New SD Express Speed Classes

Secure Digital (SD) Cards have become a most critical component to all of modern technology.  They have supplanted all competitive technologies (Sony Memory Stick, CompactFlash, MultiMedia Card, etc.) to be found in every type of portable, and even non-portable, computing device today.

A recurring challenge has been how to make sure that the performance capability of a particular SD card matches what is expected from a host device.  There is wide variance between card manufacturers and brands in terms of the speed of their SD cards.

To address this issue, for many years the SD Association (SDA) has defined a set of speed classes, primarily geared towards supporting the digital imaging industry.  A digital camera taking large RAW format digital images in quick succession needs to have much speedier write performance from its SD card than an SD card just being used to transfer a handful of files from one PC to another.  Another example:  a poor performing SD card could cause a digital video camera to drop frames, impacting the reliability of a recording.

More recently, the SDA has expanded the set of speed classes to include a new set for SD Express cards.  Recall that SD Express cards were introduced in 2018, in version 7.0 of the SD specifications.  This was a revolutionary upgrade to SD technology, which married SD protocols with PCIe and NVMe protocols.  Now, an SD card could operate closer in performance to NVMe hard drives versus traditional SD cards.

SD Express technology was improved in SD 8.0 and SD 9.0, and then again in October 2023 when the SDA released SD 9.1.  This revision of the SD specification introduced the following new speed classes specific to SD Express cards:

Therefore, the complete family of SD speed classes now looks like:

An excellent whitepaper describing the new speed classes, as well as new features of SD Express 9.1 such as power management, thermal management, and multi-stream recording, can be found here:  SD Express Speed Class.