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90 The PCB Magazine • March 2014 Copper clad circuits in the printed circuit industry have evolved into four different class- es, each having different sets of standards. For instance, IPC has separate sets of standards for rigid, flexible, high speed and high frequency, and HDI. Further, there are separate standards within these families for design, acceptance, and base materials. These standards provide logical organization to the design, process, and material considerations of each class of circuit [1] . In many applications, however, designs may utilize all four classes of boards in a single as- sembly. This presents significant challenges for the designers, fabricators, and materials sup- pliers because choices made using design rules from one type of board may significantly lim- it utilization of another type of board on the same assembly. For example, a designer may be forced to use a complex and bulky connector system to link two rigid boards together due to design choices, instead of utilizing a small in- terconnect utilizing a rigid-flex circuit. The re- sult could be that the assembly does not fit into the assigned form factor. This article will focus on flexible circuit technology and specifically, on the material properties that account for the broader use of flexible circuits in high frequen- cy and high-speed applications. How Flex and Rigid Materials Differ Of the four types of circuits mentioned, flex suffers from being the least understood in the design community. There are many reasons for this, but this is mainly due to the fact that flex- ible circuits comprise a much smaller market than do rigid circuits. In addition, flex designs generally have to fit into a pre-defined form fac- tor which means that very few flex designs look alike. These realities make it a significant chal- lenge for standard design tools to adequately plug flex into their design flow. Figure 1 shows an example of a flex circuit design driven pri- marily by the form factor in which it must fit. The traditional deployment of flex in these ap- plications facilitates the need for fundamental differences in flex materials from typical rigid F e a t u r e by Glenn Oliver dupont electronics and communications Using Flex in High-Speed Applications Figure 1: an automotive flex circuit designed to fit into a tight form factor.