A Complete Guide to SD Card Specifications and Features

SD Card (Secure Digital Card)

Basic Definition

An SD Card (Secure Digital Card) is a compact, portable flash memory storage device designed for use in consumer electronics, cameras, smartphones, and industrial devices. Developed by the SD Association (SDA) in 1999 (as a successor to MultiMediaCard/MMC), SD cards use NAND flash memory to store data non-volatilely (no power required to retain data) and are available in three form factors with varying capacities, speeds, and use cases.

Core Form Factors

SD cards are categorized by physical size to fit different devices:

1. Standard SD Card

Dimensions: 32mm × 24mm × 2.1mm (credit card-sized, thickest variant).
Use Cases: Digital cameras (DSLR/mirrorless), camcorders, laptops, and industrial equipment (e.g., security cameras).
Features: May include a write-protect switch (physical slider to prevent accidental data deletion).

2. Mini SD Card

Dimensions: 21.5mm × 20mm × 1.4mm (smaller than standard SD, rarely used today).
Use Cases: Older mobile phones, portable media players, and small cameras (largely replaced by microSD).
Compatibility: Works with standard SD slots via an adapter.

3. Micro SD Card (TransFlash)

Dimensions: 15mm × 11mm × 1mm (smallest and most widely used form factor).
Use Cases: Smartphones, tablets, action cameras (GoPro), drones, handheld gaming consoles (Nintendo Switch), and IoT devices.
Compatibility: Includes a “microSD to SD” adapter for use in standard SD slots; variants like microSDHC and microSDXC match the capacity/speed specs of full-size SD cards.

Capacity Classes

SD cards are grouped by maximum storage capacity, defined by the SDA:

Class	Full Name	Capacity Range	File System	Key Use Case
SDSC	SD Standard Capacity	Up to 2GB	FAT16	Older devices, basic cameras
SDHC	SD High Capacity	2GB – 32GB	FAT32	Consumer cameras, smartphones
SDXC	SD eXtended Capacity	32GB – 2TB	exFAT	4K video, high-capacity storage
SDUC	SD Ultra Capacity	2TB – 128TB	exFAT	8K video, professional media

Note: SDUC is a newer standard (2018) and only supported by high-end devices (e.g., professional cameras, latest smartphones).

Speed Classes & Ratings

SD card speed is critical for tasks like video recording (especially 4K/8K) and burst photography. The SDA defines several speed rating systems:

1. Speed Class (C)

Measures minimum sequential write speed (in MB/s) for standard video recording:
- Class 2 (C2): ≥2 MB/s (SD video, basic stills)
- Class 4 (C4): ≥4 MB/s (720p video)
- Class 6 (C6): ≥6 MB/s (1080p video)
- Class 10 (C10): ≥10 MB/s (1080p/4K video, burst photography)

2. UHS Speed Class (U)

For UHS (Ultra High Speed) bus-enabled cards (faster data transfer interface):
- UHS-I: Bus speed up to 104 MB/s (most common)
- UHS-II: Bus speed up to 312 MB/s (uses extra pins for faster transfer)
- UHS-III: Bus speed up to 624 MB/s (rare, high-end professional use)
UHS Speed Class ratings (minimum write speed):
- U1 (UHS Class 1): ≥10 MB/s (4K video, burst mode)
- U3 (UHS Class 3): ≥30 MB/s (4K/8K video, high-bitrate recording)

3. Video Speed Class (V)

Optimized for high-resolution video recording (defines minimum write speed for sustained video capture):
- V6: ≥6 MB/s (1080p)
- V10: ≥10 MB/s (1080p/4K)
- V30: ≥30 MB/s (4K/60fps)
- V60: ≥60 MB/s (8K/30fps, ProRes video)
- V90: ≥90 MB/s (8K/60fps, raw video)

4. Application Performance Class (A)

For app storage (e.g., running apps from microSD cards on smartphones/tablets):
- A1: ≥10 MB/s write, ≥1500 IOPS (read), ≥500 IOPS (write)
- A2: ≥10 MB/s write, ≥4000 IOPS (read), ≥2000 IOPS (write) (faster app loading)

Key Technologies & Features

1. NAND Flash Type

Most consumer SD cards use TLC (Triple-Level Cell) NAND (balance of cost and capacity).
Professional/high-end cards use MLC (Multi-Level Cell) or even SLC (Single-Level Cell) NAND for higher endurance (more program/erase cycles) and speed.
Budget cards may use QLC (Quad-Level Cell) NAND (lower cost, lower endurance).

2. Write-Protect Switch

Standard SD cards include a physical slider that locks the card to read-only mode (prevents accidental deletion or formatting). MicroSD cards lack this switch but may support software-based write protection.

3. Error Correction (ECC)

Built-in Error-Correcting Code (ECC) detects and fixes data corruption, critical for reliable storage (especially in industrial/security applications).

4. Password Protection

SD cards support SDMI (Secure Digital Music Initiative) and AES encryption for password-protected data (via software like SD Card Formatter or device-specific tools).

Common Use Cases

1. Photography & Videography

DSLR/mirrorless cameras: SDXC UHS-II V90 cards for 8K video, raw photo burst shooting.
Action cameras (GoPro): microSDXC U3 V30 cards for 4K/60fps video.
Camcorders: SDHC/SDXC UHS-I cards for 1080p/4K recording.

2. Mobile Devices

Smartphones/tablets: microSDXC A2 cards for expanding storage (apps, photos, videos).
Gaming consoles (Nintendo Switch): microSDXC U3 cards for game storage (supports speeds up to 104 MB/s).

3. Industrial & IoT

Security cameras/DVRs: High-endurance SD cards (e.g., SanDisk Industrial, Samsung PRO Endurance) for 24/7 continuous recording (up to 100,000 hours of use).
IoT devices (smart home sensors, drones): microSDHC cards for local data logging.

4. Computing

Laptops/desktops: SD cards for portable storage or booting lightweight OS (e.g., Linux).
External card readers: For transferring data between SD cards and computers (UHS-II readers for maximum speed).

Choosing the Right SD Card: Key Considerations

Device Compatibility: Check if your device supports SDHC (up to 32GB), SDXC (up to 2TB), or SDUC (up to 128TB).
Speed Requirements:
- 1080p video: Class 10/U1/V10.
- 4K/60fps video: U3/V30 (minimum) or V60/V90 (for high-bitrate).
- Raw photography/burst mode: UHS-II V90 (fast write speeds).
- App storage: A1/A2 rating (for smartphones/consoles).
Endurance: For continuous recording (security cameras), choose high-endurance cards (MLC/SLC NAND, rated for 1000+ P/E cycles).
Brand Reliability: Opt for reputable brands (SanDisk, Samsung, Kingston, Lexar) to avoid counterfeit cards (common in low-cost unbranded options).