MLC Caters for Number of Applications but Industrial Ones Often Require SLC

The rising pressure on price, including in the industrial and embedded market, has caused many storage solution manufacturers to resort to cheaper MLC and TLC NAND flash memory technology. However, some users are taking the opposite route and are using SLC – especially in smaller and medium-sized form factors that require less than 32GB of storage capacity.

"The decision as to whether an MLC memory solution is sufficient or an SLC is needed is not a simple one to make, because there are many factors that play a role here," explains Roger Griesemer, head of business unit flash products at Swissbit AG.

In principle, two profiles can be distinguished: The first is ‘extreme data retention’ applications, in which data is primarily read and hardly ever written. The other is ‘high endurance’ applications, which involve high numbers of write operations and are used for example in video surveillance cameras. An example of an ‘extreme data retention’ application is the car navigation system. Theoretically, map data is only written once to the SD memory card, and is thereafter only read. It’s a similar scenario with SSDs used solely as a boot drive or recovery drive, for example in ATMs. In these cases, MLCs with their lower ‘write endurance’ are often quite sufficient.

"However, there are other practical factors that will have an effect," cautions Griesemer. "Applications in vehicles will often involve exposure to very high temperatures, which impair data retention times considerably. If a device is expected to retain data reliably at high temperatures for years, this can only be achieved with special data refresh methods, which the firmware executes using clever analysis techniques."

Such applications will often require the use of SLC NAND, as the refresh algorithms also strain endurance. This is especially the case when the application does not solely involve read operations and is part of a system that has not been rendered uninterruptable, in which case the memory needs to exhibit excellent behavior in the event of a power failure. This is the case in 99% of all applications in which safety and security are factors, for example in transport, medicine, communications networks, industrial computers, and also in automation.

Whether a MLC storage solution is sufficient also depends on storage density, controller technology, and firmware algorithms. With larger form factors, such as 2.5" SATA SSDs with storage densities exceeding 64GB, high-quality MLC NAND flash will also often satisfy requirements, as the degradation of memory cells can be spread over a large area. However, it is recommended that the load of the SSD always be verified. S.M.A.R.T. values and design-in service from Swissbit make it possible to calculate the expected service life of the SSD in use. For lower storage densities and smaller form factors however, an MLC solution is usually not sufficient. In these cases, data needs to be spread over a smaller area and it is not possible to utilize controllers that are as large or powerful, thus reducing service life considerably. This usually necessitates the use of SLC NAND flash.

"Especially since the quality of MLC NAND flash has declined constantly in recent years. The new MLC generations no longer achieve the same levels of reliability than the older ones. After all, cost pressure affects not only application manufacturers, but also storage solution manufacturers," explains Griesemer. "We’ve found that many customers are now stricter with their specifications and demand memory quality matching the second-to-last generation or SLC solutions, because they have had some bad experiences with current MLC solutions. For them though, it is proving increasingly difficult to acquire high-quality storage solutions."

This is because the mass market is trending away from SLC to MLC and TLC flash technologies.

A shortage of reliable industrial storage
In terms of overall market share, TLC currently holds around 60-75%, MLC 20-35%, and SLC 3-5%.

"This is making SLC and increasingly MLC – especially in industrial-purpose quality – a niche product. Accordingly, an increasing number of suppliers, including recently Western Digital, have abandoned the industrial market and are exclusively concentrating on server and client applications. The embedded market is too heavily fragmented for them, making it too costly to serve. Especially in the field of 2.5" SSDs, providers often don’t assume a clear market position, making it difficult for customers to determine who is an industrial supplier and who isn’t."

It’s not only dependent on the quality of the products themselves, but also on whether their long term availability is secured, whether a fixed bill of material exists, and whether there is a stringent PCN process.

At present, Swissbit is a memory expert and manufacturer of memory products in Europe that has an explicit long-term focus on the industrial and embedded markets. Around the world, there are a few suppliers from the USA and from Asia.

"But none offer such a flexible and customer-focused industrial range," remarks Griesemer.

In order to produce the much lower quantities with often specific and higher requirements at marketable prices, Swissbit has been employing a flexible system of in-house production in Germany for years. The memory specialist has made investments in its technical consulting team and in a planning tool that ensures efficient management with extended quality management and improved quality assurance.

Solutions for cost-sensitive applications
In addition to the wide SLC portfolio, Swissbit also offers solutions positioned between SLC and standard MLC for cost-sensitive applications. Thanks to tested and selected MLC flash raw materials, the EM-MLC (endurance managed MLC) model of the 2.5" SATA SSD X-500 series achieves endurance levels that are three to ten times higher than with conventional MLC, while maintaining the same data retention.

"EM-MLC memory offers extremely attractive performance relative to price (cost per gigabyte) and very high data reliability. This makes it the ideal solution for cost-sensitive industrial applications that don’t use a large number of write operations but still have increased requirements, for example a larger temperature range, high resistance to shock and vibrations, and transparent diagnosis functionality," explains Griesemer.

An additional innovative solution is an MLC that is operated in ‘reliable mode.’ It offers ten times the write endurance, while only doubling the cost. This is currently available from Swissbit on a project basis.

"Regardless of whether a customer selects an SLC solution or one of our relatively new MLC models, with Swissbit they are always on the safe side when it comes to quality and a low TCO," says Griesemer. "For this, we only select high-quality, industrial-grade materials and semi-finished products that we subject to stringent quality checks and process using state-of-the-art production methods. We also use clever firmware, NAND flash management algorithms, to ensure maximum security and reliability. Long-term availability and established PCN processes are a matter of course for us as an industrial supplier."