What is inside of SD card?

Brief History of SD Cards

SD cards were invented in August 1999 by SanDisk, Panasonic (Matsushita), and Toshiba as an improvement on previous storage formats like MultiMediaCards (MMCs) https://en.wikipedia.org/wiki/SD_card. The SD initially stood for “Secure Digital” and offered more storage capacity than earlier formats such as CompactFlash.

SanDisk led the invention of the SD card format, focusing on the chip design and flash memory technology. According to SanDisk co-founder Eli Harari, the goal was to “provide an ideal storage medium for the mobile phone, digital camera and digital audio applications” https://blog.westerndigital.com/invention-sd-card/.

The SD format built upon flash memory technology that was invented earlier by Fujio Masuoka at Toshiba in 1980 https://en.wikipedia.org/wiki/Memory_card. By offering a compact form factor, durable design, and higher capacity, SD cards quickly became a widely adopted standard for portable consumer devices.

SD Card Sizes

SD cards come in a wide range of storage capacities, from around 1GB to 1TB. The different storage sizes are denoted by the SD card format, with each format supporting a specific capacity range:

  • SD – Up to 2GB
  • SDHC (High Capacity) – More than 2GB up to 32GB
  • SDXC (Extended Capacity) – More than 32GB up to 2TB

The SD format supports standard SD cards up to 2GB in size. The SDHC format was introduced to support cards between 2GB to 32GB. Then SDXC was created to support even higher capacities from 32GB up to enormous 2TB cards. The physical size of the SD card remains the same across these formats – what changes is the maximum capacity supported.

Most new SD cards today are available in the SDHC and SDXC formats, with common sizes being 16GB, 32GB, 64GB, 128GB, 256GB, 512GB, and 1TB. The huge storage capacities make SD cards extremely useful for high resolution photos, 4K/8K videos, games, and other large files.

Source: https://www.kingston.com/en/blog/personal-storage/microsd-sd-memory-card-guide

Inside the SD Card

The inside of an SD card contains a small printed circuit board protected within a plastic housing. The main components on the circuit board are the flash memory chips that store the data, and a controller chip that manages the memory and interfaces with the device using the card. The controller connects to a series of metal contact points along the bottom edge of the circuit board, which are exposed through the opening in the SD card plastic casing. When inserted into a device, these contact points align with pins inside the SD card slot to establish an electrical connection for transferring data.

The flash memory chips are the storage medium that holds all of the photos, videos, documents, music and other files saved to the SD card. Flash memory is a type of non-volatile memory, meaning it retains data even when power is removed. The controller chip handles reading and writing data to the flash memory chips. It also includes the logic required for the SD card to communicate using the standard SD protocol with cameras, phones, computers and other devices. The controller ensures reliable data transfers and proper interfacing with the host device.

Together, the flash memory chips and controller chip are the key electronics inside every SD card that make it function as a portable, removable storage device. The components are miniaturized and tightly integrated onto the small circuit board protected inside the durable plastic casing.

Flash Memory

Flash memory is a type of non-volatile memory that is made up of EEPROM (Electrically Erasable Programmable Read-Only Memory) chips (What Is Flash Memory? Types, Working, Benefits and …). It stores data in a non-volatile way, meaning the data remains even when the power is cut off. This makes flash memory well-suited for storage devices like SD cards where data needs to persist when the device is powered down.

Flash memory works by using floating gate transistors to store charge on a floating gate. The presence or absence of charge on the floating gate determines the logical state of 0 or 1 for binary data storage. To write data, a high voltage is applied to the control gate of the transistor, allowing electrons to tunnel through the thin oxide layer onto the floating gate. To erase the data, a voltage of the opposite polarity is applied which removes the electrons. This allows flash memory to be electrically erased and reprogrammed as needed (What is Flash Memory? | Definition from …).

The Controller

Inside every SD card is a small microchip called the controller. This acts as the brain of the SD card, managing all of the flash memory and handling read/write requests (See SD Host Controllers).

The controller chip also performs crucial functions like wear leveling, security, and error correction. Wear leveling helps distribute writes evenly across the flash memory to prevent any single cells from wearing out faster. The controller handles all the security features like encryption and access control. And it also corrects any errors that may occur during reading or writing data.

So in summary, the controller is a tiny computer chip that serves as the interface between the flash memory and host device. It enables the reliable and smooth functioning of the SD card. Without the controller, the raw flash memory in SD cards would not be independently usable.


SD cards use several interfaces that allow them to connect to different devices. The most common interfaces are:

  • SD – This is the full-size SD card interface. SD cards were originally designed in this size, which measures 32 mm × 24 mm × 2.1 mm.

  • MiniSD – A smaller version of the SD card, miniSD measures 21.5 mm × 20 mm × 1.4 mm. MiniSD allowed SD cards to be used in smaller devices like mobile phones.

  • MicroSD – An even smaller version, microSD measures 15 mm × 11 mm × 1 mm. MicroSD cards are commonly used today in smartphones, tablets, and other small devices. According to Delkin, microSD is the most common SD card interface today.

These different interfaces allow SD cards to be used across a wide variety of devices, from digital cameras to mobile phones and tablets. The small size of microSD in particular has helped drive the adoption of SD cards in compact, portable electronics.

Speed Class Ratings

SD cards are rated by speed classes to denote their minimum speeds. The SD Association introduced speed classes like 2, 4, 6, and 10 to standardize these ratings.

For example, a Class 2 card has a minimum write speed of 2 MB/s, while a Class 10 card has a minimum write speed of 10 MB/s. Higher speed classes indicate faster performance.

There are also UHS speed classes for ultra high speeds. UHS-I cards are rated as U1 for 10 MB/s minimum and U3 for 30 MB/s minimum. UHS-II cards can reach even faster speeds.

According to the SD Association, “The Video Speed Classes defined by the SD Association are V6, 10,30,60 and 90. V6 and V10 can be applied to High Speed and UHS Bus IF product family. V30 can be applied to UHS-I and UHS-II product family” (source).

So in summary, speed classes and ratings help indicate the guaranteed minimum performance to expect from an SD card.


SD cards have no moving parts, unlike traditional hard drives, which makes them more durable in some ways.

However, SD cards are still susceptible to damage from magnets, x-rays, and liquid. The flash memory chips and circuitry inside can be ruined if exposed to these elements.

Some users report SD cards working perfectly after several years of storage unused in a drawer, while heavy use in devices can degrade cards faster. Overall, SD cards are designed for many years of use if handled with care.

Frequent users should aim to replace their SD cards every 2-3 years for optimal performance and to avoid data loss from gradual wear. Industrial SD cards built for security cameras and other write-intensive uses will typically last 5+ years.

While durable against drops and vibration, SD cards should be transported in cases and treated as delicate electronics. A damaged card has potential for irrecoverable data loss.

Security Features

SD cards contain built-in security features to protect the data stored on them. Some of the key security functions include:

Encryption – SD cards support encryption standards like the Content Protection for Recordable Media (CPRM) system. This allows the data on the card to be encrypted to prevent unauthorized access if the card is lost or stolen.

Passwords – Many SD cards allow you to set a password that must be entered before accessing the data on the card. This prevents unauthorized users from being able to view the contents if they get access to the physical card.

Secure Erase – The SD standard includes a feature called Secure Erase that allows the card to be “digitally shredded” if needed. This erases all data by overwriting the memory cells, preventing recovery of deleted data.

Overall, these security protections are designed to provide effective safeguards against someone being able to access your personal data on an SD card without authorization. Proper use of passwords and encryption provides a strong defense against data theft or misuse.

Use Cases

SD cards have many common uses across consumer electronics. Some of the most popular use cases include:

Digital cameras, phones, handheld gaming devices

SD cards allow you to greatly expand the storage capabilities of devices like digital cameras, smartphones, and handheld gaming systems. Nearly all digital cameras use SD cards as their storage medium, as do many mobile phones and portable gaming devices like the Nintendo Switch. SD cards enable you to store thousands of photos, videos, apps, and games on these devices.

Laptops, tablets, printers, media players.

Many laptops and tablets have SD card slots that allow you to insert an SD card to expand the available storage space. This lets you store more files, photos, videos, and programs on your laptop or tablet. SD cards can also be used with printers to print directly from the SD card. Media players like MP3 players can use SD cards to store music files. Overall, SD cards provide removable, reusable storage that can augment many electronics.