Design007 Magazine


Issue link:

Contents of this Issue


Page 54 of 121

JULY 2021 I DESIGN007 MAGAZINE 55 • Broadside-coupled striplines should ide- ally be used on a core – Try to avoid having prepreg between them. is is required to control the Z-axis alignment between the two signal layers • Specify the design trace-to-trace spacing for correct modeling • Give the impedance lines a different aperture (usually 0.0001" difference) than non-impedance traces – Makes it easier to adjust line widths if needed • Specify reference planes Minimizing Impedance Costs Using the following guidelines will help to minimize impedance costs: • Specify impedance only on layers that really require this • Route all controlled impedance traces onto the same layer • Specify a ±10% tolerance when possible • Designate one target impedance value to be tested per layer • Couple power/ground on adjacent layers when possible – Allow for modifying the construction to meet overall tolerance Conformance Tests 100% Testing All impedance coupons are 100% tested when specified by the customer. Aer com- parisons against specified values, the measure- ments are electronically stored. Serialization Serialization is a traceability process for con- trolled impedance jobs. Impedance is typically measured with a TDR (time domain reflectom- eter) by adding test coupons to the panel. Before the impedance has been tested, the coupon and PCB have a serial number marked on them. Understanding the cost drivers in PCB fab- rication and early engagement between the designer and the fabricator are crucial ele- ments that lead to cost-effective design suc- cess. Following your fabricator's DFM guide- lines is the first place to start. DESIGN007 Anaya Vardya is president and CEO of American Standard Circuits; co-author of The Printed Circuit Designer's Guide to… Fundamentals of RF/Microwave PCBs and Flex and Rigid-Flex Fundamentals; and author of Thermal Management: A Fabricator's Perspective. Visit I-007eBooks. com to download these and other free, educa- tional titles. He also co-authored "Fundamentals of Printed Circuit Board Technologies." Figure 2: Controlled impedance test systems such as this Polar Instruments example can perform a series of impedance tests automatically. (Source: Polar Instruments)

Articles in this issue

Links on this page

Archives of this issue

view archives of Design007 Magazine - Design007-July2021