High-Power Thermal Management: Two-Layer IMS Design Considerations

As power electronics become more compact and power-dense, two-layer IMS PCB designs are helping improve thermal management, reliability and EMI performance in high-power applications.
by Chris Parker

As electronic systems become more compact and power-dense, effective thermal management has emerged as a primary bottleneck to performance, efficiency, and long-term reliability. These increasing thermal demands are exposing the limitations of conventional FR-4 materials and accelerating the adoption of advanced substrate constructions, such as two-layer insulated metal substrates (IMS), in high-power applications.

Although FR-4 remains a cost-effective and widely used solution for low-power designs, its inherently low thermal conductivity and reliance on dense thermal via networks limit its suitability for advanced power systems based on gallium nitride (GaN) and silicon carbide (SiC) semiconductors. While GaN and SiC power transistors can withstand higher junction temperatures than traditional silicon devices, this capability offers little benefit unless the rest of the PCB assembly can reliably operate under the same conditions.

In contrast, two-layer IMS PCB constructions integrate a thin, thermally conductive dielectric with a metal baseplate, enabling efficient heat transfer directly beneath power components. This architecture lowers junction temperatures, enhances mechanical stability, and supports electromagnetic shielding and space-efficient routing strategies.

This article examines the growing adoption of IMS constructions in high-power, mission-critical applications and analyzes the structural and material properties that lead to measurable advantages in performance and reliability. Key design considerations unique to IMS technology are also discussed. Finally, a Texas Instruments case study illustrates how switching to an IMS board construction can reduce junction-to-ambient thermal resistance by up to 45% compared to an equivalent FR-4 design operating under similar conditions. Together, these insights provide a practical framework to support informed material and design decisions in high-power circuit applications.

Evolution and Adoption of IMS in High-Power Module Design

IMS technology gained widespread adoption in the 1990s as telecommunications rapidly expanded. DC-DC converters – particularly quarter-brick and half-brick power modules – became standardized around fixed mechanical footprints (2.28 × 1.45 inches and 2.28 × 2.20 inches, respectively), enabling compact, board-mounted, isolated power conversion across a wide operating range, from approximately 50 watts to over 600 watts. The combination of standardized external footprints and flexible internal architectures made IMS a natural fit for brick converters, enabling substantial increases in power density and reliability without altering established system-level form factors.

Internally, these converters rely on high-current copper planes, dense thermal via arrays and increasingly, IMS substrates. By enabling MOSFETs, GaN and SiC switches, magnetic components and rectifiers to transfer heat directly into a metal baseplate, IMS eliminates the thermal bottlenecks inherent in conventional FR-4 constructions. The metal base acts as an efficient heat spreader, provides improved mechanical stability under shock and vibration, and supports the stringent thermal performance requirements of the telecom and datacom sectors.

By the early 2000s, IMS board constructions had expanded into the defense and aerospace sectors to address the more demanding requirements of radar and RF power systems. These applications emphasized electromagnetic interference (EMI) containment, long-term reliability, and resistance to wide-range thermal cycling. Over the past two decades, IMS adoption has continued to expand across a wide range of high-power applications, including:

• Defense and radar power supplies. Where large, continuous copper ground planes on IMS substrates provide inherent RF shielding and predictable EMI performance.
• Directed-energy and avionics platforms. These require high breakdown voltages, mechanical rigidity, and stable operation across extreme temperature swings.
• Industrial power converters (DC-DC, AC-DC, DC-AC). Often implemented with an IMS power stage paired to an FR-4 control board for optimal thermal and routing performance.
• Motor drives and inverters. Where direct attachment of power semiconductors to the metal base enables efficient heat extraction and improved long-term reliability.

How the Choice of Metal Shapes IMS Performance

Approximately 80% of IMS applications employ aluminum baseplates, valued for their favorable cost, light weight, ease of machining, and effective thermal spreading capability. Aluminum’s coefficient of thermal expansion (CTE), typically around 22–24ppm/°C, is acceptable for most power electronics and is generally well accommodated by compliant dielectric layers and typical semiconductor packages. In high-stress, high-reliability environments, however, roughly 20% of IMS constructions utilize copper baseplates.

Typically, aluminum is selected when the combined copper thickness is less than 10% of the aluminum base plate thickness. When copper foil thickness exceeds 10% of the aluminum base plate thickness and includes large internal ground planes, copper is selected to maintain part flatness and reduce internal stress caused by the relatively high CTE of aluminum.

Copper baseplates offer two key advantages.

• Superior CTE Matching. Copper’s CTE (~16–17ppm/°C) closely matches that of copper circuitry, plated vias, and many package leadframes. This tighter CTE alignment significantly reduces thermomechanical shear stress at the copper-dielectric-metal interfaces during thermal cycling. The benefit is particularly pronounced in heavy-copper constructions, designs subjected to rapid thermal transients (such as GaN and SiC switching), embedded-die architectures, and aerospace or defense modules that must withstand thousands of thermal cycles over decades of service.

• Higher Thermal Conductivity. With a thermal conductivity of approximately 400W/m·K – roughly twice that of aluminum (~200W/m·K) – copper enables faster heat spreading and more uniform temperature gradients beneath power devices. The superior thermal performance of copper is essential for high-density converters and mission-critical systems where localized hot spots can degrade reliability.

While copper baseplates introduce higher material cost and weight, the resulting gains in mechanical stability, resistance to warpage, and long-term solder joint reliability often outweigh these trade-offs in demanding applications. For this reason, copper remains the substrate of choice when maximum reliability, controlled thermal expansion, and robust thermal cycling performance are non-negotiable.

Why the Second Layer Matters

Two-layer IMS boards represent more than an incremental improvement over single-layer substrates; they directly address several fundamental challenges inherent in high-power, high-frequency and high-density electronics. Engineers typically introduce a second layer to improve EMI resistance, enhance circuit routing flexibility and boost thermal performance in compact power systems.

In military, aerospace, and radar applications, the second layer in a two-layer IMS stackup is often implemented as a continuous internal ground plane. This buried copper layer serves as an effective RF shield, providing significant electromagnetic performance benefits. This also establishes a low-impedance, low-inductance reference plane that suppresses switching noise from fast GaN and SiC power devices, reducing both radiated and conducted EMI. It also improves signal integrity by providing well-controlled return paths for high-frequency and high-speed signals.

In addition, the internal ground plane enhances the system’s immunity to noise by electrically isolating sensitive control and signal circuitry from high-energy power stages, supporting compliance with stringent military and aerospace electromagnetic compatibility (EMC) requirements.

Radar and directed-energy platforms frequently combine this internal ground plane with dense via stitching around electrically noisy regions. These vias electrically connect the top copper layer to the internal ground plane, forming a vertical Faraday barrier that confines electromagnetic fields within defined boundaries. The result is a cleaner, more controlled RF environment that remains stable across temperature extremes, load variations, and switching conditions.

For systems characterized by rapid voltage transitions (high dv/dt), RF sensitivity or space constraints, the inclusion of an internal ground plane offers a highly effective method of EMI control and high-frequency stabilization, without introducing significant complexity to the overall system design.

In applications such as industrial power supplies, motor drives, telecom brick converters and high-density power modules, the benefits of a second IMS layer extend beyond EMI containment. A buried copper layer enables functional separation of circuitry within constrained footprints while simultaneously improving thermal performance.

The second layer allows designers to physically separate power and signal functions within the board. Low-current, noise-sensitive control and signal traces can be routed on the internal layer, while high-power traces remain on the top layer, where thicker copper and wider traces are easier to implement. This approach supports space-efficient layouts, enabling increased routing density while maintaining creepage, clearance and thermal conduction path requirements.

In more advanced constructions, portions of the top dielectric can be selectively removed to expose copper features on the second layer. These localized “copper wells” allow power components, such as MOSFETs, diodes, LEDs or bare die, to be mounted nearer to the metal baseplate. Reducing the thermal path length lowers effective thermal resistance, decreases steady-state operating temperatures and improves long-term reliability.

In these architectures, the second layer serves as a multifunctional element in both the electrical and thermal strategies. Depending on system requirements, it may serve as a shield, a low-current routing layer, a heat spreader, or a component mounting surface. This flexibility is particularly valuable in applications that must balance high power density, EMI compliance and mechanical robustness within a compact, space-constrained PCB assembly.

Design Considerations: Copper Weight, CTE Constraints and Baseplate Selection

Selecting copper weight in a two-layer IMS design requires careful balancing of thermal performance, manufacturability and mechanical reliability. While heavier copper improves current-carrying capacity and heat spreading, it also increases mechanical stress within the dielectric and metal stackup, particularly when paired with aluminum substrates.

Copper weight limitations. In most two-layer IMS constructions, the internal copper layer is limited to 4oz using standard processes. Designs exceeding 4oz are technically feasible but become more design-dependent and may require additional operations.

Heavy copper effectively stiffens the copper-dielectric stack, reducing the dielectric’s ability to accommodate differences in thermal expansion between layers. The use of thick copper layers may require a transition to a copper base plate to minimize internal stress.

When higher copper weights are required. Applications such as motor drives, high-current DC-DC converters, and pulsed-power systems may require copper thicknesses greater than 4oz. on one or both layers. In these cases, maintaining reliability often requires changes to the IMS’s mechanical structure.

Copper baseplates are particularly advantageous for thick-copper designs because their CTE (~16–17ppm/°C) closely matches that of copper foil. This alignment significantly reduces shear stress during thermal cycling compared to aluminum baseplates, which have higher CTE values (~22–24ppm/°C) and expand more aggressively, increasing the risk of distortion in heavy-copper constructions.

Copper thickness rule of thumb. A practical engineering guideline is to limit the total copper thickness on any layer to roughly 10% of the metal baseplate thickness. When the copper thickness exceeds this ratio, internal stresses become difficult to manage, the risk of warpage increases and dielectric integrity may be compromised.

Dielectric Selection: Thickness, Via Structure and Thermal Design Principles

Dielectric selection is central to IMS performance, directly influencing thermal behavior, electrical isolation, and mechanical stability. Two-layer IMS designs typically employ one of three dielectric classes: high-temperature (HT), high-performance (SFL), and standard-performance (SPL).

Among these, HT dielectrics are the variety most commonly used in two-layer IMS constructions due to their high breakdown voltage, good thermal capability and excellent reliability in harsh operating environments.

Selecting dielectric thickness requires balancing thermal performance against voltage-isolation requirements. Thinner dielectrics reduce thermal resistance and improve heat transfer to the metal baseplate, while thicker dielectrics increase breakdown voltage and provide additional margin during high-potential (HIPOT) testing. Standard thickness ranges are typically 3-12mils for HT dielectrics, with custom thicknesses reserved for high-volume or highly specialized applications.

Designers typically qualify thicknesses using test voltages on the order of twice the maximum operating voltage, plus a safety margin, while avoiding operation near the material’s intrinsic breakdown strength to preserve long-term reliability.

Via structure. Via structures are another critical element in IMS design. Plated through-hole vias provide vertical heat transfer, carry current between layers, and enable stitched shielding for EMI control. Filled vias improve planarity and enhance thermal conduction, while arrays of parallel vias increase current-carrying capacity and reduce localized heating.

Thermal design principles. From a thermal standpoint, IMS substrates significantly outperform FR-4 laminates because heat is rapidly conducted through the dielectric into the metal baseplate. This efficient heat-spreading capability enables higher current densities and more compact routing. Conventional FR-4 trace current calculators often underestimate the current-carrying capacity of IMS constructions because they don’t account for enhanced heat dissipation through the metal core and adjacent copper planes. As a result, IMS designs can often use narrower traces or thinner copper to achieve equivalent current ratings, an important advantage in space-constrained power modules.

Compared with FR-4 constructions that rely on dense thermal via arrays to move heat toward the baseplate, TCLAD thermally conductive dielectrics spread heat across the full metal baseplate area. The result is lower, more uniform junction temperatures, rather than the localized conduction paths that via arrays provide. This approach also eliminates the incremental drilling, plating, and via-fill operations that dense-via FR-4 boards require, reducing fabrication cost and shortening lead time at production volumes.

System-Level Considerations: When the Entire Thermal Stack Matters

Overall heat dissipation is governed by the entire thermal stack, not just the IMS baseplate and dielectric. Cooling strategies, interface materials, package design, mechanical clamping and heat sink geometry all influence system-level thermal performance. Accurate thermal modeling must account for every interface in the thermal path, including copper layers, dielectrics, solder joints, thermal interface materials (TIMs) and mounting hardware, to avoid overly optimistic temperature predictions.

Some IMS designs use selective dielectric removal to expose copper features on the second layer, creating localized “copper wells.” These structures shorten the thermal path to the metal baseplate, reduce parasitic loop inductance, and improve heat dissipation for power semiconductors or LEDs, while simultaneously freeing routing area on the top layer. This approach improves performance by incorporating features commonly associated with embedded-die techniques, blurring the boundary between IMS construction and advanced packaging technology.

In many practical systems, an IMS-based power board is paired with a separate FR-4 control or logic board, interconnected through press-fit pins, board-to-board connectors, or soldered headers. This architectural separation keeps temperature-sensitive electronics away from high heat flux regions while allowing the board to fully leverage the thermal advantages of IMS construction.

In high-voltage applications, partial discharge inception voltage (PDIV) becomes a critical design consideration. Partial discharges can initiate within voids, interfaces or material imperfections in the dielectric when localized electric fields exceed a critical threshold. Although these discharges may not cause immediate failure, repeated partial discharge activity accelerates dielectric aging. To ensure long-term dielectric reliability, designers evaluate PDIV under combined electrical and thermal stress conditions representative of real operating environments. Elevated temperatures reduce dielectric strength and increase discharge activity, making PDIV testing especially important for wide-bandgap power systems that operate at higher voltages, switching speeds, and junction temperatures. Proper control of dielectric thickness, material selection, and void-free processing is therefore essential to maintaining sufficient PDIV margin throughout the life of the IMS assembly.

Together, these system-level considerations enable engineers to optimize IMS-based designs for thermal performance, electrical isolation and long-term reliability in demanding power electronics applications.

IMS vs. FR-4. To quantify the thermal performance differences between conventional FR-4 and IMS PCB constructions, Texas Instruments conducted a comparative study. The evaluation used two-layer FR-4 and two-layer IMS PCBs with similar layouts, as illustrated in the cross-sectional views in Figure 3.

The thermal performance of each board was evaluated using six Texas Instruments three-phase GaN inverters (LMG3410R050) driving a motor in a high-power, high-switching-frequency operating environment.

In both cases, a thermal interface material (TIM) was applied between the PCB and an anodized aluminum heat sink to ensure consistent contact and minimize thermal resistance. All testing was performed under natural convection conditions.

Both the FR-4 and IMS boards delivered a continuous RMS current of 3 amps to drive a 200-volt, three-phase motor. Operating conditions were maintained until steady-state temperatures were reached.

Table 2 summarizes the thermal resistance contributions along the heat-dissipation path for both FR-4 and IMS implementations, including vias, dielectrics, interface materials and the heat sink.

Conclusion

This study demonstrates the significant thermal performance advantages of TCLAD IMS technology over conventional FR-4 PCB constructions. Experimental results show that the FR-4 PCB exhibits approximately 45% higher average junction-to-ambient thermal resistance than the IMS PCB, confirming the superior heat dissipation capability of IMS for high-power, high-density electronic applications. These findings highlight IMS as an effective solution for improving thermal management, enhancing system reliability and extending the operational life of power electronic devices.

As power electronics continue to evolve toward higher voltages, faster switching frequencies, and increased power density, the inherent thermal, electrical, and mechanical limitations of FR-4 substrates become increasingly restrictive. Two-layer IMS architectures provide a compelling alternative by offering enhanced thermal management, robust electrical isolation, improved EMI containment and greater mechanical stability, attributes that are particularly critical for GaN- and SiC-based power systems operating near their performance limits.

Nevertheless, achieving optimal IMS performance depends on careful material selection and design optimization. Dielectric properties, copper layer configuration, thermal interfaces and system-level constraints must be considered holistically to balance thermal efficiency, electrical reliability and manufacturability. Leveraging application-specific expertise in these areas is essential to fully realize the benefits of IMS technology in real-world power electronics designs.End of article content

Chris Parker is product engineering manager at TCLAD (tclad.com); chris.parker@tclad.com.