Solid state memory cards, as modular and non-volatile storage solutions, are widely used in various electronic systems, bringing expanded functionality, rich features, and higher user convenience to these systems. In order to connect these storage cards to the host device, compatible connectors are required to ensure reliable mechanical and electrical connections. These connectors must maintain consistent performance under the increasingly compact constraints of devices, where physical footprint and signal integrity are key design factors. Before inspecting the storage card connector, it is necessary to first establish a basic understanding of the storage card architecture and its role in embedded and consumer electronic devices.
Overview of Storage Cards
A memory card is a small solid-state storage device used to store digital data such as photos, videos, music, and documents. They are based on flash memory chips, a type of storage chip developed in the 1980s that uses floating gate transistors to retain data even after a power outage. Due to the absence of moving parts, flash memory has fast access, high durability, and the ability to be electrically erased and rewritten.
Over time, storage cards have undergone significant evolution, with their development trend mainly focused on reducing size and increasing storage capacity. The most commonly used formats nowadays are:
SIM card (user identity module): SIM card stores user credentials and mobile network data, and supports voice call, SMS, mobile Internet and other functions. Each SIM card contains an IMSI (International Mobile Identity) used for authentication with cellular networks. SIM cards have been reduced from the outdated full-size format (credit card size) to mini SIM (25 x 15 mm), micro SIM (15 x 12 mm), and nano SIM (12.3 x 8.8 mm). In mobile devices, nano SIM card has a high space utilization rate and occupies a mainstream position. It is worth noting that mini SIM cards are now commonly referred to as SIM cards or standard SIM cards.
SD card (Secure Digital Card): SD cards are widely used in consumer electronics for data storage and transmission. They expand the internal storage space of smartphones, digital cameras, game consoles, and tablets. Current storage card formats such as SDHC, SDMC, and SDUC can provide higher storage capacity and faster transfer speeds. Its physical appearance (standard SD, miniSD, and microSD) and standardized pin layout make it the preferred form of removable storage media.
Smart card: The size of a smart card is similar to a credit card, but it has an integrated circuit chip that enables additional functions such as identity verification, encryption, secure identification, and payment processing. They can interact through physical contact or use RFID or NFC standards for contactless operation. These cards are commonly used in financial, medical, and access control systems and require compatible card readers in the main device.
Evolution of Memory Card Capacity over Time Image
Figure 1: Evolution of storage card capacity over time. (Image source: Same Sky)
Insertion type of storage card connector
As mentioned earlier, both SIM cards and SD memory cards rely on a mechanical interconnect system to establish a stable electrical interface with the host device. This interface must ensure the stability of signal connection and support smooth insertion and removal of cards, even under frequent use or mechanical stress. To meet these requirements, modern connector designs adopt various optimization mechanisms to adapt to different application scenarios and mechanical limitations:
Push in/Pull out: Simple design - insert when pushed in, pull out when pulled out. No locking buckle or spring. Suitable for low vibration or semi permanent installation.
Press to insert/eject (auto eject): Spring mechanism - insert once, eject again. Provide tactile feedback, suitable for high-frequency usage scenarios in consumer electronic devices.
Articulated: The card is secured in place using a locking cover. Specially designed for high vibration environments, such as automobiles or industrial systems, where safety fixation is crucial.
Each design has its own advantages and disadvantages in terms of durability, space utilization, and usability, and the final choice depends on the mechanical and environmental requirements of the application.
Understand the memory card connector contacts
Both SD cards and SIM cards establish communication connections with the host device through conductive pads (i.e. pins or contacts) on the card surface. These pins are aligned with the corresponding contacts in the connector socket to complete the circuit connection. Each pin is assigned a specific function in a standard layout (known as pin arrangement).
The physical connection is achieved through a dedicated socket installed on the main device PCB. The connector is specially designed to accommodate specific card formats, such as microSD or nano SIM, to ensure reliable alignment and signal transmission.
SD card contact configuration
The SD card supports higher data transmission rates through multiple interface standards. A standard full-size SD card typically comes with 9 contact pins, while the microSD version typically has 8. Higher speed variants, such as UHS-II or SD Express, may include additional pins or even a second row of pins to handle higher signal bandwidth. Modern SD cards support the following communication protocols:
SPI (Serial Peripheral Interface): A simple, low-speed interface commonly used in embedded systems or hobbyist systems.
SD bus interface: From default speed and high speed to UHS-I, UHS-II, and UHS-III, each level supports higher transmission rates.
PCIe/NVMe: This interface is widely used in SD Express memory cards, supports ultra high speed data access, and is suitable for professional level video processing, large file transfer, and SSD level performance requirements.
The connector design must be compatible with the pin configuration and supported protocols of the card to ensure functional and data integrity.
SIM card contact configuration
SIM cards are typically equipped with 6 to 8 contact pins, depending on the card format and application requirements. The following are the contact functions of an 8-pin SIM card:
VCC: Power Input
GND: Grounding reference
CLK: Clock signal used for data synchronization.
I/O: Bidirectional serial data cable
VPP: Programming Voltage
RST: Signal reset
RFU: Reserved for future use (usually two pins, idle in standard applications)
The 6-pin SIM card omits two RFU contacts and only retains the necessary pins: VCC, GND, I/O, CLK, RST, and VPP.
Some SIM card connectors may include one or more additional pins to detect if the SIM card is inserted. These pins are part of the connector, not the SIM card itself. When inserting a SIM card, these pins will be mechanically short circuited to ground. They enable the host system to detect card insertion/removal events and respond accordingly without interfering with signal contact.