What Is Soft Thermal Gel for Heat Transfer?

Studies show that up to 55% of electronic component failures are related to heat. Furthermore, rigid thermal pads used for heat dissipation can cause fragile bare chips to crack or deform multi-layer PCB boards under excessive pressure. In this situation, soft thermal gels for heat transfer become an ideal solution.

The Chemistry and Physics of Soft Thermal Gel

The main characteristics of soft thermal gels are low modulus (flexibility) and high stability.

Composition Characteristics

Standard thermal grease is a simple fluid mixture that is highly susceptible to leakage during thermal cycling, while thermal gels utilize a special polymer matrix:

• Cross-linked polymer structure: This material is based on a polymer whose molecular chains are interconnected. This prevents the material from flowing or migrating, even when the components expand due to heat.

• Stability: This cross-linked network ensures that the material remains in place, providing reliable heat transfer performance throughout the product's lifespan, a key advantage over traditional thermal pastes.

Softness Characteristics

The softness of the material is measured using a Shore 00 durometer:

• Low modulus: Most high-performance soft gels have a Shore 00 hardness value between 20 and 45, making them extremely flexible.

• Low compression force: This means that the material requires very little force to conform to every microscopic surface imperfection, virtually eliminating the risk of component damage or PCB warping.

Engineering data: A standard 2.0 mm gap pad may require 5 psi of pressure to deform by 30%, while a similar soft thermal gel requires less than 0.5 psi to achieve 100% surface wetting.

Soft Thermal Gel vs Thermal Pads and Pastes

Understanding when to choose soft thermal gel over traditional thermal pastes or gap pads requires examining their performance differences in real-world conditions.
            

Soft Thermal Gel vs Thermal Paste (Grease)

Soft Thermal Gel vs. Thermal Gap Pads

Thermal Gap pads are chosen for their ease of handling, but this comes at the cost of high mechanical stress and compromised thermal contact.

• Pad Disadvantage: They require significant force to achieve the surface wetting needed to minimize Rcontact. This pressure is detrimental to fragile components.

• Gel Advantage: Soft thermal gel achieves superior wetting automatically, and its flowability allows it to cover the entire interface, even complex contours.

The Bond Line Thickness (BLT) Advantage

The key to achieving low thermal resistance (Rth) is to minimise the bond line thickness (BLT).

• Thermal Pads: are limited by their fixed nominal thickness (e.g. 1.0 mm).

• Soft Thermal Gels: Because gels flow, automated dispensing can achieve the absolute minimum BLT dictated by the gap height. This consistent, thin layer is why gels often yield a better overall Rth than pads with the same raw conductivity.

Key Engineering Use Cases for Soft Thermal Gel in Heat Transfer

Protecting Fragile Components

The most direct reason for choosing soft thermal gels for heat transfer is to protect component safety. Stress is present during heat transfer, and the gel can eliminate this stress, maintaining the mechanical integrity of the components while providing high performance.

Vibration Damping

Automotive components are subjected to extreme shock and vibration standards during operation. In this environment, soft thermal gels can act as both a thermal bridge and a vibration damper.

High-Tolerance Stacked Structures

In PCB stacked structures containing both tall and short components, the gel conforms to the component surfaces, preventing localized stress concentrations.

Common Applications of Soft Thermal Gel for Heat Transfer

LED and Lighting Thermal Management

LED drivers require exceptional longevity, often rated for 50,000 hours.

• Requirement: The material must resist the thermal expansion stress from the metal-core PCBs.

• Advantages: Soft thermal gels absorb this thermal cycling stress without detaching or cracking, preventing thermal degradation that leads to light flicker or color shift over the product's lifespan.

Automotive and Electric Vehicle Applications

The EV industry is a prime user of dispensable thermal materials.

• Application: Soft Thermal Gels are used in high-power inverters, onboard chargers, and increasingly, as EV battery gap filler between battery cells and cooling plates.

• Benefit: Their low density and ability to cure in place make them ideal for mass-production automation lines.

Industrial and Telecommunications Equipment

These applications demand high reliability in harsh environments.

• Application: 5G Remote Radio Units (RRUs) and industrial power supplies often involve vertical mounting.

• Requirement: The TIM must maintain vertical stability. Soft Thermal Gels with high thixotropy are essential to ensure the material stays exactly where it is dispensed, reliably cooling chips over a decade of outdoor service.

How to Select and Dispense Soft Thermal Gels

Selecting the appropriate thermal gel and using it correctly during the manufacturing process requires considering both the manufacturing process and the cost of use.

One-Part vs Two-Part Thermal Gels

• One-Part (Pre-cured): These are already stable gels. They require no mixing and are best for non-critical applications where rework is common.

• Two-Part (Curing): Requires mixing two components (A and B) at the nozzle. This creates a highly stable, cured elastomer. It offers the best long-term reliability and absolute vertical stability but is more difficult to rework.

Calculating the Dispense Pattern

The way the soft thermal gel is dispensed is critical to avoiding trapped air bubbles:
Air Avoidance: Dispensing patterns like the "Spiral," "Cross," or "Serpentine" are preferred over solid squares. These patterns allow air to escape easily as the heat sink or enclosure is pressed down onto the component.
Throughput and Manufacturing Costs
Automation ROI: The capital investment for automated dispensing equipment (robot arms, volumetric pumps) is quickly justified. Eliminating the manual labor required for cutting and placing hundreds of thermal gap pads drastically improves manufacturing throughput.

Frequently Asked Questions

Q: Can I reuse soft thermal gel?
A: No. Once compressed, the gel's structure is compromised, and it will not reliably conform again. It must be completely wiped off and re-dispensed during rework.
Q: Does thermal gel dry out over time?
A: High-quality silicone or acrylic thermal gels are formulated to resist dry-out and separation for the lifespan of the device. This is a key advantage over cheaper thermal pastes.
Q: Is soft thermal gel electrically conductive?
A: Almost universally, no. Thermal gels use electrically insulating ceramic fillers (like aluminum oxide) and are safe for direct contact with conductive traces and pins.
Q: Is soft thermal gel the same as thermal paste?
A: No. Soft gels are engineered to limit oil mobility (less pump-out), often thixotropic and easier to automate; pastes are lower cost but more prone to migration.
Q: How often should I re-qualify gel in production?
A: After any process change or supplier change; spot checks annually or if ΔT drift > ~5°C in field monitoring.
Q: Do gels outgas and contaminate optics?
A: Some do. Use silicone-free or low-VOC gels for optics and ask for TML/CVCM data.
Q: Can soft thermal gel be used for CPU and GPU cooling in computers?
A: Yes. Soft thermal gel works excellently for CPU/GPU cooling, especially in laptops where vibration resistance and long-term stability matter.
Q: Can I mix different brands of thermal gel?
A: Not recommended. Different formulations may not be chemically compatible,mixed gels can phase-separate or cure improperly.
Q: What storage conditions does thermal gel require?
A: Proper storage methods can extend shelf life and maintain performance. It is recommended to strictly follow the instructions for use and storage provided in the product manual.

Soft thermal gels are a mature and reliable thermal interface material technology. When the specifications are chosen correctly, soft thermal gels offer a perfect balance of conformability, low mechanical stress, and ease of manufacturing.
Whether you are designing the next generation of electric vehicles, developing high-power LED systems, or designing industrial control equipment, Sheen Technology can provide comprehensive thermal management solutions designed to meet the most demanding requirements. Visit our website or contact our technical support team to learn how soft thermal gels can be reliably applied in your heat transfer projects.