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Flex-Oct2018

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48 FLEX007 MAGAZINE I OCTOBER 2018 often assumed that it is differ- ent than a rigid board. The fact is that laying out a flex circuit board is like a rigid board with just a few differences. The set- up of the software layers and the output files are the same as a rigid board; however, dif- ferences exist in the cover and stiffener layers with some basic design rules to keep in mind. It is essential to understand that a flex circuit will flex in a way that means key features— such as vias, terminating traces, and sharp angles—need to be kept away from bending regions. A flex circuit is generally built with a polyimide material that is more difficult to process, so traces, vias, annular rings, pads, and spacings should be kept as large as possible. The ques- tion is often asked, "How small a trace or via can be used?" The answer is that the smaller the trace or via, the more difficult the manu- facturing process will be, which ultimately will affect reliability. On a rigid board, solder mask is applied to the outer layers to protect the copper features. On a flex board, the outer copper features are usually protected by a cover layer. Therefore, in the design process, the cover layer and sol- der mask layer are created the same way. The final difference in the design process is the stiffener. Stiffeners are used to add support to certain regions of a flex circuit board. The stiff- eners can be in multiple regions of a flex circuit and located on either side of the board. If all the stiffeners are represented in one file, it is important to identify in the fabrication draw- ing which side of the board the stiffeners need to be applied. Otherwise, a separate layer will need to be created for the top and bottom stiff- eners of the flex circuit board. When additional support for a specific area on the flex circuit board is required, or protection is needed for attached components or connec- tors, the best option is to include a stiffener in the design. This will eliminate the circuit from moving and protect the integrity of the solder joints. It is crucial to remember that the stiff- ener is best placed on the opposite side of the component it is supporting. There are numer- ous types of stiffeners to choose from, includ- ing polyimide, FR-4, stainless steel, aluminum, etc. The thickness of the stiffener depends on how the board will be used. The thicker the stiffener, the more support it provides. If the board is being used in small or tight spaces, the thickness of the stiffener may be an issue and a thinner stiffener may be required (Figure 1). Now that the designer has completed a flex circuit, the next task is to lay out a rigid-flex. With a rigid-flex design, the level of confu- sion and apprehension increases significantly. Engineers often think that the flex portion of the board is glued or somehow attached to the rigid section of the board (Figure 2). Rigid-flex is built like all rigid and flex boards with the method of layers stacked on top of layers. When it comes to designing rigid-flex, the approach is the same as the other circuit boards. The main difference is that certain regions of the rigid layers will be blank in the design file. The board manufacturer will recognize this as a flex region and will plan the board accordingly. Unlike a rigid board, a flex circuit has a lot of variation, so having a detailed fabrication draw- ing to accompany the design is very important. The fabrication drawing should call out all the details to ensure that nothing is overlooked by the manufacturer. The worst thing is to have Figure 1: Four-layer flex circuit with stiffener sections on the back side to support the surface-mount components.

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