By Fred Calnan and Timothy Byrd
CommsDesign Jul 31, 2001

Given the rapidly evolving world of communication equipment designs, it's often easy to forget about the nuts and bolts of system packaging. What are the primary issues that need to be considered when packaging electronics for a communication system? Whether it is an IP router, telco switch, or wireless base station many of the design issues are the same.

So, now you're asking, where do I start to improve my system packaging. Here are 10 tips that will get you going.

1. Agency requirements.
   A primary driver for a telecom package design is the application for the enclosure. Will it be a central office (CO) installation? What countries will it be sold in? These factors drive the agency compliance requirements, including NEBS, CE, and FCC. The designer needs to be familiar with the requirements from different agencies so the necessary allowances can be made to ensure compliance.

   Once the agencies have been determined, the system engineer can determine the level of fire, electromagnetic interference (EMI), and vibration protection needed. The choice of materials, appropriate shielding, and adequate structural design will follow accordingly.

2. Don't forget size
   Designers should take into account what marketing's view is for the overall package size, bench top, and rack-mount (such as 2 or 13U). Telecommunication electronic systems are often designed to be mounted in a rack. Racks are sold in a variety of configurations such as open or closed. Other settings that vary from rack to rack include height, widths, depths, and the presence or absence of integral cooling systems.

   Many of these variations are controlled by the installation or industry specifications, while others are dictated by the electronics and thermal constraints of the system. These constraints also influence the chassis U height. Generally, it is preferable to keep the chassis height as small as possible to allow multiple products in a rack. Often, however, the total heat generated within a rack is controlled by industry specifications so the 5-kW system you squeezed into a 10U package does not guarantee that you will be allowed to stuff 15 kW into a rack.

3. Maintenance interface.
   All chassis have some type of system maintenance interface. This can be quite sophisticated, involving remote Ethernet access or modem dial up. Normally, some level of local annunciation is also required. The system maintenance interface can act as an aid to locating a problem system rapidly in a central-office (CO) environment. Integrating an alarm panel into the chassis requires careful consideration early in the design process. It is not always acceptable to utilize a card slot for the alarm board when pushing the envelope on port density, because all slots may be required for connection interfaces and I/O. This can drive some innovative solutions and chassis customization.

   In some cases, an alphanumeric or graphics display, keypad, and various LED or light-bar configurations are required. If these items are needed, easy access for maintenance should be considered. Ideally these items should be easily removable for swap out in the case of a failure. This also applies to fan trays, filters, and power supplies. In addition, it is advantageous to enable the removal of the backplane without dismantling the chassis, although this is a tough requirement in a lot of designs.

4. It's all about power.
   The main consideration here is the incoming voltage and overall wattage of the system. The overall power requirements need to be estimated by combining the loads on each board as well as the fan power requirements. The form the power supply will take is very important for the chassis designer to determine early on. Is it a board-mounted module or a plug-in module that takes up space in some number of slots? Will the power supply be scaleable to the next system configuration release? What are the efficiency and power factor requirements?

   Power distribution in the chassis is a key consideration, and for high-power systems, a 48-V distribution scheme reduces current without running into any agency restrictions.

   A rough estimate of the space for the rectifier module can be estimated from the overall wattage - high-end designs can achieve about 10 W per cubic inch. DC-DC converters will be required to bring voltages to the 3.3- or 5-V level on the boards, and these can have power densities up to about 120 W per cubic inch. To address high-availability issues, redundancy and hot swap are common requirements, as well as separate external power connections.

5. Making the connection.
   The connections to the electronics need to be carefully considered. These connections include board to backplane, power supply to backplane, external wiring to faceplate, and other internal wiring connections like fans, and LEDS.

   The demand for storage in a system chassis continues to grow, making considerable additions to the overall chassis design requirements. If larger amounts of storage are required, a separate shelf is typically designed to accommodate the disk drives. Normally these disk drives require easy accessibility for replacement, so the shelf will have guides and a latching arrangement similar to a normal card cage.

6. Red alert! Raise Shields!
   Electromagnetic compatibility (EMC) describes a products ability to operate in an environment satisfactorily without interfering with other equipment and without being overly susceptible to radiated emissions from external equipment. The chassis or enclosure acts as a barrier to prevent ingress or egress by attenuating the offending radiation. The total shield effectiveness is a combination of the effectiveness of the absorption loss and the reflection loss of the enclosure, absorption being the only effective way to dissipate low frequency energy, less than 100 Hz.

   Since they are better at absorption, aluminum and steel are the common choices for shielding enclosures. Aluminum is the better choice for high-frequency signals, which are attenuated by reflection. However, structural and cost issues often prevent the use of this weaker, more expensive material, so the designer must make the appropriate trade-off. Overall, the shielding effectiveness is reduced as the frequency of the radiation increases, so state-of-the art designs dealing with gigahertz signals need careful consideration by the chassis designer.

   To compensate for internal heat generation, enclosure openings are necessary for cooling but these should be kept to a minimum. Gap lengths should be kept to 1/50th of the wavelength of the radiation. The use of EMI gaskets to lower interference resistance across joints is almost always necessary with gigahertz signals.

7. Taking on thermal issues.
   Thermal analysis is an extremely important part of the overall design, and a detailed computational fluid dynamic (CFD) analysis should be performed, particularly if there is a high power density for the design. Chassis with 3 W/cubic inch power density will require average airflow rates in the order of 300 ft per minute minimum.

   The first step in assessing thermal constraints is a system-level analysis where a solid model of the air space is imported into a finite element analysis software package. The mesh and boundary conditions are defined, and then a CFD analysis is performed that generates air velocity, temperature, pressure, and film coefficients.

   The second stage is a board level analysis, which is performed by modeling a board with components placed on it. The film coefficients from stage one are applied to provide accurate convection information, and the output from this phase is a temperature plot of the component cases and board surfaces. If placement modifications are not sufficient to bring case temperatures to acceptable levels, a system modification is made, and the process is repeated. Typical system modifications can include increasing air velocity or flow by adding/re-sizing fans or strategically placing baffles to guide the airflow.

   Final thermal verification can include the generation of simple physical models that simulate the required heat load locations and measuring the results to obtain verification of the analysis.

8. Have it your way.
   High functional density on the individual boards and in the overall box will often eliminate using standard card cages, necessitating customization. Primary drivers of customization are heat sink and airflow requirements, board placement, power supply placement, and interconnects.

   In a custom system the backplane or mid-plane design may have no standard spacing and custom face plates will need to be designed to cover the front of the cards. Standard ejectors can normally be engineered into the custom face-plate, taking account of whether a micro-switch and or electrostatic discharge (ESD) pin is required to accommodate hot swap, a critical capability for today's high-availability comm systems.

9. Keeping costs down.
   Typically cost targets for the manufacture of the chassis are secondary to the more important requirements discussed above but once the requirements have been met, the costs must be considered carefully.

   Will the design have a number of different configurations? If so, on low card density configurations the chassis cost becomes a larger percentage of the overall cost per port. A scaleable design may be required. If extruded structural members are used, the size is more easily scaleable than with a sheet metal chassis, but for high-volume products extrusions are an expensive solution. Utilizing extrusions certainly reduces the engineering effort initially and is a good solution if fewer than 1,000 units are to be built.

   Sheet metal is the most economical approach. As volume grows custom tooling can be designed to form and bend the metal and drive the manufacturing costs even lower. Using rivets instead of screws will further reduce manufacturing costs.

10. What about ergonomics?
    For many companies the visual impression created by the design is important because it is the first impression their customer gets of the product. However, a lot of effort can be expended by an industrial design team in creating a visually appealing box and very often these systems go in locked cabinets or some CO location, so this aspect of the design should be kept in perspective.

    Easy access to the field replaceable components is an important consideration, and the designer should make as many subassemblies as possible field replaceable. This could include fan trays, power supplies, I/O cards and back-planes.

Timothy Byrd is the mechanical systems engineering manager at Tality Corp. He has 20 years of experience in the design and analysis of systems in the communication market space. He holds a BSME from the University of Lowell and can be contacted at tbyrd@tality.com.