Top 20 Considerations For Selecting Thermal Interface Materials

Introduction

Heat is often considered the limiting factor in the advancement of electronics systems. Lower thermal resistance will drive, not follow, future electronic designs. These solutions must be cost effective, user friendly and developed quickly. Thermal interfaces are usually an afterthought to designs but play a huge factor in the performance and reliability to a device’s operation. Thermal interface materials can be used for heat dissipation thus providing a cost-effective method allowing engineers the flexibility to reduce overall size of the package. Thermal interface materials can help reduce heat sink dimensions as well as the need for larger cooling fans. Thermally conductive materials can be applied in low and high volumes with relative ease. Furthermore, they can act as a dielectric insulator between components to prevent arcing.

Considerations

1. As a start, the components on either side of the thermal joint must be identified. Is it a semiconductor, insulated gate bipolar transistor (IGBT), metal�oxide�semiconductor field-effect transistor (MOSFET), heat sink, cold plate, or something else? The actual components on the board may dictate the type of interface material. If it is a central processing unit (CPU) or graphics processing unit (GPU), a high-performance product would be used. If you are trying to interface components of varying heights, a gap filler would be used. If you are trying to insulate components to prevent arcing, an insulator would be used. Available thermal interface materials (TIM) are:

Phase Change Materials (PCM)
Thermal interface material that softens or liquefies under heat (50 to 100�C) to fill voids on surfaces and lower thermal resistance between surfaces.

Gap Fillers
Products that act to fill a sizeable gap between surfaces (usually 0.254 to 0.508 mm [0.010″ to 0.200″]) and lower thermal resistance while replacing air gaps between surfaces with thermally conductive material.

Putties
Extremely soft gap fillers that can be compressed over 50 percent of their original thickness to optimize thermal performance.

Greases
Thermal greases are materials that act to reduce thermal resistance between mating surfaces and can offer lower bond line thickness between two surfaces.

Insulators
Thermal products that electrically insulate between two surfaces while lowering the thermal resistivity.

High-Performance Material
Phase change and grease products that have lower thermal resistance usually below 0.1�C-in²/Watt (0.65�C-cm²/Watt).

2. It is important to determine the amount of heat, usually in Watts, that needs to be dissipated. If you are trying to dissipate a high amount of heat, either a high-performance material or high-performance gap filler could be used. This would eliminate looking at the lower performance materials. The low thermal resistance products are much better if a large amount of heat must be removed from a package.
3. What is the type of clamping device at the thermal joint? Some common ones are bolts, spring clips, Belleville washers, etc. If bolts or screws are being used to join components, the gap fillers or insulators could be used. If it is a fixed gap, the minimum and maximum gap size must be considered. This varies by application and is determined by package design. If the use of a high-performance material is preferred, spring clips or Bellville washers are recommended so that constant, even pressure is applied.
4. What are the interface footprint and dimensions? With larger footprints, a gap filler is preferred so that all voids and surface irregularities are filled. Smaller footprints do not have this concern so high-performance products could be used. In general 25.4 x 25.4 mm (1″ x 1″) is the determining size. There are exceptions to this rule. Basically, if you have a large footprint, say 76.2 x 101.6 mm (3″ x 4″) for example, the gap fillers are better at filling the voids across the entire area without concern for surface flatness and polish. If you have a 0.508 mm (20 mils) thick gap filler, it will make contact at 0.508 mm (20 mils) in one area and 0.381 mm (15 mils) in others. With a high-performance material that is only 0.127 mm (5 mils) or thinner, you may be able to interface from 0.025 to 0.076 mm (1 to 3 mils), but will have difficulty if the gap is 0.102 to 0.127 mm (4 to 5 mils) as the PCM will not remain at 0.127 mm (5 mils) once it flows and greases tend to push out, which hampers clean contact. This is more of a concern on large areas than small areas (for example, 12.7 x 12.7 mm [0.50″ x 0.50″]) where the likelihood of such a variance is remote.
5. To deal effectively with the heat at the joint, the temperature range at the interface must be known. The gap fillers are rated for -40 to +200�C. The high-performance materials are rated -25 to +125�C in most cases. Therefore, the material and type of application must run within the same parameters.
6. The clamping pressure at the thermal interface is a factor in determining which material should be used. If the clamping pressure is low 0.14 to 0.34 MPa (20 to 50 PSI), a gap filler performs well. If the pressure is higher (greater than 0.34 MPa [50 PSI]), the high-performance materials may work better. There are exceptions to this application’s specific approach. If too much pressure is applied to a silicone-based gap filler, combined with the addition of heat, a “weeping” of the silicone may occur where it can squeeze out and migrate to other components on the board. If too little pressure is used with a high-performance product, the thermal resistance across the interface will not optimize. If a design is based on a thermal resistance level of X and the actual thermal resistance is Y, heat may not be removed as expected and thermal failures and other thermal issues could result.
7. Is there a pressure limit in the thermal joint due to component attachment conditions such as Ball Grid Array (BGA) and solder limits to prevent fatigue failure? PCB bending stress is another consideration that needs to be addressed. Harder materials may cause warping of the board if placed on a BGA or another delicate component. In this case, there are several materials available that are soft and conformable. They will not stress the board as much as other materials and still can perform fairly well at very low pressures (less than 0.07 MPa [10 PSI]). Putties and very soft gap fillers are recommended for BGA’s since they lower the stress put on leads and the balls are better than traditional gap fillers.
8. The material type, surface finish and flatness of the mating surfaces in the thermal joint are important. With highly polished or machined surfaces (i.e., military, aerospace applications), either type of material, gap fillers or high performance, will work. If a casting or extrusion is used, the gap fillers are better because they can fill voids and other surface imperfections more effectively.
9. Does the interface material require electrical isolation or conduction? There are truly electrically conductive thermal materials available, such as graphitic substances. The gap fillers are electrically isolating. The phase changes and greases are neither rated isolating or conductive. If applied thickly, they will act as an insulator provided there are no voids in the application. If there are voids when applying and metal touches metal, it will provide a conductive path in some instances.
10. Will the interface material be reusable or reworkable? Although it is recommended to replace the interface material after pressure and heat have been applied to it, the gap fillers can be reused in some instances, as a general rule. High-performance materials must always be removed and replaced with new material. The PCM’s are more easily removed when they are still warm, rather than at room temperature.
11. Is the interface in a horizontal plane or vertical mounting plane? The gap fillers are normally better for vertical mounts and high-performance products would work, but consultation with the factory is recommended.
12. If vibration is a concern, such as in mobile applications, gap fillers are better at absorbing shock than the high-performance materials.
13. If it is a space application or to be used in a vacuum can some products be post baked to meet the NASA out-gassing specifications? Yes, materials that meet such requirements are available.
14. If you must meet requirements for silicone extractables, such as Bellcore, there are products available that naturally meet Belcore but are still silicone-based products.
15. In many applications a UL flammability rating is required. Many TIM products are HB and V0 rated. The insulators are 94V0 rated as are most of the high-performance products. There are so many possibilities it is advisable to contact an expert familiar with what is available.
16. Die cutting can lower the cost of the material. For example, squared corners instead of radii can share like rails on the steel rule die and this saves on losses associated with parts that have notches or corners with radii.
17. If the thermal pad requires pressure sensitive adhesive or a naturally tacky side for assembly there are several materials that are naturally tacky. All other products require the introduction of pressure sensitive adhesive to hold parts in place during assembly. Note: The adhesive is not meant for permanent bonding and must be used with some sort of clamping device as well. It is only meant to hold it in place during assembly. Adhesive is available on one side of the part to prevent misuse. Be aware that the adhesive is not thermally conductive and can impede thermal performance depending on material thickness.
18. If the thermal pad needs to stay attached to one side of a joint upon disassembly, some materials can be treated so only one side stays on the heat sink and does not stick to component during disassembly.
19. For some applications the thermal pad must “slide” into the assembly such as a card cage. There are materials that are ideal for these applications that require a sliding motion in and out of a rack because of their natural lubricity. There are also materials available that a carrier added to allow for sliding removal and insertion. The graphitic material is also excellent for sliding in and out of racks due to its naturally lubricious properties
20. Some final considerations relate to any special packaging requirements, such as kiss-cut, rolls or special liners. Special packaging and/or configurations can be supplied as needed to ease use of TIM products. If it is a price-sensitive application you may want to look at the lower performance products as this will reduce cost. If it is a low volume application, price may not be sensitive and a higher performance product may be acceptable if not desired to optimize package performance.

 

Conclusion

Although there are other factors that may come into consideration for selecting thermal interface materials this article concentrates on the top 20 most important factors. Details on product aging, compression set, thermal cycling and other factors can be provided by manufacturers if deemed important for your application.