A well-performing thermal interface material (TIM) fulfills a variety of application requirements within the broad range of electronic modules and systems incorporated in medical imaging and other medical electronics systems. Understanding how high-performance metallic and similar types of TIM materials may be applied for reliable performance is an important thermal and mechanical design need.
Thermal Interface Material Function
Thermal interface materials are a low-cost but critical factor that determines system life and reliability for those components that dissipate some energy as heat due to inefficiencies. The TIM is intended to minimize the thermal barrier that exists between the surface of a microprocessor, for example, and an attached heat-sink. Measurement of this barrier is generally expressed as a value for thermal resistance between two mating surfaces (semiconductor case and heat-sink base). Minimizing the resistance value reduces potential for semiconductor overheating, resulting in increased reliability during system operation and increased life. Excessive operating temperature is generally acknowledged as the cause of early component failure. The amount of power dissipated as waste heat due to inefficiency at the device level can range from a fraction of a watt to tens or hundreds of watts, depending on the specifics of the semiconductor and the intended operation.
Medical Electronics Applications for TIMs
A range of applications for thermal interface materials may exist within a single complex medical electronics system. Categories of semiconductor devices that are heat sources within medical electronics generally can be identified as:
- Microprocessors, graphics processors, and other integrated circuits for computing, sensing, and image processing;
- Power supplies and power semiconductors (mounted on a printed circuit board or as a system power supply);
- DC-DC converters, board level;
- RF power devices in gradient amplifiers in CT scanners;
- Detector components in CT scanners;
- Diode lasers and laser bars for surgical laser systems;
- LED, halogen, and incandescent light sources for surgical lamps and operating theater lighting fixtures;
- Motor controllers, drivers, and related components for scanner servo motors and drive motors;
- Motor controllers for liquid pumps and filtration systems; or
- Imaging system tube and related power components.
Thermal design for medical imaging and surgical systems includes extensive use of liquid cooling within large systems. Heat dissipation and thermal solutions exist across a wide range of different types of heat source interfaces. Selection of wellperforming TIMs is critical for the reliable system design and product life key to medical electronics systems.
Categorizing Thermal Interface Material Application Requirements
Applications for TIM materials must first be examined to identify the general category of such a material needed to solve a heat dissipation problem. Across the wide range of TIM material types available, thermal performance and reliability over the life of the electronics system are important criteria. Metallic TIMs are one category of many available TIM types, and distinctions are made by identifying several principal characteristics of an intended design application.
Principal considerations for selection of thermal interface materials include the following.
Primary: Identify the primary requirement that will define the type of TIM to be considered. These are examples of what constitutes major defining characteristics of an application requirement in order to get started:
- Gap-filling: Is there a large distance [e.g., > 0.20mm (0.008")] to fill at the interface between the case of a heat source and a heat-dissipating component, such as a heat-sink, or other metal surface? This is a very common application. Both solid and liquid-dispensed gap-filler TIMs are available with many different characteristics, thicknesses, filler materials, and relative cost differences.
- Adhesive attachment: Must the heatsink be attached with an adhesive (as compared to mechanical fasteners)? If so, this suggests a thermally-conductive adhesive or a material coated with an adhesive on both sides.
- Electrical isolation: If the heat-sink must be electrically isolated, this eliminates several major categories of TIM types that do not meet the requirements of a dielectric, with creep and cut-through requirements. Examples of types eliminated are graphitic and metallic TIMs, compounds and materials containing a metallic particle loading, and others.
- Electrically conductive: Conversely, if a highly-electrically conductive interface is necessary, this suggests evaluation of metallic foils, other metallic TIMs (such as phase-change compounds coated onto an aluminum foil), and graphitic TIMs.
- Very low thermal resistance across the interface: This primary requirement defines the need for very high thermal performance, ideally with mechanical fasteners and the ability to apply a high-clamping force to achieve very minimal thickness. Typically, highly-thermally conductive materials are also highly-electrically conductive, and this suggests evaluation of highly filled dispensed compounds (such as the traditional thermal greases), phase-change materials, metallic foils coated to promote surface wetting, and metallic foils and graphite forms of pads.
Secondary: A second set of application characteristics will influence specific material selections within a major category of potential TIMs once the primary characteristic (above) has been identified and a category of TIM selected. These secondary characteristics are important to overall product thermal performance, product life, and reliability in the specified application conditions. Examples include:
- Device operating temperature envelope, ambient temperature and humidity, and other environmental conditions;
- Fastener clamping force and type, number, and layout of fasteners (determining uniformity of force applied);
- Mating surface flatness and roughness;
- Type of dispensing equipment available or considered;
- Mounting attitude (vertical versus horizontal);
- Shock and vibration;
- Chemical and constituent compatibility of mating surfaces; and
- Other relevant requirements, including health and safety factors.
This general description indicates a two-step process. The primary requirement defines an overall selection for choosing which major category of TIM material to test and evaluate. The secondary requirements provide a finetuning based on application specifics. This suggests evaluation of different suppliers’ materials of one or two types, within the context of the primary TIM categorization.
An evaluation scheme that summarizes this analysis procedure for the many different TIM material requirements and characteristics is shown in Figure 1.
Design decisions can influence some of the trade-offs possible within this overall evaluation scheme. A decision to move away from use of a thermally-conductive adhesive, for example, may be made in order to improve overall thermal performance of the heat-sink plus TIM combination, or if the amount of power dissipated suggests a need for a larger and heavier heat-sink with mechanical fasteners.
Eleven generalized categories of TIM materials are shown in Table 1. This compilation represents a set of general targets for type of material, basic chemistry or material employed, relative thickness ranges, and types of reinforcing carriers available. This compilation is intended as a general guide, as there are thousands of materials available globally from several hundred vendors that fit within the general term “thermal interface material.” This table is arranged in a general ordering, from the lowest-performing and thickness of materials to the highest thermal performance categories.
Included in the 11 categories are graphite sheet materials, which serve a function as heat spreaders but have extremely poor through-plane thermal conductivity. These are most frequently termed nonetheless by many vendors as TIM materials, which is misleading.
Also included is reflowed metal solder, used by several integrated circuit (IC) manufacturers as a so-called “TIM1”. This indicates the interface location is within a semiconductor package, providing a direct thermal path from the backside of the IC die to the underside of the package lid. This category is not typically considered in the general categorization of TIM materials, but is important here, given the thermal challenges and space and volume limitations for semiconductor packaging for implantable medical devices.
Generally, increased thermal performance (i.e., reduced thermal resistance through the interface) is achieved with the use of these categories of TIM materials:
- Highest-performing silicone-based thermal greases
- Phase-change thermal interface materials, liquid dispensed
- Low-melting point alloys (typically with indium as one constituent within an alloy)
- Metallic TIM preforms, patterned
- Reflowed solders (in this discussion, used as a TIM1 within a semiconductor device package)
The relative cost for higher-performing materials in these categories generally increases with increasing thermal performance.
TIM Performance Characteristics
Table 1 illustrates a rank ordering of relative thermal performance for various classes of thermal interface materials. Note the importance of type of material or chemistry, as well as the general relationship of thick materials versus thin materials in this table of improving thermal performance (from first to last category).
The relative bulk thermal conductivity value for each material category is not shown in this table, as other characteristics tend to dominate relative thermal performance through the range of polymeric and non-metallic material types. Dominant characteristics that determine thermal performance for a given TIM include:
- Relative thinness of material;
- Relative clamping force applied, to achieve minimum thermal path; and
- Degree of surface wetting achieved, to reduce or eliminate interfacial contact resistance.
The highest-performing thermal material types are the fully-metallic material categories—when significant mechanical clamping force can be applied or the use of a reflowed lowmelting point solder (such as an indium alloy) is considered. This can be demonstrated in a standardized testing procedure, utilizing an established ASTM test methodology with increasing clamping force applied under laboratory conditions.
Figure 2 illustrates thermal resistance test results per the industry standard test methodology for a metallic TIM preform manufactured as a patterned indium metal foil, versus a high-performance silicone-based thermal grease. Other test methods are also available; the ASTM test standard methodology is utilized to enable direct comparative testing of two or more material types under identical laboratory conditions.
A further examination of thermal performance characteristics of certain material types for an array of integrated circuit applications, the most common for medical electronic equipment of all types, is also available in industry literature.
This article was written by Seth Homer, Product Manager for Strategic Accounts, Indium Corporation, Clinton, NY, and Dave Saums, Principal, DS&A LLC, Amesbury, MA. For more information, Click Here .