SLC or Single Level Cell is the highest grade of NAND Flash currently available and is used for industrial. By industrial we mean; aerospace, defence, transportation, energy, telecoms, infotainment, medical marine, offshore, manufacturing type applications. SLC is highly reliable but expensive. SLC typically comes with a five-year warranty. SLC products are typically bespoke manufactured.
MLC or Multi-Level Cell is one of the most common grades of NAND Flash. This is also used in industrial applications and is cheaper than SLC. However, there is a trade-off between speed, reliability and durability. MLC is suitable for products that may need a change of storage every 18-24 months and are not heavily used. MLC can operate at wide temperatures and can be customised if industrial grade.
TLC or Triple Level Cell is a version of MLC, is used for the consumer market and is the lowest grade of NAND flash. The endurance and cost are low and it should not be used in any applications running operating systems or storing critical data. TLC is commonly found on the high street and online is widely available and cheap to manufacture.
Bridging the gap
Some companies have produced a type of NAND which bridges the gap between SLC and MLC. Performance is generally better than MLC but not as good as SLC. Manufacturer ATP, which uses aMLC, is a good example. The key differentiator with aMLC is the programme erase cycles (PE), i.e. the number of PE cycles is a number that a given device can sustain until problems occur. Each cycle causes a small amount of physical damage to the card which will accumulate over time until eventual failure. PE cycles for MLC range from 2-10K, aMLC 20k and SLC 60-100K, depending on the manufacturer.
One of the underlying differences between industrial and consumer products is that industrial grade product is highly customisable. Vendors use advanced technology and algorithms to manipulate the NAND Flash. Examples include wear levelling, which moves write cycles around the chip so that cells wear evenly; on-device duplication, which reduces the volumes of data written and so lowers wear; redundancy, which reserves a portion of the device’s capacity to replace cells as they fail; and write optimisation, which stores data writes so they can be made in large chunks to reduce the number of write operations. Also, the controller used in industrial applications is far more effective than consumer grade. Companies such as Phison, Toshiba SMI and Hyperstone specialise in SD card controller technology.