The demand for thermal management materials and adhesives is driven by the unwanted and potentially harmful heat generated by ever-shrinking electronic components and systems in all areas of the electronics market, including medical. In recent years, more functionality has been included in a single device and the size of each new device has gotten smaller. Miniaturized components in tight packages with high power output create the challenges that design engineers are now facing.
Thermal management needs are also expanding in hot niche markets like batteries and LEDs, where increased complexity, density, and intensity are causing manufacturers to look for new materials and designs for better thermal conductivity, dissipation, and insulation.
Factors in Thermal Management
There are three important basic factors that need to be considered in any approach to thermal management. These include:
- Good heat sink or heat pipe design and proper airflow
- A high, thermally conductive interface material that is as thin as possible
- Elimination of voids along the interface material so that no trapped air remains between the interface material, power device surface, and heat sink surface
Supporting high-power integrated circuits (ICs) on today’s printed circuit boards (PCBs) requires working within the limitations of thermal conductivity, coefficient of thermal expansion (CTE), weight, and rigidity. Balancing these different considerations is tricky. For example, where copper can be very useful in thermal management, it is not able to manage a board’s CTE, and it substantially increases board weight.
Thermal Interface Materials
As the miniaturization trend continues in medical electronics and other industries, the importance of good thermal interface design has led to development of new thermal interface materials Choosing a thermal interface material that will work in an application, an engineer might consider power density, heat dissipation, bond line thickness, processing requirements, and reworkability.
Thermal interface materials can be broadly categorized as one of the following types:
- Polymer matrix composites — different types of carbon fibers combined with a variety of thermosetting and thermoplastic resins, including epoxy, cyanate ester, liquid crystal, nylon, polycarbonate, acrylonitrile butadiene styrene, polybutylene terephthalate, and polyphenylene sulfide
- Metal matrix composites — silicon carbide particle-reinforced aluminum, beryilia particle-reinforced beryilium, carbon fiber-reinforced aluminum, copper-tungsten, copper molybdenum, aluminum silicon, and Invar silver
- Carbon/carbon composites — carbon nanofibers, vapor-grown carbon fibers, nano-graphene platelets, pyrolitic graphite, and other carbon/ carbon mixes
Advanced thermal interface materials can provide better thermal management characteristics as well as improvements in weight and strength and are being integrated into a range of thermal management solutions, including insulating, hybrid, and nonwoven papers; insulating solders; grease; phase change materials; and conductive adhesives.
Insulating hybrid and nonwoven papers. Lightweight carbon composite laminates, aramid papers, and nonwovens can all be used for heat insulation/ dissipation. Easily die-cut to complex shapes, they provide excellent high temperature, abrasion, and chemical resistance; smooth surfaces; high torsional rigidity and stability; and high or low conductivity for absorbing, reflecting, or conducting heat.
Solder. New formulations in solder are delivering added thermal control to today’s high-performance, high-energy, and high-heat applications. Advances in solder thermal management include:
- Lead-free die attach solder alloys
- Active solder — lead-free solder with titanium or rare-earth elements added
- Eutectic bonding/soldering where silicone is alloyed with metals such as gold or aluminum to offer better heat dissipation/management
Thermal grease. The traditional interface material in electronics is thermal grease. Available in silicone or non-silicone varieties, thermal grease provides thermal resistance through excellent gap filling and an extremely thin bond line. It is reworkable, and it is easy to apply, including automated dispensing. It provides good reliability at a low cost. Thermal grease also can be applied as a thermal interface pad, where the grease is impregnated in the pad. The thermal pad offers the same wetting capability as thermal grease, but can be die-cut to custom shapes for drop-in-place assembly. Issues with grease can include contamination (for silicone grease), pump-out during thermal cycles, and imprecise or inconsistent application.
Phase change materials. Phase change materials are often used to supplement some of the issues related to grease. These materials are solids at room temperature but change to liquid once the excess heat of a device pushes the material past its melting point. Typically composed of a coating of phase change compound on an aluminum or polyimide substrate, new phase change materials can be coated directly onto a release liner without using a substrate. This improves performance by creating a better flow when the phase change material is in the liquid stage, as well as better gap and void filling. The interface is thinner without the substrate, resulting in more efficient heat transfer.
The phase change material doesn’t pump out of the interface like grease and it is more manufacturing friendly. There is no messy application or cleanup necessary, and in many cases, phase change materials provide more reliable thermal management over the long run than grease.
Thermally conductive adhesives. Adhesives provide unique options for thermal management. They are often the best choice where components are not connected by mechanical attachment, or where the micro-movement of substrates requires adhesion for a component to maintain contact with the substrate. They are often used with semiconductor packages as an interface between a chip and a heat spreader. Thermally conductive adhesives can be configured as:
- Interface pads — conformable adhesive pads that are easy to handle and provide high conductivity
- Liquids — usually epoxies, which provide an ultrathin bond line and easy integration into manufacturing dispensing equipment
- Tapes — high mechanical strength plus good surface wetting and excellent shock absorption
In addition to semiconductor applications, thermally conductive adhesives are popular in medical electronics, where they are used for attachment in addition to thermal management. It is possible to use an adhesive that combines thermal and electrical conductivity, for example, as an electrical ground to a board. Or conversely, to use an adhesive that is thermally conductive but electrically insulating.
Using an adhesive for thermal management requires considering potential trade-offs in bond strength versus heat dissipation where thick application increases the bond but decreases heat dissipation. It is also important to consider how much filler is in the adhesive. A lot of filler provides high shear strength but lower flexibility. Finally, the CTE between the component, substrate, and adhesive must be calculated. All of the possibilities need to be assessed in regard to suitability for the manufacturing process and cost.
Working with an Experienced Converter/Materials Supplier
Working with an experienced thermal management materials and adhesives converter and industrial assembly supplier is essential to choosing the right thermal management materials for a particular application.
From identification and selection of the appropriate materials and adhesives, to slitting, layering, laminating, precision die-cutting, and packaging of the finished product, an experienced thermal management materials and adhesives converter provides the design, prototyping, testing, and manufacturing knowledge required for success.
When working with customers who are designing with thermal management in mind, finding the right adhesives and materials is often a process of elimination. The more knowledge of how much heat the component generates, its place within the overall product, and other thermal management details, the shorter the process of selecting and matching appropriate adhesives and materials.
Medical electronics OEMs should look for converters to provide precision die-cutting, multi-layer laminating, and slitting to tight tolerances; access to a range of thermal management solutions; and testing capabilities.
Fabrico, for example, selects from servo driven rotary die-cutting, CNC die-cutting, laser die-cutting, and water jet die-cutting to meet the complex specifications of thermal management for electronic components. For example, a servo-driven rotary die-cutter can maintain tight tolerances ranging from 0.015 to 0.005 in. at speeds up to 500 fpm, and is ideal for the complex, multilayer die-cutting and lamination that a thermal interface pad or tape may require.
For complex foam tape die-cutting, water jet technology provides clean edges with no distortion. Laser die-cutting, kiss-cutting, slitting, and laminating can also be used in converting for medical applications. If a grease or liquid thermal interface material is selected, the converter can provide and plan for easy integration into the manufacturer’s process with dispensing recommendations and solutions.
A converter with a fully equipped test laboratory can ensure that customer materials meet designed-in specifications before they move to the factory floor, often eliminating the need to test materials at the customer’s facility. A complete test lab offers:
- Accurate and precise part dimension measurement and verification
- Adhesive/release liner to determine converting properties and high-speed application characteristics
- Material strength measured to ensure that material meets application requirements
- Static shear testing to measure the cohesive strength of the adhesive to withstand a fixed load over time
- Material weight measurement to determine adhesive coating weight
- Microscopic imaging to determine differences between adhesive and material over time
- Dielectric testing to determine a material’s electrical insulation properties
- Thermal testing for materials and adhesives
- Resistance and voltage testing to provide a complete profile of the electrical properties of a material or adhesive
Another important consideration is to ensure that the converter has strategic relationships with world-class materials suppliers, such as 3M™, Loctite®, and Adhesives Research to assist its customers in selecting the best material for the intended use and to expedite materials sourcing. Whether adhesive films or liquids, all critical material properties must be considered in every project, including chemical, thermal, and moisture resistance.
This article was written by Christian S. Yorgure, Ph.D., Manager of Business Development at Fabrico Medical, Rochester, NY. Fabrico, Light Fabrications, and Trient Technologies are divisions of EIS, a Genuine Parts Company. For more information, click here .