Today there are many applications that require the deployment of a serious computer. Depending on the application and situation, there are various ways to implement such computer. Larger industrial systems often can benefit from a standard PC or IPC. And when available space is much more limited, complete IPC boards such as PC104, 3½" or 5¼" can still be a viable choice. However, when even those are too large or unpractical, or when specific cooling requirements apply, or when the cable mess with such boards is undesired, more customized solutions should be considered.
Since it is quite a challenge to develop a complete processor board, many boards have come to market that implement a complete PC on a modular board. These module PCs are designed to be placed on top of a carrier board, which provides the practical implementation of the solution, such as signal processing, filtering and connectors. These are generally known as "Computer On Module". The most common, widespread and universal variants of these modules are those by the ETX and XTX standards.
These modules have four 100-pin connectors to connect with the baseboard. These connectors are typically called X1 to X4. The difference between the ETX and XTX modules is all about the functionality on X2; on ETX modules, this connector provides the legacy ISA bus, while XTX utilizes this connector for PCI-express, SATA, additional USB2.0 and some other ´modern´ PC interfaces.
More information about the ETX standard can be found on the ETX - Industrial Group website.
The baseboard for an ETX or XTX system must provide four 100-pins Hirose connectors to connect with the CPU module. The mechanical fixation of the module is done with four M2.5 mounting holes and standoffs. These Hirose connectors are available in two sizes; the standard connector results in a board to board spacing of 3mm, while the alternative connector gives a spacing of 9.5mm. Using this alternative leaves some 7.5mm of space between the baseboard and the bottom of the module, this is a comfortable amount of space for things like power supplies and such. With the standard connectors, this space is just ~1mm, which means the area below the module is effectively lost.
Modules that are designed and supplied according the ETX standard lack a complete cooling solution. That is because the standard requires a thermal interface located 8mm above the module PCB. The system designer must add his own cooling solution on top of this heatspreader. This heatspreader is typically 2mm thick, which means the component height of the parts on the module should remain below 6mm. In the latest version of the ETX standard (3.0.2), the thickness requirement for the heatspreader is removed, which leaves the manufacturer free in his choice of thickness and component height, as long as the heatspreader remains at 8mm above the PCB. However, because of mechanical stability, it is unlikely that manufacturers will deviate from the old 2mm standard.
In real life, many suppliers of ETX modules do provide complete heatsink solutions for their modules, which can be used instead of the heatspreader. The disadvantage of such solutions is of course that the complete solution becomes quite dependent on the particular supplier because each supplier has a different opinion about these heatsink solutions.
Often the quality of the standard heatspreaders is lacking due to excessive stresses on the CPU module. Therefore it can be beneficial to design a complete cooling solution, which also incorporates the heatspreader function. Of course this requires some attention to the design of the thermal interface stack between the heatsink and the CPU, but when done right, that also gives a better thermal performance than the typical heatspreaders. This way, the choice of module manufacturer remains as free as possible because the custom solution can be adapted to any module layout quite easily.
Some custom ETX heatsinks and a thermal stack
Examples of projects that have been archived using ETX modules:
Alternative CPU modules:
Besides ETX and XTX, many COM-Express modules are being offered these days. Especially in the high-end area of Core Duo processing, this newer standard seems to take over.
That is quite unfortunate because ETX has some important advantages over COM-Express:
COM-Express also has an advantage; in contrary to the average ETX module, these boards usually do provide reasonably robust mounting holes for the thermal solution. However in our designs that is not an issue because we have designed a thermal interface which overcomes all ETX tolerances without stressing the board.