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52 FLEX007 MAGAZINE I APRIL 2018 hole sizes, physical separation of features, positional accuracy, etc.). In these cases, the manufacturer can often employ methods to deal with the requirement on a localized basis. In the case of other features, the tolerance may be less critical, significantly less criti- cal, or even non-critical. An important thing to keep in mind is that flexible materials are not as dimensionally stable as rigid materials, and while local features may be held in tight tolerance relative to each other, features from end to end may be less predictable. Given that flexible circuits are normally installed in some 3D form after assembly, the tight tolerances on planar measurements are often not necessary. If there are questions about a tolerance callout, the designer should contact the manufacturing engineer. It is always best to solve the problem before it becomes a problem. Unclear Layer Designation (Rigid or Flex) The purpose of a product specification is to provide clear, unambiguous instructions on the product's construction. In the case of a multilayer circuit design, this is vitally important. The relationship of internal circuit layers relative to one another has become increasingly important in not only assuring that correct interconnections are being made, but also in product performance, especially with controlled impedance designs and sig - nal integrity issues. Several different systems have been developed over the years to help assure that there is no uncertainty in the order of the circuit layers in the final con - struction. The fabricators engineering staff can provide recommendations if needed. Note the thickness and construction of each core in Figure 1. Cover Layer Requirements Not Properly Called Out or Defined Coverlayer and cover coat are terms nor- mally reserved for flexible circuit construc- tions and they are by default a defining struc- tural element of both flex and rigid flex cir- cuits. Coverlayers serve as a flexible solder mask of sorts, protecting the delicate circuits from damage and potential wicking of solder ensure that the customer gets the product that is best suited to the application. The following are discussions on matters of high importance to achieving first-pass success in securing quality flexible circuits from a flex circuit vendor. It is important to provide some information about the operational requirements for the flex circuit, especially if the circuit is to be used in a dynamic flexing application, such as for a disk drive read/write head assembly. The reason for this is the circuit vendor needs to provide a plan for proper layout strategy for manufactur- ing; a plan which accounts for the grain direc- tion of the copper foil during the manufactur- ing process. This is because there is a measur- able difference in terms of flexing performance between the machine and transverse directions of the copper foil. Fabrication Specification Details After the basic circuit design layout is com- pleted, the next most important piece of infor- mation required is the fabrication specification. This document communicates to the fabricator all the pertinent details for the physical con- struction of the circuit and what is needed and expected in the final product. If this informa- tion is incomplete or inaccurate, or if a cus- tomer has requirements that cannot be reason- ably met by a competent manufacturer, time will be unnecessarily lost, at a cost to both the customer and the vendor. For this reason, it is vitally important that the fabrication specifica- tions are checked and rechecked before putting them out for bid. In the sage words of the mas- ter carpenter, "Measure twice, cut once." Manufacturing Tolerances Manufacturing system operators need not only the dimensions of the part they are to manufacture, but also the tolerance for the important features of the product. With flexible circuits, this is something that must be done with thought, care, and consideration of the realities of flexible circuit materials. With some features, design tolerances may be critical for the performance, fit, or further processing of the product (line widths, spaces,

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