Excerpt: The Printed Circuit Designer’s Guide to…Flex and Rigid-Flex Fundamentals


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Designing Flex Circuits for First-Pass Success, Part 1

The design process is arguably the most important part of the flex circuit procurement process. The decisions made in the design process will have a lasting impact, for better or worse, throughout the manufacturing cycle. In advance of providing important details about the actual construction of the flex circuit, it is of value to provide some sort of understanding of the expected use environment for the finished product.

The electronics industry serves several different markets that do not always share the same product acceptability or reliability expectations. For this reason, the electronics industry, through IPC and other standards organizations, has developed a classification system that specifies what is expected of products for different classes. The system of classification is not intended to be a measure of quality. Rather, quality is a matter of conformance to a set of established requirements for a product in a given application. Therefore, quality products can be created in each of the classifications within the system. It is generally accepted that there are three classes of product. These have been defined by IPC standards as follows:

  • Class 1 – Consumer products and products for non-critical applications where cost is normally the primary driver.
  • Class 2 – Higher-order products in terms of quality and reliability expectations, including telecommunications, computers and general industrial.  
  • Class 3 – High-reliability applications including military, aerospace, automotive and medical products.

By defining the class of the product being designed, the purchaser is letting the manufacturer know what added controls to apply to the manufacturing process and the level of care they will need in the inspection process to 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 manufacturing; a plan which accounts for the grain direction of the copper foil during the manufacturing process. This is because there is a measurable 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 completed, the next most important piece of information required is the fabrication specification. This document communicates to the fabricator all the pertinent details for the physical construction of the circuit and what is needed and expected in the final product. If this information is incomplete or inaccurate, or if a customer has requirements that cannot be reasonably 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 specifications are checked and rechecked before putting them out for bid. In the sage words of the master 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, 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 critical, 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 signal 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 construction. The fabricators engineering staff can provide recommendations if needed. Note the thickness and construction of each core in Figure 1.

FlexConsFig1.jpg 

Figure 1: Example of four-layer flex construction.

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