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Basics of solid-state memory technologies in consumer electronics (Part 3)

08 Oct 2012  | Thomas Coughlin

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Part 1 starts off with look at the history of flash memory and its basic operations. Part 2 explores bit errors, the causes of flash memory cell wear, and SLC vs MLC tradeoffs.

Flash memory in consumer electronics
Although most consumer electronics devices in the past used NAND in a card format, there is a progression away from that model as flash content increases.

It appears that the flash content of earlier systems motivated users to store all of one type of data in one flash card, and all of another into another card. As one card was filled, it would be taken to the office for eventual offloading to a hard disk drive, while the other was still in use.

Today many consumer electronics devices contain such an abundance of flash that this flash can be split into zones where one zone suits one purpose and another suits another purpose, with the threat of the flash becoming filled diminishing over time. This of course assumes that the required capacity of the files also doesn't increase in time.

Flash memory environmental sensitivity
The key advantage that flash memory devices have over electro-mechanical storage, such as hard disk drives or optical discs, are their extreme ruggedness. From time to time, stories appear attesting to this. SanDisk once issued a press release about a photographer who shot pictures of the demolition of a bridge using a remote-controlled camera. When the camera was struck by flying debris, the lens was shattered, but the flash card remained intact, yielding close-up photos of the explosion.

Another incident involved the fiery crash of a jetliner whose flight recorder was severely burned. Even the epoxy packages on the flash chips were melted. A reverse-engineering firm was able to reattach wires to the flash chip, and subsequently read out the data that enabled them to find the cause of the crash.

How much abuse are these chips capable of withstanding?

Standard specifications on a chip of any sort are operating temperatures of -40° to +125°C, with storage temperatures of up to 150°C. NAND flash chips follow this convention. The maximum storage temperature is set to one that the package can withstand for extended periods without losing its hermeticity, as moisture is lethal to a chip. The chip inside can withstand temperatures up to about 450°C before the silicon melts to the point that the impurities start to move around, changing the operation of the device. The epoxy will melt at a far lower temperature of over 300°C, but the chip will still keep its data after the package is long gone.

Shock and vibration specifications on semiconductors are similar, in that they are specified conservatively to account for weaknesses in the package, rather than in the silicon chip itself. Typical specifications are for high levels of constant acceleration that is used to test if the bonding wires will pull off the chip, which once again is a test of the package rather than of the silicon itself.

Memory reliability specs to estimate product lifetime
There are few specifications that must be considered to understand failure in a flash chip. This is particularly important to understand if the engineer wants to design a controller rather than to purchase one from a controller vendor.

The most important specification is endurance. This is a measure of how many erase/ write cycles can be performed before a bit failure is likely to occur. The guaranteed minimum for most makers of SLC flash is 105 or 100,000 erase/write cycles. For two-level MLC the guaranteed minimum is almost always 104 cycles, or one tenth the number of SLC.

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