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SEPTEMBER 2019 I DESIGN007 MAGAZINE 75 3. How Can Designers Ensure a Smoother Production Process? For example, how do they indicate wheth- er it really doesn't matter if a component is coated or not? When design and production work seamlessly well together, the outcome will nearly always be successful, particularly when a designer understands the intricacies of conformal coatings. One area where design- ers can help their production colleagues is to specify where coatings are optional, or where there are "don't care" areas, in the engineer- ing drawing. For instance, consider LEDs on a circuit board. When the spectral output of the LED is important to the product function, and coat- ing the LED would interfere with that func- tion, then the LED should not be coated. If the LED is on the assembly as a test select, and the presence of conformal coating (assum- ing transparency) does not interfere with the assembly function, then it may be coated. In specifying the optional coating, the designer provides more flexibility to the coater in as- sembly operations. It is best practice to specify the areas that need to be coated and the areas that don't as well as the "don't care" areas to help the coating process run as smoothly as possible. 4. Can a Designer's Instructions Be Easily Misconstrued? We might need to duck for cover here, but the answer is yes! Engineering drawings should avoid specifying "100% coverage," as this means numerous things to different peo- ple. The only real way to get 100% coating is to have assemblies with no connectors or un- coated components, and they must be either dip-coated or vapor-deposited. In practice, you can only inspect coatings in areas that are vis- ible to the inspectors. Inaccessible areas can- not be inspected. 5. How Is Coating Thickness Specified? Cured conformal coating thickness varies by chemistry. It's also advisable to be aware of what these coverage requirements do to your resulting application method. Adequate cover- age—particularly on corners, sides, or under leads—can be a challenge in thin-film applica- tions; meanwhile, thick-film processes can in- crease the flow characteristics of a coating ap- plication and can be more difficult to manage around no-coat areas. When specifying the thickness of a coating on an engineering drawing, the industry prac- tice is to measure coating on a flat, unencum- bered area of the assembly, and not on items like component leads. As many modern as- semblies are very component-dense, it is of- ten difficult to find a flat, unencumbered area of the assembly. Consequently, it is a common and accepted practice to use witness or pro- cess control coupons for such measurement. Designers will need to know that the nomi- nal thickness measured on flat, unencumbered areas or witness coupons will have no relation to the thickness of coating achieved on the cor- ner of a discrete, or the leads of ICs, where the thickness might be one micron or less with a nominal thickness of 25 or 50 microns. In a re- cent study performed by the IPC titled "Confor- mal Coating: State of the Industry," there were some very eye-opening results for many folks who assumed they were getting 25 microns ev- erywhere. Again, the key takeaway is nominal coating thickness is a process indicator only; the actual coverage on leads and components is far more important to the reliability of the assembly, which should be understood by all parties. The goal is the greatest degree of cov- erage possible, and cross-sections should be performed to understand how the application process is delivering this critical to success pa- rameter. The witness coupon will only tell you if your process has changed significantly. 6. How Is the Best Method of Application Determined? There isn't necessarily a best method to ap- ply a conformal coating. Choosing the most pertinent application method for a particular assembly will depend on which existing equip- ment is available to the manufacturer, the coat- ing processes in use, the average time interval between the start of production of one unit, and the start of the next and the overall design

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