Printed Circuit Design & Fab - January 2009 - (Page 22)

LAMINATE MatErialS taBlE 2. Mid Dk and Df measurements with varying glass taBlE 3. Mid Dk and Df material properties at styles and resin contents. cOnStructiOn reSin Dk 3.900 3.800 3.670 3.510 Df 0.0110 0.0115 0.0114 0.0114 0.0120 0.0118 0.0119 0.0125 0.0124 0.0126 0.0133 0.0132 % 2 ghz 5 ghz 10 ghz 2 ghz 5 ghz 10 ghz Mid Dk, Df 0.004" 1 X 2116 45 3.920 3.910 Mid Dk, Df 0.004" 1 X 3070 49 3.820 3.800 Mid Dk, Df 0.004" 1 X 3313 54 3.700 3.680 Mid Dk, Df 0.0043" 106/1080 61 3.540 3.520 varying frequencies. cOnStructiOn freq (ghz) Material Data Sheet Mid Dk 2113 prepreg Mid Dk 106 Prepreg 2 Dk 5 10 2 Df 5 10 3.77 3.75 3.75 0.0116 0.0122 0.0120 3.63 3.62 3.60 0.0122 0.0129 0.0127 3.25 3.23 3.23 0.0141 0.0150 0.0147 Mid Dk 1x2113 3.5 mil Core 3.77 3.75 3.75 0.0116 0.0122 0.0120 Mid Dk 2x106 3.5 mil Core 3.41 3.39 3.38 0.0133 0.0140 0.0138 taBlE 4. A comparison of four materials at 50% resin content. reSin SyStem Dk Hi Tg Phen Hi Tg Dicy Mid Dk/Df Low Dk/Df @ 2 ghz Df lOSS DB/in 0.460 0.385 0.103 0.054 Dk 4.05 3.90 3.78 3.72 @ 5 ghz Df lOSS DB/in 0.0228 0.0174 0.0121 0.0066 0.528 0.395 0.271 0.146 Dk @ 10 ghz Df lOSS DB/in 0.530 0.790 0.531 0.311 4.05 0.0229 3.90 0.0174 3.77 0.0119 3.72 0.0070 4.16 0.0196 3.92 0.0169 3.79 0.0115 3.76 0.0061 taBlE 5. test methods used for determination of Dk and Df material properties. methODS Two Fluid Cell Parallel Plate IPC Stripline Bereskin Stripline SpecificatiOn IPC 2.5.5.3 IPC 2.5.5.9 IPC 2.5.5.5 Sample thickneSS 15 - 120 mils 20 - 62 mils Size 1 MHz 1 MHz - 1.5 GHz 2 - 15 GHz 1 GHz - 35 GHz 2" x 2" 4" x 4" frequency range accuracy Dk ± 1% ± 1% ± 1% ± 1% ± 0.5% Df ± 10-3% ± 10-3% ± 10-3% ± 10-3% ± 10-4% 5 - 250 mils 3.2" x 3.2" 58 - 62 mils 2" x 2.75" 8 GHz - 12.4 GHz 3 - 200 mils 2" x 2.75" Split Post Cavity IPC 2.5.5.13 is very difficult to define on a data sheet, but it should be considered. The rate of moisture uptake of the material is dependent on several factors which include: ambient environmental and process temperatures, time of material exposure during fabrication, influence of wet PCB manufacturing processes, prepreg, etched laminate, PCB storage conditions, inner layer and PCB thickness, PCB packaging, resin type and resin content, PCB design and construction, all of which allow, introduce or trap moisture in the PCB. These factors influence the amount of moisture that might be in a PCB. The definition of each point of ingress cannot be easily modeled and is not easily predicted. like 2116, 1652 and 7628 and 65% to 75% for 106 and 1080, which are typically used to bond innerlayer cores together. Typical thin core constructions in the 0.003 inch to 0.005 inch thickness range are around 50% for single-ply glass cores and in the 60% resin content range for two-ply glass cloth cores. Different laminate thicknesses require different glass styles, and subsequently, different resin contents to achieve the desired thickness. Using a mid-Dk material operating at 2 GHz, from the example above, one can determine the values for the electrical properties for a single ply of 1080, 2116 and 7628 laminate and 106 and 1080 prepreg. taBlE 1 reveals how these values change with resin content. by the components mounted on the board. Selection of a material with a minimal response to increased temperatures should be considered when operating temperatures are elevated. FiGurES 2 and 3 demonstrate that using a lower Dk and Df product provides a more stable performance over a wider range of temperatures. As an example, for a Mid Dk resin system between 23° C to 50° C, the difference in permittivity, Dk and loss tangent, Df is about 0.5% and 10% respectively. The change in permittivity is small while the difference in loss tangent is significant. The approximate increase in attenuation using Equation 1 is ~10% or -0.266 dB/in., versus -0.295 dB/in. Effects of Differences in Construction Laminate properties are dependent on construction because each construction is made using different glass styles and resin contents. The designer should be aware of the differences in Dk and Df values between single-ply and two-ply laminate constructions for the same thickness. A typical data sheet lists only one construction and one resin content and does not show the range of construction options. taBlE 2 compares single-ply construction to a two-ply construction showing that the electrical properties are significantly different within a given thickness. These differences do become critical when bandwidth is at a premium. On an existing design, substituting a single-ply core for a two-ply core construction to save cost will result in differences in the laminate electrical properties. One common mistake that is made is a substitution of a single-ply laminate or prepreg for two-ply dielectric openings to save money and to offer a reduced cost PCB without consideration of the changes in electrical performance. The result may be a nonfunctional or poorly functioning system. Compare the properties of laminate used in a design built using a 2 x 106, 3.5 JANUARY 2009 Effects of Temperature The reported values on the laminate data sheet are typically derived from testing at room temperature; however, the electrical performance of the substrate will also vary with the operating temperature of the PCB. Most high-end designs typically incorporate components that develop a significant amount of localized heat during operation. The designer should be aware of the influence of the material’s increased temperatures due to localized heat generated 22 Effects of Moisture As designers continue to push the design envelop and operating frequencies continue to increase, humidity dependant loss should be considered.2 The rate of diffusivity and the equilibrium moisture content can be calculated using Ficks’ Law of Diffusion for a given laminate using a specific resin system and thickness. Laminate data can be used for comparison between material choices and to make predictions about the potential influence during system operation. Moisture dependant electrical performance data PRINTED CIRCUIT DESIGN & FAB

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Printed Circuit Design & Fab - January 2009 Contents Our Line Market Watch Around the World Happenings ROI The Signal Doctor Positive Plating Final Finish Forum Making Sense of Laminate Dielectric Properties Design and Fab Tips for Improving Solder Mask Registration Automating the DDRx Interface Verification Process Printed Circuit Design & Fab Annual Buyers Guide: Special Suppliers Section Printed Circuit Design & Fab Annual Buyers Guide: Guide to Products and Services Off the Shelf Marketplace Ad Index BGA Bulletin

Printed Circuit Design & Fab - January 2009

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