Embedded Open Modular Architecture
With recent embedded processors becoming mainstream and powerful enough for general-purpose computing, the Embedded Open Modular Architecture is an initiative to create interoperable hardware standards for mass-volume systems based around embedded processors. The first initiative is to re-use the PCMCIA form-factor, in a similar way to Conditional Access Modules.
The re-use of industry-standard connectors and sockets is a very common practice. Typically in the Embedded world, SO-DIMM is the form-factor of choice to re-purpose, because CPU, RAM, NAND Flash and even some micro-connectors can fit onto the 35mm x 68mm size. Other suitable form-factors include MXM and PCMCIA (CardBus) and potentially ExpressCard, although the limited pincount of ExpressCard makes it much less attractive.
This form-factor is suitable for factory-only installation. The 35mm x 68mm size also makes it very tight to fit more than the CPU (up to 25mm sq), NAND Flash (up to 20mm) and more than two RAM ICs (appx 20mmsq each), as well as PMICs. However, some companies such as Gumstix have created a de-facto modular standard for their own products such as the Overo range, using the more expensive POP RAM to make space even for on-board WIFI SIP modules. The majority of SO-DIMM form-factor modules, such as Magniel's OMAPMOD, are double-sided.
All of these options are fantastic for prototyping and for the low-to-mid-range volumes, especially for Hardware enthusiasts. However, when it comes to placing designs based around SO-DIMM and other similar form-factors of this size into mass-volume production, the space-saving decisions that had to be made immediately become a problem. Many Chinese Factories are simply not equipped to handle the higher number of layers required, nor the smaller PCB track sizes, and simply do not have access to the plasma-based equipment needed to solder Package-on-Package ICs.
Use of the PCMCIA 85mm x 55mm form-factor for purposes other than the CardBus standard is not without precedent: [Conditional Access Modules] do not comply with the CardBus electrical and electronic specification. The size and design however is highly suited to hot-swapping and provides convenient carrying and storage. It therefore makes sense to put the entire embedded computer onto the card. The size is large enough to fit CPU (up to 25mm), up to 4 RAM ICs (typically 20mm sq), a NAND Flash IC (up to 20mm) as well as PMICs and other components - even single-sided, thus reducing the cost.
The specification is at Embedded_Open_Modular_Architecture/PCMCIA.