Design007 Magazine

PCBD-Apr2016

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50 The PCB Design Magazine • April 2016 Thoughtful design will pay huge dividends down the line, and you will have friends for life among your production colleagues if you make their jobs just that little bit easier! So having started the conversation about the im- portance of making sound early-stage design decisions, for my third column, let's look at some of the issues that the production depart- ment is likely to face when applying confor- mal coatings. We'll start with board layout. By the "simple" act of placing connectors and compo- nents that must not be coated along one edge of the assembly, the conformal coating ap- plication process will be simplified. This might allow dip coating to be explored as a potential alternative methodol- ogy, speeding applica- tion times and reduc- ing costs. Also, avoid large arrays of discrete components, which can pose a huge coating challenge due to the high levels of capillary forces present. The net result is often areas of no coverage or protec- tion on the board as well as areas of excessive thickness prone to stress-cracking, de-lamina- tion and other coating defects. Similarly, tall components present challenges of their own by the creation of shadowed or hard to reach areas. Splashing is another associated problem. The trick is to avoid placing tall components next to 'must-coat' components in order to avoid this eventuality. Adhesion results with conformal coatings can vary from supplier to supplier and this can create problems when applying solder resists. A quick and very effective solution to this is by specifying a surface energy of >40 dynes/cm on incoming bare boards and ensuring that each batch is religiously tested and rejected if they do not meet this minimum value. Conformal coatings are usually liquid when applied, and will flow with a combina- tion of gravity and the capillary forces present. Whether you are masking or relying specifically on selective conformal coating, leaving a buf- fer of at least 3 mm clear between the area to be coated and un- coated areas will make the production process easier. That old adage, "If some is good, more is better" doesn't neces- sarily hold true with conformal coatings, which are designed to be applied at the thickness specified on the datasheet. Exceed- ing the recommended thickness is unlikely to provide better protec- tion, but may intro- duce a range of production issues ranging from dramatically increased cycle time to solvent-en- trapment, stress-shrinkage, de-lamination and cracking, to name just a few! If you need to apply a thick coating, two thin coating applications are better than a sin- gle thick one. Additionally, if you need more thickness than specified, use a coating that is designed to be applied thickly or consider a res- in product. Conformal coatings should not be used as under-fill materials, as they generally contain no filler and have relatively high Z-axis thermal coefficients of expansion. Make this mistake and you could see the lifetime of ball grid array (BGA) and quad flat no-lead (QFN) terminations by Phil Kinner eLectroLuBe Design Tips for Easier Conformal Coating sensiBLe Design

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