Dissecting SanDisk's Ultra Plus SSD
The very first thing you notice when you pick up an Ultra Plus is its weight. And not because it weighs a ton. Rather, it feels like it's packed with goose down or silly string. Seriously, this is as light as a 2.5" SSD can get. That might be really convenient if you're going to send it into space, where every gram counts. However, it's a little disconcerting the first time you hold it in your hand (even for a seasoned veteran of solid-state storage like myself). Removing the four screws hiding behind the rear label allows us to pop the drive open and see the reason for the Ultra Plus' abnormal lightness.
The 2.5", 7 mm z-height chassis hides a truly diminutive PCB. Not much bigger than an mSATA drive, it'd be difficult to make a solid-state device any smaller with a full-sized SATA interface. It's also worth noting that the top housing is metal, but only in the same way that the tin foil used to wrap bubble gum is metal. Look closely and you can tell that the top housing of our sample is bent up a little bit. It's best not to get too boisterous with a chassis sporting the structural integrity of al dente pasta.
It does the job SanDisk needs it to, though, and frankly matches the style employed by most other SanDisk SSDs. It doesn't get many style points. We know what's on the inside counts most though, so let's take a closer look at the PCB.
This 256 GB Ultra Plus has four NAND emplacements. Two on the front, two on the back. The 19 nm Toggle-mode NAND comes from the Chinese fabs that SanDisk and Toshiba share. It utilizes SanDisk's All Bit Line architecture, which theoretically allows for more NAND parallelism, since one memory device typically only has the ability to access half the bit lines simultaneously (a half bit line architecture). Naming aside, the NAND is supposedly rated for 3000 P/E cycles, putting it on par with the majority of consumer drives today.
The thermal demands of mobile computing mean that SSDs tend to get quite toasty at times. To keep this from becoming an issue, the Ultra Plus throttles if its flash gets too hot. The effect is a reduction in write speed as the hardware cools down, though it doesn't kick in until past 60 degrees Celsius. You'll probably never see SanDisk's safety mechanism trigger on a desktop; it's more targeted to notebooks. If it does become necessary, though, the NAND-saving feature could be a real boon. Flash characteristics change at higher temperatures, and endurance can suffer as a result. Write and erase operations require more power than read ops, and therefore thermal throttling is made effective by reducing write speed.
The silk screening is a bit difficult to decipher, but Marvell's 88SS9175-BJM2 controller is located just above the Samsung LPDDR2 DRAM package. A few drives employ DDR3 data caches, but the older Marvell processor's ARM architecture doesn't support the faster memory interface.
This 256 GB PCB has 128 MB of DRAM on-board. The 128 GB variant includes just 64 MB. As you can see in the picture above the DRAM and controller packages both get a touch of BGA under-filling.
Unlike the newer 88SS918x-series SSD processors from Marvell, the older design is limited to the SATA 3.0 specification. It's not a huge deal; however, version 3.1 allows TRIM commands to be queued, preventing halts while the drive sorts everything out. In 3.0, read and write commands are still queued, while TRIM is put off until the pause can be tolerated. The 3.1 spec changes that, though you'll probably never notice the difference. Most SSDs receive the TRIM command from the operating system and hold onto it for a while. Then, once the drive is good and ready, it TRIMs. Unless you force the operation, an SSD takes its sweet time, waiting until the drive drops to idle to do its business.
The back of the PCB holds a smattering of surface-mount components and the other two NAND emplacements.
