Flashback: the rise (and fall?) of the microSD card

The recent shopping spree had us looking at great deals for microSD cards and with that came the sad realization that many of us no longer have a phone with a memory expansion slot. This sent us on a walk down memory lane to revisit the history of the microSD.

A few years ago we did a retrospective of the memory card, covering MMC, SD, Memory Stick and the like. Today we want to focus on microSD exclusively because – for better or for worse – this is the card that won the format wars.

That’s ancient history now, we have an article from over a decade ago that chronicled the rising adoption of microSD. With very few exceptions it was the memory card format of choice for most makers. Looking back at it, it was an easy win – MMC and SD (and even the short-lived miniSD) were too big and really only Sony was pushing Memory Stick.

The percentage of smartphone makers that adopted microSD by 2010

microSD, sometimes referred to as “TransFlash”, was launched in 2004. The first phone to use the new card format was certainly a Motorola – there are a few models that came out in 2004, but evidence points towards the Motorola E398 being the first.

The E396 was capable of MP3 playback and came with a 64MB card in the box. Even with heavy compression you couldn’t fit many songs on it, but you could always pop it out and replace it with a new card.

This phone has an important place in history as it served as the basis for the Motorola ROKR E1 – the first phone with iTunes support. Apple had a controlling 75% market share of digital music sales in 2005 and that strongly depended on the success of the iPod. However, Steve Jobs saw the danger that phones posed to his pocket music player and wanted to get into that market as well. The ROKR was a failure, but the phone that followed was, let’s just say, a huge success.

microSD is a smaller version of the SD card. There are some minor differences (other than size we mean), but they are so small that a passive adapter can convert from microSD to full-size SD. This was useful for plugging the card into a computer to load it up with songs or to download the photos and videos you shot on your phone.

This relationship meant that microSD improvements went in lockstep with the SD card’s evolution. The first big change came in 2006 with the introduction of the SDHC – HC for “High Capacity” – standard.

Previously, cards were capped at 2GB capacity. SDHC expanded that to 32GB and made support for FAT32 mandatory. This file system allowed for not just large cards but for large files too (up to 4GB).

The next big jump was in 2009 with the SDXC format, “eXtended Capacity”. These bumped the limit to 2TB and switched to exFAT, an evolution of the FAT32 file system that allows files to grow beyond 4GB.

A few years ago the SD specification was updated with SDUC, “Ultra Capacity”, which supports cards up to 128TB. It will be a long, long while before that limit is reached. In fact, even decade plus old SDXC format is yet to become a limiting factor as the biggest microSD cards currently on the market have 1TB capacity.

The world's first 1TB microSD card arrived in 2019 with a price of $450

Capacity is the most important measurement of a microSD card, but there are a few others that you should be aware of. The “speed class” is very important for some applications as it guarantees a minimum sequential write speed. The speed class is usually indicated on the card itself if you know how to read the icons.

The simplest rating is just something like “class 2”, which is marked on the card as a 2 inside a C. This means a guarantee that the card won’t ever go under 2MB/s. There are C2, C4, C6 and C10 classes. The faster the card, the faster you can copy files to it.

Some real-time applications like video recording are heavily dependent on sustained write speed, so much so that there is a dedicated class for it. It goes from V6 to V90, meaning from 6MB/s (enough for standard definition video) all the way to 90MB/s (which you need for 8K footage).

Here is a handy chart from the SD Association that shows the relationship between sequential write speed and video resolution. Note that this is just a guideline since different cameras use different codecs at different bandwidths.

SD speed classes required for a given video resolution (and frame rate)

The original SD format had provisions for transfer speeds up to 12.5MB/s, which was later increased to 25MB/s. The data bus was further upgraded with UHS-I (“Ultra High Speed”), which raised the speed limit to 104MB/s.

A complete overview of SD speed classes

UHS-II is a major departure from the original format as it adds an extra row of pins. This further increased the transfer speeds to 156MB/s in full duplex mode and 312MB/s in half duplex (i.e. data flows in both directions or only one direction, respectively). Putting an extra row of pins on the large SD cards was easy enough, however, the size of microSD posed a challenge.

UHS-II microSD cards do exist, but they are rare and pricey. Even more rare seem to be devices that actually support UHS-II microSD cards. Even without UHS-II the cards are good enough for high resolution video capture, but the rise of smartphones introduced a new challenge.

More speed requires more pins - enter UHS-II and SD Express

So far we have talked about the memory card as storage for multimedia – MP3s and videos. Those remain its most popular uses. A more interactive use is to store apps and games, which grew in size and complexity over time.

These are not good applications for the cards, however, as they are slow in another way. Video is recorded sequentially, so only sequential speeds matter. Apps and games need fast random access and most cards just aren’t designed for that.

Some are better than others though – the SD Association introduced the Application Performance Class. Both describe speed in terms of IOPS, random Input/Output operations Per Second. The first class is called A1 and it guarantees 1,500 IOPS for reads and 500 IOPS for writes. A few years later came A2, which increased the targets to 4,000 IOPS for reads and 2,000 IOPS for writes.

The latest development is SD Express, which just follows the lead of NVMe SSDs and adopts the PCIe data bus. The original specification allowed for a single PCIe 3.0 lane and transfer speeds up to 985MB/s. Then came support for a single PCIe 4.0 lane (or two PCIe 3.0 lanes) going at up to 1,970MB/s. The highest speed possible right now is achieved with two PCIe 4.0 lanes – a whopping 3,940MB/s.

SD Express requires the extra pins similar to UHS, which hinders adoption on the tiny microSD cards. And like we said, devices that support the extra pins are rare.

The Steam Deck can run games from a microSD card, however, Valve equipped it with only a UHS-I slot. This means transfer speeds not much higher than a spinning hard disk (better seek times, but nowhere near as good as an SSD). The Nintendo Switch also has only a UHS-I slot.

The rise of the SD and microSD cards

microSD cards are still fairly popular, their tiny form factor has earned them a place in action cameras, drones and so on. And they have found a use in handheld consoles, even if the larger SD cards (especially the SD Express kind) would have been a better pick.

Their popularity on smartphones is declining, however. How come? We want to assign partial blame to streaming services – how many MP3s and video files do you have on your phone? What about your friends? With fast 4G and now faster 5G and falling costs of mobile data, streaming has gone from viable to the preferred option. Spotify, Netflix, YouTube and the like mean you don’t need all that storage on your phone.

Mobile gaming is now bigger than PC and console gaming combined, but that won’t boost adoption of microSD for the reasons discussed above. A game large enough to have trouble fitting in the internal storage will also be too demanding to run off the card.

Another culprit is the increasing capacity of built-in storage. 128GB seems to be the average right now and most people say they need 128-256GB. With that there isn’t much need for expandable storage.

We know some of you absolutely hate that most makers have stopped equipping their phones with microSD slots, especially in the flagship class. Unfortunately, the average consumer seems to care about the card slot about as much as they do about compact phones. The same goes for the average smartphone maker.