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66 The PCB Design Magazine • December 2016 from 5kg bulk cans to 250g packs). Generally, unless you intend to use the resin and hardener in a single run, precautions like those outlined above should be taken to ensure that a dry at- mosphere is maintained within the containers and that they are well sealed between runs/us- age, otherwise you are going to suffer an awful lot of waste product! And then we come to the all-important mix- ing stage: Do you mix manually or make use of a special mixing machine? Manual mixing is perfectly adequate for small-volume/short pro- duction runs or when developing prototypes. For these smaller jobs, the best approach is to use flexible plastic resin packs that provide mea- sured quantities of resin and hardener in sepa- rated compartments of the pouch. When you are ready to use the pack, you simply remove the separating clip and 'massage' the pouch to mix the components thoroughly together. For larger volumes and/or longer runs, man- ual mixing could introduce variations in the mixed volumes of resin and hardener, which will lead to variations in cure times as well as compromising the quality of the cured product. In this case, machine mixing is the better alter- native as the mix ratio is set within tight toler- ances and maintained throughout the run. Again, when running two-part polyurethane resins through mixing and dispensing machines, it will be necessary to protect both components from moisture, either by fitting desiccant traps to the product holding tanks or by flushing the tanks continuously with dry nitrogen. Resin systems—be they epoxy, urethane or silicone—differ with regard to the curing condi- tions they require to obtain an optimum cure. Generally, most resins will cure at room tem- perature (20–25°C) over a period of 24 hours; however, some will require much higher tem- peratures to cure successfully, while others may benefit from a post cure—applying an elevated temperature to the encapsulation or potting area after it has achieved a partial cure. The curing process for epoxy resins is gener- ally slow, though the manufacturer can obtain a range of different cure speeds and cured proper- ties by altering the hardener chemistry. However, beware that for epoxy resins, a fast cure can also mean a very exothermic reaction. Depending upon the size of the unit being potted and the amount of resin used, it might require some cool- ing to control the resultant exotherm. In the case of polyurethane systems, which have a lower exo- therm during cure, even for fast cure systems, it's the humidity you must consider, as environments with high humidity might cause the formation of bubbles and craters on the surface of the resin. After curing, and particularly for those units that have been heated to cure the resin, a pe- riod of carefully controlled cooling may be nec- essary to minimise the development of stress points between the unit, the resin and the com- ponents. Curing is a process affected by many variables and consequently it can be very diffi- cult to predict the outcomes with any accuracy. For example, the temperature at which a resin is cured will affect not only its cure speed, but also the quality of the end-result, so it's as well to carry out some trials before committing to a specific cure rate and/or cure temperature. Moreover, it is always advisable to check the product's technical data sheet to obtain the rec- ommended guidelines for curing; should these not meet with your requirements or you foresee any conflicts between the curing process and your production procedures, be sure to contact your supplier's technical support team for fur- ther advice. Remember, they have a wealth of experience at their disposal and most technical support teams that I have had any dealings with will relish a challenge! PCBDESIGN Alistair Little is technical director for Electrolube's Resins Division. UNDERSTANDING THE PRACTICALITIES OF RESIN APPLICATION AND CURING " Do you mix manually or make use of a special mixing machine? "