Printed Circuit Design & Fab - January 2009 - (Page 26)

LAMINATE MatErialS FiGurE 8. Dissipation factor versus resin content at 2 GHz. FiGurE 9. Dissipation factor versus resin content at 5 GHz. taBlE 6. A comparison of the Dk test values using two methods: the Bereskin teSt methOD Split Post Cavity Dielectric Constant (Dk) Bereskin Stripline Dielectric Constant (Dk) Stripline and the Split Post Cavity methods with a 50% resin content laminate. frequency (ghz) ppO epOXy lOW Dk epOXy miD Dk epOXy lOW Dk/Df 2 GHz 5 GHz 10 GHz 2 GHz 5 GHz 10 GHz 3.63 3.62 3.60 3.63 3.61 3.60 3.65 3.65 3.63 3.67 3.66 3.65 3.70 3.70 3.68 3.72 3.71 3.70 3.62 3.58 3.59 3.63 3.59 3.59 FiGurE 10. An example of a 16-inch strip line using a Mid Dk epoxy resin system. taBlE 7. A comparison of the Df test values using two methods: the Bereskin Stripline and the Split Post Cavity methods with a 50% resin content laminate. teSt methOD Split Post Cavity Dissipation Factor (Df) Bereskin Stripline Dissipation Factor (Df) frequency (ghz) ppO epOXy lOW Dk epOXy miD Dk epOXy lOW Dk/Df 2 GHz 5 GHz 10 GHz 2 GHz 5 GHz 10 GHz 0.0099 0.0102 0.0110 0.0140 0.0141 0.0141 0.0084 0.0089 0.0089 0.0120 0.0127 0.0125 0.0095 0.0096 0.0096 0.0129 0.0131 0.0131 0.0044 0.0050 0.0053 0.0060 0.0066 0.0071 laminate test methods used to determine the bulk dielectric properties of materials cannot be used to take into account the localized effects of construction. Differences in Resin Systems PCB designers and engineers must be careful to note that each resin system will have different Dk and Df values depending on resin content. A common mistake is making a substitution from one resin system to another without fully understanding the differences in material performance. This often results in boards that do not function at all. One example of this error is the substitution of a High Tg lead-free Phenoliccured resin system versus a High Tg non lead-free Dicy cured resin system. This error can occur when the electrical properties are not fully investigated and the performance of the Phenolic-cured system falls short. FiGurES 6 and 7 demonstrate the differences between four resin systems. taBlE 4 compares the four materials at 50% resin content. Note the significant difference in material performance between resin systems. By using equation 1 we can evaluate the difference and benefit of using lower Dk/ 26 Df laminate materials. A simple estimated bandwidth analysis at 5 GHz gives the following results: -0.527 dB/in, -0.395 dB/in, -0.271 dB/in and -0146 dB/in respectively. In theory, the line length could be 3.6 times longer depending on the dielectric material used. Differences in Test Methods The methods commonly used for testing laminates are Parallel Plate, Two Fluid Cell, Split Post Cavity Resonator, IPC X-Band Stripline, Bereskin Stripline and Full Sheet Resonance methods. All of these methods have limitations that are dependant on the sample thickness, frequency used for the test, accuracy of the test method and repeatability of the measurements.3,4,5 Because of the number of different test methods and resin contents used by laminate suppliers, it is extremely difficult to make an accurate comparison between laminates using the data sheet. The applicable test frequency range, sample requirements and accuracy of the results vary depending on the method used. The test fixture also plays a role since air gaps between the samples and the fixture can skew the results. And indeed, the dif- ferences in fixtures for the same method produce significant differences in test values due to air gaps. The first two methods listed in taBlE 5 are commonly used for standard epoxy laminates, but they are not capable of providing adequate information for HSD (High-Speed Digital) designs above 2 GHZ. The IPC Stripline Method is well suited for HSD but has a limited range of test frequencies. The Bereskin Stripline Method has the advantages of the IPC Stripline test method but offers a wider range of test frequencies. The Bereskin Stripline method has been demonstrated to correlate well with both FEA techniques and actual PCB board testing.5 By employing equations 1 and 2 with the data collected using the Bereskin Stripline Method, one can determine the approximate attenuation and compare the results. This type of analysis is critical to understanding how the laminate data compares to simulator generated data and actual PCB measurements. The Split Post Cavity method appears to offer a wider range of test frequencies and better tolerances; however, the values may not correlate as well with actual PCB board test data. In the IPC test method7 IPC-TM-650 2.5.5.5 Section 1.3 (Stripline Test), it is clearly stated “…users are cautioned against assuming the method yields permittivity and loss tangent values that directly correspond to applications. The value of the method is for assuring consistency of product …” It is clear that the only true way to understand the usefulness of laminate data produced by these methods for design purposes is to perform adequate correlation studies between the method(s), actual measured test vehicles and software analysis/simulations. To further define how these methods produce significantly different data sets, taBlES 6 and 7 compare the Dk and Df test values using two methods: the Bereskin Stripline and the Split Post Cavity methods JANUARY 2009 PRINTED CIRCUIT DESIGN & FAB

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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