Thermal interface materials (TIM) are typically placed between two components in an electronic system to enhance their thermal behavior, such as heat flow between an integrated circuit and a heat sink. Proper thermal management minimizes any thermal fluctuations at these interfaces, which in turn reduces structural damage from overheating, leading to reliable, long-term performance of the electronic device over time.
In the last few decades, many TIMs have been developed to manage the thermal boundary resistance between these layers or components. These TIMs include solid pads, thin reinforced TIMs, and wax-based phase change materials. In recent years, dispensable thermal interface materials have become popular for heat-management applications in the modern manufacturing and assembly of electronic devices. TIM materials are dispensed under slight pressure and hold their shape in the end-use environment. TIMs are typically dispensed onto an electronic component from a package — for example, thermal grease is packaged in a syringe, which makes application easy.
When properly selected and integrated, TIMs can result in long-lasting, high-performance products that meet or exceed customer expectations.
This whitepaper highlights factors that must be considered when selecting the best TIMs and packaging and dispensement systems.
Selecting the most appropriate TIM for an assembly application requires a thorough understanding of performance expectations of the device, the end-use environment, and manufacturing production needs. Dispensable TIMs are available for many types of applications — the two most popular are thin bond line applications and gap-filler applications. Dispensable TIMs may all appear the same, but each has mechanical and chemical behaviors that have been developed for very specific applications and typically cannot be interchanged for another material.
The first step is to identify the type of thermal interface application you are working on. Is it a gap filler application? Gap-filler TIMs are designed to provide low thermal resistance by increasing thermal conductivity. They must be able to absorb all the variances in gap size and be mechanically stable, so they do not flow out or pump out. No clamping force is required.
Or, is the application intended to create a thin bond line? Typically, thin bond line applications are meant to break down the contact resistance between two components or materials — for example, metal on plastic or metal on silica. Surfaces that come into contact with each other have a surface roughness that creates a high thermal resistance. The purpose of the TIM material in these situations (typically thermal grease) is to simply fill microchannels and pores at the contact, breaking up the resistance. In rare cases the thin bond line applications can create gaps if the distribution pattern of the TIM is inaccurately dispensed, or the pressure is too high.
For any thermal interface, it is important to understand exactly what type of material is required — is it to break up contact resistance or fill a gap between two components? Companies also wonder if the gap filler material can serve both roles — will it hold its shape in the gap, but can it also be squeezed down into a very thin bond line? This is not the case. Each TIM material is formulated to have a very specific viscosity and other precise mechanical characteristics, such as particle size and density, to deliver specific heat-transfer behaviors for the application. After the most suitable TIM has been determined, the next step is determining the most effective way to dispense the TIM from its packaging to the component.
Thermal Grease Thermal grease is typically the TIM choice to create a thin bond line for breaking up contact resistance. Only a small amount of pressure is required to position the grease in place. Greases are applied under a constant load, such as springs or clamps, which apply just enough pressure to keep the grease securely in place.
Gap-Filler TIMs Gap fillers are designed to be stable, with minimal flow under low pressures. The goal is to fill gaps and break up the contact resistance between heat source (for example, an integrated circuit) and heat sink.
Gap-filler applications require a TIM that holds its shape and does not require a constant load to stay in place. These materials: • Provide low thermal resistance by having high thermal conductivity • are compliant in order absorb variations in gap size • are mechanically stable—will not flow out or pump out.
For thin bond lines, thermal grease is the most common dispensable thermal interface material. These materials are nonvolatile but may contain solvents (which will be shown on the manufacturer’s data sheets). Thermal greases do not cure and are ready for immediate use.
Thermal conductivity is a common way to evaluate TIM materials. Much of the thermal resistance at an interface is caused by the bulk conductivity of the TIM material. For thermal greases and bond line applications, a balance must be struck between high conductivity and how thin the bond line can be.
Mechanical stability and reliability are other critical considerations for both thermal greases and gap fillers. It is paramount to avoid any pump out or flow out at the thermal interface, which could lead to catastrophic failure.
Most suppliers of TIM materials maintain very robust durability data that shows material performance under various heat, humidity, pressure, and shock conditions, which is critical information to have for determining which TIM is best for the application.
For thin bond lines: • Materials are designed to compress to as thin a bond line as possible. • Conductivity is an indication of performance, but must be balanced with bond line thickness. • The interface and grease are clamped down under steady, low pressure using springs, screws).
Greases require a controlled dispensing system for efficiency and quality. A variety of systems are available, ranging from simple, low-cost dispensing guns to sophisticated automated systems.
A performance advantage of grease over gap-filler TIMs is that, due to its lower viscosity, it is easier to dispense.
Gap Filler Applications There are two types of gap fillers: cured and noncured. • Cured gap fillers use volatile materials that are cured in place. Two separate TIM materials — A and B — are mixed together and cured in place to form a solid elastomer. • Noncured gap fillers are made from a single nonvolatile material and do not required curing in place. These materials remain in a stable, high-viscosity state. Once they are dispensed, they do not migrate from the interface unless the applied pressure is too great. Cured gap fillers are ready for immediate use.
Dispensable thermal interface materials provide manufacturing advantages that speed up production, reduce operator involvement, improved quality, and simplify product design. For example, dispensable TIMs (one part or two part) can be dispensed from the same systems that dispense other materials, such as epoxy. The system does not need to be readjusted or recalibrated for new materials, saving time and money.
With automatic systems, fewer operators are needed to perform dispensing — the equipment dispenses exactly the right amount of material, quickly and expertly. Dispensing systems vary from table-top units that require operators to larger automated systems that require less labor to operate. Selecting a system depends largely on the production volume needs.
Thermal interface materials also require only slight pressure or stress because they are viscous and flow easily. This simplifies the product design process because the more-complex systems that would be required for mitigating larger amounts of force on printed circuit boards or solder joints would not be required.
Packaging is a key part of the dispensing process. What is the best way to get the TIM from its packaging to the component?
For smooth operation and production, dispensing should be considered early in the design for manufacturing (DfM) process, after the TIM has been selected. What equipment is needed? What are the packaging options?
Packaging varies from supplier to suppler—most common are 30-cc or 50-cc syringes for noncuring type TIMs and 50-cc syringes for cured TIMs that require an A-B mixture. A-B mixtures also require a static mixer that can combine both materials as they are dispensed from their cartridges.
Suppliers offer a variety of packaging methods, ranging from small packaging (cartridges or syringes) to large pails for high-volume needs on automated dispensing systems. Custom packaging is also available, however, even small changes in packaging can create negative impacts on the compatibility of A and B materials, so appropriate due diligence is required to determine this before production starts.
Dispensing systems range from syringes (TIM forced out by air pressure or piston) to larger-scale automated table-top systems. Which system is best for a specific application depends on material selected, assembly design, and the volume of production. Dispensing processes can be robotic, delivering the precise amount of TIM to the component with no error. This is typical for higher-volume needs, where the TIM is dispensed from larger packaging such as cans or pails.
Low-cost dispensing options using cartridges or syringes are a good choice for low-volume production or lab use and are easy to operate (pneumatically driven). The amount of material dispensed is controlled by time and pressure, with the TIM directly dispensed from its cartridge through a hosing assembly.
Options include intelligent control vacuum options to prevent oozing. One drawback to consider during the design phase is that, depending on volume, frequent changing of cartridges leads to large volumes of packaging waste, so environmental sustainability should be considered.
The more sophisticated table-top dispensing equipment is available in pneumatic or screw-driven options. An x-y table can be programmed to move and dispense TIM in exact amounts for any pattern of dispensement.
Dispensing large volumes of highly viscous TIM requires considerable force, so stronger packaging is typically needed to withstand these forces. For example, cans that are not rated for these higher pressures will burst if used incorrectly.
Valve systems can also be used to dispense TIM. Typically they face the same challenges and performance requirements that other dispensing methods do. For valve systems, a ram is used to dispense the TIM from the can, providing superior control. There is a large variety of dispensing valves to consider, according to the size, frequency, and volume needs of the TIM “shot.” Some valve systems are made with wear-resistant components when using more abrasive TIM materials that are filled with ceramic particles to help achieve higher thermal conductivity.
Yet another variety is the “snuff-back” high-pressure dispensing valve, which uses time and pressure to dispense the TIM and also has a snuff feature that creates a “pull back” so TIM material does not ooze, resulting in less material waste, higher quality, and better dispensement control.
Several factors must be considered to select the best TIM material and packaging and dispensement systems. These materials and systems are ideal for thin bond line and gap-filling applications in electronic systems to manage thermal flow. When properly selected and integrated, these materials and technologies will result in long-lasting, high-performance.products that meet or exceed customer expectations.
Christian Miraglia is the applications Engineering Manager for Fujipoly America Corporation, based out of its Customer Engineering Resource Center in San Jose, CA. He is a graduate of the New Jersey Institute of Technology and has been with Fujipoly for over 15 years.
Visit https://www.fujipoly.com/usa/products/sarcon-thermal-management-components/thermal-gap-filler-products/ to learn more.
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