Ruggedizing the SD Card



Departing from the standard shape and dimensions of SD cards is key to improving the physical ruggedness of SD devices that will find use in harsh environment applications.

Products meant for harsh environments, like those found in the military, aerospace and industrial sectors, are often designed to meet certain government or industry standards.  These standards help to ensure interoperability and consistent, reliable operation for these products in the environments in which they will operate.  One of the trends in the military and aerospace market is the move towards the adoption of commercial or COTS solutions.  These products too are often designed to industry standards, but it is important to note which industries or markets were the driving forces behind these standards.

Figure 1: For many military, aerospace and industrial applications, standard SD cards, rated for operation from -25°C to +85°C, would not meet the operational requirements of products designed for these markets. (Courtesy ATEK Access Technologies)

Figure 1: For many military, aerospace and industrial applications, standard SD cards, rated for operation from -25°C to +85°C, would not meet the operational requirements of products designed for these markets.

In many cases, products originally designed for commercial and even consumer markets are being adopted for use in more demanding applications.  Such has been the case with Secure Digital (SD) cards.  SanDisk, Matsushita and Toshiba founded the SD Association in 2000.  The three companies developed the SD card as an improved version of the MultimediaCard (MMC).  SD cards and the smaller form-factor version, microSD cards, were used as memory storage and data transfer devices for digital cameras, video cameras, MP3 players and mobile phones. 

As SD cards (including SDHC and SDXC versions) have found their way into more demanding applications outside of the consumer and commercial electronics space, manufacturers have responded by releasing specialized versions that are more robust than consumer SD cards.  The most common enhancements to make the SD cards better suited for more demanding applications include the following:

Industrial Temperature Operation

Standard SD cards are rated for operation from -25°C to +85°C.  For many military, aerospace and industrial applications, a lower end of -25°C would not meet the operational requirements of products designed for these markets.  In response, manufacturers have released SD cards rated for operation across the industrial temperature range of -40°C to +85°C.

SLC NAND Flash

Single Level Cell (SLC) NAND flash has only two states per cell.  The cell is either a “0” or a “1.”  Most consumer SD cards, however, utilize Multi-Level Cell (MLC) NAND flash. MLC NAND flash has four states per cell “00,” “01,” “10” or “11.” MLC allows for larger memory capacity devices for a given die size.  This lowers the cost per byte, but does so at a cost.  MLC NAND flash has lower write speeds and a lower number of program/erase cycles than SLC NAND flash.  For critical applications, especially those that are write-intensive, SLC is a more robust choice, so manufacturers focused on military, aerospace and industrial applications are offering SD memory with SLC NAND flash.

Fixed/Controlled BOM

The main components that make up an SD card’s electronics include the NAND flash memory, the SD controller IC and the firmware that controller is running.  With most consumer SD cards, any or all of these three could change from lot to lot, resulting in varied capacity, write performance (throughput), wear leveling and error checking performance.  In response, manufacturers are now offering SD memory devices with fixed/controlled bills of materials (BOMs).  Any change to one of these controlled BOM items would require a product change notification.

Physical Robustness

The above enhancements can all be accomplished while using the same form factors as called out in the SD card specifications.  But where the above enhancements improve the robustness of the device with regards to operational temperature, number of write cycles and consistency of performance, they do not address the physical robustness of the cards.

Figure 2: The Datakey DFX RUGGEDrive™ memory token is an example of an SD memory device that uses a proprietary form factor to improve physical ruggedness.  The RUGGEDrive token measures approximately 2 inches in length by ¾ inches in width by ¼ inches thick and utilizes solid over-molded construction.

Figure 2: The Datakey DFX RUGGEDrive™ memory token is an example of an SD memory device that uses a proprietary form factor to improve physical ruggedness. The RUGGEDrive token measures approximately 2 inches in length by ¾ inches in width by ¼ inches thick and utilizes solid over-molded construction. (Courtesy ATEK Access Technologies)

Some companies have attempted to ruggedize the SD card while still maintaining the card dimensions called out in the SD specification. However, this approach severely limits what can be done to make a card more physically robust.  For many military, aerospace and industrial applications, a robust physical design is desired.  Achieving this ruggedness requires a departure from the standard form factors (Figure 2).

Advantages and Disadvantages

Departing from the standard SD form factors can significantly improve the physical ruggedness of the memory device. However, it is important to note that using a more physically robust proprietary form factor has both advantages and disadvantages. Some potential advantages include:

Limited Support and Testing Requirements

When the standard form factor is used, the designer needs to consider all devices that will fit in that socket. For the SD card form factor, this could include MMC cards, SD cards, SDHC cards, SDXC cards and more. There are also scores of manufacturers, each with many models. While the standard should help ensure compatibility without having to test using all of the cards in existence, it is likely that some compatibility issues could exist with some cards. When a non-standard form factor is used, this exposure is greatly reduced.

Increased Security

Security is all about putting up barriers. While using a non-standard form factor would certainly not qualify as a robust security step on its own, this simple physical layer implementation affords two benefits:

  • The proprietary socket on the host can make it more difficult for someone to introduce malware or otherwise affect or access the host. This is especially true when the proprietary memory devices have limited or controlled availability, where the memory devices are not available for purchase by the general public.
  • If an SD memory device that uses a proprietary form factor is lost, it’s not possible to access the on-board data by plugging it into consumer computers, tablets or smartphones , as the proprietary-form-factor device doesn’t plug into such devices.

Additional Revenue Source

When a standard SD card is used, users have the ability purchase any SD card they wish from any source they wish, such as online or from a local store. While this is convenient for the user, it transfers potential income from the manufacturer of the system to the local or online retailer. When a proprietary form factor is used, the manufacturer can be the sole source for the approved memory device. Depending on volumes, this can be a significant source of income for the manufacturer.

Still, there are some disadvantages to consider, including:

Increased Price

Increased ruggedness and security comes with a price. A proprietary design that is physically more robust is certain to have a higher price point than a standard consumer SD card.  For some cost-sensitive designs, this will preclude the use of devices that use more robust proprietary form factors.

Limited Availability

One of the benefits of using standard SD cards is their local availability. If users have a failure, they can easily acquire a replacement by driving to the nearest discount, electronics or drug store. When a proprietary product is used, users will most likely have to contact the manufacturer of the system or their sales and distribution channel to get a replacement device.

Required Adaptor

When a proprietary form factor is used, there may be times where it is necessary to transfer data on the device to a PC or other device that only has standard connectors. In order to connect the proprietary memory device, an adapter must be used that interfaces with one of the standard ports.

Conclusion

Consumer versions of memory devices, like SD cards, which are small in size, are great for use in small consumer electronic devices, such as cameras and smartphones. However, memory devices in military, aerospace and industrial applications could be exposed to vibration, dirt, moisture, physical and electrical shock, temperature extremes, and rough use. Therefore, these applications often require a more durable solution. Abandoning the standard shape and dimensions of an SD card can significantly improve the device’s physical ruggedness.

SD Card Destructive Testing Video

ATEK Access Technologies performed a test and produced a video documenting the tests, comparing the amount of force it takes to make a series of SD cards non-functional (Figure 3). The test looked at three different brands of SD cards—SanDisk, Samsung and Delkin Devices—as well as the Datakey DFX RUGGEDrive™ memory token. ATEK used a digital force gauge to apply an increasing force to the SD card or DFX RUGGEDrive memory token to the point where an audible click or cracking sound could be heard. The presumption was that the click would be the sound of the enclosed IC cracking. After hearing the click, each device was tested to confirm that it was no longer functional. The testing showed that a robust proprietary design can be many times more durable than products that conformed to the standard form factor.

Figure 3:  A test performed by ATEK Access Technologies and documented here helps make the case that a robust proprietary SD card design offers durability greater than that of products that conform to the standard form factor.

Figure 3: A test performed by ATEK Access Technologies and documented here helps make the case that a robust proprietary SD card design offers durability greater than that of products that conform to the standard form factor. (Courtesy ATEK Access Technologies)


paull-plitzuweit_180x195Paul Plitzuweit is the senior product manager for the Datakey product line at ATEK Access Technologies. The Datakey line consists of rugged portable memory products, including the RUGGEDrive™ line that delivers USB flash drive or SD card functionality in a more robust and secure design than consumer alternatives. Paul can be reached at pplitzuweit@atekcompanies.com.

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