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Design007-Dec2018

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70 DESIGN007 MAGAZINE I DECEMBER 2018 Last month, I began my new series of col- umns on encapsulation resins by selecting five frequently asked customer questions regard- ing resin chemistries and properties, and typical resin a p p l i c a t i o n s and their limi- tations. In this column, I'm taking this introduction a step further by listing what I believe are the top five tips for circuit designers and manufacturers who seek to ensure that the in-service r e l i - ability and longevity of their electronic assem- blies and products are fully addressed. Five Top Tips 1 First, think very carefully about the sort of environment your PCB is likely to encoun- ter. It is easy to over-engineer a product so that it will survive the very worst of conditions, but worst conditions may only be fleeting or transient. Therefore, a resin solution with a lower temperature performance specification will often cope. Take temperature extremes as an example; your application may experience occasional temperature spikes of up to 180°C, which you might feel deserves treatment with a special resin. However, such excursions may only be short-lived. Under normal operating conditions, the PCB might only be subjected to a maximum temperature of 120°C, opening up a wider choice of resin types and methods of application. Similarly, the required chemical resistance of your chosen resin will depend on the duration and/or extent of the chemical contamination. For example, there is a considerable dif- ference in terms of the extent of chemi- cal damage between a thin layer of a con- taminating chemical on the resin surface that is wiped off within five min- utes, and 500 ml of a chemical present on the resin surface for one hour or more. Com- plete immersion takes the requirements to an even higher level. Furthermore, the range of chemicals that a PCB might eventually be exposed to is often quite limited, and almost certainly not the broad range that is frequently listed at the design stage just to be on the safe side. 2 Environmental factors that normally affect a PCB include temperature, chemi- cal attack, physical shock (vibration), and thermal shock. The trick is to decide which of these is likely to have the greatest impact upon your PCB and then concentrate on mak- ing an appropriate resin choice. Each of the three main resin types (epoxy, polyurethane, and silicone) have strong points as well as weaknesses. Silicone resins have the broadest continu- ous operating temperature range of any of the resin chemistries, so they are a natural choice Top Five Tips to Protect PCBs from Harsh Environments Sensible Design by Alistair Little, ELECTROLUBE

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