CircuiTree - March 2009 - (Page 20) Flexible Thinking By Joe Fjelstad Back to Basics Part 23 – Bending and Flexing Design Concerns T he next two or three installments of this “Back to Basics” series will focus on the fundamental purpose of flexible circuits after electrical and electronic concerns have been met: bending and flexing the circuit and the design features and practices that will help assure the circuit performs to expectation for the life of the product. As has been mentioned many times over the years, most flexible circuits are designed simply to allow the user to form the circuit to fit the shape of the package in what are called flexto-install applications; however, there are still a great many applications that require some dynamic flexing from hundreds, thousands, or even millions or billions of times over the life of the product. In fact, if one looks closely at the assembly process, the act of placing the flex circuit into the final assembly may require that the circuit be bent or manipulated several times in the process. Thus, even if people think they are immune to understanding the mechanical issues of dynamic design, they may well be misguiding themselves. The truth is that dynamic concerns are basic to the product. about the uncertainties of shock and vibration impact, the practice of avoidance should be routine. When routing conductors through bend, fold, and flex areas, the traces should be routed at 90 degrees or perpendicular to the bend line. This is an intuitively natural approach to a circuit routing scheme that is generally prescribed by most design standards. That said, the rule is somewhat fungible and over the years has been not infrequently violated as a matter of convenience. Whenever possible, conductors should also be routed on a single metal layer through bend and fold areas to enhance flexibility. When not possible, the conductor should be staggered from side to side to avoid the I-beam effect, which was discussed in an earlier installment of this series. The concept of neutral axis (see Figure 1) is very important to dynamic flex circuit design and flexing performance. In theory, the neutral axis concept is predicated on the observation that the center It is worth remembering that even static flex circuits can be dynamically cycled by virtue of their application and operating environment such as experienced in all types of transportation including, planes, trains, boats, and automobiles from shock and vibration. Figure 1 Keeping Copper in Neutral Axis Is a Desirable Design Practice Making It Possible to Minimize Cyclic Strain and Significantly Extend Flex Cycling Endurance To help illustrate, it is worth remembering that even static flex circuits can be dynamically cycled by virtue of their application and operating environment. Dynamic flexing is not necessarily limited to low frequency, high amplitude flexing but also very high frequency, low amplitude cycling such as experienced in all types of transportation including, planes, trains, boats, and automobiles from shock and vibration. If dynamic flex design rules are not considered or are ignored, the potential for unexpected cyclic fatigue failure of a flex circuit in such an application is a legitimate concern; thus, some attention to the few simple rules is encouraged as they are arguably good practices for all flex circuit designs. One of the first caveats for designers is that they avoid placement of through-holes in bend areas. This important design practice is sometimes lost in the process. Plated though-holes disrupt the uniformity of the circuit traces and can result in local stress and strain that could be cause for potential failure. There are exceptions for every rule, and, for some static design applications, it may be possible to successfully place plated vias within a bend area, provided they are properly strain relieved, have a full coverlayer, and have a bend radius that is sufficiently large. However, avoidance of through-holes is much more critical in dynamic applications, and, in light of the earlier comments 20 March 2009 • circuitree.com of virtually any item bent is practically immobile, with compressive and tensile stresses being absorbed by the outer layers of material. Thus, if the copper metal circuits are kept to the center of the design, the flexing life should be enhanced; test data has proven this to be true. In summary, employment of these few simple design practices will greatly improve chances for meeting product performance objectives relative to flexing a bending. There are other concerns as well, and these will be discussed in the next installment of this series. ■ Joseph Fjelstad, founder and president of Verdant Electronics and co-founder of SiliconPipe, is an author and innovator in electronic interconnection and packaging technologies. Download his flex circuit book free at www.flexiblecircuittechnology.com. E-mail: jfjelstad@siliconpipe.com Are You Getting CircuiTree Weekly? Our weekly e-Newsletter recaps the week in the electronics industry and provides links to FREE educational resources you won’t find anywhere else. Don’t miss another edition. Sign up today at www.circuitree.com/enews. http://www.flexiblecircuittechnology.com http://www.circuitree.com/enews http://www.circuitree.com
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