Accueil
System mechanics: Accommodating and protecting the components
Electronics Packaging: Everything a system needs
Having presented an overview of an electronics packaging system in the first part of this series, we devote this second part to a closer look at the first significant single element - the mechanical structure from which the system is constructed. The mechanics both provides protection against many stresses and environmental influences and is also the principal framework for accommodating electronic and electrical components.
The mechanical structure
The 19" standard as basis
The basis for the mechanical design of all electronics packaging systems is the 19" standard (IEC 60297) or the IEEE1101.x standard (Fig. 1). These specifications define the interfaces between the individual components such as connectors, PCBs, subracks, cases and cabinets, which enables the customer to obtain system components from a variety of sources. The 19" standard was created in the mid-1970s and has become established as the most commonly-used industry standard for the construction of such systems. IEEE1101.x is the American counterpart to the European IEC 60297. In 2000 a process of adaptation took place between the IEC and IEEE standards that removed almost all of the remaining minor differences between them. A metric standard (IEC 60917) was defined in 1988, though this has only been adopted in telecommunications.
Fig. 1. The 19" standard: basic dimensions for subracks
The IEC 60297 series of standards specifies, in its various component documents, the mechanical design of components, PCBs, subracks and frames/cabinets for 19" systems. These standards are concerned with the physical structures and specify the dimensions of height, width and depth.
• IEC 60297: This standard provides the basis for 19" design. The 19" dimension corresponds to a front panel width of 482.6 mm. Sub-units of width are specified in HP (horizontal pitch) = 5.08 mm (2/10") and height is given in U (height units) of 44.45 mm (1 3/4").
• IEC 60297-3-100: Describes the main dimensions for front panels, subracks, chassis, frames and cabinets.
• IEC 60297-3-101: Describes the dimensions for modular subracks and the PCBs inserted into them.
• IEC 60297-3-102: This part document is an addition to the previous and specifies mechanical devices for the extraction and insertion of PCBs using insertion/extraction handles.
• IEC 60297-3-103: This last part specifies the coding and alignment pin for plug-in units (which comprise front panel + board + connector).
• IEC 60297: This standard provides the basis for 19" design. The 19" dimension corresponds to a front panel width of 482.6 mm. Sub-units of width are specified in HP (horizontal pitch) = 5.08 mm (2/10") and height is given in U (height units) of 44.45 mm (1 3/4").
• IEC 60297-3-100: Describes the main dimensions for front panels, subracks, chassis, frames and cabinets.
• IEC 60297-3-101: Describes the dimensions for modular subracks and the PCBs inserted into them.
• IEC 60297-3-102: This part document is an addition to the previous and specifies mechanical devices for the extraction and insertion of PCBs using insertion/extraction handles.
• IEC 60297-3-103: This last part specifies the coding and alignment pin for plug-in units (which comprise front panel + board + connector).
Cases and subracks
Electronics packaging systems may be realised as subrack/plug-in systems, desktop systems or tower systems. In desktop and tower versions the element of visual appearance is of greater importance than it is for subrack or chassis-based systems housed in a cabinet or rack. Internal construction is however fundamentally the same for all types. Where the system mechanics are based on a particular product platform, such as those of Schroff, the customer can select individual mechanical components (Fig. 2) from a comprehensive product palette, such as horizontal rails, side panels, rear panel and accessories for EMC shielding etc, or alternatively can opt for a ready-made chassis fully preconfigured for the electronics system to be used (e.g. CompactPCI, VME64x etc.). It is however equally simple to create a bespoke system whose width, height and depth - and also colour and equipment - differs from the standard design as listed in the catalogue.
Fig. 2. The mechanical structure of a system is built up from individual components
Front panels and handles
The front cover of a system is formed by one or more front panels. On the one hand they make a strong visual impact as the 'face' of the system. The cut-outs required for connectors, LEDs, handles and switches, plus printed elements or company logos also have a visual style function. Additionally, however, the front panel is an important element as regards the provision of electromagnetic (EMC) shielding.
Since each individual plug-in board in a system normally has its own front panel, relatively long vertical slits (Fig. 3) occur on the front of the unit between the various panels, which, without suitable shielding, would significantly affect the electromagnetic compatibility of the components inside the system. For example, in a system with a processor clock frequency of 1 GHz the electromagnetic wavelength (lambda = λ) is approximately 0.3 m. The critical length at which significant electromagnetic interference or radiation occurs is equivalent to lambda/10. In this example, then, the maximum permissible dimension of any slit or gap is 30 mm. Longer slits must be avoided and any gaps fitted with suitable seals - EMC gaskets in either textile or stainless steel - which provide a conductive link between the various front panels.
Since each individual plug-in board in a system normally has its own front panel, relatively long vertical slits (Fig. 3) occur on the front of the unit between the various panels, which, without suitable shielding, would significantly affect the electromagnetic compatibility of the components inside the system. For example, in a system with a processor clock frequency of 1 GHz the electromagnetic wavelength (lambda = λ) is approximately 0.3 m. The critical length at which significant electromagnetic interference or radiation occurs is equivalent to lambda/10. In this example, then, the maximum permissible dimension of any slit or gap is 30 mm. Longer slits must be avoided and any gaps fitted with suitable seals - EMC gaskets in either textile or stainless steel - which provide a conductive link between the various front panels.
Fig. 3. Long vertical slits between front panels affect EMC shielding performance
Shielded front panels should also be fitted with an alignment pin that fixes the precise position of the inserted board within the system or subrack. The EMC shielding of the individual panels is obtained from the pressure of the gaskets against one another. Without a defined fixing point (alignment pin) on the front panel (e.g. 21 x 4 HP panels), the forces between the compressed EMC gaskets would cause each panel to be pushed sideways, thus making the insertion of the last plug-in unit impossible.
Depending on the connectors used, high forces may have to be overcome when inserting or extracting a plug-in unit. Appropriate handles are used to match this force when inserting or extracting the boards; these may be simple trapezium handles or specialised types with an integrated gearing mechanism including locking and a microswitch (e.g. for CompactPCI, VME64x, MicroTCA and AdvancedTCA).
Depending on the connectors used, high forces may have to be overcome when inserting or extracting a plug-in unit. Appropriate handles are used to match this force when inserting or extracting the boards; these may be simple trapezium handles or specialised types with an integrated gearing mechanism including locking and a microswitch (e.g. for CompactPCI, VME64x, MicroTCA and AdvancedTCA).
The role of the cabinet
19" cabinets or 19" racks are mostly used in situations where a variety of electronics packaging is required, e.g. for a control system or one that involves a series of measuring or monitoring devices. Closed cabinets not only accommodate the systems; they also provide certain protective functions such as EMC shielding and IP protection. In such a situation the systems contained in the cabinet do not then need to be provided with special protection themselves. The cabinet can also take care of other environmental influences such as ambient temperatures or dynamic loads (shock and vibration). Finally, where a cabinet is used it is possible to use intelligent cabling concepts, cooling systems, access control and power distribution systems.
Environmental influences: EMC shielding, IP protection and mechanical loading
Further standards describe the environmental influences that may act on a system and how their magnitudes can be deduced and tested. The environment standard IEC 61587 defines the following parameters:
Part 1: Climatic and mechanical tests and safety aspects
Part 2: Seismic tests (static and dynamic loads)
Part 3: Electromagnetic shielding performance tests
A further task of the mechanical aspect is to protect the system from dust and water. The IEC 60529 standard sets out the levels of IP protection for cabinets and cases against ingress of dust or water and likewise the protection of personnel against dangers inside the enclosure. IP protection specifications consist of two figures, e.g. IP 20. The first indicates the protection against foreign bodies (from fingers inserted through to the entry of dust), and the second digit represents the degree of protection from water.
Prior to the introduction of IEC 61587, the German defence equipment standard (VG 95373 part 15) or special military standards were also used for EMC tests (Fig. 4). To ensure that all its testing is verifiable and can be replicated, Schroff collaborates with independent certified laboratories such as the Institute for High-Voltage Engineering at the University of Karlsruhe.
Part 1: Climatic and mechanical tests and safety aspects
Part 2: Seismic tests (static and dynamic loads)
Part 3: Electromagnetic shielding performance tests
A further task of the mechanical aspect is to protect the system from dust and water. The IEC 60529 standard sets out the levels of IP protection for cabinets and cases against ingress of dust or water and likewise the protection of personnel against dangers inside the enclosure. IP protection specifications consist of two figures, e.g. IP 20. The first indicates the protection against foreign bodies (from fingers inserted through to the entry of dust), and the second digit represents the degree of protection from water.
Prior to the introduction of IEC 61587, the German defence equipment standard (VG 95373 part 15) or special military standards were also used for EMC tests (Fig. 4). To ensure that all its testing is verifiable and can be replicated, Schroff collaborates with independent certified laboratories such as the Institute for High-Voltage Engineering at the University of Karlsruhe.
Fig. 4. Electromagnetic shielding test rig
Static and dynamic loads
Static loads arise primarily from the weight of components contained in a system. Individual specifications (VME, CompactPCI etc) set out, for example, the maximum weight for plug-in boards, so that the total load of a system may be calculated. The values for these loads determine, on the one hand, the materials used in a system's construction (steel plate, aluminium etc), and on the other, whether the components are bolted together, welded, or in fact should consist of a single unit.
Should the system be exposed to dynamic loads, e.g. by being situated close to rotating machines or used in railway or transport applications or on ships or aircraft, the design should include suitable protection against shock and vibration influences. In such applications shock absorbers are often employed. Products that are to be used in regions with high seismic activity must also be tested to what is known as the Bellcore standard for earthquake resistance.
Should the system be exposed to dynamic loads, e.g. by being situated close to rotating machines or used in railway or transport applications or on ships or aircraft, the design should include suitable protection against shock and vibration influences. In such applications shock absorbers are often employed. Products that are to be used in regions with high seismic activity must also be tested to what is known as the Bellcore standard for earthquake resistance.
Materials and recycling
Electronics packaging systems consist of up to about 98 % metals such as aluminium, steel or stainless steel. Various finishes such as AlZn, raw or passivated aluminium and various paint and print finishes are also usual. Since the metals are used in a relatively pure state, or with surface alloys, they can be recycled without problems at the end of the product's life cycle. The plastics used, e.g. for guide rails, handles and insulation strips, should likewise be recyclable. The main plastics used are PVC and PBT. The backplane, PSUs, fans etc can also be appropriately recycled.
Since the middle of 2006 it has no longer been permitted to market or distribute end products with dangerous materials (The German electrical and electronic equipment act 'ElektroG', including WEEE - Waste Electrical and Electronic Equipment, and RoHS - Restriction of Hazardous Substances). While certain systems used in industry are not subject to these guidelines, Schroff made the strategic decision early on to modify all its products to conform to RoHS. Thus for example as early as in 2004 Schroff was using a system of lead-free soldering of components. Environmental and recycling considerations are now consistently applied by Schroff to production, packaging and transport.
Since the middle of 2006 it has no longer been permitted to market or distribute end products with dangerous materials (The German electrical and electronic equipment act 'ElektroG', including WEEE - Waste Electrical and Electronic Equipment, and RoHS - Restriction of Hazardous Substances). While certain systems used in industry are not subject to these guidelines, Schroff made the strategic decision early on to modify all its products to conform to RoHS. Thus for example as early as in 2004 Schroff was using a system of lead-free soldering of components. Environmental and recycling considerations are now consistently applied by Schroff to production, packaging and transport.
The influence of mechanics on cooling systems
The mechanical design of a system, and the shielding provisions in particular, have a direct bearing on the cooling (waste heat removal) of an electronics packaging system. What is best in terms of EMC shielding - a system that is as closed as possible - has a negative effect on cooling. For cooling, the parallel arrangement of the PCBs and the defined vertical airflow thus obtained are normally such that an optimised air-cooling system based on fans is sufficient. Further specific requirements for the cooling of boards or of the overall system are set out in the specifications for CompactPCI, VMEbus, MicroTCA and AdvancedTCA. The third part of this series will discuss this topic in detail.
Notes on the author:
fa908, 07/2009