SD Express marks a significant change for removable storage. It moves the common SD card from its traditional bus to the much faster PCI Express (PCIe) interface, using the NVMe protocol for communication. This gives SD cards performance similar to internal solid-state drives (SSDs).

This guide compares the foundational specifications: SD 7.0, SD 7.1, and SD 8.0. Understanding these differences is important for system designers and consumers looking to use this technology. We will look at their features, the performance they offer, and the design challenges they present.

Key Design and Performance Factors

Host Implementation

A host device (like a laptop or camera) does not use one single "SD Express controller." Instead, it combines its existing SD host controller with a standard PCIe root port. A physical switch, or Mux, directs signals. The "SD-First" initialization flow is essential. The host uses the SD protocol to detect the card. If the card reports PCIe support, the host switches power and signals to its PCIe interface. This maintains full backward compatibility with older SD cards, which is a key requirement.

Performance, Power, and Heat

The speed of SD Express introduces power and heat levels not seen in traditional SD cards. Cards can draw up to 1.80W, which creates a significant thermal challenge in a small, passively cooled form factor. Performance is directly tied to heat management.

If a card's temperature exceeds 75°C, the host system must reduce its power state to prevent overheating. This is known as thermal throttling. The SD 7.0/8.0 specifications define these power states:

• P3 (Performance): Up to 1.80W (for card temperatures < 75°C)
• P2 (Power-Saving): Up to 1.44W (for card temperatures < 85°C)
• P1 (Cooling): Up to 0.72W (for card temperatures < 90°C)

This means the theoretical peak speeds (like 3938 MB/s) are not guaranteed. The actual sustained speed depends entirely on the host device's ability to dissipate heat. A well-designed laptop slot with airflow will achieve better sustained performance than a sealed, bus-powered external reader. This variable performance was a major factor leading to the changes in the SD 9.1 specification.

The Path to SD 9.1 and Beyond (Context: Oct 2025)

The variable performance of SD 7.0 and 8.0, while fast, created an unpredictable user experience. It was difficult to know if a card would be fast enough for a specific task, like 8K video recording, in a specific device. This led to the development of SD 9.1.

The newer SD 9.1 standard addresses this directly by introducing defined speed classes (e.g., E150, E300, E600). These classes guarantee a *minimum sustained write performance* (e.g., 150 MB/s for E150) by using a more advanced power and thermal management API. This allows a card and host to communicate and agree on a sustainable speed based on the host's thermal budget. Other new features, like boot support, are also positioning SD Express to compete with embedded memory in computers and game consoles.

Thermal Management Solutions

Given the high power draw and resulting heat, managing thermals is a primary design challenge for host devices. Simply having a slot is not enough; the slot must be designed to dissipate heat effectively to prevent throttling and maintain performance.

Common solutions being implemented in host devices include:

• Heat Spreaders: Many host devices (like laptops and cameras) now link the metal SD card slot casing to a larger internal heatsink or chassis ground plane. This uses the device's own body to pull heat away from the card.
• Passive Materials: The use of ultra-thin, highly conductive materials, such as flexible graphite sheets, allows heat to be spread from the card to other cooling components within a tight space.
• Active Cooling: In high-performance systems like gaming devices or workstations, the SD Express slot may be placed in the direct path of an internal fan to provide active airflow, which is the most effective solution for sustained P3 performance.

The diagram below illustrates the basic concept of using a heat spreader to move thermal energy away from the card.

Host Controller Solutions and Market Adoption

Off-the-Shelf Bridge Controllers

Implementing SD Express support does not require a completely new, single-purpose chip. For many devices, especially PCs and external readers, manufacturers use "bridge controllers." These chips bridge the USB or motherboard PCIe bus to the SD Express card's interface.

As of late 2025, several companies provide off-the-shelf bridge solutions for hosts and readers, including:

• BayHub Technology
• Realtek
• Genesys Logic
• JMicron

These bridge controllers are vital. They translate signals from a host's existing USB or PCIe bus to the SD Express interface. This allows manufacturers to add SD Express support to laptops, motherboards, and external readers without needing a new, custom-designed main processor (SoC) with native support. This modular approach has significantly helped speed up the adoption of the standard.

Market Adoption

Adoption of SD Express is steadily growing. The first wave of support appeared in high-end laptops and motherboards aimed at creators who need to transfer large files quickly. We are now seeing adoption in next-generation digital cameras and video recorders designed for 8K and high-frame-rate capture.

External SD Express readers, typically using USB 3.2 Gen 2 or Thunderbolt, are also common. These devices offer a convenient way to get maximum speed from these cards, though they are often subject to thermal throttling during very long sustained transfers due to their compact, bus-powered designs.

Technical Implementation Challenges

Beyond heat, host designers face other technical hurdles when implementing SD Express, particularly with the SD 8.0 (PCIe 4.0) specification.

Voltage Regulation and Sequencing

An SD Express host slot must be a "jack-of-all-trades." It needs to provide 3.3V power for legacy SD, UHS-I, and UHS-II cards. However, the PCIe interface used by SD Express runs at 1.8V. The host controller must first detect the card type using the 3.3V SD interface and then, if an SD Express card is found, perform a careful power-down and power-up sequence to switch the card's main power rail to 1.8V before initiating the PCIe link. This requires precise power sequencing logic.

Signal Integrity for PCIe Gen 4

Running a 16 GT/s (gigatransfer per second) PCIe Gen 4 signal is extremely demanding. This high-frequency signal is vulnerable to degradation from:

• Connector Crosstalk: The pins on the small microSD and full-size connectors are very close together, increasing the risk of interference.
• Trace Length and Routing: The traces on the host's motherboard must be routed very carefully, often with specific length-matching, to ensure the differential signals arrive at the same time.
• Signal Loss (Attenuation): The signal weakens over distance. In devices like laptops, the signal may need to travel from the main processor, through a bridge chip, and then to the slot, all of which can degrade the signal. Designers must use high-quality materials and may need to add "re-driver" chips to boost the signal strength.

These challenges are why many first-generation SD 8.0 hosts may initially only support the more stable PCIe Gen 3 speeds, even if the card itself is Gen 4 capable.

Frequently Asked Questions (FAQs)

Will my old SD cards work in an SD Express slot?

Yes. SD Express host slots are fully backward compatible with all previous SD cards (like UHS-I and UHS-II). The host will detect the older card during initialization and operate it using the standard SD protocol at its maximum supported speed.

Can I use an SD Express card in my old (non-Express) SD slot?

Yes, but you will not get any of the "Express" speed. The SD Express card will detect the non-Express host and fall back to its legacy SD interface, which is typically UHS-I. This limits its speed to a maximum of 104 MB/s.

Why am I not getting the 3938 MB/s speed advertised on my SD 8.0 card?

That speed is a theoretical maximum for a PCIe Gen 4 x2 interface. To reach it, you need both a card and a host device that support PCIe Gen 4 x2. Many hosts, especially early ones, only support Gen 3 or a single Gen 4 lane, limiting speeds to 985 MB/s or 1970 MB/s. Most importantly, performance is limited by heat. If the card gets too hot, the host will "throttle" it to a lower power state (P2 or P1) to cool down, which reduces speed.

What is the main difference between SD 8.0 and the newer SD 9.1?

SD 8.0 defines the *maximum theoretical performance* based on the PCIe interface (e.g., up to 3938 MB/s). SD 9.1 addresses the *variability* of that performance by introducing "SD Express Speed Classes" (like E150, E300, E600). These classes guarantee a *minimum sustained write speed* (e.g., 150 MB/s for E150), which is critical for applications like uninterrupted 8K video recording.

Conclusion

The SD 7.0, 7.1, and 8.0 specifications represent a fundamental shift for SD cards, moving them from a legacy bus to the modern PCIe and NVMe protocols. This change unlocks performance that rivals internal SSDs, opening up new possibilities for high-resolution video, gaming, and even system booting.

However, this performance introduces significant engineering challenges, particularly in managing power consumption and heat dissipation within a tiny, familiar form factor. The variable speeds that result from thermal throttling highlighted the need for more predictable performance, which is precisely what the subsequent SD 9.1 standard was designed to address.

These foundational specifications built the bridge from traditional, slow removable storage to a new class of high-speed, high-performance media. As hosts and cards continue to mature, SD Express is positioned to become a new standard for creators, professionals, and enthusiasts.