All Designers Need to Know About Memory Card Connectors

Solid-state memory cards serve as modular, non-volatile storage solutions in a broad range of electronic systems—facilitating extended functionality, feature sets, and user convenience. To interface these memory cards with host devices, compatible connectors are required to provide reliable mechanical and electrical coupling. These connectors must maintain consistent performance under increasingly compact device constraints, where physical footprint and signal integrity are critical design factors. Prior to examining memory card connectors, it's important to first establish a foundational understanding of memory card architectures and their role within embedded and consumer electronics.

Memory cards overview

Memory cards are compact, solid-state storage devices that hold digital data such as photos, videos, music, and documents. They rely on flash memory—developed in the 1980s—which uses floating gate transistors to retain data without power. With no moving parts, flash memory offers fast access, high durability, and the ability to be electrically erased and rewritten.

Over time, memory cards have evolved significantly, with trends focusing on reduced size and expanded capacity. Today’s most common formats include:

• SIM Cards (Subscriber Identity Module): SIM cards store user credentials and mobile network data, enabling functions like voice calls, SMS, and mobile internet. Each SIM contains an IMSI (International Mobile Subscriber Identity) used to authenticate with cellular networks. SIMs have shrunk from the obsolete full-size format (credit card size) to mini-SIM (25 x 15 mm), micro-SIM (15 x 12 mm), and nano-SIM (12.3 × 8.8 mm). In mobile devices, nano-SIM dominates due to its space efficiency. It is also important to note that mini-SIM is now commonly referred to as SIM or standard SIM.
• SD Cards (Secure Digital): SD cards are widely used for both data storage and transfer in consumer electronics. They expand the onboard memory of smartphones, digital cameras, gaming consoles, and tablet PCs. Current variants—like SDHC, SDXC, and SDUC—offer higher capacity and faster speeds. Their mechanical form factors (standardSD, miniSD, and microSD) and standardized pinouts make them the go-to format for removable memory.
• Smart Cards: Smart cards resemble credit cards in size but embed an integrated circuit chip for added functionality such as authentication, encryption, secure identification, and payment processing. They may interface through physical contacts or operate contactlessly using RFID or NFC standards. These cards are often used in financial, healthcare, and access-control systems, requiring compatible readers in host devices.

Figure 1: The progression of memory card sizes over the years. (Image source: Same Sky)

Insertion types for memory card connectors

As previously outlined, both SIM and SD memory cards depend on a mechanical interconnect system to establish a stable electrical interface with the host device. This interface must facilitate consistent signal contact while enabling smooth card insertion and extraction, even under frequent use or mechanical stress. To address these requirements, modern connector designs implement various engagement mechanisms optimized for different use cases and mechanical constraints:

• Push-In / Pull-Out: Simple design—push to insert, pull to remove. No latch or spring. Best for low-vibration or semi-permanent installations.
• Push-Push (Auto-Eject): Spring-loaded mechanism—push once to insert, push again to eject. Offers tactile feedback and is ideal for frequent use in consumer devices.
• Hinged: Uses a locking cover to secure the card in place. Designed for high-vibration environments like automotive or industrial systems where secure retention is critical.

Each style offers trade-offs in durability, space, and usability with the choice coming down to the application’s mechanical and environmental needs.

Exploring memory card connector contacts

Both SD and SIM cards establish communication with a host device through a series of conductive pads—known as pins or contacts—on the card surface. These pins align with mating contacts in the connector socket to complete electrical circuits, with each pin assigned a specific function in a standardized layout called a pinout.

The physical connection is made via a dedicated socket mounted on the host device’s PCB. Connectors are tailored to match the specific card format, such as microSD or nano-SIM, to ensure reliable alignment and signal transmission.

SD card contact configurations

SD cards have advanced to support higher data rates through several interface standards. A typical full-size SD card features 9 contact pins, while microSD versions usually have 8. Higher-speed variants, such as UHS-II or SD Express, may include extra pins or even a second row to handle increased signal bandwidth. Modern SD cards support the following communication protocols:

• SPI (Serial Peripheral Interface): A simple, low-speed interface commonly used in embedded or hobbyist systems.
• SD Bus Interfaces: Ranging from Default Speed and High Speed to UHS-I, UHS-II, and UHS-III, with each step enabling progressively higher transfer rates.
• PCIe/NVMe: Utilized in SD Express cards, this interface supports ultra-high-speed data access—ideal for professional-grade video, large file transfers, and SSD-class performance.

Connector design must align with the card’s pin configuration and supported protocol to ensure full functionality and data integrity.

SIM card contact configurations

SIM cards typically feature 6 to 8 contact pins, with the exact count depending on the card’s format and application requirements. Here are the contact functions for an 8-pin SIM card:

• VCC: Power supply input
• GND: Ground reference
• CLK: Clock signal for data synchronization
• I/O: Bidirectional serial data line
• VPP: Programming voltage
• RST: signal reset
• RFU: Reserved for future use (usually two pins, not active in standard applications)

6-pin SIM cards simply omit the two RFU contacts and retain only the essential lines: VCC, GND, I/O, CLK, RST, and VPP.

Some SIM connectors include one or more additional pins for card presence detection. These pins are a part of the connector—not the SIM card—and are mechanically shorted to ground when a card is inserted. They enable the host system to sense card insertion/removal events and respond accordingly, without interfering with signal contacts.

Figure 2: A comparison of various SD and SIM card pinouts. (Image source: Same Sky)

Smart card contact configurations

As already noted, smart cards are credit card-sized devices with an embedded processor and memory (RAM, EEPROM, ROM). They connect to host systems either through contact pads or contactless RF communication. Smart card sockets provide a secure, removable interface for reliable data exchange and frequent use. The contact configuration for smart cards matches that of an 8-pin SIM card discussed above.

Memory card applications continue to grow

The arrival of SIM cards revolutionized mobile device connectivity by enabling secure user identification and network access. Similarly, the evolution of SD cards significantly boosted device capabilities by offering secure, removable storage for data transfer and expansion. Originally developed for mobile phones and portable electronics, memory card applications have since expanded across a wide range of advanced technologies, including home security, IoT devices, wearables, drones, gaming consoles, and beyond.

Conclusion

Modern memory card technology supports high-performance data storage and connectivity across a wide range of devices. To accommodate various SD, SIM, and smart card formats, a broad selection of interconnect solutions is available, including connectors, ejectors, extenders, and adapters. These components come in top- and bottom-mount options and ultra-low-profile designs, making them ideal for compact and space-limited applications. Same Sky’s interconnect solutions and memory card connectors are offered in a variety of configurations to meet the needs of virtually any design requirement.