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PCBD-Oct2016

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66 The PCB Design Magazine • October 2016 leave components vulnerable to mechanical shock and vibration. Cured resin hardness is measured on the Shore A (softer polyurethanes) and D (harder epoxies) scales. Of course, there are applications in which the encapsulation resin must have an extremely low hardness—in order to maintain flexibility at low temperatures, for example. This is par- ticularly critical for devices that not only have sensitive components but which are also fre- quently exposed to rapid changes in tempera- ture. Moreover, as some devices may need to be reworked, it will be necessary to remove the resin. In general, encapsulation resins can be difficult to remove; however, specialist formu- lations are available that allow the bulk of the resin to be cut out of the unit for this purpose. Colour What many might consider to be the least important property—colour—could actually be critical for certain applications. Optically clear resins may be desirable for a variety of reasons and this property is of particular importance when it comes to the potting of LED lighting fixtures, as the cured resin will obviously need to retain its clarity for the life of the unit. It must also be UV resistant, particularly for out- door LED fixtures. Colourless resins are also use- ful for prototyping applications as the encapsu- lated components are easily viewed during and after environmental and mechanical testing. As well as providing physical protection, co- loured/opaque potting and encapsulation res- ins are also useful for identifying certain circuit areas. Moreover, they conceal what lies beneath the encapsulant surface, providing an effective foil against counterfeiters or those wishing to copy a circuit layout, and helping you to pro- tect your intellectual property. Viscosity The viscosity of a two-part mixed resin will influence the way it flows around the compo- nents that are to be potted. Too low a viscosity and the resin will find every gap in the board; too high and the resin will not flow around and under components, creating voids and leading to potential weak points, which may shorten the service life of the components. Thixotropic resins that regain their high viscosity once dispensed are ideal for 'glob-top' applications where a select component or area of a PCB needs to be protected, as opposed to encapsulating the entire unit. Where a low dielectric constant is required (such as for RF applications), some resin systems may include specialist fillers to obtain the lowest possible value. These fillers can make the resin harder to mix and dispense due to the resultant high viscosity, and specialist formulations that avoid fillers altogether may be needed. Flame Retardancy Fillers can also be added in order to render potting resins flame retardant up to the highest UL-94 V-0 standard. Aluminium hydroxide fill- ers are commonly used, resulting in low smoke emission and a low level of toxic fumes; howev- er, the necessary high filler volumes will increase the viscosity of the resin as well as its density, incurring both dispensing and weight penalties. Bromine compound fillers used in smaller vol- umes have a lesser effect on resin viscosity and density but these can give rise to higher levels of smoke and toxic fumes. However, new phos- phorus based flame retardant technologies are being introduced, offering the promise of lower viscosity and low toxicity. Cure Time Cure times for two-part resins vary enor- mously. As a rule-of-thumb, a good 24 hours at room temperature will be required to obtain WHY ARE RESIN PROPERTIES SO IMPORTANT? " In general, encapsulation resins can be difficult to remove; however, specialist formulations are available that allow the bulk of the resin to be cut out of the unit for this purpose. "

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