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Everything you need to know about NAND Flash

What is NAND Flash?

NAND Flash is a type of non-volatile storage technology that does not require power to retain data. An everyday example would be a mobile phone, with the NAND Flash (or the memory chip as it’s sometime called) being where data files such as photos, videos and music are stored on a microSD card. NAND flash chips are roughly the size of a finger nail and can retain huge amounts of data.

Some examples of NAND flash are below in an SD card, left and a USB, right.

What is NAND Flash used for?

NAND Flash is all around us – in our smartphones, in modern televisions and in the computers and tablets in our homes and offices. It is also found outside, in all sorts of equipment from traffic lights to digital advertising panels, passenger announcement systems and displays. Anything that has artificial intelligence (AI) and needs to retain data is likely to contain NAND Flash.

NAND Flash is available in different grades; industrial and consumer. There are significant differences between industrial and consumer grade NAND, so it is important to consider the impacts these differences have on reliability, endurance, compliance and total cost of ownership (TCO) before selecting the NAND Flash that is fit for purpose in your application.

Manufacturing process

The way that NAND Flash is made can have a dramatic outcome on the performance characteristics.

NAND Flash goes through over 800 different manufacturing processes and it takes around 30 days to make just one wafer (which is the size of a large pizza, typically 300mm in diameter, see image below). NAND factories or ‘FABS’ are huge, run 24/7, 365 days a year for maximum efficiency and are 100 times cleaner than hospital operating theatres. FABS can make different types of NAND Flash (SLC, MLC, 3D - more on that below) and come in different sizes. Some of the largest FABS can make over 100,000 wafers a month.

Once the wafer is divided, or cut into separate chips, these are analysed and graded as 1,2,3 or 4, with 1 being the highest quality. The chips are then shipped to a vendor, such as Kingston Technology, who use the NAND in their own products.

The highest quality chips will have the best performance characteristics and therefore a price premium.

 

 Types of NAND Flash

In the simplest terms, the data stored on NAND Flash is represented by electrical charges that are stored in each NAND cell. The difference between Single-Level Cell (SLC) and Multi-Level Cell (MLC) NAND is in how many bits each NAND cell can store at one time. SLC NAND stores only 1 bit of data per cell. As their names imply, 2-bit MLC NAND stores 2 bits of data per cell and 3-bit MLC NAND stores 3 bits of data per cell. 3-bit MLC is referred to as Triple-Level Cell (TLC) and Quadruple-Level Cell (QLC). The fewer bits per cell, the smaller the capacity, but data is written and retrieved faster and the NAND chip has a higher endurance level so will last much longer. In summary, SLC is the fastest and has the highest endurance but lower capacities (typically up to 64GB). TLC is much slower, with low endurance, but has a much higher capacity threshold.

The graph below shows bits per cell and their key characteristics.

New kid on the block – 3D NAND

There is a new player in the NAND Flash market, 3D NAND. Although the concept of 3D NAND is not new (it has been around for almost a decade) it has only made its way into the market in a big way in the last couple of years. It is arguably the biggest NAND development since its inception in the 1980s but getting 3D NAND to work consistently has been an issue. However, these challenges have now been overcome and 3D NAND will doubtless be the NAND Flash of choice for years to come. 

In layman’s terms, 3D NAND is the stacking of memory chips on top of each other. Some manufacturers call this V (for vertical) NAND. The aim of this NAND is to make applications and devices run faster and more efficiently, hold more information and use less energy. The first company to launch 3D NAND was Samsung, a household name and by far the world’s largest NAND Flash manufacturer, with over 40% of the global NAND market.

Getting 3D NAND to work consistently in wide operating temperatures (-40°C to 85°C) has been a challenge for some manufacturers who supply the industrial markets. The standard operating temperature of 0°C to 70°C is adequate for consumer grade, workplace, server, and data centres but for applications working in harsh environments or remote places this temperature range is not enough. Industrial storage providers have now had time to analyse and test 3D NAND to the point where it consistently works in wide operating temperatures and it is now available for industrial applications.

Differences between NAND Flash types

There are pros and cons for each type of NAND Flash. SLC holds less data but is very fast and has the highest endurance. It is, however, the most expensive. Other types of NAND can hold more data and are cheaper but have much lower endurance levels and are often slower. So, when deciding which NAND flash is suitable for an application it is important to carefully consider these characteristics and their impact on application performance.

Probably the key differentiator between NAND types is the endurance or Programme Erase (P/E) cycles. Due to the way NAND Flash memory works, it is required that a cell is erased before it can be written to. The process of erasing and then writing a cell is called a P/E cycle. Each time a cell is erased it is damaged or worn out, so there is a lifetime for each cell. This is exacerbated by the makeup of NAND Flash. A cell can only be erased as a block and is written as a page. The size of each block and page is dictated by the design of the NAND Flash chip, but a block consists of many pages. If the information in a cell changes, it is written to a different cell and the old data is marked as ‘ready for deletion’. Then, the ‘good’ data in a block is moved elsewhere and the whole block erased.

So, often even if data in a cell does not change it will still go through a P/E cycle. Cells are worn as equally as possible using wear-leveling technology. This whole process increases the number of P/E cycles and ensures that the NAND Flash does not fill up with redundant data. The lifetime of the device is defined as endurance and is proportional to the P/E cycles of the NAND Flash. Remember earlier about storing bits of data in each cell?

In comparing the different types of NAND the typical P/E cycles are as follows:

  • SLC 60,000
  • MLC 1,500 to 3,000 (lower endurance for consumer/higher for industrial)
  • 3D TLC 500 to 3,000 (lower endurance for consumer/higher for industrial)  

As can be seen, there is a considerable difference in NAND endurance and selecting the wrong type will have an impact on application performance.

3D XPoint (pronounced cross point) is a new kind of memory jointly developed by Intel and Micron. Intel and Micron claimed that 3D XPoint would be up to 1,000 times faster and have up to 1,000 times more endurance than NAND Flash, as well as having 10 times the storage density of conventional memory. Early products are certainly faster and more durable than NAND and denser than conventional memory, but they haven't yet quite lived up to the full extent of these claims.

3D XPoint can be found in Intel’s Optane range of products which is gradually filtering into the desktop and laptop market.

Bridging the Gap with SLC mode

SLC mode is a hybrid of 2 bit per cell MLC using intelligent firmware to emulate the storage states of SLC. This results in increased endurance on MLC (20-30K P/E cycles) at only at a fraction of the cost when compared to SLC. For industrial applications where cost and reliability are of equal importance, this offers a good middle ground. In the marketplace, SLC mode is often referred to as pSLC and some manufacturers have their own brand name, such as iSLC and aMLC, but essentially these are all the same thing.

The comparison table below highlights the differences of the main types of NAND Flash and their key characteristics.

Differences between consumer & industrial grade NAND

Industrial grade NAND is typically SLC, SLC mode, and MLC and until more recently 3D TLC. Industrial grade is highly customisable and works in wide operating temperatures of between -40°C to 85°C and is used in the aerospace, defence, transportation, medical, marine, energy, and infotainment sectors to name just a few.

Consumer grade cannot be customised and operates in temperatures between 0°C to 70°C. it is typically found in phones, laptops, tablets, PCs and televisions.

Beyond 2020

3D NAND is a huge development and undoubtedly has its benefits. But is it truly ideal for industrial applications? The answer is that it really depends on what it is being used for.

As 3D NAND becomes more mainstream in industrial sectors there is some concern that the manufacture of SLC and MLC will cease, so engineers are looking to switch to 3D NAND technology as a result. SLC and MLC will not disappear overnight, in fact, supply and demand are still strong for both given their unique performance characteristics and customisation attributes.

There is certainly no immediate need to switch to 3D NAND as the technology is still relatively new to the industrial market and will evolve over the next few years as specialist industrial manufacturers, such as Innodisk and ATP Electronics, push the boundaries of this technology. It is certainly worth considering alongside SLC and MLC products but sits at the lower end of the pricing, endurance and performance spectrum.

SLC v MLC v TLC
The video below explains the differences in these types of NAND Flash -

 

Downloads

Established in 1990, Simms International plc is a privately-owned, specialist distributor of world-class memory and storage solutions to hardware and infrastructure providers, value-added resellers and online retailers. The company offers a broad portfolio ranging from the latest technology to legacy products, providing NAND flash solutions and DRAM module technology designed for applications that demand the highest levels of reliability and endurance.

31 May 2019, 7:58