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Aspect ratio is a term commonly used to define the ratio of width to height.

The term also refers to the ratio of a longer dimension to a shorter one.

With this definition, how does aspect ratio relate to EMC and PCBs? When providing ground stitch connections in a PCB using multipoint grounding to a metallic enclosure, the distance spacing between ground stitch location must be determined in all directions: x- and y-axis [1].

RF currents that exist within the power and ground plane network tend to couple to components, cables, peripherals, or other electronic items within the assembly. This undesirable coupling may cause improper operation, functional signal degradation, or EMI. When using multipoint grounding to a metal chassis, by providing a third wire ground connection to the AC

mains, RF ground loops become a significant design concern. This configuration is typical with personal computers. An example of a single-point ground connection for a personal computer is shown in Fig. 2.26.

Because the edge rates of components are becoming faster, multipoint grounding is becoming a mandatory requirement, especially when I/O interconnect is provided in the design. Once an interconnect cable is attached to a connector, the device at the other end of the interconnect may provide a remote RF path to a third wire AC ground mains connection, if provided for its respective power source. The power source for the load may be completely different from the power source from the driver (e.g., the negative terminal of a battery versus AC). A large RF loop between I/O interconnects can cause undesirable levels of common-mode energy to be observed as either radiated or conducted emissions between the two ends of the cable.

How can we minimize RF loops that may occur within a PCB assembly?

The easiest way is to design the PCB with many ground stitch locations to chassis ground, if chassis ground is provided within a multilayer assembly.

The question that arises is, How far apart do we make the ground connections from each other, assuming the designer has the option of specifying where the ground stitch location will occur when implementing this design technique?

The distance spacing between ground stitch locations should not exceed ˣ/20 of the highest frequency, or harmonic of concern, not just the primary frequency of interest. If many high-bandwidth components are used,

multiple ground stitch locations are typically required. If the unit is a slow edge rate device, connection to chassis ground may be minimized, or distance spacing between ground locations may be increased. This separation should be related not to the clock rate but to the highest harmonic frequency of the circuit.

The reason this distance spacing is selected is due to characteristic

features of dipole antennas. An efficient antenna can exist with dimensions down to ˣ/20 of the highest generated frequency, or harmonic. A finite impedance exists within the ground structure, which is one side of a dipole antenna. The signal trace contains RF energy, which is the driven element of the dipole. Thus, between two locations on the PCB, an antenna

structure is developed. The concept of multi-point grounding is to minimize the dipole effect created between signal traces and a return path. The

smaller we make the distance spacing between two locations, the higher the resonant frequency of the antenna, resulting in less EMI being

developed internal to the PCB assembly.

For example, ˣ/20 of a 64-MHz oscillator is 9.2 in. (23.4 cm). If the straight-line distance between two ground stitch connections to a 0V-reference in either the x- or y-axis is greater than 9.2 in. (23.4 cm), a potential efficient RF loop exists. This loop could be the source of RF energy propagation, which could cause noncompliance with international EMI emission limits.

Unless other design techniques are implemented, suppression of RF currents caused by poor loop control is not possible and containment measures, such as sheet metal must be implemented. Sheet metal is an expensive cost that might not work for RF containment. Aspect ratio is illustrated in Fig. 2.27, where the distance spacing between ground locations is less than ˣ/20 in all directions.

Figure 2.27: Example of aspect ratio.

This chapter deals with the basics of EMC related to printed circuit boards.

Implementation of layout concepts has not yet been discussed. Since the subject of aspect ratio using a multiground methodology has been covered, we now consider a design technique for use on a PCB with multipoint

grounding.

With many chassis ground connections, how does one incorporate a cost-effective technique for making numerous ground points without use of screws for mechanical securement? Alternative techniques and material are available, with an overall cost less than the screw-secured connection, once labor costs are factored in. An example of a creative technique for providing numerous ground stitch locations, using only several screws for mechanical securement, is shown in Fig. 2.28. The material illustrated is an EMI-conducted cloth gasket on a neoprene sponge core. Other material may be used, such as beryllium copper fingers. Many manufacturers provide conductive material in any size, shape, and configuration

imaginable. This technique has been applied numerous times with extreme success.

Figure 2.28: Creative technique for providing multipoint ground

connection.

Use of this design implementation must be made long before component placement is finalized on the PCB and before mechanical design of the enclosure. What we want to achieve, for example, is 20 or more ground stitch points from the PCB to the chassis, in addition to the required screw connections. Between the PCB and metal mounting plate, or enclosure housing, a physical distance is present. This distance is due to the use of

pem-studs or equivalent securement provided in the chassis enclosure.

The height of the PCB above the chassis housing is typically 0.25 in. (0.64 cm). Within this spacing, placement of conductive gasket occurs using a mylar sheet of plastic as an alignment and support bracket. Holes are

punched into this mylar sheet. The gasket, in the shape of an I-beam 0.125 in. (3.2 mm) square is inserted into each punched hole. The mylar sheet is then located over existing pre-specified screw-secured mechanical

locations. These locations provide securement of the PCB to the mounting plate for structural requirements. These screw locations may be predefined by the architecture of the product. Relocation of these screw securement points may not be possible. For personal computers, screw locations are fixed for compatibility between vendors.

After component placement, before routing, multipoint ground locations should be identified. A plated through-hole pattern is provided on the bottom layer of the PCB. However, an actual through-hole is not

incorporated. On the bottom of the PCB, a non-solder mask circular pattern is made, as if this location were an actual through-hole pad for screw

securement. The diameter of this pattern on the bottom of the PCB must be very large, approximately 0.25 to 0.50 in. (0.64 to 1.28 cm)! A large pattern is needed to accommodate for tolerance connection between the gasket and PCB. With a sponge core, the conductive material will spread out in all directions. The oversize pattern of the ground point

accommodates for the slop in the mechanical connection and allows for optimal bonding between the gasket and PCB. This large pattern also prevents discrete or active components near the ground location from being shorted out by the conductive material. Multiple vias connections from the ground plane internal to this multilayer PCB must be provided to the ground pad, without disrupting the ability of the autorouter to perform its task. The height of the gasket must be greater than the physical dimension of the standoff. This additional height requirement permits compression of the gasket once the PCB is installed and screw-secured.

Once all ground connections are determined and the PCB is installed into the enclosure, screws are provided to mechanically secure the PCB to the chassis. The PCB presses down on the conductive gasket. The gasket will spread out on both the top and bottom side of the mylar sheet, thus

assuring a low-impedance bond connection between PCB and chassis. If a different PCB layout is required with this same chassis, but component placement and ground point requirements become different, hole locations in the mylar sheet can be redefined, thus preventing a redesign of the enclosure. This is where cost savings become significant. It is cheaper to punch holes in a mylar sheet than to retool a sheet metal chassis or re-layout a PCB.

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Table of Contents

2.3: MAGNETIC FLUX AND CANCELLATION

2.8: RF CURRENT DENSITY DISTRIBUTION

2.9: GROUNDING METHODOLOGIES

2.10: GROUND AND SIGNAL LOOPS (EXCLUDING EDDY 2.13: SLOTS WITHIN AN IMAGE PLANE

Chapter 2 - Printed Circuit Board Basics

Printed Circuit Board Design Techniques for EMC Compliance: A Handbook for Designers, Second Edition by Mark I. Montrose

IEEE Press © 2000 Recommend this title?