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40 The PCB Design Magazine • June 2015 tral axis will be located somewhere between the ground plane and the signal plane. That means there will be stress as compression on the ground plane and stress as tension on the signal plane. If a different structure is considered, such as a three-layer stripline circuit with a copper layering scheme of ground-signal-ground, then the neu- tral axis can be very close to the signal conductor layer. The top and bottom ground layers will have stress as tension and compression respectively, but the signal layer in the geometric center of the cross-section may have very little or no stress. Due to this, the stripline circuit may be formed ef- fectively without damage to the inner signal con- ductor; however, it is likely to cause some damage on the outer ground plane layers. A thin stripline circuit would minimize the stress on the outer layers and minimize the risk of fracturing. There are many fabrication variables to con- sider as well. One is that nickel is very brittle and can easily initiate cracking of the conductor layers. Another issue is copper plating over the laminate copper, which has a potential to have a copper grain boundary difference and may be an issue for cracking when the circuit is bent. Plated through-holes in the bending area are problem- atic and the type of copper used on the laminate can be critical. If more information on bending, forming and flexing of rigid circuits is required, it is best to dis- cuss these issues with your material supplier, or contact a technologist familiar with flexible cir- cuit technology. Many times, bending, forming and flexing a high-frequency circuit board will follow the same basic principles of flexible cir- cuit technology. Even though the two technolo- gies differ electrically, the mechanical aspects are similar. PCBDESIGN THE ART OF BENDING AND FORMING PCBS continues CliCk To View Video Interview Product specialists Michael Bode and geoffrey hazlett join guest Editor Dan Feinberg to talk about the company and its products and solutions. They also discuss some of the technologies being enabled by Polar, including controlled impedance and insertion loss testing. Polar Talks Impedance and Insertion Loss Testing by Real Time with... IPC APEX EXPO 2015 John Coonrod is a senior market development engineer for Rogers Corporation. To read past columns, or to reach Coonrod, click here. lightning speed laminates

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