Beyond Thermal Conductivity: Exploring Polymer-based TIM Strategies for High-power-density Electronics

Estimated reading time: 12 minutes

Abstract
As power density and thermal loads continue to increase, effective thermal management becomes increasingly important. Rapid and efficient heat transfer from power semiconductor chip packages is essential for achieving optimal performance and ensuring long-term reliability of temperature-sensitive components. This is particularly crucial in power systems that support advanced applications such as green energy generation, electric vehicles, aerospace, and defense, along with high-speed computing for data centers and artificial intelligence (AI). In these domains, even minor thermal inefficiencies can lead to hotspots, performance degradation, system failures, and significant safety risks, underscoring the need for robust thermal management solutions.

Thermal interface materials (TIMs), particularly TIM2 materials, play a vital role in facilitating efficient heat transfer from the package lid (integrated heat spreader) to a heatsink, cold plate, or other cooling components. This article examines the underlying principles of TIM2 design and performance, including heat transfer mechanisms, material chemistry, dispensing techniques, bondline control, and long-term thermal performance. The focus is on gap fillers, particularly two-part (2K) silicone-based formulations, and their contributions to manufacturing process efficiency, effective heat dissipation, and long-term reliability.

Why TIM2 Matters

In thermal design, TIM2 refers to a thermally conductive material applied between the exterior of a semiconductor package, typically the integrated heat spreader (IHS), and a cooling component such as a heatsink, vapor chamber, or cold plate. Unlike TIM1 and TIM1.5, which make direct contact with the chip, TIM2 bridges the secondary thermal interface and must accommodate greater variability in gap size, mechanical tolerances, surface roughness, and contact pressure.

Figure 1: Typical TIM1, TIM1.5 and TIM2 locations in integrated circuit (IC) assembly.

The primary function of TIM2 is to fill microscopic surface irregularities and minimize thermal contact resistance at both interfaces between the IHS and the cooling solution. In high-power-density systems where effective heat dissipation is critical, TIM2 performance directly influences junction temperature, thermal cycling reliability, and overall device lifespan. A poorly optimized TIM2 layer can force system designers to throttle chip performance to prevent overheating, undermining the very goal of advanced high-speed, high-power devices.

The Heat Is On: How the Market is Responding

The thermal interface materials market is undergoing rapid expansion, with multiple industry research firms projecting sustained double-digit growth in the coming years:

• Market valued at $4.1 to 4.6 billion in 2024, with projections ranging from $7.5 billion to $12.4 billion by 2030–2034, reflecting a compound annual growth rate (CAGR) of 11–12%.
• This growth is the result of the widespread adoption of electric vehicles (EVs) and high-performance computing, the expansion of telecom infrastructure (notably 5G), and the proliferation of consumer electronics.
• While the Asia-Pacific region currently leads in consumption, demand for TIM solutions is also rising steadily across North America and Europe.
• TIM2 materials used between packages and heatsinks account for a large share of the market. However, TIM1 materials, applied at the chip level, are gaining momentum due to advances in semiconductor packaging technologies.

This surge in demand is driven not only by the escalating thermal output of modern electronic systems but also by the growing recognition of the critical role TIMs play in system-level thermal management.

TIM2 Thermal Science: Heat Transfer and Thermal Resistance

The primary function of any TIM is to minimize thermal resistance between two surfaces. For a TIM2 interface, the total thermal resistance (R_th) is given by:

R_th = R_bulk + R_contact1 + R_contact2

Where R_bulk, the intrinsic material resistance, is defined by Fourier’s law: R_bulk = BLT / k, where BLT is the bondline thickness and k is the thermal conductivity (W/m·K). R_contact1 and R_contact2 are the interfacial thermal resistances at the TIM-surface interfaces, and in practice, their combined value is often much greater than R_bulk.

While a high thermal conductivity (k-value) is desirable for TIM materials, it does not ensure low thermal resistance. Without adequate surface wetting and mechanical compliance (the ability to conform to surface irregularities), interfacial contact resistance can dominate the thermal path. That is why a TIM’s capacity to fill surface irregularities and microscopic voids is often more important to overall thermal performance than its intrinsic thermal conductivity.

Notably, many thermal modeling tools used in package design overlook interfacial resistance, focusing instead on bulk resistance alone. This simplification can result in thermal solutions that underperform in real-world conditions, where inadequate heat dissipation forces chip throttling, undermining the performance advantages of high-speed, high-power devices.

Closing the Gap: The Rise of Two-Part (2K) Silicone TIMs

Gap fillers are a type of TIM2 material engineered to effectively bridge variable gaps between components and heat spreaders, providing both thermal conductivity and mechanical compliance. Among these, two-part (2K) silicone-based gap fillers have emerged as a preferred solution for many high-performance, high-power systems where long-term reliability is essential.

What makes 2K silicone gap fillers particularly valuable is their unique combination of excellent conformability, reworkability, and long-term stability, even under demanding operating conditions. Supplied as two separate components, a base and a curing agent, they are mixed just prior to application and cure in place to form a soft, thermally conductive elastomer. Once cured, the material conforms well to complex geometries and surface irregularities, filling air gaps and reducing interfacial thermal resistance.

A key benefit demonstrated in real-world applications is their reworkability: after curing, the material can be removed and reapplied without damaging the assembly. This makes 2K silicone gap fillers ideal for environments where repair or replacement may be required.

More Perks of 2K Silicone TIMs

2K silicone gap fillers offer a broad range of performance and processing advantages that make them well-suited for a variety of thermal management applications:

• Dispensability: They are compatible with automated, high-precision robotic dispensing systems, enabling efficient, consistent, and scalable high-volume production.
• Cure-in-place behavior: Unlike traditional thermal pads, these materials cure without requiring compressive force during assembly, simplifying integration and reducing mechanical stress on components.
• Customizable rheology: Their flow properties can be tailored to meet specific needs, including vertical surfaces, narrow gaps, and large areas.
• Mechanical compliance: Once cured, they form a soft, conformable elastomer that adapts to complex surface topographies and accommodates variable gap sizes caused by thermal cycling.
• Thermal conductivity: Formulated with thermally conductive fillers, such as ceramic particles, these materials offer tunable thermal performance, with k-values ranging from 1.5 to 6 W/m·K, depending on the formulation.

Art of the Flow: How Rheology Shapes Dispensing Performance

Extensive experience with 2K silicone gap fillers has shown that their rheological behavior plays a critical role in both dispensing and final assembly.

• Shear-thinning behavior: These materials are formulated to exhibit non-Newtonian shear-thinning properties, meaning their viscosity decreases under shear stress. This allows them to flow easily through narrow dispensing nozzles under pressure. Once the shear force is removed, they quickly regain viscosity to prevent slumping or dripping, especially on vertical surfaces.
• Thixotropic tuning: The rheology of 2K silicone materials can be tailored to meet specific application needs. High-viscosity formulations are ideal for large, vertical bondlines, while lower-viscosity versions work well for thin-layer coverage or narrow gaps.
• Influence of filler morphology: The shape of thermally conductive filler particles influences flow behavior. Spherical fillers improve flow and reduce viscosity, while high-aspect-ratio fillers, such as flakes or fibers, enhance thermal conductivity by creating more continuous heat paths, though often with increased viscosity.
• Filler optimization strategies: Advanced formulations carefully balance particle size, shape, distribution, volume loading, and surface treatments (e.g., silane coatings) to maximize packing density and minimize interfacial thermal resistance. Hybrid filler systems that combine different particle shapes and sizes can further enhance thermal conductivity while maintaining processable flow viscosities.

Fine-Tuning the Dispensing Process

Proper mixing and dispensing of 2K silicone gap fillers are critical to achieving reliable performance and long-term durability. The two components, base and curing agent, are best combined using static mixers, then applied using volumetric or time-pressure dispensing systems. To ensure process stability and product performance, several key parameters must be carefully controlled:

• Mixing ratio: Precise ratio control is essential to achieve complete curing and optimal mechanical and thermal properties.
• Dispense pattern: Spiral, bead, or dot patterns are chosen based on the surface area and the geometry of the gap being filled.
• Cure time and temperature: Cure profiles can be adjusted for room-temperature curing or accelerated thermal curing, depending on production requirements.
• Equipment design: The pumps and valves in the dispensing system must be designed to handle abrasive fillers to ensure stable and consistent flow.

Automated dispensing systems enable consistent bondline thickness, minimize voids, and ensure reliable wet-out, delivering optimal thermal contact and long-term durability.

Addressing Surface Roughness through Material Design

MacDermid Alpha gap fillers are specifically formulated to maximize both conformability and surface wetting. By engineering them with low hardness and viscoelastic properties, they can conform to surface roughness, compensate for variations in planarity, and fill voids without the need for high assembly pressure. Surface wetting is optimized to improve contact with mating surfaces, reducing contact resistance (R_contact). This, in turn, lowers overall thermal impedance, particularly when combined with a well-tuned dispensing pressure and pattern.

Bondline Thickness and Assembly Pressure

Bondline thickness (BLT) is determined by the size of the gap, assembly tolerances, and specific application requirements. Because 2K silicone gap fillers cure in place, they resist squeeze-out under clamping pressure (unlike greases). For applications that demand precise thickness control, spacer beads or mechanical stops are commonly used.

For gap fillers, BLTs typically range from 200 µm to several millimeters. This wide range allows them to bridge both small and large gaps while accommodating varying assembly tolerances. Their versatility makes them suitable for diverse applications that require reliable thermal contact to be maintained across uneven surfaces, from high-performance computing and telecom infrastructure to EV battery modules and power converters.

Outgassing, Ionic Contamination, and Electrical Safety

Beyond thermal and mechanical performance, TIM2 gap fillers must meet stringent requirements for chemical stability, cleanliness, and safety, particularly in high-reliability or safety-critical applications.

• Outgassing: Volatile compounds can condense on nearby components, risking electrical and optical failures. To assess this risk, TIMs are tested using total mass loss (TML) and collected volatile condensable materials (CVCM) in accordance with ASTM E595 standards. Low-outgassing formulations are critical for applications like satellite electronics, infrared (IR) targeting systems in missile guidance, and automotive cameras used in advanced driver-assistance systems (ADAS).
• Ionic contamination: Mobile ions such as sodium (Na⁺), potassium (K⁺), and chloride (Cl⁻) can cause corrosion and leakage currents in high-voltage systems. To minimize this, high-purity raw materials and tight process controls are used to keep ionic content below accepted industry thresholds, typically below 10 ppm.
• Electrical insulation: Many TIM2 materials must also insulate electrically. Silicone-based formulations with ceramic fillers offer both high thermal conductivity and electrical resistance. Dielectric breakdown strength and volume resistivity are key metrics for validating electrical insulation performance across operational voltage ranges.
• Specialty solutions: To further mitigate outgassing and contamination, specialty solutions are available that absorb and trap residual volatiles over time within sealed environments. These materials complement low-outgassing TIMs and enhance long-term reliability in high-vacuum and precision electronic assemblies.

Compliance with IPC, UL, and JEDEC standards ensures that TIM2 materials meet both thermal and electrical requirements for demanding applications.

Thermal Cycling and Long-Term Reliability

Cured 2K silicone gap fillers deliver outstanding long-term reliability, maintaining elasticity and contact at the interface even after thousands of thermal cycles. Key characteristics contribute to this durability:

• Low modulus: Their inherent softness absorbs stresses caused by coefficient of thermal expansion (CTE) mismatches during thermal cycling, protecting delicate components from damage.
• Pump-out resistance: Unlike thermal greases, cured silicone gap fillers remain securely in place throughout thermal cycling, preventing the mess and thermal performance issues associated with pump-out.

Long-term performance is validated through accelerated aging tests, including thermal cycling from -40°C to 150°C, high-temperature storage, and humidity resistance testing.

TIM2 in Automotive Applications

The automotive sector places unique demands on TIM2 materials. Advanced driver assistance systems (ADAS), for instance, must meet strict automotive industry standards for performance, safety, and durability, ensuring long-term reliable performance under challenging and dynamic operating conditions. Key requirements include:

• Thermal cycling: Automotive electronics face extreme temperature fluctuations, from -40°C cold starts to 150°C under-the-hood conditions. TIM2 materials must resist cracking, shrinkage, and delamination over thousands of thermal cycles.
• Long-term reliability: Designed for 10- to 15-years of service, automotive electronics require TIMs to maintain elasticity and thermal conductivity under continuous exposure to thermal and mechanical stresses.
• Vibration and shock: TIM2 materials, particularly gap fillers, must remain compliant and securely in place despite exposure to ongoing shock and vibration.
• Electrical safety: With high-voltage components alongside sensitive mixed-signal circuits, electrically insulating TIMs are critical for preventing short circuits and ensuring functional safety.
• Outgassing and cleanliness: Volatile emissions can impair optical components in cameras and LiDAR. Low-outgassing formulations that meet VOC standards are essential for preserving optical clarity and reliability.
• Manufacturability and tolerance compensation: Assembly variability can create uneven gaps. 2K silicone gap fillers are ideal for this task, flowing into irregular gap geometries and curing in place.

By meeting these demanding requirements, 2K silicone gap fillers have become a preferred solution across a range of applications. Beyond ADAS modules, battery control units, radar systems, and camera electronics in modern vehicles, they are also used to enhance reliability in sectors such as high-performance computing, telecom, aerospace, defense, and green energy.

Closing the Gap with Advanced TIM Solutions

Looking ahead, ongoing innovation in rheology, filler architecture, and material chemistry will continue to expand the performance of TIMs. However, to fully reap the benefits of these advancements, designers must look beyond bulk thermal conductivity and adopt a system-level perspective, one that accounts for the contact mechanics at the interface.

In today’s power-dense electronics, there is no one-size-fits-all solution for thermal management. Whether designing automotive control units, rugged aerospace systems, or compact power modules, selecting the right TIM is critical to achieving optimal performance, reliability, and manufacturability.

MacDermid Alpha understands that every application presents unique thermal challenges. That is why we have developed a comprehensive portfolio of TIM1 and TIM2 solutions to meet diverse needs across industries such as automotive, power electronics, high-performance computing, telecom, and industrial automation.

TIM2 solutions include:

• Thermal gap fillers: Including 2K silicone-based formulations engineered for automated, high-volume manufacturing, with outstanding thermal performance, mechanical compliance, reworkability, and long-term reliability
• Thermal pastes and gels: Offering excellent wetting, low bondline thickness, and ease of rework
• Thermal pads: Ideal for legacy systems where reusability and mechanical cushioning are required

TIM1 solutions include:

• Indium-based materials: Delivering ultra-high thermal performance and reliability for chip-level thermal management
• Silver and copper sintering materials: Low-pressure, high-yield die-attach solutions ideal for silicon carbide (SiC) power semiconductors operating in extreme temperatures

Our experts work closely with OEMs, Tier 1 suppliers, and fabless companies to develop customized strategies that align with specific performance and manufacturing goals. Organizations seeking support with material evaluation, design optimization, or complex thermal challenges can benefit from addressing thermal interface requirements early in the design process, as rising power density and thermal constraints increasingly shape design decisions.

Padmanabha Shakthivelu is director of product management at MacDermid Alpha Electronics Solutions. Nico Bruijnis is director of thermal interface materials at MacDermid Alpha Electronics Solutions.

This paper original appeared in the September 2025 Design007 Magazine.