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62 DESIGN007 MAGAZINE I JUNE 2019 on basic requirements, such as ensuring that the correct storage conditions for the product are being maintained. Operate good stock ro- tation so that the resin is not left in storage for a prolonged length of time, and always make sure that containers are kept well sealed. Second, it is vital to ensure that the correct mix ratio for two-component resins is achieved. For medium- to large-production volumes, the use of a mixing machine is recommended to minimise the risk of variability in the mix ra- tio across a production run. It is important that all pipework used with these machines is bled properly so there isn't any air present in the system, which could result in mix ratio varia- tions or air entrapment in the resin. It is always wise to carry out regular checks on the amount of each component being dis- pensed if a machine is used. And if manu- al mixing is preferred, such as for short runs and/or prototyping, it is advisable to use cali- brated balances to measure each component. Only pour the amount of resin required for the job in hand and be sure to reseal containers without delay. Cleanliness is vital to achieving satisfactory potting or encapsulation. Metal enclosures, for example, must be thoroughly cleaned before resin application to remove any corrosion, dirt, grease, or metalworking fluids that might be present as these will reduce the resin's abil- ity to adhere to the metal surface. Similar pre- cautions need to be taken with plastic-mould- ed housings to remove any residual mould re- lease agents. Make sure that the PCB itself is clean because any dust or other contamination on the surface will get trapped. The resin will adhere to the contamination rather than to the surface of the board or components, resulting in a weak spot and a potential point of failure. The useable life, gel, and curing times of a resin are all based on it being cured at room temperature. It is possible to reduce these times by the application of heat, but it is best to do this after the material has gelled to en- sure a consistent cure. The amount and type of resin in a unit will determine the potential exotherm, which is the rise in temperature of the resin resulting from the chemical reac- tion between its two components on mixing. The larger the volume of resin used, the larger the exotherm. Where larger volumes need to be encapsulated, it is best to conduct this in two or more stages to minimise any potentially damaging exotherm. Check the technical data sheet for any curing recommendations for el- evated temperatures but remember that every oven/curing chamber is different, so the same resin in the same volume may require a differ- ent curing profile in two different ovens. Environmental Considerations Resins can protect an electronic assembly against a wide variety of environmental influ- ences, whether they are due to climatic condi- tions or potential attack from any chemicals to which the assembly may be exposed. One of the biggest concerns is to keep water out of the assembly from light moisture due to high am- bient humidity or full immersion in saltwater. Electronic assemblies are now being de- ployed and operated in environments that would not have been contemplated only a de- cade or so ago, and each year, more extreme environments are being specified by designers. For example, there are resins suitable for use in aquatic environments, both fresh and saltwa- ter, from a light splash to full immersion. And resins can now be found protecting a variety of submerged applications from swimming pool lights to sensors used for monitoring pollution in rivers to telecommunication cable joints deep beneath the ocean surface. Resin formulations are also likely to be influ- enced by geographical location. Polyurethane resins, for example, are particularly sensitive to moisture and have to be modified for areas with high humidity and/or rainfall. Converse- ly, silicone resins will require a certain amount Cleanliness is vital to achieving satisfactory potting or encapsulation.

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