Metallic TIM Testing and Selection for IC, Power, and RF Semiconductors

Thermal interface materials (TIMs) provide an important performance and reliability mechanism for operation of semiconductor devices. Thousands of products developed and sold as TIMs exist in many different forms, which can be classified into fourteen different general types, and more such materials are in development globally. The large number of such existing materials and the continuing desire to develop new types and new forms are driven by several factors, one of which is the continuing need to improve overall operating reliability of electronic systems. This presentation is intended to provide an overview of TIM function, requirements, and reliability factors, followed with an outline of recent developments in several types of metallic TIMs and related design goals for development, testing, selection, and use in several market application areas.

Thermal Management Trends

The continuing trend for device miniaturization and increased functionality and operating speed for processors and other integrated circuits typically leads to increases in heat dissipation and, importantly, increased heat flux. A second trend impacting system thermal design for power semiconductors and integrated circuits is the need in certain types of equipment to operate in increasingly harsh environments, including higher ambient temperatures, reducing thermal design headroom.1 In RF and microwave systems, the transition from silicon to gallium nitride (GaN) RF devices typically frequently results in higher device heat flux, as GaN RF die are typically smaller and, even with identical power dissipation as a preceding silicon device generation, results in higher heat flux.

A recent industry trend has been a focus solely on selection of thermal materials based on bulk thermal conductivity rating and thermal resistance.

These are trends which influence requirements and selection for thermal materials at each level of package and system design. Developments are continuing in a less well-known type of TIMs that is based on the use of metallic foils and preforms. Metallic preforms enable an electrical path to be maintained for certain types of devices with an electrically-live baseplate or exposed contact, as well as meeting system design targets for thermal performance, ease of handling and placement, system reliability, rework, and other criteria.

Function

The primary purpose of thermal management design for electronic systems is to effectively dissipate heat generated by semiconductor devices, batteries, power supplies, and similar elements. Thermal management requires continuous heat dissipation from powered devices and maintenance of device and system temperatures within a specified range. Other important temperature-dependent factors in system operation include ambient operating and storage temperature ranges, humidity and environmental conditions such as prevalence of dust or corrosive gases, altitude, shock, vibration, and need for ruggedizing for operation in harsh conditions.
A critical determinant of device reliability is proper control of device operating temperature within a specified range. Thermal interface materials provide a critical component in the heat removal path from a semiconductor to an ultimate heat sink.
A recent industry trend has been a focus solely on selection of thermal materials based on bulk thermal conductivity rating and thermal resistance. It is important to note that while a tested thermal resistance value in an application is a critical determinant of system performance, the TIM selected must also meet requirements for factory rework, field rework, and future semiconductor upgrade capability as important practical factors in system design and maintenance.

Mechanical Performance Requirements

Introduction of a thermal interface material between a semiconductor or other heat-dissipating component and an attached metal heat sink or liquid cold plate is required to provide the best contact and thermal path between the two surfaces. The ideal interface is direct metal-to-metal contact between two surfaces across the contact area. While providing the most efficient thermal interface, the excessive cost of machining to a mirror surface with perfect flatness and parallelism is impractical for virtually all types of electronic system designs. The practical alternative is the use of a TIM designed to drive fill any gap due to surface roughness, lack of parallelism, or inadequate clamping force applied. There are two major types of attachment which must be considered first: (a) adhesives; and (b) non–adhesive TIMs designed for use with mechanical fastening of the heat sink or cold plate. All of the following discussion refers to the second category, assuming non-adhesive TIMs and the use of mechanical fasteners.

The majority of commercially available thermal interface materials utilize polymeric compounds in different forms, using many different chemistries.

The ideal TIM would be a preform or compound that can be placed or dispensed, meeting these criteria:

  • Applicable in a very thin layer sufficient to fill any gap created by lack of parallelism between two mating surfaces. [Note that all TIM materials are typically of lower bulk thermal conductivity than the adjoining metal surfaces, which are typically copper, aluminum, or a CTE-matched composite or laminate.]
  • Relative compliancy, to allow the TIM to move and conform to surface variations in the mating surfaces.
  • Easily placed, dispensed, or screened in semi-automated handling equipment, where a known thickness can be reliably and repeatedly placed to a pre-determined thickness.
  • If a polymeric compound, relatively thixotropic to ensure that no unwanted run-out occurs, even in vertical mounting orientation and high temperature conditions.
  • Not susceptible to pump-out due to mechanical pumping action of the package baseplate; this is applicable to power semiconductor modules that have a large baseplate area, where electrical switching creates conditions equivalent to power cycling, driving mechanical expansion and contraction for materials within the module.
  • No outgassing, no separation of a silicone oil carrier (known as bleed out or bake out), no potential contaminants that can redeposit on solder joints, optical elements. Organic compounds may not contain ionic contaminants.
    Performance of a TIM can be affected by several general design factors, including the integrity of the mechanical design, such as
  • Number of mechanical fasteners and placement, to eliminate potential for bowing and warpage of one or both surfaces.
  • Clamping force applied. The majority of TIM types will demonstrate an improvement in thermal resistance value with increased clamping force; typical thermal performance graphs in vendor data sheets are intended to show thermal resistance versus clamping force, given the importance of this factor in achieving optimal thermal performance.
  • Surface flatness and surface roughness of mating surfaces.
  • Mounting attitude.
  • Ambient temperature, maximum operating temperatures at the baseplate (i.e., interface surface), humidity, and other environmental conditions.

Reliability and Failure Modes

The majority of commercially-available thermal interface materials utilize polymeric compounds in different forms, using many different chemistries. Several common failure modes are common with polymeric TIMs in power semiconductor and some IC applications. These include:

  • Pump-out, as described above, during operation of semiconductor modules with large surface area, due to temperature-induced expansion and contraction.
  • Dry-out or migration of certain carriers, typically a silicone oil.
  • Outgassing of silicone (and potential redeposition on other components).
  • Run-out of an organic compound lacking sufficient thixotropic behavior. This phenomenon may also be induced due to high operating temperatures (i.e., >150-200°C) and/or operation in a vertical mounting orientation. Run-out can occur with a silicone-based thermal grease or certain gels and phase-change compounds.

Failure modes of this type are not uniformly found with all chemistries for thermal interface compounds. However, the importance of the function provided by the TIM for semiconductor life and reliability has caused certain major power semiconductor manufacturers and electronic systems manufacturers to test candidate materials in elevated temperature environments, typically in vertical orientation of the module and TIM under test, in a clamped condition, and to perform such other tests as necessary. Some companies have also specified that no silicone-containing compound may be used.

Storage, Handling, and Application Requirements

Basic requirements for a TIM in practice include factors not directly related to minimizing thermal resistance across an interface. Protection from shipment damage, storage and shipment temperatures, ease of delivery and placement (especially in high volume assembly operations), and ability to rework are common examples of practical design specifications for a thermal interface material.

Many applications require a TIM that can be easily reworked, referring to the ability to separate the two components when necessary. This does not suggest that a TIM that has been removed can be reused in the same application; a new TIM of the same type should be placed prior to reassembly. Rework requirements vary by industry segment and cost factors, type of equipment, and the value of one or more components. Rework requirements generally do not apply to very high volume, cost-sensitive devices, but can be critical in certain areas within aerospace, satellite, phased array radar, traction, and similar equipment markets. This reflects significantly higher cost of semiconductors, such as for radar transmit/receive modules, in these markets. There are different types of rework requirements and when a rework requirement is specified for an assembly, this can restrict the types of materials that may be used. The ability to separate and remove a heat sink or cold plate, without requiring preheating or other processes, can for such system designs eliminate consideration of certain TIM types. Cleaning of surfaces must be undertaken during rework and any need for a solvent or other chemical may also restrict the types of TIM considered.

Potential for damage to a TIM, assembly processes, storage temperatures, and other factors may result in specifications for protection of the material, depending on the type of chemistry and format, or be driven by quality assurance processes. Packaging of thin films, compound coatings on carriers, and care in handling requirements are also important to maintain product integrity, given the importance of the for function of a TIM. A requirement that a TIM manufacturer provide a release liner with a tab is common some TIM preforms with a compound that may be easily damaged if exposed, as an example. The release liner provides protection for the compound or film during packaging, shipment, and handling, after a TIM is applied to a heat sink baseplate by the heat sink manufacturer. After shipment to the system manufacturer and assembly, the tab on the release liner provides a visual indicator to determine during assembly quality checks that the release liner was removed during system assembly.

Metallic TIM Developments

The summary above outlines typical requirements for thermal interface materials, broadly. The use of metal foils eliminates certain failure mechanisms found with polymeric compounds in various forms, such as compound separation, run-out, and outgassing. While flat metal foils manufactured of indium, indium alloys, and even copper alloys have been used at TIMs for decades for certain types of applications, this practice has not been well known across the industry. Indium foil (commonly known as “shims”) have been applied as a TIM for flange-mount RF semiconductor packages, with mechanical fasteners, for decades. These shims act as a TIM and also provide an electrical path through the baseplate or flange, typically manufactured with copper or copper-molybdenum (and similar materials).

Cleaning of surfaces must be undertaken during rework and any need for a solvent or other chemical may also restrict the types of TIM considered.

Flat metallic foil TIMs do not offer potential for compression or a relative compliancy to adapt to non–flat surface conditions, excessive surface roughness, or lack of parallel interface surfaces. Applications typically have utilized relatively high clamping forces to maximize surface contact across all surfaces.

Recognizing the need for improvements, in particular for large surface area modules where a relatively large gap is to be filled with a TIM, innovative patterning has been developed primarily for indium and indium-alloy foils. Further work has now also applied these specialized types of patterning to a wider number of metals and alloys, expanding the range of devices and markets where such TIMs can be used, to meet specific cost, rework, environmental, and other factors.

The application of patterning is intended to allow the use of a relatively thin foil with a bulk thermal conductivity that is substantially greater than that of traditional polymeric TIM types, to fill gaps that are as much as twice the nominal thickness of the foil prior to application of a given pattern. Table 1 (above) lists bulk thermal conductivity for several TIMs that have been developed and evaluated, manufactured with several metals and metal alloys. The bulk thermal conductivity of traditional silicone-filled thermal greases typically are in the range of 0.5 – 7.0 W/mK, in commercial products. [Bulk thermal conductivity of certain other materials that are often termed as TIMs, such as graphite heat spreaders, is advertised to be available in ranges up to 1,500 W/mK; however, these typically are highly anisotropic materials intended for use as heat spreaders, not as TIMs. Typical through-plane (Z–orientation) thermal conductivity for such materials is typically as low as 5.0 – 15.0 W/mK.]2

As an example of a typical application, a large isolated gate bipolar transistor (IGBT) power semiconductor module is supplied with industry standard baseplate dimensions of 122.0mm by 62.0mm. The module baseplate flatness combined with the specified flatness of the liquid cold plate mounting surface may indicate a potential gap of as much as 0.15mm, depending on the specifications provided by the manufacturer of each component.

Metallic TIM Thermal Performance

Test data is shown in Figure 1, illustrating the thermal performance difference achieved with the Application of the patterning to an indium metal foil, compared to the same thickness of indium foil without patterning. Comparative data is included for a well-known silicone-based thermal grease, tested in the same test stand with identical conditions. [Thicknesses of the grease, stenciled in place to a known thickness, and the foil thickness are shown in the key; the foil thickness for the patterned metallic TIM is prior to application of the patterning.] The importance of clamping force applied is demonstrated by the data in Figure 1. For clamping force less than approximately 42 PSI (2.9 bar), the silicone thermal grease provides better thermal performance, measured as thermal resistance per unit area at a given pressure, using the standard ASTM D 5470-12 measurement procedure.4 For system applications where a clamping force greater than 42 PSI can be applied, the use of the patterned metallic TIM demonstrates an important performance improvement. The alloy used in this example is 99.99% indium, for both the flat foil and the patterned foil. Other metals and alloys will yield different thermal resistance values.

The importance of comparative thermal resistance data generated per ASTM D 5470-12, in controlled laboratory conditions with all variables such as varied surface roughness, uneven or non-parallel contact surfaces, and similar, is to evaluate one material directly to another, as shown in Figure 1. This is an important step for down selection of a very small number of thermal interface materials to test in the intended package or system assembly, where there may be less control in an application. The use of the standardized test conditions under ASTM D 5470-12 is the first step in evaluation.

[Note that in Figure 1, above, comparative data for flat and patterned metallic TIMs includes reference points marked as “A” and “B”, illustrating the reduction that can be attained in clamping force required for comparable thermal resistance values. This reduction is achieved with the addition of patterning to a flat foil of the same thickness. A significant reduction force required can be important for devices with certain types of mechanical restrictions.]

A commonly used second step for selection of TIMs applied to the case of a semiconductor (typically referred to as TIM2) is to test in-situ under given conditions of an application. An example is testing with an IGBT module or a flange-surface flatness value may not be known or where a minimum number of fasteners may allow some degree of warpage of the baseplate under clamped conditions. Figure 2 illustrates in-situ test results conducted with a GaN RF device in an RF industry standard flange-mount package with a CTE-matched metal baseplate and two fasteners. GaN RF devices built into a prototype module by the systems design company have smaller die area and higher heat flux values than previous silicon die generations; this higher heat flux requires a higher performance TIM2. This data, generated in testing with prototype modules, compares the performance of the silicone thermal grease that has customarily been used versus the thermal performance of a patterned metallic TIM, with two different clamping pressures and two different modules tested.6,7

Operating Conditions and Reliability Data

The use of a metallic TIM, as a flat foil or as a patterned and compressible material, has an additional benefit for certain types of applications where an electrical path is desired through the device baseplate and the TIM. The flanged GaN RF devices tested by an OEM, with thermal performance results as shown in Figure 2 are an example of this type of application.

Metallic TIMs also may be operated in harsh conditions which may not be appropriate for organic TIM types. Elevated operating temperatures and use in liquid immersion cooling are examples of operating environments which may preclude consideration of other types. Operation of electronic devices at high temperatures, such as are found in geothermal and oil and gas exploration tool markets, require that all materials and components be tested at the specified maximum operating temperature. Table 2 is a compilation of suggested maximum operating temperatures for metallic TIMs manufactured with different metals and alloys.6,7

These suggested maximum values illustrate how specialized TIMs can be selected to meet very specific requirements. Operating temperature in excess of 175°C for power semiconductors (not common in computing systems) is in the range typical of geothermal and oil/gas exploration electronic systems.1 Silicon carbide semiconductors capable of operation at higher temperatures drive the need for development of TIM and encapsulation materials operating in the same temperature range.9

An example of a specialized metallic TIM for use in applications with high clamping forces and for burn–in and test requirements where repeated contact to devices under test must be made reliably is the development of indium metal clad with aluminum, with patterning applied. The aluminum cladding is the TIM2 surface which contacts the device under test, for burn-in requirements, leaving no residue or marking on the surface of the semiconductor.10

Power cycling data for several different TIM types, including both polymeric and metallic TIMs, is shown in Figure 3. Testing has continued during development using different evaluation tools for reliability of these metallic TIMs, with comparisons to traditional TIM types. The purpose of this evaluation is to demonstrate performance under extreme conditions, such as power cycling and high humidity environments, as performance beyond time zero is increasingly important in evaluating TIM reliability.

Summary

New forms of thermal interface material types continue to be developed to address critical needs for reliability and product life for semiconductors. Industry trends continue to require new developments for very specialized thermal interface materials to meet new requirements, such as higher operating temperatures, operation in harsh environments, elimination of potential for outgassing and silicon migration. Development of patterned metallic TIMs, demonstrating improvements in thermal performance and reliability in testing, when compared to commonly-used TIM products, has been discussed. A range of metals and alloys, thicknesses, and patterns have been developed to meet increasingly specialized requirements for TIM2 applications for semiconductors.

REFERENCES
1. Pathak, A.D., “High Temperature DC to DC Converter Operates in +215°C Environment, Meeting Demands of Down-Hole Oil Exploration Market,” IMAPS International Conference on High Temperature Electronics Network, Cambridge UK, July 6-8, 2015.

2. Saums, D.L., “Developments in Advanced Thermal Materials,” IMAPS France 10th Workshop on Thermal Management and Micropackaging, La Rochelle, France. February 4-5, 2015.

3. Wilson, G., “Sn+ Metallic Thermal Interface Materials for Superior Thermal Management,” Proceedings of IMAPS France 10th European Micropackaging and Thermal Management Workshop, La Rochelle, France, February 2015. “Heat-Spring” is a Registered Mark of Indium Corporation, USA. “Indalloy” is a registered mark of Indium Corporation, USA.

4. ASTM International, ASTM D5470-12, Standard Test Method for Thermal Transmission Properties of Thin Thermally Conductive Solid Electrical Insulation Materials, ASTM International, Philadelphia PA USA, 2012.

5. Lasance, C.; Murray, C.T.; Saums, D.; Rencz, M.: Challenges in Thermal Interface Material Testing, Proceedings of Semi–Therm Symposium 22, Dallas TX USA, March 13-15, 2006.

6. Jarrett, R.N., et al., “Comparison of Test Methods for High Performance Thermal Interface Materials,” Proceedings of Semi-Therm Symposium 23, March 2007.

7. For additional information on TIM testing with ASTM D 5470-12 methodology, transient testing techniques, and the use of thermal test vehicles, see also: C. J. M. Lasance, A. Poppe (eds.), Thermal Management for LED Applications, DOI 10.1007/978-1-4614-5091-7_8, Springer Science+Business, New York, 2014. Refer to Ch. 8, Sections 8.8 to 8.12.

8. Nelson, C.; Galloway, J.; Fosnot, P., “Extracting Thermal Interface Material Properties with Localized Transient Pulses,” Proceedings of Semi-Therm Symposium 30, San Jose CA USA, March 2014.

9. Sili, E., et al., “Study of the Electrical and Thermal Properties of a Silicone Elastomer Filled with Silica for High Temperature Power Device Encapsulation,” IMAPS International Conference on High Temperature Electronics, Cambridge UK, July 6-8, 2015.

10. Sanchez, J.A., “Challenges of Thermal Interface Materials in Test of IC Packages,” IMAPS Advanced Technology Workshop on Thermal Management 2013, Los Altos CA USA, November 5-7, 2013.

 

BY TIM JENSEN, SR. PRODUCT MANAGER (PRESENTING AUTHOR) AND DAVID L. SAUMS, PRINCIPAL (CORRESPONDING AUTHOR) INDIUM CORPORATION

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