Issue link: https://iconnect007.uberflip.com/i/271756
92 The PCB Magazine • March 2014 PCB materials. One fundamental difference about flex materials is that the base dielectric generally does not contain glass reinforcement. Most flexible circuits contain various grades of polyimide as the dielectric to provide both mechanical integrity and flexibility. Some may refer to this polyimide by the DuPont brand name Kapton. The flexible copper clad lami- nate, or flex core, provides the "backbone" to the circuit and usually is made with Advanced Kapton that has higher elastic modulus. An- other fundamental of flex is that soldermask is generally not used to cover the outer layers of the circuit. This is because soldermask is usually very brittle and will crack when flexed. Instead of soldermask, adhesive is coated on one side of a thin, conformable, elastic standard Kapton layer. This is called a coverlay to distinguish it from soldermask, since it is processed different- ly. In a similar way, a layer used to bond copper clad flex cores together with adhesive coated on both sides of a polyimide layer is called a bondply instead of a prepreg. This is because the polyimide in the center does not flow. Only the adhesive coated to the polyimide flows. Not only are the dielectrics that go into flex circuits made of different materials than are rigid, but the processes used to make the base dielectrics are also significantly different. Some insight into this difference can be seen in the cross-sections of 50 ohm controlled impedance lines in Figure 2. Most rigid copper clad lami- nates are made by impregnating resin into a glass cloth. This means that the composite dielectric is created at the same time it is laminated. This is not the case for flex materials. Flex dielectrics are manufactured on large rolls of coated film and laminated to copper as a separate step. The advantage of this manufacturing method is that the thickness of these cast films are very consis- tent. This additional process step makes it less efficient to make constructions more than 100 um thick. While 100 um thick dielectric layers are considered to be "very thin" and thus chal- lenging to make consistently using a glass/resin lamination process, this thickness range is con- sidered to be "thick" for flexible circuit designs and by comparison relatively easy to manufac- ture. Significant differences also exist between the copper typically used in flex and in rigid circuits. Electrodeposited (ED) copper is by far the most common type of copper used for rigid circuits. This type of copper is made by "grow- ing" copper on a large electrified drum that ro- tates in a solution. The result is very consistent, but with fine copper grains when viewed mi- croscopically. Another way to make copper foil is to start with a large ingot and run it through successively closer rolls. This rolling process an- neals the copper making it more ductile by cre- ating relatively large copper grains, leading to USING FLEX IN HIGH-SPEED APPLICATIONS continues Figure 2: 50 ohm signal lines with rigid (a) and flex (B) materials. a b