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48 The PCB Design Magazine • February 2017 by Alistair Little ELECTROLUBE SENSIBLE DESIGN So far in these columns, we've looked at the various reasons for circuit potting and en- capsulation using resins, outlined the various resin properties and how they relate to particu- lar types of circuit protection, seen how to ap- ply these resins in a production environment by deciding on which mixing and application techniques are appropriate to specific produc- tion needs, and finally, how best to achieve a satisfactory cure. But, I hear you ask, which type of resin is best for my project and production methods? How are my choices affected by the physical constraints of the board or area to be covered? What resins are suitable for rework? These and other issues I plan to cover in this column. Basically, you have a choice of three main types of resin: epoxy, polyurethane and silicone. Each has distinctive properties and your choice will be guided by how these properties can be exploited to meet your particular production circumstances and application needs. Let's first take a look at production volumes. Are you pro- totyping, delivering a short production run, or manufacturing on a much larger scale? Your an- swers to these questions will determine whether you use manual or machine-based application methods, and this will pose a further question: Which resin types are compatible with these ap- plication techniques? For example, if you are potting prototypes or small-volume runs you will most likely use manual methods, while much larger-volume runs will best be served by automated, machine- based mixing and dispensing systems. Should you be looking to work up from the prototype