FLEX007

Flex-July2018

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50 FLEX007 MAGAZINE I JULY 2018 Design Matters Surprisingly, the design of the circuit mat- ters as well when choosing an adhesive sys- tem. If the circuit will be single-sided (one layer of copper) or double-sided (two layers of copper), then either adhesive system will perform equally well, all other criteria being equal. However, if the design will be a multi- layered circuit, then the characteristics of the adhesive have a significant effect. Small plated through-holes less than 8 mils in diameter may also have an effect on the performance of the circuit if the wrong adhesive is chosen. As you can see, there is a lot to consider when deciding on an adhesive-based system for your flexible circuit. Now, let's discuss some of the properties for both types of systems and see how they relate to the discussion above. Acrylic Adhesive: Pros and Cons Acrylic adhesive is the best choice for dynamic applications because it stays mallea- ble after it is cured. That is its most valuable characteristic. That is, unless, it must be flame retardant. In order for acrylic adhesive to com- ply with UL94 V.0, brominated flame retardants must be added which reduces the reliability for dynamic flexing. It reduces the thermal sta- bility as well. Acrylics do not perform well at higher temperatures either. The material starts to soften in the 180–200°C range, which can cause delamination of the insulating layers as well as the copper traces. Acrylic doesn't per- form well for wire bond designs either. The softer adhesive tends to absorb the ultrasonic wire bonding energy causing difficulties in the assembly process. And finally, it has a higher coefficient of expansion. This causes move- ment in the Z-axis which means that it is not a good choice for multilayer circuits because of the damage that could be done to plated through-holes. Epoxy Adhesive: Pros and Cons Epoxy adhesive, on the other hand, is not a good choice for dynamic operations because it cures hard. However, that makes it a bet- ter choice for a lot of other applications. For instance, it has the best bond strength at high during the lifetime of the product. Think of a flip phone, a printer, or an MRI machine. A flexible circuit used in a static operation can be thought of as a derivative of the traditional PCB, albeit lighter and more malleable. How- ever, in a dynamic application the circuit must be, not only flexible, but robustly so. We must be able to exploit all of the inherent advantages of the materials to their fullest extent. In either of these cases the adhesive plays an important role. Let's review another factor that affects the choice of adhesive. Identify the Environment The environment where the product will reside during the majority of its lifecycle also enters into the adhesive choice. Will the envi - ronment be excessively hot or cold? Will the circuit be immersed in water or chemicals? Does the circuit need to be flame retardant or not? These are all questions that may make a difference in the adhesive system chosen for the circuit. For instance, if the circuit will or can be immersed in water, we'll want to choose an adhesive system that is the least absorptive. If the circuit will be used in an application that will see it covered in chemi - cals or biological fluids, we'll want the most chemical-resistant system. If the circuit will see higher than ambient temperatures, we'll want an adhesive that performs well at those temperatures without any degradation of the lamination. Does the circuit need to per - form to UL94 V.0? That is, must the circuit be flame retardant as in automotive or aerospace applications? All of these concerns must be addressed when choosing the adhesive sys - tem for your flexible circuit. These are all questions that may make a difference in the adhesive system chosen for the circuit.

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