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March 2014 • The PCB Magazine 93 the name rolled-annealed (RA). The improved ductility and larger copper grains make the cop- per more and less likely to crack due to bending. For this reason, many flexible circuits utilize RA copper instead of ED copper. Another difference between RA and ED copper is how the surface finish must be considered with respect to design and fabrication. Figure 3 shows a low magnifi- cation visual microscopic view of the outer cop- per surface of ED and RA copper clad laminates. Note that there is directionality in the RA cop- per which is why flex fabricators must always consider the machine direction (MD) in line with the rolls and the orthogonal transverse di- rection (TD). If one refers back to Figure 2, the impact of this directionality can be seen in the copper profile. The ED copper in Figure 2 (A) is much rougher while the telltale grooves of the smooth RA copper can be seen in Figure 2 (B). Behavior of Flex Materials at High Frequencies Traditionally, the electrical properties of flex materials were not considered to be criti- cal since flexibility and copper adhesion were of paramount concern. The dramatic increase in bandwidth requirements and the miniatur- ization of devices has brought properties like dielectric constant and loss tangent to the fore- front of concern. Higher frequencies and speeds require tight control of impedance with very small loss budgets. These current applications require consistent dielectric thickness, low di- electric constant and low dielectric loss. Dielectric Constant and Loss Tangent At frequencies above 1 GHz, the term "di- electric constant" is actually a misnomer. This property is not constant with frequency in this region for most organic materials used in print- ed circuits. The reason for this is that the mol- ecules in these polymers vibrate when RF or mi- crowave energy is applied. The property must be described with a complex quantity called relative permittivity. The real component most closely aligns to the concept dielectric constant and describes the energy stored in the dielectric by capacitance. The imaginary component de- scribes the energy lost in the dielectric. If you take the tangent of the ratio of these two com- ponents, you get the loss tangent of the mate- rial. Materials with high loss not only are prob- lematic since more energy is absorbed, but the relative permittivity will change a lot more with frequency. This phenomenon called dispersion is not unique to flex materials. Typical epoxies used in rigid circuits have very similar behavior electrically to the acrylic adhesives more typi- cally used in flex circuits. Due to differences in processing preferences, epoxy is more common- ly used as a flex adhesive in Asia while acrylic is more commonly used in North America and USING FLEX IN HIGH-SPEED APPLICATIONS continues Figure 3: (l) optical microscope images of electrodeposited and (r) rolled annealed copper. ed ra md td