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Examining whether to gel or pad

New electronic product generations bring new challenges associated with thermal management and there are two general types of thermal interface materials: gels (or dispensable gap fillers) and gap filler pads which can be used.

However, while both displace air voids and ensure proper heat transfer, each one has distinct advantages depending on the application.

Demand for effective thermal interface materials is rising in direct response to changing needs in the electronics packaging market.

The objective of thermal management in electronics packaging is the efficient removal of heat from the semiconductor junction to the ambient environment and this process can be separated into three phases:

1. Heat transfer within the semiconductor component package

2. Heat transfer from the package to a heat dissipater (the initial heat sink)

3. Heat transfer from the heat dissipater to the ambient environment (the ultimate heat sink)

The first phase is generally beyond the control of system-level thermal engineers because the package type defines the internal heat-transfer processes. In the second and third phases, the packaging engineer’s goal is to design an efficient thermal connection from the package surface to the initial heat spreader and on to the ambient environment.

Achieving this ambition requires a thorough understanding of heat-transfer fundamentals, as well as knowledge of available interface materials and their key physical properties.

When evaluating thermal interface materials, engineers look to identify high-performance products that meet the thermal, design, manufacturing and cost challenges inherent in each customised application.

Gap filler pads and gels

Both pads and gels offer effective means of thermal management. While pads have a longer proven track record, recent advances in gels have, in some cases, surpassed their performance. The following are ways that newly engineered gels compare with gap pads in matters of critical importance to design engineers.

Both gels and pads are conformable to a degree, but the maximum configurability of a gap pad is less than that of a gel 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 set their dispensing equipment for the same flow rate batch-to-batch to maintain a consistent volume and avoid wasting any material. Newer gel technologies achieve a more repeatable and higher flow rate that improves throughput and reduces waste.

Identifying the amount of heat (Watts) in need of dissipation will determine the thermal conductivity performance required of the application’s gap filler. The higher the value, the more heat the material can theoretically dissipate. Industry-wide, the thermal conductivity of gap pads and gels can range from 1 to 10 W/m-K (Watts per meter Kelvin).

While this issue might be the great unknown in real-world applications because of the newness of advanced gels, rigorous accelerated aging tests can be used to aid long-term reliability. Three different aging treatments can be performed on a fixture with the gel compressed between two stainless steel panels: a dry heat soak at 125° C; heat and humidity at 85° C and 85% relative humidity; and temperature cycling from -40° to 125° C. Future state, a thermal shock element is added, while vibration testing is also performed based on the GMW3172 standard.

In the curing process, cross-linking refers to relatively small molecules joining much larger polymer chains in the thermal interface material. Although both pads and gels cure via the same cross-linking mechanism, in a pad there are more cross-links, which leads to a stronger cure. Curing contributes to the viscoelastic properties of gels, which improves their form stability over a non-cured system.

Assembly and cost dynamics

The opportunity for automation is a significant advantage when it comes to gels. While pad placement can be automated to some extent, the equipment and fixtures required is typically specialized and may not readily adapt from one job to another.

Production speed will be application-dependent, but to illustrate the potential advantage of gels, consider a specific customer example. This particular customer was contemplating a switch from pads to gels and tested both materials to gauge the difference in throughput. The study revealed that it took an operator 18 seconds to apply one pad manually, but this process reduced to just four seconds using a gel and an automated process.

The argument favouring gels grows even more convincing if there are multiple dispense locations on a single part. An automated/robotic gel dispenser can hit each location in one cycle, contrasting greatly to the manual, individual application of gap pads.

When placing a gap pad, the operator needs to know its orientation. There is a top side and bottom side, and in many instances, there are left-right and/or up-down orientations. Whereas manual pad application introduces more risk of human error, with gel application the metered gel is simply dispensed onto a specific location.

Precision and shape

One benefit of gap pads is that they can be cut to the exact shape of the customer’s part, whereas gel adopts its post-compression shape. The specific application will drive the degree of precision required, as well as determine the acceptability of gel expansion beyond the application surface.

The choice of gel shape can help determine and control the ease of application and any resulting spread. For instance, a dot shape is the easiest to dispense and will result in a roundish cross-section. An X-shape or serpentine 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 gel may be able to achieve those geometries better.

To maximize thermal performance, the interface material must contact the entire target area on both the component and heat-sink surfaces without air entrapment. A simple dot pattern provides adequate coverage, the shortest cycle time and the least chance of introducing air into the thermal interface material. The more complex the profile is, so the greater the probability for introducing air (e.g., serpentine and spiral).

Broadly speaking, gels tend to be less expensive on a volume basis against comparable gap pads. 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 manual pad application (depending on shape and geometry).

Dispensing equipment investment can start at $10,000 to $30,000 for low-volume table-top 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.

Above: Gels v pads - dispensing patterns compared

Packaging solutions

Gel packaging starts in 10 cc cartridges, which are suitable for use as samples, or for manual low-volume applications. The next tier up is a variety of pneumatic-dispensed cartridges ranging from 30 to 600 cc. These cartridges require simple dispensing equipment, such as a high-pressure air line with a regulator and nozzle. An operator can dispense manually or there could be some type of robot-assist mechanism. The largest packages, supporting the highest throughput volumes, are one- or five-gallon pails, which require a pneumatic pump to push the material into a secondary metering valve.

While a geometrically complex die-cut part pad has costs associated with development and production, there are other options if a customer has a lower budget and wants to handle pad cutting in-house. For instance, pads are available in sheet form that can be readily cut or trimmed prior to application. In terms of gel dispensing, the provision of more control allows customers to make changes on the fly without having to modify a drawing, perform a first-article inspection, or complete formal engineering change procedures.

Gap filler pads have long been the go-to choice for many design engineers. However, recent advances means that thermal gels can provide superior performance, easier manufacturing and assembly, and a lower cost in certain high-volume applications; particularly as electronic design applications get smaller, more fragile and more complex. Ultimately, maintaining an open mind to using the latest gels is a consideration that could pay off in performance, manufacturing efficiency and cost savings.

Author details: Jonathan Appert is a Research and Process Development Engineer, Parker Hannifin Corporation, Chomerics Division

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Jonathan Appert

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