November 4, 2010

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.

Thinner printed-circuit-board (PCB) materials have some obvious mechanical advantages in maintaining low profiles and light weight in mobile and portable electronic designs. But using thinner laminates requires a number of considerations, since the thickness of the dielectric material will impact the conductor thickness of a controlled-impedance line such as the 50-Ohm lines commonly used at high frequencies. Before selecting a thinner PCB laminate material to save circuit height or weight, it may help to review the types of thinner laminate materials currently available, their key characteristics, and how each compares when used in practical manufacturing and application environments.

In addition to lighter weight and lower profiles, thinner laminates are attractive at millimeter-wave frequencies of typically 30 GHz and above. Thinner laminate materials help prevent unwanted modes of signal propagation at the shorter wavelengths associated with millimeter-wave frequencies. Of course, other factors, such as the dielectric constant and dissipation factor (loss) of the materials, can impact how they are used at those higher frequencies.

As frequencies increase, wavelengths get shorter and circuit features become finer. This usually encourages the use of PCB laminate materials with lower dielectric constant (Dk), since increasing the dielectric constant for given impedance conductor at a given frequency has the effect of also shrinking the circuit dimensions. While this can have benefits when designing and fabricating circuits for lower-frequency use, it can result in extremely small and challenging circuit tolerances at millimeter-wave frequencies. Because using thinner PCB laminates also leads to finer circuit dimensions, thinner laminates should be carefully chosen to gain the best benefits when used for millimeter-wave circuits.

Let’s look at a few examples of the effects of laminate thickness and Dk on circuit dimensions. When 50-Ohm microstrip conductors are fabricated on laminate materials having different thicknesses and Dk values, the thickness of the conductors can vary widely. In all cases, the dielectric materials were laminated with 0.5-oz. copper. For example, with a 5-mil-thick laminate with Dk of 2.2, the conductor width of a 50-Ohm microstrip line is quite wide, at 14.8 mils. For a laminate with the same thickness, but higher Dk of 4.5, the conductor width narrows to 8.9 mils.

With thinner laminate materials, those linewidths can shrink dramatically. For example, with a 2-mil-thick laminate with Dk of 2.2, the conductor width of a 50-Ohm microstrip line is now only 5.6 mils. And if a 2-mil-thick laminate with Dk of 4.5 is used, the conductor width of a 50-Ohm microstrip line is now only 3.3 mils. A wider conductor usually translates into better fabrication yields and lower losses, although loss is also a function of the dissipation factor of the laminate material.  

Thinner laminate materials and those developed for use at millimeter-wave frequencies are typically based on some resin material, used with or without a filler. Resins include hydrocarbon systems, liquid-crystal-polymer (LCP) materials, and polytetrafluoroethylene (PTFE) systems with woven-glass filler, ceramic filler, and woven-glass and ceramic filler. PTFE-based laminates are generally well known among high-frequency circuit designers for their low Dk values and extremely low loss compared to other laminate materials. The various fillers serve to add mechanical integrity to the PTFE materials, but at the cost of increasing the dielectric constant and the dissipation losses.

In thin sheets, the moisture absorption of these materials is quite low, which supports consistent electrical performance under varying environmental conditions. PTFE-based laminates also have good peel strength of the copper laminate from the dielectric surface, which can be critical when fabricating the narrow conductor widths of high-frequency circuits. Although PTFE laminate materials are typically characterized by high values for the coefficient of thermal expansion (CTE) in the z (thickness) axis, one of the benefits of thinner boards of this material is that the effects of the CTE in the z-axis is minimized. PTFE laminates are also characterized by stable Dk and loss factors over broad frequency ranges, making them suitable candidates for broadband circuit designs.

Of course, PTFE-based materials are also known for their special manufacturing requirements, such as special drilled hole wall preparation prior to plating because of the natural thermoplastic properties of the material. The use of woven-glass layers can improve the mechanical stability of PTFE laminates compared to those with random glass fillers. But some designers have experienced what is referred to as a “weave effect” when using woven-glass PTFE at higher frequencies, due to micro-variations in the dielectric constant as a result of uneven density of glass within the PTFE dielectric. Because this effect is more pronounced for thinner materials and at smaller wavelengths, designers should consider the possible impact on their applications.

When CTE is a critical requirement, PTFE laminates with ceramic filler can reduce the CTE in the z-axis to improved levels, as might be required for multilayer circuits in which stable plated through holes (PTHs) are required to interconnect the different circuit layers. Of course, the type of filler in a laminate can also play a role in manufacturing circuits for millimeter-wave applications. The particle size of the filler material will set a limit on how close PTHs can be drilled for signal and ground connections, which can be critical at millimeter-wave frequencies.

Losses in high-frequency PCBs generally can be traced to the dielectric material, to the conductors, or to radiation losses. The dielectric losses are material dependent, and can be compared for different materials by means of the dissipation factor values. Radiation losses are typically a result of design choices, such as the use of stripline, microstrip, or coplanar-waveguide (CPW) technologies, although the use of laminates with high Dk values can help minimize radiation losses in microstrip circuits.

At frequencies of 30 GHz and higher, conductor losses can be exacerbated by the roughness of the conductor and the integrity of the interface between the conductor and the dielectric material. At the extremely short wavelengths of millimeter-wave frequencies, a rough conductor surface is essentially a longer path for the millimeter-wave signals to follow compared to a smooth surface. For example, insertion-loss performance at millimeter-wave frequencies can be improved through the use of low-profile copper conductors or reverse-treated copper conductors. In addition, the use of low-loss plating finishes, such as silver can also help to minimize insertion loss when using thin laminate materials at millimeter-wave frequencies.