SMT007 Magazine

SMT007-Oct2018

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64 SMT007 MAGAZINE I OCTOBER 2018 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 parti- cle size of the paste and that the screen will be able to cope with the thickness of paste required. Moreover, if you are using auto- mated 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 pres- sure exerted on the interface that minimises 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 trans- fer 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 conduc - tive fillers to boost their thermal perfor- mance, 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 deter- mined 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 ther - mal 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 inter- face due to poorly mating surfaces, inaccurate application of a thermal interface medium, or gaps wider than calculated can reduce the effi- ciency 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 For example, if the product touch-dries rapidly, it may not be suitable for stencil printing as the cured product may block the screen.

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