Another problem is the growing number of contacts of NAND flash chips which can be as high as 600 pins for the highest-capacity chips. This is a problem for the manufacturers that want to make the chips smaller. The solution is in improving the system-in-package and package-on-package technologies as well as in introducing the through silicon via technology when holes are created in the die with a laser and then filled with copper.
Transitioning to serial interfaces may also be a good solution of the problem of the large number of pins. As a replacement for embedded flash, the first generation of flash with a serial interface is going to come out in the next year. It is called Universal Flash Storage:
UFS memory will not only make the data-transfer rate higher but also change the operation logic of flash drives in Internet gadgets, smartphones, etc. The UFS standard supports multi-tasking, creates a flexible request queue (borrowed from the SCSI concept), scales up from 300 MBps and higher, and allows building a chaining topology to reduce the minimum required number of the controller’s or even the host-controller’s channels. In fact, the UFS standard may grow up to be a full-featured computer interface rather than an interface for embedded NAND flash only. In fact, NAND UFS is something like single-die SSDs.
Despite the ratified JEDEC specifications (JC-64.8) there are still no commonly accepted methods of measuring the reliability and other consumer properties of SSDs. It is impossible to compare products from different brands, which confuses the consumer.
So far, the following things are certain. All SSDs are divided into two classes: client and enterprise. Each class has its own requirements. For example, the characteristics of client SSDs will be correct for an operating temperature of 40°C and for a daily operation of 8 hours or less. Enterprise SSDs must retain their specified properties during all-day operation at up to 55°C. The allowable data retention (in the turned-off state) is 1 year for client SSDs (at 30°C) and 3 months for enterprise SSDs (at 40°C).
This means that the controller has to rewrite, from time to time, even cells that are not written to. For example, it has to renew data in read-only archives and do so four times more frequently in enterprise systems than in client computers. The functional failure rate for both types of SSDs is not higher than 3% while the UBER parameter is 1015 and 1016 for client and enterprise SSDs, respectively.
There will be a different system of ratings for each product class but it will be based on the TBW (terabytes written) value, a decimal value close to the 100% wear of the specific SSD under the specified conditions and at the allowable level of errors.
The service life of an SSD is measured directly by writing until the SSD fails due to the exceeded limit of rewrite cycles or is calculated by extrapolation. In the latter case we have to rely on the manufacturer’s honesty in performing the math.
For today, the JEDEC standard only defines the operation load for measuring the service life of enterprise-class SSDs. It is based on synthetic benchmarks rather than on emulating some real-life applications. The whole user-accessible capacity of the SSD is loaded at that. The load for client-class SSDs is still under development. All the specifications are provided for 512-byte sectors. The load and rating system for 4KB sectors is under development.
Summing up the facts presented in this article, we can give you the following synopsis. The development of social networks and related services provokes users’ interest in multimedia-supporting digital devices which need rather large displays and amounts of memory. Tablet PCs suit this purpose very well, so they have a chance to capture a share of the notebook market as well as to create their own market niche, stimulating the growth of the flash memory and SSD markets. In two years the cost of storing data on SSDs is expected to drop to $1 per gigabyte. Moreover, the 2012-model-year SSDs will be much faster than today’s $100 products. And in six years more, SSDs will exceed traditional HDDs in terms of total storage space. It is going to be very interesting to watch the new technologies and devices emerge on the market during that process. Stay tuned to us, as there’s a lot of exciting things ahead that need to be tested in X-bit's labs!