SSD Form Factors/Interfaces – Effects on ruggedization

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The choice of SSD form factor is generally guided by the physical space and interfaces available in a system. When designing a new Military-grade system, engineers focus on SWaP (Size, Weight, and Power). That is, until the storage planning phase. Focus quickly changes to capacity and smallest possible footprint, accidentally forgetting POWER (and therefore HEAT). Everyone wants the fastest, smallest, highest capacity drive (me too!), but remember to take into consideration your application and operating environment. A four-seater Ferrari isn’t going to get you and three friends through the Rubicon trail!    

Understanding your application performance requirements, power available, and heat management techniques available will ensure application success. It'll also save you money and headaches! In a controlled environment, like a data center, it is easy to meet fastest, highest capacity, smallest possible footprint. When we start to deploy in harsh environments with high temperatures and high vibration, a small footprint means less physical space for heat management and ruggedization, which means these functions must be implemented AROUND and OUTSIDE the SSD. The faster the write speed, the more power is required. The more power, the more heat is dissipated. Without a proper heat management strategy, the SSD controller has to start power throttling (reducing performance) to continue operating.

Footprint absolutely matters. Considerations become - at the performance required in the operating environment, if a smaller form factor SSD is chosen: 

• How much extra space is needed for heat management?
• What connectors/cases are going to be used?
• Does extra engineering effort still save footprint/costs, or should different SSD form factor be considered?
• If highest capacity is priority, can cheaper commercial-grade NAND Flash be used at the cost of reduced performance at high/low temperatures, decrease reliability, and shortened SSD lifecycle

The 2.5-inch/U.2 form factor is the “standard” and most common SSD form factor. It is 69.80mm wide x 100.2mm long, and available in multiple heights - 7mm/9.5mm/15mm/custom. Capacity and often ruggedization (if done properly) is increased with height. There is space in the case to provide multiple heat-management techniques, and space for more rugged connectors. There are rugged connectors that use same footprint as standard SATA connector, leaving more space for ruggedization and/or flash (more info below). User have the choice of SATA or PCIe. SATA offers a generally sufficient speed, at low power. PCIe provides a much faster storage solution, at the cost of higher power consumption, and therefore heat dissipation. 

M.2 is another very popular form factor, it provides high performance and capacity while minimizing footprint. M.2 is the most compact SSD form factor, only 22mm wide, and available in lengths of 30mm, 42mm, 60mm, 80mm, or 110m. Users have the choice of SATA for low power, or PCIe for highspeed. The M.2 is used in many rugged deployments when reducing footprint is paramount. A consideration is operation in very high temperatures or high vibration operating environments, there is limited ruggedization that can be done on the M.2 itself. Heat blankets and other heat dissipating techniques can be done outside the SSD. For high shock and vibration, some vendors create rugged enclosures for M.2 SSDs. This often leads to solutions that are messy, and negates the benefit of a small footprint, sometimes growing larger (and more expensive) than a 2.5-inch drive.

The mSATA standard is about one-eighth the size of a 2.5inch drive. The FF adopts the PCI Express Mini form factor and connector - but requires a SATA controller. Generally, mSATA drives are used in ultra-thin and mini devices, or as a secondary drive.  

SSDs are available in a number of other form factors, varying in size, and therefore capacity and ruggedization. 1.8-inch and 3.5-inch SSDs follow the model of the 2.5-inch, with the ability to do more inside the case. EDSFF has so far been successful in data centers, however does not offer much ruggedization for military deployments.