US What a microSD card is
A microSD card is a removable flash memory card in the smallest of the three SD card sizes. It measures 15 × 11 × 1 mm, about the size of a fingernail, and follows the specifications of the SD Association, the industry body that also defines full-size SD cards.
The format began in 2004 as SanDisk’s TransFlash card and was adopted by the SD Association as microSD in 2005, which is why some devices and listings still call it a TF card. Inside, a micro SD card is a small solid-state drive: NAND flash memory plus a controller that handles wear levelling and error correction, packaged with eight contacts on one face.
microSD vs SD
microSD and full-size SD cards use the same bus and the same command set, so the difference is mostly physical. A full-size SD card is 32 × 24 × 2.1 mm and has a sliding write-protect tab. A microSD card is about a fifth of the area, has no write-protect tab and fits a simple passive adapter that turns it into a full-size SD card.
The adapter only works one way: a microSD card fits an SD slot, but a full-size card never fits a microSD slot. Performance is set by the card’s bus and speed class, not by its size, so a microSDXC card and an SDXC card with the same markings perform much the same.
Faster bus versions change the contacts. UHS-II and UHS-III cards add a second row of pins, and SD Express cards carry a PCIe and NVMe interface. Both still work in an ordinary slot, but only at UHS-I speed, so a fast card needs a host that supports its bus.
microSD capacity classes and speed markings
The capacity class decides which file system a card uses and which hosts can read it. A host that supports a newer class reads the older ones, but not the other way round.
| Card type | Capacity | Default file system | SD specification |
|---|---|---|---|
| microSD | Up to 2 GB | FAT12 or FAT16 | SD 1.0 and 1.1 |
| microSDHC | Over 2 GB up to 32 GB | FAT32 | SD 2.0 |
| microSDXC | Over 32 GB up to 2 TB | exFAT | SD 3.0 |
| microSDUC | Over 2 TB up to 128 TB | exFAT | SD 7.0 |
The capacity limits work exactly as they do for full-size cards, which our guide to SDHC vs SDXC covers in more detail. Speed is marked separately on the label.
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Speed Class (2, 4, 6, 10). The minimum sustained write speed in MB/s. Class 10 means at least 10 MB/s.
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UHS Speed Class (U1, U3). At least 10 MB/s or 30 MB/s sustained write on a UHS bus.
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Video Speed Class (V6 to V90). The minimum sustained write in MB/s for video recording. V30 suits most 4K recording.
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Application Performance Class (A1, A2). Minimum random read and write IOPS, which matters when an operating system or apps run from the card.
Industrial microSD cards
Consumer cards are built for photos and music: written occasionally, read often, used at room temperature. Embedded systems use them very differently, writing logs and updates around the clock in hot enclosures, and that is where consumer cards fail. Industrial microSD cards are designed for that job.
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Temperature rating. Qualified across the industrial temperature range, typically −40 to 85 °C, against roughly −25 to 85 °C for many consumer cards.
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Flash type and endurance. SLC or pSLC flash and a stated endurance rating, often many times that of a consumer TLC or QLC card of the same capacity.
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Power-fail protection. Firmware that keeps the card’s mapping tables intact when the power drops mid-write.
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Health monitoring. Lifetime and spare-block reporting, so a card can be replaced before it wears out.
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Fixed bill of materials. The controller and flash stay the same across orders, so a qualified card behaves the same years later.
microSD in embedded and edge systems
On embedded boards, microSD is most useful as removable boot and recovery media. Most of our Arm SBCs have a microSD slot, and many also carry on-board eMMC, so an operating system image can be loaded, updated or recovered without special tools. Many rugged Jetson computers also include a microSD slot for logs or data transfer.
For the main operating system in a deployed system, eMMC or an SSD is usually the better choice. They are faster, are soldered or screwed down rather than held by a spring contact, and are designed for constant writes. Where a card has to stay in service, use an industrial card, a slot with a positive retention cover, and a file system set up to minimize writes.
Quick answers on microSD cards
microSD cards: common questions
Is a TF card the same as a microSD card?
Yes. TF stands for TransFlash, the original SanDisk name. TF cards and microSD cards are the same size and work in the same slots.
Can a microSD card be used in an SD slot?
Yes, with an adapter. A passive microSD-to-SD adapter makes the card the size and shape of a full-size SD card. No electronics are involved, so speed is unchanged.
Are all micro SD cards the same?
No. They share a size, but capacity class, speed class, flash type and temperature rating all vary. Check that the host supports the card’s capacity class first.
How long do microSD cards last?
It depends on how much is written. A card in a camera can last many years. A consumer card running an operating system or logging data may wear out within months; an industrial pSLC card lasts far longer.
What is the largest microSD card?
Around 1 to 2 TB today. microSDXC cards are specified up to 2 TB, and the microSDUC class allows up to 128 TB as cards reach that size.
We fit industrial memory cards, eMMC or SSDs to every embedded system we supply as standard, matched to its write load and temperature, and load and validate the image before it ships.