Printed Circuit Design & Fab - March 2009 - (Page 40) LamInate fIGure 4. Comparison between different materials and thickness. eral reasons, including the fact that in high frequency applications where consistent Dk is critical, moisture absorption can be a problem. Water has a Dk value of about 70, and even a small amount of moisture absorbed into a high frequency circuit or a controlled impedance circuit can change the electrical performance. The second application example is a microstrip circuit that will need to be formed to a specific shape around a mandrel that has a 0.5-inch diameter. The electrical concerns are primarily low insertion losses (< 0.05 dB/in), tightly controlled impedance (50 ohms +/- 5%) and operating considerations at 900 MHz. Typically, an application operating at 900 MHz is not considered high frequency and more traditional FR-4 materials could normally be used. There are however, other electrical concerns that drive material selection to a high frequency grade material in this example. The need for tightly controlled impedance translates to a substrate that must have tightly controlled thickness and Dk values. Traditional FR-4 materials will not control these attributes nearly as well as high frequency circuit materials. Of course, the etching of the conductor width will need to be tightly controlled at the circuit fabricator to meet the controlled impedance targets. Overall, the concern of low insertion loss can be rather complicated. There are many issues that can affect this circuit property such as: connectors, signal launch design, plating finish on the copper conductors, dissipation factor of the substrate, copper roughness, circuit geometry and some assembly processes. When choosing the material that will give the best advantage for insertion loss, a material with low dissipation factor, smooth copper and low moisture absorption would be optimum. Also, the circuit design should use a relatively thick substrate. In this case, there are additional concerns with the mechanical challenge of forming a circuit around a mandrel (fIGure 3). Typically, a thinner substrate with no glass reinforcement will be better for forming the circuit and will generate less strain on the copper layers. Also, the copper type should be rolled wrought or rolled annealed copper, which has a grain structure that is optimum for elongation in the x-y plane.1 Another consideration could be the type of plating finish applied to the copper conductors of the microstrip. If ENIG (electroless nickel / immersion gold) is used, then the nickel is brittle and can be problematic for bending. Also, the ENIG process will typically deposit a thickness of approximately 100 microinches to 200 microinches of nickel and about 5 microinches of gold. With the operating frequency at 900 MHz, the skin effects will force the signal to use about 87 microinches of conductor. That means the signal energy will predominately use the nickel layer, and nickel will cause an increase in conductor losses and ultimately insertion losses. This is due to the fact that nickel is less conductive than copper and has a permeability value that is much greater than copper. The permeability value will adversely affect the magnetic fields of the propagating waves. 40 For this application, designers should choose a high frequency material that has a tight control of the Dk and thickness tolerances, low dissipation factor, smooth copper, non-glass reinforcement, low moisture absorption and a non-nickel/gold finish. To determine the optimum thickness, there will need to be a trade-off between mechanical and electrical properties. For a one-time bend, there is a rule of thumb that states the strain on the copper should be approximately 2% or less. fIGure 4 shows a comparative table of different materials and thicknesses in regards to the mechanical stress values and insertion losses. In Figure 4, it can be seen from the first model to the second that the thickness of the substrate decreases. The decrease in thickness improved the mechanical stress significantly. A decrease in the substrate thickness will force a decrease of the conductor width in order to maintain 50 ohms. The decreased conductor width will increase the conductor loss, and ultimately, the insertion loss. When going from the second to the third model, a different material was selected that had a lower dielectric constant and lower dissipation factor while still maintaining the same substrate thickness. The lower dielectric constant dictates an increase in conductor width to maintain 50 ohms. The increase in conductor width lowers conductor losses, and the decrease in dissipation factor lowers dielectric losses. With lower conductor and dielectric losses, insertion loss also decreases. It can be seen that the stress number is a little higher than would be desired, but when looking at other models, this appears to be the best-case scenario. In reality, this may be good enough for the actual application. If in practice this model does not result in good, repeatable bends, the mandrel may need to be increased in size to lower the stress. When using high frequency materials in a PCB design, it is important to understand the primary performance requirements of the final product so that the best material can be selected for use. Most high frequency materials are used in specific applications where low electrical loss and uniform electrical performance across frequencies is needed. In addition, low moisture adsorption and excellent chemical resistance are salient properties of many high frequency materials and can be advantageous in specific environmental applications. PCD&F Ed. Note: In Part 2 of this series on high frequency materials, fabrication processes will be investigated. reference 1. John Coonrod, “Bending and Forming High Frequency Printed Circuits” , IPC Printed Circuits Expo, APEX and the Designers Summit, 2007 . John coonrod is a market development engineer with Rogers Corporation; john.coonrod@rogercorp.com. MARCH 2009 PRINTED CIRCUIT DESIGN & FAB
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