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Design007-June2019

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66 DESIGN007 MAGAZINE I JUNE 2019 Design changes that occur: • In the conceptual stage cost nothing • During the design stage requires just a little extra time • During the test stage means that you have to regress one stage • During production—or worse yet, in the field—can cost millions to fix and possibly damage the company's reputation Having the project completed on time and within budget means that costs are cut by re- ducing the design cycle and generating higher profits due to shorter time-to-market and an extended product life cycle. The key to ensuring product reliability all be- gins in the design phase: 1. Controlling the impedance of single- ended and differential signals and matching the trace impedance to the driver prevents reflections, which cause crosstalk and electromagnetic emissions 2. Establishing design constraints based on simulation will ensure that the rules will be followed by downstream tools and validated to conform by the various design rule checkers (DRCs) 3. Providing a low AC impedance pat, for high current switching devices minimizes radiation from microstrip signals and from fringing fields of embedded stripline signals A poorly designed substrate with inappropri- ately selected materials can degrade the elec- trical performance of the signal transmission, increasing emissions and crosstalk, and can also make the product more susceptible to ex- ternal noise. These issues can cause intermit- tent operation due to timing glitches and in- terference, dramatically reducing the product's performance and long-term reliability. Selecting the most suitable dielectric mate- rial for the substrate is one of the most im- portant factors in establishing a reliable prod- uct. There are literally thousands of materials to choose from. However, when each material is used for the right target application, the re- sultant PCB will have the lowest possible cost while still satisfying the design and perfor- mance goals of the project. Selecting the best material for an application is often a daunting task, but armed with the right tools, one can quickly sort through the vast array of choices to make an informed decision. The electrical properties of dielectric mate- rial can be described by two terms: 1. The dielectric constant (Dk), or relative permittivity (Er), is the ratio of the amount of electrical energy stored in a material by an applied voltage; it describes how the material increases the capacitance and decreases the speed in the material 2. The dielectric loss or dissipation factor (Df), or loss tangent (tan δ), is a parameter of a dielectric material that quantifies its inherent dissipation of electromagnetic energy Figure 3 depicts the profile for dielectric ma- terials with a Df < 0.005. The iCD Materials Planner has five default profiles ranging from basic FR-4 to ultra-low-loss materials as in Ta- ble 1. This enables the designer to compare di- electric materials based on the manufacturer, fabricator, frequency, dissipation factor (loss), and dielectric constant. Table 1: Loss profile ranges.

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