Developments in Small Form Factors

The Benefits of COM-Based Single Board Computers

Recent discussions have debated the advantages of COMs, such as COM Express, versus stackable SBCs, such as PC/104. Here’s how to get the benefits of both at the same time.


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The benefits of the single board computer (SBC) are well established. An SBC provides a complete solution off-the-shelf. No electronic design or manufacturing are needed, making access to the technology available to customers ranging from giant multinational corporations to one person working in their garage. Getting the board running is usually a relatively painless activity. While integrating the SBC into the final product may require some mechanical design activity, such as enclosure design, cable design and the establishment of assembly and test procedures, an SBC is a tremendous shortcut for anyone wanting to provide computer control of their product. 

In addition, most SBCs today offer some sort of I/O expansion possibility, whether through the vendor’s proprietary system or through an industry standard. The most common industry standard forms of I/O expansion are slot cards, such as ISA / PCI / PCI Express; sockets, such as PCI MiniCard and PCI Express MiniCard; and stackable systems, such as PC/104 and SUMIT. The availability of these expansion systems means that a customer can usually buy whatever I/O is needed for the application from a supplier, again eliminating the need for any circuit design and manufacturing (Figure 1).

Figure 1
The AURORA is a typical off-the-shelf embedded small form factor SBC. This board offers an Atom Z530 1.6 GHz processor, rugged SODIMM memory, gigabit Ethernet, USB, SATA, multiprotocol serial, LVDS and SDVO features in the PC/104 form factor. It has both PC/104 and SUMIT expansion capabilities for adding I/O modules. This off-the-shelf board provides an instant solution for small to medium volume applications.

SBCs do have their limitations however. Since SBC vendors want to fit as many applications as possible, their designs usually follow industry-standard, general-purpose physical formats. In some cases a standard format may not fit the customer’s application. In addition, some rugged applications may call for the elimination of cables or the use of special I/O connectors. In these cases, a typical off-the-shelf SBC may be difficult to use.

Furthermore, even though most vendors follow industry standards, those standards only specify the mechanical outline of the board, not the types of I/O or the locations of the I/O connectors. Therefore, no two SBCs are exactly alike. If a customer wants an increase in processing power, or the currently designed-in product becomes obsolete, significant changes may be required in the customer’s physical design to accommodate the different configuration of a replacement product.

Regarding cost, although the ability to mix and match I/O from multiple vendors is attractive, a multi-board design usually is not cost-effective for high volume applications, and it also may increase the size of the system beyond the packaging limits of many applications.

A few other important but often overlooked drawbacks exist for SBCs. First, when new processors are introduced, the first products in which they appear are typically computer-on-module products not SBCs, which means that SBC users must wait longer for the opportunity to incorporate them into their products. Secondly, most SBCs use heat sinks on the top side of the board for processor cooling. These heat sinks are inefficient since they conduct heat to the surrounding air. If the SBC is in a sealed box, the cooling problem becomes even worse, limiting the upper temperature at which the SBC can operate reliably.

Because of their many advantages, SBCs are used in a wide variety of applications in all industries. SBCs find use in applications in which customers do not want to or are not able to design their own electronics. In addition, they are used when the product’s physical design can accommodate their shape, and their method of I/O expansion, without significant compromise. Due to the aforementioned disadvantages, SBC applications are usually limited to lower volume applications (less than 1,000 per year), although there are certainly many exceptions to this upper limit.

COM: Optimizing for High Volume and Custom Designs

The Computer-on-Module (COM) concept is, in many ways, the reverse of the SBC concept. Where the SBC is an off-the-shelf complete solution, a COM is essentially a large component, requiring the customer to design a baseboard to provide power to the COM and bring out the I/O. Where each SBC offers its own set of I/O features and connectors, most COMs adhere to a standard set of connectors and features. A system using a COM can usually swap out that COM for another one relatively easily, making performance upgrades vastly easier and improving the ability to protect the product from obsolescence.

The easy interchangeability of COMs results in important benefits of performance scalability and design reusability. A designer can use different COMs in the same base product to achieve multiple price and performance levels, serving more customers with just one product design.

Another advantage of a COM-based design is that the design exists in two “layers”: The COM layer provides the processor circuit, and the baseboard layer provides everything else. Stacking these two layers together provides a compact solution whose physical outline can be smaller than a single board computer where everything is on the same layer.

Customers who use COMs in their designs are, by definition, customers who have the ability to design and manufacture their own baseboard. Since such a design effort is usually only justified for a high volume application, COMs tend to be used in large quantities and are therefore extremely cost-competitive. 

The large-volume business model for COMs has a flip side however: It often presents difficult hurdles for small volume customers who may not be able to meet the vendor’s minimum order requirements, wait for the build-to-order production lead time or obtain satisfactory design-in support. 

Also, the effort to design and build a custom baseboard is often more than many customers are able or willing to accept. In today’s competitive business world, companies want to focus their resources on their competitive and unique abilities to maximize their payback on research and development. A medical instrument company would prefer to hire a scientist with expertise in their particular discipline over a circuit board designer.

Combining COM + SBC: The Best of Both Worlds

What if a customer could obtain the benefits of both SBCs and COMs without suffering the disadvantages of either? To summarize: What if a customer could achieve the smallest possible outline for the computer board, achieve scalability in performance and protection from obsolescence, take advantage of off-the-shelf expansion I/O boards, and avoid having to do any custom design? This is the objective of the COM-based SBC. 

In the COM-based SBC, the processor is in the form of a COM, and all the other circuitry is on the baseboard, just like a traditional COM-based solution. The main difference, however, is that the COM-based SBC takes the form of a familiar industry standard format so that it can serve a broad range of customers. In addition, the COM-based SBC provides sockets for adding industry-standard expansion modules, allowing easy tailoring to each customer’s application (Figure 2). 

Figure 2
A COM-based SBC utilizes layers to condense the electronics and thermal solution into a smaller outline than a typical COM baseboard.

From the customer’s perspective, a COM-based SBC is exactly like a traditional SBC, except for the important and valuable distinction that the customer is not tied to a particular processor. The benefits of performance scalability and protection from obsolescence, so important to any company, are instantly achieved by using a COM-based SBC. 

Note that these benefits are also available to the SBC vendor: One baseboard turns into many products, while the vendor can also get to market sooner and more economically by avoiding the complex and risky processor circuit design.

At first you might think that a COM-based SBC is taller or bulkier than a single board computer, where everything is on the same plane, or PCB. However, virtually all SBCs use heat sinks, and the height of the heat sink is almost always significantly higher than the height of an installed COM module. In a large number of embedded applications, a COM module uses conduction cooling to dissipate its heat directly to a wall of the system enclosure, and this conduction cooling “heat spreader” is thin as well. So in many cases a two-layer COM-based SBC is actually thinner than an SBC.

Conduction cooling presents an opportunity for two more valuable advantages of COM-based SBCs. First, in a traditional SBC, the heat sink and the expansion are on the same side (top) of the board. Often this results in physical interference or undesirable compromises, such as making the SBC larger or installing an I/O board over the heat sink. But by putting the cooling system (heat spreader) on the opposite side from the expansion site, a COM-based SBC avoids interference problems and can be smaller. 

Second, since the heat in a conduction-cooled design is transmitted directly to the system enclosure without passing through the air inside, the entire inside of the enclosure is kept cooler than with a heat sink cooling approach. This leads to higher upper operating temperature capability plus reduced rates of heat-induced failure (Figure 3).

Figure 3
Conduction cooling vs. heat sink.

The COM-based SBC takes advantage of all the benefits of a traditional off-the-shelf SBC plus all of the benefits of a COM-based solution, while avoiding the pitfalls of both. Furthermore, when designed properly, the COM-based SBC eliminates many problems that plague embedded designs. 

Example—COM-based SBC

Diamond Systems offers a COM-based SBC named Magellan. This product consists of a 3-layer “sandwich”: The “application layer”, or baseboard, on top; the COM layer (in this case a COM Express module) in the middle; and a conduction-cooling heat spreader on the bottom (Figure 4).

Figure 4
The Magellan SBC from Diamond Systems utilizes a COM Express processor module to reduce size and provide performance scalability. The customer can choose from among five different processors while retaining the exact same footprint, features and connectors.

In addition to the standard set of I/O included with COM Express (gigabit Ethernet, USB, SATA, VGA, LCD, Audio), Magellan provides a second gigabit Ethernet, four multi-protocol serial ports, and a wide-range input power supply. The heat spreader on the bottom allows for the most efficient dissipation of heat from the processor while leaving the entire top side free for “triple play” expansion sockets for PCI-104, SUMIT, and FeaturePak I/O modules plus a board-mounted USB mass storage device.

In this case the entire product takes the outline of the COM Express module. By using a COM for the processor and adopting a two-layer design, Magellan offers the highest level of features available in a small 125 x 95 mm package. Providing the same features in a single board would require the outline to be twice the size. 

When Magellan was first introduced, it was offered with the choice of Atom Z510 1.1 GHz and Core 2 Duo LV 1.6 GHz processors. Now, because of the COM-based design, it can be offered  with the additional options of Atom Z530 1.6 GHz, Atom N455, and Atom dual core C525 processors, without any additional design effort. This gives Magellan customers more flexibility than they would obtain with a traditional SBC, while providing greater protection from obsolescence.  

Diamond Systems
Mountain View, CA
(650) 810-2500