Dry-to-the-Touch Thermal Grease Cuts Installation Mess

Aug 1, 2002 12:00 PM
By Prakash Khatri and John Ziemski, AOS Thermal Compounds, Eatontown, N.J.

A dry-to-the-touch thermal grease product offers the lowest thermal resistance at low closure force - with handling ease.

Most designers now use thermal grease products to resolve the most demanding applications. Despite considerable drawbacks in the use of thermal grease as an interface material, most engineers still believe the advantages of its use — in particular, its low thermal resistance — far outweigh the disadvantages (Fig. 1). Micro-faze^® is a dry-film thermal grease that has great promise in becoming totally user-friendly for thermal engineers and designers. It's the first nonsilicone thermal grease created to solve contamination and migration problems that are typically associated with silicone-based products. The nonsilicone compound is a synthetic-based thermal grease used to ensure quick, efficient heat transfer and dissipation.

Three important goals guided the development of Micro-faze:

• Design a thixotropic and dry-to-the-touch thermal grease by incorporating an appropriate polymer.
• Coat the new product onto a substrate to create sheets and rolls that can be die-cut.
• Develop a product that will perform as well as or better than actual thermal grease (Table 1, on page 45).

Once the physical properties were amenable to a solution, the focus shifted to end-user practicality. This essentially entailed adaptation of an interface material that can be die-cut, is easy to handle, and possesses an exact amount of film for subset on two sides protected with release liners. The subsets selected were aluminum foil and Kapton^®. Foil was chosen for its thermally conductive property, and Kapton for its electrical insulation value. Therefore, two distinct products have evolved, depending on end user requirements (Table 2).

Dry-Film Thermal Grease

In essence, Micro-faze (Fig. 2) is thermal grease coated onto a substrate to provide exceptional thermal conductivity; or coated onto Kapton for electrical insulation and thermal conductivity. The product is easy to use and has the time-proven performance of thermal grease.

The composition of this product is that of a nonsilicone, non-wax-based, dry-to-the-touch thermal grease that's naturally tacky. It easily adheres to a component or heatsink without an adhesive or other non-thermally conductive material that can degrade thermal performance. It can also be die-cut to whatever size or shape desired for thermal pad handling and convenience.

The product is an excellent replacement for phase-change materials. Because it is a thermal grease, the product requires no actual phase change to take place before it begins working. Instead, heat transfer takes place at any temperature.

Thermal Interface Materials

To better understand the Micro-faze technology, it's useful to review the nature of thermal greases compared to related thermal management developments, such as silicone elastomer pads and wax-based phase-change materials. Table 3, on page 46, lists advantages and disadvantages of thermal interface materials.

Performance Data

Micro-faze technology is available in two forms: Type A (aluminum substrate) and Type K (Kapton substrate), coated on both sides with specially formulated nonsilicone thermal grease that's naturally tacky but dry-to-the-touch. It's not wax-based and offers the lowest thermal resistance. With its advantages, the Micro-faze product outperforms thermal pads and phase-change materials (Fig. 3 and Table 4).

Micro-faze product advantages include the following:

• Retains all proven values of thermal grease in film form.
• Requires minimum force for total interface contact.
• Allows total “wetting action” to fill all voids without changing phase.
• A positive coefficient of thermal expansion and thixotropic properties increases the wetting action for total interface contact.
• Heat transfer begins immediately and can take place at any temperature, particularly for cold-plate applications.
• Essentially a “drop-in-place” product that is easy to use and handle in any manufacturing environment.
• Naturally tacky and requires no adhesive or other nonconductive material used that may affect thermal resistance.
• Microscopically changes to fill all voids on part surfaces.
• Thixotropic nature prevents run out.

Micro-faze K provides heat transfer properties and high insulating capabilities. Major applications for Micro-faze include: power modules, IGBTs, dc-dc converter modules, solid-state relays, diodes, power MOSFETs, and large area applications for power supplies and other custom enclosure heat dissipating surfaces.

	Advantages	Disadvantages
Elastomeric Thermal Pads A silicone rubber containing heat-conducting particles, such as zinc oxide, aluminum oxide, aluminum nitride, or boron nitride.	• Not messy • Saves time through ease of installation • Eliminates problem of applying exact the amount of grease with each application	• Do not flow as freely as grease and will invariably deform when a sufficient compressive load is applied to conform to surface irregularities. • At low pressure, it cannot fill voids between the surfaces, causing high thermal interface resistance.
Phase-Change Materials combine grease-like thermal performance with pad-like handling. Solid at room temperature, but behave like thermal pastes or greases once they reach their phase-change, or melt temperature.	• Easy to install	• Material is under constant pressure, so each thermal cycle and subsequent phase changes may introduce voids that cannot be refilled.
Thermal Grease employs dispersed, thermally conductive ceramic fillers either in silicone or hydrocarbon oils to form a paste. Joint integrity is maintained using spring clips or mounting hardware.	• When enough grease applied to one mating surface is in contact with a second, the grease fills the voids in between and eliminates interstitial air. • Allows thinnest joint to form as both mating surfaces come into contact at their high points, resulting in lowest thermal resistance.	• Excess grease can flow out beyond the edges • Can be messy
Table 3. Advantages and disadvantages of thermal interface materials.

With these Micro-faze advantages, you no longer have to sacrifice thermal performance for convenience.

Kapton is a trademark of Dupont and Micro-faze is a trademark of AOS Thermal Compounds.

No Creep, Nonsilicone Technology

The first nonsilicone “no creep” heatsink compound was developed to eliminate the problems of separation and contamination. After several thermal cycles, silicone compound can dry out, causing cracking and separation (see pin holes and worm tracks in picture below). As air gaps form, components lose interface contact, resulting in a loss of thermal conduction. Nonsilicone compounds stay in place for the full operational life of your hardware.

For more information on this article, CIRCLE 335 on Reader Service Card

Acceptable Use Policy blog comments powered by Disqus