As advances in electronic products have required higher power in smaller packages, the challenges associated with thermal management have become more intense. Two general types of thermal interface materials – gels (or dispensable gap fillers) and gap filler pads – are commonly used by design engineers for displacing air voids and ensuring proper heat transfer. Each has distinct advantages depending on the application.

Following is an analysis of key performance and manufacturability characteristics in both conventional gap pads and new advances in dispensable gel materials.

Heat Transfer Fundamentals

The objective of thermal management in electronics packaging is the efficient removal of heat from the semiconductor junction to the ambient environment. Thermally conductive materials are used to eliminate air gaps or voids from adjoining rough or uneven mating surfaces. Because the thermal interface material has a greater thermal conductivity than the air it replaces, the resistance across the joint decreases, and the component junction temperature will be reduced.

When evaluating thermal interface materials, design engineers seek to identify high-performance products that meet the thermal, design, manufacturing and cost challenges inherent in each customized application. Several material types have been developed in response to the changing needs of the electronics packaging market in applications such as telecommunications equipment, consumer electronics, automotive electronics, LEDs/lighting, power conversion, power semiconductors, desktop computers/laptops and servers, handheld devices, memory modules and vibration dampening. In this analysis, we are focusing on pads and liquid or gel-like materials.

Gap Filler Pads: The Basics

Thermally conductive gap filler pads offer excellent thermal properties and high conformability at low clamping forces. Key features and benefits include:

  • High conformability
  • Ultra-low deflection force
  • High-tack surface to reduce contact resistance
  • Minimal component stress
  • Reduced “hot spots” on printed circuit board
  • Thicker pads can improve vibration dampening
  • Can “blanket” multiple components

Gels: The basics

This schematic represents two surfaces in contact and heat flow across the interface without (left) and with thermal interface material applied.

Next-generation gels are highly conformable, pre-cured (in most cases), single-component compounds – and feature a cross-linked gel structure that provides long-term thermal stability and reliability. Key features and benefits include:

  • One-component dispensable: Eliminates multiple part sizes/numbers; aids in automation
  • Fully cured: Requires no refrigeration; no mixing or additional curing; no settling in storage
  • Highly conformable at low pressure: Applies minimal stress, making it effective for delicate components
  • High surface tack

Pads vs. Gels – Understanding Key Differences

Thermally conductive gap filler pads are cut-to-shape and applied manually to offer thermal properties and high conformability at low clamping forces.

Both pads and dispensable materials have proven to be effective means of thermal management in many industries and applications. While pads have a longer proven track record, recent advances in gels have closed the performance gap and gels have, in some cases, surpassed the performance of pads. Following are ways newly engineered gels compare to gap pads in matters of critical importance to design engineers:

Performance Characteristics

Conformability – Both gels and pads are conformable to a degree, but the maximum configurability of a gap pad is less than that of a dispensable due to its solid structure.

Flow rate – The goal when developing gels is to achieve the highest and most repeatable flow rate. Customers want to be able to set their dispensing equipment for the same flow rate batch-to-batch to maintain a consistent volume of material and avoid waste. Newer dispensable technologies achieve a higher, more repeatable flow rate that improves throughput and reduces waste.

Thermal conductivity – Identifying the amount of heat [Watts] in need of dissipation will determine the thermal conductivity performance needed in that application’s gap filler. This is usually calculated and measured in Watts per meter Kelvin, or W/m-K. The higher the value, the more heat the material can theoretically dissipate. Both pads and gels are capable of dissipating a large amount of heat, generally from 1 W/m-K to 10 W/m-K, depending on the specific product used.

Pads are cut from sheets of various options of material carriers with peel liners to expose the tack surface.

Long-term reliability – While this issue might be the great unknown in real-world applications because of the newness of advanced gels, rigorous accelerated aging, thermal shock, and vibration tests have shown that gels are capable of achieving long-term reliability.

Manufacturing/Assembly/Cost Dynamics

Automation – The opportunity for automation is a significant advantage for gels because dispensing systems are quite versatile. While pad placement can be automated to an extent, the equipment and fixturing required to do so is typically quite specialized and may not be readily adapted from one job to another. Even with dispensing equipment programming time built into the setup cost, the process can be cost-efficient compared to the time required to design and produce pad-application tooling and the additional effort to then qualify the application process.

To maximize thermal performance, the thermal material must contact the entire target area on both the component and heat sink surfaces without air entrapment. In order to achieve this, a proper dispense pattern is critical. Taking part considerations into account, the next process design task is to specify the dispensed material pattern. A simple dot like the first pattern provides adequate coverage, shortest cycle time and least chance of introducing air into the thermal interface material. The more complex the profile, the greater the probability for introducing air (e.g., serpentine and spiral).

Throughput – Speed in part production is application-dependent, but to illustrate the potential advantage of gels we will cite a specific customer example. This particular customer was considering a switch from pads to gels and ran a test of both materials to gauge the difference in throughput. Their study revealed that it required an operator 18 seconds to apply one pad, including handling the pad, placing it properly and then moving on to the next component. Using a dispensable and an automated process, those same steps required only four seconds.

The argument favoring gels grows even more convincing if there are multiple dispense locations on a single part. An automated/robotic dispenser can hit each of the locations in one cycle, whereas with a gap pad, the operator would have to apply a pad to each part individually.

When designing in a dispensable thermal interface material, there are several considerations to keep in mind when determining the appropriate product. The main purpose of the material is to conduct heat, but with a dispensable gel, there is more to the selection process than simply evaluating thermal conductivities. These tables show various considerations related to the volume of parts involved in an application. (See larger image.)

Ease of application – When placing a gap pad, the operator needs to know the pad’s orientation. There is a top-side and bottom-side to the pad, and in many instances, there are left-right and/or up-down orientations. Manual application introduces more risk for human error. With gel applications, the metered material is simply dispensed onto a specific location.

Precision – A benefit of the gap pad is that it can be cut to the exact shape of the customer part, whereas the gel takes the shape of how it spreads out once it is compressed. The specific application will drive the degree of precision required, as well as determine the acceptability of whether the material extends beyond the surface of what it is being applied to.

Shape complexity – The shape a gel is dispensed into can help determine and control the ease of manufacturing and shape of the resulting spread. For instance, a dollop or “Hershey Kiss” type of shape is the simplest to dispense and will result in a roundish cross-section. An X-shape or serpentine dispense pattern results in a square cross-section at the expense of throughput. Very complex or thin shapes may not necessarily be well accommodated with die-cut gap pads; a dispensable may be able to achieve those geometries better.

Cost – On a broad spectrum, gels tend to be less expensive on a volume basis when comparing them with a gap pad product of a similar performance level. Experience with multiple applications suggests that about 5,000 parts per year is the threshold where it becomes more economical to use gels and an automated dispensing system versus pads that are manually applied for the same application. Shape and geometry play heavily into that calculation however.

Dispensing equipment investment can start at $10,000 to $30,000 for low-volume, tabletop units that require an operator. Increased sophistication and added features like camera recognition for quality control bring the equipment cost to $80,000 to $120,000 plus installation and training for a fully automated system.

Packaging – Gel packaging starts in 10-cc cartridges, which are suitable for manual dispensing for use as samples or very low volume applications. The next tier up is a variety of pneumatic-dispensed cartridges ranging from 30cc to 600cc. Those require simple dispensing equipment, which typically may include a high-pressure air line with a regulator and nozzle to connect it to the cartridge. An operator can dispense it by hand or there could be some type of robot-assist mechanism.

THERM-A-GAPTM GEL 37 is a dispensable thermal gap filler that can be applied fully cured in custom geometric shapes depending on the application.

The largest packages, supporting the highest throughput volumes, are one- or five-gallon pails. These require a pneumatic pump to push the material into a secondary metering valve.

Pad packaging is essentially as complex as the customer part. A geometrically complex die-cut pad has costs associated with developing and producing the pad to that shape. However, there are other options if a customer wants to handle cutting the pad on their own and/or has multiple shape requirements with a lower budget. For instance, pads are available in sheet form that can be readily cut or trimmed prior to application.

Liquid/gel dispensing gives the customer more control, which is often an advantage. Customers can make changes on the fly without having to change a drawing, perform a first-article inspection and complete formal engineering change control procedures involved in modifying the design of a part, for example.

Conclusion

Gap filler pads have long been the go-to choice for many design engineers, but recent advances in thermal gels, which are highly conformable, pre-cured single-component compounds, can provide superior performance, a greater ease of manufacturing and assembly, and a lower cost in certain high-volume applications; particularly as electronic design applications get smaller, more fragile and more complex.

Maintaining an open mind to using high-performance gels is a consideration that could pay off in performance, manufacturing efficiency and cost savings.

This article was written by Jonathan Appert, Research and Process Development Engineer, Parker Chomerics (Woburn, MA). For more information, visit here .


Aerospace & Defense Technology Magazine

This article first appeared in the February, 2021 issue of Aerospace & Defense Technology Magazine.

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