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50 The PCB Design Magazine • February 2017 stage to a full production run, then the differ- ences between manual and machine mixing and dispensing will have to be taken into ac- count when choosing your resin. When mixing and dispensing manually, a longer useable life and gel time will generally be required compared with machine mixing, which is a faster process that doesn't necessar- ily need these properties. The process could in- volve the use of two different resins with differ- ent mixed properties, but having similar cured properties in order to take full advantage of the two different mixing conditions. Useable life is defined as the time that the mixed resin can flow and remain just workable; the gel time is the interval between mixing and the point at which the resin has just set and cannot flow. In the latter state, the resin is still relatively soft and can often be reshaped by applying slight pressure. However, note that the larger the vol- ume of mixed resin, the shorter the useable life and gel time. If a rigid to semi-rigid encapsulation is accept- able, then the slower curing rate of a two-part epoxy resin would provide the desired longer useable life and gel time for manual mixing. Ep- oxy resins are also suitable for machine dispens- ing but caution should be exercised if the resin contains abrasive fillers as these will accelerate the wear of dispensing equipment. To mitigate this problem, epoxy resins are available without fillers. Should a more flexible cured finish be re- quired, then polyurethane or silicone based res- ins would be suitable alternatives to epoxies. Two-part polyurethane resins tend to have a lower exotherm (less heat is generated) com- pared to epoxy resins, which may be advan- tageous when potting delicate components. While silicone resins tend to be less popular than either epoxy or polyurethane resins, they are highly flexible on cure and they tolerate continuous high operating temperatures—often in excess of 180°C. When the resin is mixed, next comes the question: How do you want it to be applied? If the parts of the circuit to be coated are defined, discrete areas, then generally either a dam-and- fill system is employed to isolate the areas that need to be potted or a more thixotropic resin can be used. Some single-part epoxies and es- pecially silicone resins have these advantageous thixotropic properties. On the other hand, if an entire board needs to be covered, then resins with good flow char- acteristics should be used as the resin will be de- signed to flow from a dispensing point to cover the board. Most two-part epoxy resins—even those containing fillers—are not only easily mixed but can flow between components and devices with limited spacing. But how much resin should you apply? As a general rule of thumb, the amount of resin to be applied must be sufficient to cover the top of the highest component of the board, and the thickness of the resin layer must pro - vide the desired level of protection. While most customers will determine the minimum thickness of resin layers for their particular ap- plications by trial and error, the relevant tech- nical data sheet will provide good guidance, and consulting your supplier will often help resolve a problem. A note here about colour: Optically clear resins may be desirable for applications such as LED lighting fixtures, as the cured resin will obviously need to retain its clarity for the life of the unit. Colourless resins are also useful for prototyping applications, as the encapsulated components are easily viewed during and after environmental and mechanical testing. On the other hand, coloured or opaque potting and en- capsulation resins conceal what lies beneath the encapsulant surface, providing an effective foil against counterfeiters or those wishing to copy " If a rigid to semi-rigid encapsulation is acceptable, then the slower curing rate of a two-part epoxy resin would provide the desired longer useable life and gel time for manual mixing. " SELECTING THE RIGHT RESIN FOR THE JOB