Printed Circuit Design & Fab - January 2009 - (Page 23)

LAMINATE MatErialS FiGurE 5. Dk and Df versus frequency. FiGurE 4. Unbalanced build. FiGurE 6. the variation of Dk versus resin system and percent resin in the laminate. mil core and one substituting a single-ply 1 x 2113, 3.5 mil core. taBlE 3 compares the data sheet and the construction options for Mid Dk material. Given this information, one should note that the 1 x 2113, 3.5 mil core electrical properties are a relatively close match in the material data sheet, while the 106 and 2113 prepreg and 2 x 106, 3.5 mil core are significantly different. This is a critical difference that must be considered when bandwidth budgets are tight. Referring to Table 3, consider a PCB which has a 10-inch long, controlled impedance transmission line operating at 2 GHz that has been designed, tested and prototyped using the 2 x 106 construction and correct dielectric properties. In an effort to reduce cost, the PCB shop might decide to convert the design to a single-ply stack up and substitute a 2113 construction for the 2 x 106 dielectric openings. The problem arises when the line widths must be reduced to match the desired impedance due to the higher Dk values of the 1 x 2113 core and 2113 prepreg (which in this case matches the data sheet). In order to match impedance, the PCB engineer would typically calculate the line width based on a 50 Ω impedance target and would arrive at ~3.6 mils on half ounce copper. However, for the 2 x 106 construction, the line width would be ~4.4 mils on half ounce copper. By reducing the line width, the conductor losses increase, and this is where the substitution becomes a problem. The resulting attenuation, due to conductor losses alone, as determined by Equation 2 would be approximately -0.280 dB/in and -0.233 dB/in respectively. Combining these with the dielectric losses, the 2 x 106 construction would result in an attenuation of approximately -3.44 dB, and the 1 x 2113 construction would result in an attenuation of approximately -3.83 dB. This is a 10% increase from the original construction and may result in system performance issues. JANUARY 2009 poses a potential risk. The local εr (Dk) on one side of the laminate in a 106/2113, 5.3 mil core construction will be different from side to side.6 Orientation of the panel during design and fabrication becomes critical and care must be taken to ensure the panel is printed on the correct side. The difference in the Dk and Df values from the 2113 to the 106 side is approximately 12% and 19% respectively. The resulting difference in attenuation between the bulk properties and the local properties of a strip line configuration placed on the 106 side of the laminate using the construction above at 5 GHz, as deter- In another example shown in FiGurE 4, an unbalanced laminate construction also FiGurE 7. the variation of Df versus resin system and percent resin in the laminate. mined by equation 1 would be -0.36 dB/in which would have a significant impact on the band width budget. FiGurES 5 demonstrates the variation in the local dielectric properties as compared to the bulk properties. Note that the PRINTED CIRCUIT DESIGN & FAB 23 http://www.envisionrules.org

Table of Contents Feed for the Digital Edition of Printed Circuit Design & Fab - January 2009

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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