper-core structures effectively enhance board-level thermal capability; for even better CTE matching and very low thermal resistance, consider ceramic substrates. Selection should reflect power density, assembly method, dielectric withstand, mechanical strength, and cost, and be validated through thermal simulation and measurements.

There is also an aluminum-based solution in which a high-thermal-conductivity thermal adhesive is used as the dielectric and bond layer between the aluminum base and the copper foil. Its thermal conductivity is significantly better than that of epoxy-glass bonding sheets or high-thermal-conductivity epoxies, and the adhesive thickness can be set as needed.

2.1 Conventional Epoxy Substrate (FR-4)

• Thermal conductivity: 0.3–0.5 W/(m·K)
• CTE: 13–15×10⁻⁶/°C (X/Y)
• Tg: 130–170 °C
• Suitable for consumer electronics and low-power devices

FR-4 typically costs less than 10 USD/m² with mature processing, but its thermal management is insufficient for high-power scenarios and requires additional thermal measures.

2.2 Metal-Based Composites

Aluminum-Based PCB

• Structure: 6061 aluminum alloy base with boron nitride dielectric
• Thermal conductivity: 1–5 W/(m·K)
• CTE: 20–24×10⁻⁶/°C
• Cost: about 35 USD/m² (approximately 60% of copper-based)
• Example: in a 200 W charger module, IGBT junction temperature dropped from 120 °C to 72 °C, meeting UL 1973 requirements over 3000 charge and discharge cycles.

Copper-Based PCB

• Structure: oxygen-free copper base with nano-ceramic coating
• Thermal conductivity: 401 W/(m·K)
• Thermal resistance: 0.2 °C/W
• Application data: on an AI server GPU motherboard, switching to a copper-based PCB reduced the hot-spot temperature from 95 °C to 72 °C, allowed a 20% fan-speed reduction, and lowered system noise by 3 dB.

2.3 Ceramic Substrates

Aluminum Nitride (AlN)

• CTE: 6–9×10⁻⁶/°C
• Thermal conductivity: ≥ 170 W/(m·K)
• Dielectric constant: 8.8 @ 1 MHz
• Applications: 5G base-station RF modules, dielectric constant drift less than 2%, approximately 15% improvement in signal-transmission stability.

Beryllium Oxide (BeO)

• Thermal conductivity: 330 W/(m·K)
• Toxicity risk: BeO powder is carcinogenic and restricted to aerospace and other special fields.
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3. CTE Matching

In PCB design, especially for SMT boards, CTE matching is a primary consideration. Common IC substrate types include rigid organic, flexible organic, and ceramic; packaging processes include molding, pressed ceramics, laminated ceramics, and laminated plastics. Corresponding PCB materials include high-temperature epoxies, BT resin, polyimide, ceramics, and refractory glass. These device substrates typically have high thermal tolerance and low in-plane (X/Y) CTE. Therefore, first determine the device's package form and substrate material, then consider the soldering process temperature window, such as reflow peak and temperature-cycling range, and choose a PCB substrate with a CTE close to that of the device to reduce thermal-stress-induced solder failures.

CTE quantifies dimensional change with temperature, in ×10⁻⁶/°C. The basic definition is α = ΔL / (L₀ · ΔT). Lower CTE means less dimensional change during thermal cycling and better thermal stability.

Typical In-Plane CTE References

• Ceramic-substrate devices: about 5–7×10⁻⁶/°C; LCCC: about 3.5–7.8×10⁻⁶/°C
• High-Tg FR-4 or epoxy systems: about 14–17×10⁻⁶/°C (Z-axis often much higher)
• BT resin: about 13–16×10⁻⁶/°C
• PI film or flex: about 20×10⁻⁶/°C (formula dependent)
• Alumina (Al₂O₃): about 6–7×10⁻⁶/°C

Design and Selection Tips

1. 1. Device first, then board: anchor to device-substrate CTE and shortlist PCB materials with similar CTE; same-system materials aid matching.
2. 2. Mind anisotropy: organics often have low X/Y but high Z-axis CTE, especially above Tg, directly impacting PTH and solder reliability.
3. 3. Match the process window: evaluate thermal stress per preheat, reflow, and cooling profiles and system-level cycling; increase Tg, use low-CTE fillers, or optimize stack-ups if needed.
4. 4. Layout buffering: for large CTE mismatches, use stress-relief routing, pad and relief optimization, and thermal-pad tuning.
5. 5. Process–materials synergy: when perfect matching is not feasible, consider underfill, gentler ramp and soak, zoned heating, or reducing thickness gradients.
6. 6. Rely on measurements: supplier formulations vary; rely on datasheets and TMA or DMA, distinguishing below- and above-Tg CTE.

Bringing PCB CTE close to the device substrate CTE is a first-principles approach to control thermal stress. Combining anisotropy awareness, reflow profiling, and layout and process optimization improves long-term solder and PTH reliability.

Material CTE Reference Table

Note: Data are sourced from the IPC-2221 standard charts.

In design, prioritize selecting substrates whose CTE closely matches that of the chip package to reduce thermal stress.

4. CTE-Matching Design Methods and Engineering Practice

4.1 Matching Principles and Calculation Method

The IPC-2221C (2023) standard states that the CTE difference between a PCB substrate and a chip package should be less than or equal to 5×10⁻⁶/°C. Thermal stress can be estimated by σ = E · α · ΔT, where E is Young’s modulus, α is the CTE difference, and ΔT is the temperature range.

4.2 Material Modification and Structural Optimization

Flex-slot compensation design: machining approximately 0.1 mm-wide flex slots along ceramic substrate edges allows controlled micro-deformation under thermal cycling to disperse and absorb thermal stress. With proper slot placement and pitch, the effective CTE mismatch can be significantly reduced. After 500 to 1000 cycles between −55 °C and 150 °C, the thermal and electrical performance at the device–substrate interface usually fluctuates only slightly. Actual results depend on slot geometry, substrate thickness, assembly process, and package size; simulation and measurements on samples are recommended for optimization.

Nano-composite enhancement: introducing inorganic fillers such as BN nanosheets and SiO₂ sols into the resin matrix can reduce CTE and increase thermal conductivity while maintaining processability. Multi-lot evaluations indicate that with about 10 to 15 wt% total fillers and flake, spherical, or sol co-blends, in-plane CTE can be reduced to around 12×10⁻⁶/°C and thermal conductivity improved to around 0.8 W/(m·K). Formulations must balance flow, interfacial compatibility, dielectric loss, and reliability, and be verified by TMA or DSC, thermal cycling, and solder-fatigue tests.

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4.3 Typical CTE-Matching Schemes by Application

5. Engineering Design Recommendations

5.1 Substrate Selection Flow

1. Determine package CTE and power density.
2. Shortlist candidates with a CTE difference less than or equal to 5×10⁻⁶/°C.
3. Perform thermal simulation, such as Ansys Icepak or Simcenter Flotherm.
4. Evaluate cost and performance and finalize the optimal option.

5.2 Environmental Compliance

RoHS 3.0 stipulates that for halogen-free substrates, Cl is less than or equal to 900 ppm, Br is less than or equal to 900 ppm, and total halogens are less than or equal to 1500 ppm. Prioritize eco-friendly substrates compliant with IPC-1401B ESG standards.

6. Conclusion

In practical engineering, substrate selection requires balancing CTE matching, thermal performance, cost, and compliance. NextPCB offers materials and stack-ups including FR-4 or high-Tg, metal-core (aluminum, copper, heavy-copper), copper-core inlays, and ceramic (Al₂O₃ and AlN), with DFM reviews, material selection advice, and optimization of thermal paths and copper thickness. Small and medium pilot SMT assembly and reliability-related incoming and process control can be provided to help achieve the best balance among power density, size, and cost. Based on package data and process windows, NextPCB can supply CTE screening tables and thermal design heuristics to shorten verification cycles, reduce failure rates, and enhance system thermal reliability, while ensuring compliance with IPC and RoHS or REACH standards. For integrated support from proof of concept to small-batch introduction, submit your Gerber and BOM to NextPCB for instant quotes and process recommendations.

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About the Author

Arya Li, Project Manager at NextPCB.com

With extensive experience in manufacturing and international client management, Arya has guided factory visits for over 200 overseas clients, providing bilingual (English & Chinese) presentations on production processes, quality control systems, and advanced manufacturing capabilities. Her deep understanding of both the factory side and client requirements allows her to deliver professional, reliable PCB solutions efficiently. Detail-oriented and service-driven, Arya is committed to being a trusted partner for clients and showcasing the strength and expertise of the factory in the global PCB and PCBA market.

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