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42 FLEX007 MAGAZINE I APRIL 2019 some of the process touchstones that I think are important. First, there is the product concept. What spe- cific design should be used for elements of the product that make flexible circuits attractive? This question needs to be asked to make sure that flexible circuit technology is truly required for the product. Because the thinness of the cir- cuit is frequently an objective, flexible circuits are usually tapped to provide the sought-after benefit. However, sometimes—even often— a thin reinforced laminate circuit will suffice, and typically, at a lower cost. Thus, do a quick reality check; there is no need to make a prod- uct more expensive than necessary. Second, flexible circuits have the obvious ability to interconnect electronic elements (e.g., modules, displays, connectors, etc.) that are distal from one another, commonly in three-dimensional space. Moreover, these electronic elements may be required to move relative to each other when in use (e.g., disk drives, read-write heads, printer cables, etc.) or during maintenance or upgrading of a system to facilitate access to the elements of interest. Understanding the mechanical requirements associated with this movement will influence design choices, including the type of materi- als used and the type and weight of metal foil (most frequently rolled annealed copper). In addition, there are many seemingly insig- nificant details (e.g., copper foil grain direc- tion) that need to be addressed to ensure that the product will perform to expectations for the duration of its anticipated lifetime. The type and thickness of the coverlayer used in the circuit also plays into this equation as well as the objective of keeping the copper foil at the center of the construction, especially in areas designed for bending or flexing. The first topic is somewhat macro, and the second is a bit more micro and nuanced, but both topics are germane to this subject. Designing Flex Right the First Time Circling back to the bigger, yet often over- looked, items associated with the process of getting a flex circuit designed right the first time, there is a need to consider the operating environment for the end product. For exam- ple, consider the temperature excursion range and humidity expectations over its life and use as well as the processes that will be used in its fabrication. These factors will impact and likely limit the choices of materials used. Solder remains a commonly used method for assem- bly. And with higher temperature lead-free sol- ders in use today, it is necessary to choose a material that will stand up to requirements. Another matter to consider relative to field use is the mechanical stresses and strains that may be encountered by the product. Designers use flex with great frequency in dynamic appli- cations, such as those mentioned earlier (e.g., disc drives read/write heads, printer cables, etc.), and they pay special attention to the cir- cuit features that pass through the dynamic bend areas of the design. However, products that are deemed to be non-dynamic applications can and sometimes do fail due to copper fatigue failure as a result of shock and vibration endured in a field appli- cation. These are not the visible flexural cycles that can be seen with the unaided eye, but microscopic flexural cycles that are charac- teristic of vibration. While in the former case, the cycles tend to be high amplitude and mod- erately low frequency, in the latter case, the frequency tends to be high and the amplitude low. I have seen such failures and heard of oth- ers over the years. Sometimes—even often—a thin reinforced laminate circuit will suffice, and typically, at a lower cost. Thus, do a quick reality check; there is no need to make a product more expensive than necessary.

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