October 17, 2011

John Coonrod is a Market Development Engineer for Rogers Corporation, Advanced Circuit Materials Division. John has 23 years of experience in the Printed Circuit Board industry. About half of this time was spent in the Flexible Printed Circuit Board industry doing circuit design, applications, processing and materials engineering. The past ten years have been spent supporting circuit fabrication, providing application support and conducting electrical characterization studies of High Frequency Rigid Printed Circuit Board materials made by Rogers. John has a Bachelor of Science, Electrical Engineering degree from Arizona State University. www.rogerscorp.com/acm. This blog is part of Microwave Journal's guest blog series.

Bending and forming RF/microwave printed-circuit boards (PCBs) around a curved shape are sometimes part of the design process, such as when fabricating conformal antennas. While this may not be commonplace, for those times that it is necessary, it is important to know several things about the high-frequency PCB material for the project. This includes the correct type of material to use, by how much the material can be flexed without damage, and what types of mechanical and electrical effects are to be expected by bending and forming an RF/microwave PCB. Quite simply, picking the wrong PCB material for bending and forming applications can result in mechanical cracks and damage to the circuit board.

When a PCB material must be bent or formed around a mandrel to conform to a certain shape, circuit materials without glass reinforcement are preferred; materials with glass fabric tend to be more rigid and lack the flexibility needed for bending without cracking. Of course, the glass reinforcement provides mechanical stability, so some tradeoff in stability must be accepted as part of having the flexure capability in a PCB material.

Similarly, thinner PCB materials lend themselves more readily to bending and forming than thicker materials, although there is also a tradeoff in selecting a thinner material. Thicker materials offer improved dimensional stability over thinner versions of the same material, and thicker materials are usually specified when attempting to minimize insertion loss in a circuit.

PCB materials suppliers incorporate a range of different filler materials to improve electrical and mechanical stability, so choosing a circuit material for an application that requires bending and forming calls for a careful choice. For example, RO3200™ series PCB materials from Rogers Corp. are ceramic-filled laminates reinforced with woven fiberglass for structural stability, while the company’s RO3000® series laminate materials are fabricated without the woven fiberglass reinforcement.

Both series of materials are available in various thicknesses and in versions with dielectric constants of 3.00, 6.15, and 10.2 in the z-axis at 10 GHz. Both series of materials offer excellent electrical performance with temperature and outstanding structural stability. For example, RO3003™ material from the RO3000 series, with a dielectric constant of 3.00 in the z-axis at 10 GHz, has a coefficient of thermal expansion (CTE) of 17 ppm/°C in its x and y axes, closely matched to that of copper for good stability with temperature. In the z-axis, the CTE is 24 ppm/°C for good PTH reliability. In comparison, RO3203™, with a dielectric constant of 3.02 in the z-axis at 10 GHz, also features x- and y-axis CTE values closely matched to copper, at 13 ppm/°C, and z-axis CTE of 58 ppm/°C that also ensures good PTH reliability. Both materials are similar in thermal conductivity, with 0.5 W/m/K for RO3003 laminates and 0.48 W/m/K for RO3203 laminates. Both materials are available with electrodeposited (ED) copper foil in 0.5, 1.0, and 2.0 oz. options. But, while the data sheets for the two series of materials do not point this out, the absence of fiberglass reinforcement in the RO3000 series laminates makes them the more suitable of the two material series when considering high-frequency PCB materials for applications that require bending and forming.

Another consideration when bending and forming a circuit laminate is the amount of stress that will be imposed on the material’s copper layers. Because the copper will be stressed along the material’s x and y directions when a laminate is bent or formed around a mandrel, a PCB material should be specified with rolled wrought or rolled annealed copper, which has a grain structure that is well suited for the elongation in the x-y plane that occurs with bending and forming a PCB substrate. The type of plating for a PCB substrate that must undergo bending and forming is also a concern. Electroless nickel/immersion-gold (enig) plating is often used to plate high-frequency laminate materials. But nickel is extremely brittle and subject to cracking with flexure. Similarly, to minimize reliability problems in a PCB that must be bent or flexed, viaholes or plated-through holes (PTHs) should be avoided in the area of the PCB undergoing the greatest amount of flexure.

Once a PCB material has been selected for bending and flexing, it is important to determine practical limits for the bending and flexing, by means of a parameter known as a PCB material’s minimum bend radius. The minimum bend radius for a given material refers to two different types of flexures in a PCB material: for a one-time bend (often referred to as a “flex-to-install” application) and for an application in which multiple flexures will occur. In the first case, a laminate may be bent around a mandrel to create a particular conformal shape, and this occurs only once during the manufacturing process. In some cases, a PCB may be required to withstand dynamic flexing as part of an application, such as printed-wire connections in old-style clam-shell cellular telephones.

A general industry rule of thumb is to use a minimum bend radius that is 10 times the thickness of the PCB material for one-time bends and a minimum bend radius that is 25 times the thickness of the PCB material for dynamic PCB bending. Returning to the RO3003 material as an example, it is available in a range of standard thicknesses, from 0.005 in. (0.13 mm) through 0.060 in. (1.52 mm).  For a RO3002™ PCB laminate with thickness of 0.010 in. (0.25 mm), the minimum bend radius for one-time bends is 0.1 in. (2.5 mm) while the minimum bend radius for dynamic bends is 0.25 in. (6.25 mm). Further details on minimum bend radius for a variety of PCB materials can be found in Document IPC-2223 from the IPC (www.ipc.org). The document also includes recommendations on copper layer elongation for flexible circuits, such as 3% for one-time-bend applications and 0.3% for dynamic bend applications.

RF/microwave design engineers often consider PTFE-based PCB materials as flexible substrate materials, and such materials can be suitable candidates for bending and forming applications. For example, RT/duroid® 5880 from Rogers is a glass-microfiber-reinforced PTFE composite material with extremely low loss and low dielectric constant of 2.2 in the z-axis at 10 GHz. Because it does not use woven-glass-type reinforcement, it is suitable for both single-bend and dynamic bend applications, providing high reliability with the electrical performance associated with PTFE substrates.