TECH TALK

A journal of bus architecture tips & techniques

Basic Enclosure Construction 101

Is it important to know how your enclosure is built?  You bet.  You will find there are a number of construction types: extrusion-based, formed sheet metal, welded, dip brazed, milled, and Electrical Discharge Machining.  Let’s eliminate methods to build rugged and ATR (Air Transport Rack) enclosures to focus on the two most common types of construction used in a benign environment (e.g. in a lab):  extrusion-based and formed sheet metal.

Whether you are building an AdvancedTCA, CompactPCI, or VMEbus system, the enclosure will be manufactured using one of these designs or a combination of both.  Knowing a little about each type will help you fit the best solution to your requirements.

Extrusion Design
This design starts with an aluminum frame built with eight extrusions:  two front, two rear, and four depth (Figure 1).  The front and rear extrusions provide a surface on which to mount the injectors / ejectors of plug-in boards and one end of the card guides.  Tapped strips inserted into the front and rear extrusions provide a means to screw in front panels.  Two additional extrusions mounted parallel between the front and rear ones are necessary to fasten the backplane and rear end of the card guides.  The backplane extrusions are not shown in Figure 1.  Bottom, top, side panels, rack mount flanges, and other mounting provisions are easily fastened to the frame.  This design has several advantages:

• Very Modular.  It’s easy to modify a standard enclosure to meet specific requirements, e.g., reposition the power supply, ventilation, drives, card cage, etc.  The frame easily accommodates split backplanes (3U and 6U combinations), dual backplanes, and bridged backplanes.
• Low NRE (Non-Recurring Engineering).  Since extrusion-based designs are modular, there is little NRE for modifications.  Elma Electronics, for example, charges only $350 for modifying a standard design.  However, modifications may incur additional lead time.
• Cost Effective.  This style is very well priced for low volume and custom systems.
• Better Look.  Multiple side, top, and bottom plate styles are available to give a system a finished look. 

There is a disadvantage.  In large volume, this design is much higher in price than a sheet metal design.

Formed Sheet Metal Design 
An enclosure built from formed sheet metal doesn’t have a flexible internal frame.  Instead, the exterior panels provide the frame (Figure 2).  Extrusions for the backplane, card cage, and depth fasten to the panels.  There are advantages and disadvantages of which you need to be aware:

• Advantages.  They will cost slightly less than extrusion-based enclosures in low volume.  They cost significantly less in high volume.
• Disadvantages.  These designs are not modular.  Moving the position of the power supply or drive bays, for example, can incur high NRE.

Sheet metal designs are common in CompactPCI and AdvancedTCA applications that address high volume, cost sensitive industries.  

Electrical components like the backplane, fans, switches, and power supply are the same for both designs.  Extrusion-based and formed sheet metal designs meet Eurocard packaging specifications like IEEE 1101.1 and IEEE 1101.10.

Elma Electronics provides AdvancedTCA, CompactPCI, and VMEbus enclosures built from both designs.  With a wide range of products and a modular approach, Elma can meet the requirements of the most demanding applications.

Figure 1.  Example of an extrusion design

Figure 2.  Example of a formed sheet metal design

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