Stefan Mileschin

The push for smaller and thinner laptops, Ultrabooks, and tablets of late has come with some potentially undesirable side effects, namely the loss of flexibility. Of the Ultrabooks we’ve reviewed, I’m not sure any supported more than a single SO-DIMM slot for memory expansion, and many of them have all the DRAM components mounted directly onto the motherboard, all in the pursuit of reducing the z-height of the systems. In an effort to provide something of a middle ground, both Micron and TE Connectivity are offering alternatives that provide some reduction in z-height compared to standard SO-DIMMs while still maintaining the flexibility of an SO-DIMM slot.

The solution is quite simple and maintains full backwards compatibility with standard SO-DIMM slots, but to fully realize the z-height savings a modified SO-DIMM socket is required. In short, Micron is offering single-sided SO-DIMMs (with a standard 4GB capacity); since there are no components on one side of the SO-DIMM, it can lie flat against the motherboard. This is where the new SO-DIMM socket comes into play: it would have the module sit nearly flush against the motherboard so the connector would be the same but the housing would be slightly different.

To put things in perspective, a standard SO-DIMM is around 4mm thick; the new single-sided SO-DIMMs are able to reduce the z-height to 2.6mm. That’s not to say that they’re able to match surface mounted DRAM (around 1.2mm), but users and manufacturers would be able to choose between several memory configurations (generally speaking, 4GB or 8GB) and still maintain a thin profile. With surface mounted DRAM, you get the thinnest profile but completely lose out on upgradeability and if a company wants to offer two SKUs (e.g. 4GB and 8GB) it requires more effort in the manufacturing and assembly process. There’s also the potential for DRAM failures, which are simple to fix if you have a module but require a new board if you have surface mounted components.

From a high level, I’d just as soon see all modern laptops ship with 8GB standard, particularly the Ultrabooks with surface mounted DRAM, but manufacturers are always looking for ways to reduce cost and that has led to the existing crop of 4GB non-upgradeable Ultrabooks (ASUS UX21A/UX31A, Acer S7, etc.) One other item of note is that all of the reduced z-height modules from Micron will be reduced standby (1.35V DDR3L-RS). At least initially, the modules will only be shipping in 4GB capacities (currently, 8GB SO-DIMMs require dual-sided modules). Future higher density modules with monolithic devices (8x8Gb) should show up eventually, and of course all of the design elements are applicable to DDR4 when we see a shift to that some time likely next year.

This particular approach is only one of several that are apparently being tossed around in the industry, but thanks to the backwards compatibility with existing SO-DIMM slots it appears to have a better chance of succeeding. Other approaches that are being looked at right now include non-standard modules, which would require new connectors and modules and likely limited production compared to existing solutions. It’s expected other companies will also support the new connector, and availability of the new package (connector and single-sided SO-DIMMs) is expected this spring.

http://www.anandtech.com/show/6729/m...dram-solutions