CircuiTree - July 2008 - (Page 34)

[ Ask the Flexperts ] By Mark Finstad Flex Circuitry Versus Wiring Harnesses How much more reliable are flexes over wiring harnesses? I’ve heard 75 percent thrown about, but I can’t find the data to support it. While there is little doubt that ﬂex circuitry is generally more reliable than wiring harnesses, it is difﬁcult to quantify. It really depends on several different factors that are application speciﬁc, such as the complexity of the wiring, form and ﬁt requirements, and shock and vibration the wiring will experience in service. space sensitive, ﬂex is the clear winner (see Figure 1). If you have any doubt, open up your digital camera. You will ﬁnd multiple ﬂex circuits in every one. Flex circuitry allows camera manufacturers to make all of their internal electrical connections yet shrink the overall camera to an almost ridiculously small size. Because the conductive patterns of ﬂex circuitry are photo etched on thin copper foils, features such as spaces and traces can be much smaller than wires, thereby greatly increasing the connection density. Also, the ultra high dielectric strength of ﬂex materials allows conductive layers to be spaced very close together. For a failure in the ﬁeld is great. Because each ﬂex circuit is custom shaped for its intended application, each one will ﬁt into its space exactly the same every time. Shock and Vibration: Flex is the clear winner in this category due to its ultra low mass. Because a ﬂex circuit is capable of much higher wiring density than a wiring harness, it follows that the ﬂex circuit can be made much smaller with the same number of connections. The smaller wiring package of the ﬂex circuit means the ﬂex will have less mass that will therefore be less affected by shock and vibration. Flex circuits are currently used in applications where they are sub- Figure 1 Side View of a Wire Harness (Lower) That Was Converted to a Flex (Upper). The Reduction in Volume Improved the Fit and Enhanced Shock and Vibration Performance Figure 2 Tangle-Town Assembly, Left, Was Converted to Compact Flex, Right Complexity: Flex has a deﬁnite advantage in this category when compared to a similar wiring harness. Because every feature on a ﬂex circuit is deﬁned by CAD data, each and every one is exactly the same (i.e., never a wiring error). Also, on a complex wiring harness, there is always the possibility that a single wire in a bundle may be slightly shorter than the others, causing a stress concentration point where it is attached to the connector. This stress concentration point may result in a broken connection if the harness is subjected to shock or vibration. For simple wiring harnesses (less than a dozen or so connections), the ﬂex advantage is not as great. While you still gain the beneﬁt of having every ﬂex circuit being identical, it will come at a greater cost than the wiring harness. So, unless there are form, ﬁt, or service requirements that drive you to ﬂex, a simple wiring harness may be the better option. Form and Fit: When your application is 34 July 2008 • circuitree.com reference, the combined dielectric strength of polyimide ﬁlm and acrylic adhesive is over 2,000 V/mil. Individual conductive layers on ﬂex circuits are generally 0.003 in or less from each other but can still offer thousands of volts of dielectric strength between them. For these reasons, the volume consumed by a ﬂex circuit can easily be 75 percent less than a comparable wiring harness. Another advantage ﬂex has in this category is that there are no individual wires that can come loose to get pinched between a casing and a cover, occlude an optics path, or rub against a sharp edge causing a short (all of which I have seen, see Figure 2). This feature can be especially troubling when part of the wire harness is attached to a cover. Once the cover is closed it is impossible to see if all the wires are lying where they are supposed to be. If a wire becomes pinched, or if it falls into an area where there could be abrasion to the wire’s insulation, the potential for jected to shock that is measured in the tens of thousands of Gs. A wire harness would never survive this type of application. So, to answer the original question, the reliability of ﬂex over wiring harnesses is a sliding scale that is dependant on the application. In very simple applications that have just a handful of connections, used in an application that has plenty of space and no shock or vibration concerns, the two will probably perform with equal reliability. On the other end of the scale, in an ultra high G force environment, a ﬂex will perform perfectly and a wire harness will fail miserably 100 percent of the time. Most applications fall somewhere in between, so you must evaluate your speciﬁc use to determine which end of the scale you are closest to. ■ The Flexperts are Mark Finstad and Mark Verbrugge of Minco. Email: Mark.Verbrugge@minco.com http://circuitree.com

Table of Contents Feed for the Digital Edition of CircuiTree - July 2008

CircuiTree - July 2008 Contents My Line Industry Review Tech Talk Flexible Thinking Toward a PCB Production Floor Metric for Go/No Go Testing of Lossy High-Speed Transmission Lines Intelligent Design 20-Year Retrospective Ask the Flexperts Environmentally Speaking BPA Growth Curves Considering Design Variants to Maximize Process Efficiency Market Outlook Technical Product Spotlights Classified Ads Upcoming Events Ad Index

CircuiTree - July 2008

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