Selecting a thermal management material that is broadly applicable to a particular electronic assembly and its predicted operating conditions is a good starting point; however, as with many of these things, the devil is very much in the details! There are a host of available materials and methods to choose from—all of which serve a variety of purposes depending on the physical constraints of the application, such as environmental considerations, severity of duty, component layout, geometry of the assembly, etc.
Generally, the first questions you should ask yourself are, "What type of thermal management product should I apply and what should I look out for when applying it?" There are five main groups of thermal management materials that can be broken down further according to variations in their material chemistries and formulations. These include: curing and non-curing pastes, bonding materials/adhesives, encapsulation resins, thermally conductive gap fillers/pads, and phase change materials.
Non-curing pastes, for example, are ideal for applications where rework may be required. They use different base oils to provide a range of desirable properties, such as the wide operating temperature range offered by silicone-based products. Recent advances in non-silicone technologies have seen the introduction of products offering higher thermal conductivity with significantly reduced oil bleed and evaporation weight loss.
Generally, non-curing products should be applied as thinly as possible with minimal excess. The product must also be well mixed to avoid oil bleed. The golden rule is to not be tempted to apply thicker layers. It doesn’t improve the results and may even prove problematic. Remember, the thickness of the thermal interface material becomes the rate-determining step; the thicker it is applied above the minimum amount required, the slower the rate of heat transfer will be.
If rework is likely to be unnecessary over the life of the assembly, then you might consider using a curing/bonding thermal management product. However, for curing products or those that have a solvent for application purposes, you must consider the working time of the product. For example, if the product touch-dries rapidly, it may not be suitable for stencil printing as the cured product may block the screen.
To bypass this problem, check that the screen mesh size is suitable for the particle size of the paste and that the screen will be able to cope with the thickness of paste required. Moreover, if you are using automated dispensing equipment, the dot profile and quantity of paste should be considered to ensure that the minimum amount of material can still be applied.
A bonding thermal interface material may be required if a heat sink needs to be held firmly in place without the use of fasteners. In this case, it is important to get the bond strength right or the heat sink will likely become dislodged if the assembly is subject to shock or high-amplitude vibration. Another alternative is to use thermal gap pads, which are pre-cut to size and manually mounted without mess or waste, and without the need for dispensing equipment. Gap pads do not move during thermal cycling, so they tend to not suffer pump-out, which is common with some thermal pastes.
The thing to remember with pads is that they provide a thicker interface layer and tend to have a higher thermal resistance. Pads work best for applications where there is a pressure exerted on the interface that minimizes the bond line and ensures maximum contact with the gap pad. This pressure forces the pad material into the air pockets, which effectively reduces the thermal resistance.
Another option for managing the transfer of heat away from electronic devices is to use a thermally conductive encapsulation resin. These products are designed to offer protection from environmental attack and allow heat generated within the device to be dissipated to its surroundings. Encapsulation resins often incorporate thermally conductive fillers to boost their thermal performance, while the base resin, hardener, and other additives can be altered to provide a wide range of options.
Where encapsulation resins are concerned, the entire PCB is likely to be covered and the amount of resin needed will be determined by the protection level required as well as other factors, such as the weight and volume gains contributed by the resin. You will also need to ensure that there are no air voids within the cured resin because this will compromise its electrical properties and thermal transfer performance. As with all resins, check that the mix ratio is adhered to and the product is mixed well using an air-free mixing method.
Next, you should consider the substrate and intended bond line thickness. What is the condition of the substrate? Is the gap size at the interface known? Contact surfaces vary, as do contact pressures. The most important thing is to have no air trapped at the interface because air is a poor conductor of heat. Even minute amounts of air entrapment at the interface due to poorly mating surfaces, inaccurate application of a thermal interface medium, or gaps wider than calculated can reduce the efficiency of thermal transfer.
Determine whether your application is one that requires a thinly applied thermal interface material, such as a paste, or whether a thicker gap filler is required, which would normally be applied to a thickness greater than 500 microns. With any thermally conductive material, you can ensure that the interface between device and heat sink is completely filled and that all air is displaced by applying a quantity of the compound to the center of the mating surface of the device or the heat sink. Next, bringing the two together will displace any excess material as mating pressure increases.
Finally, consider your preferred method of application. Will you use manual methods, such as syringes, or be semi- or fully-automated methods with state-of-the-art dispensing equipment? Screen printing may be another option to consider. As with the materials themselves, if you are making decisions on application technique, always seek expert advice to help and guide you to the correct product and the easiest application method. The goal should be a streamlined production process and provision of efficient heat transfer both in initial application and final use.
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