An Introduction to Form Factors for PCI Express®
By Al Yanes, PCI-SIG Chairman and President

PCI Express (PCIe®) has been widely adopted in a number of applications that range from small, power-constrained IoT sensors and mobile devices to servers and networking and communications equipment. Given the unique requirements of each of these applications, PCI-SIG has delivered multiple form factors and connectors that maintain backward compatibility with previous technology generations. Keep reading to learn more about our portfolio of CEM, PCI Express M.2 and U.2 form factor and connector solutions, and the varying benefits that each delivers to the industry.

CEM
CEM – pronounced like “chem” in “chemistry” – stands for Card Electromechanical specification. It is by far the longest standing PCIe form factor, with the first iteration delivered in 2000. The CEM baseline spec is used as a reference for multiple mechanical form factors throughout the industry, and CEM add-in cards are used in everything from graphics applications to data centers. It supports forward and backward compatibility to deliver a consistent end user experience.

CEM is available in different connector sizes (CEM x1, x2, x4, x8 and x16) all of them supporting the four different speed (2.5GT/Sec, 5.0 GT/Sec, 8.0 GT/Sec, and 16.0 GT/Sec) versions of the currently released PCI Express specifications to deliver the right I/O for multiple applications. It also supports scalable power options up to 75W (edge), 2x4 enables 300W (CEM 4.0) / 600W (CEM 5.0). CEM can also be used for hybrid computing and to attach external cable solutions.

PCI Express M.2™ Connector
The PCI Express M.2 connector is a next-generation form factor designed for ultra-light, thin and power efficient mobile platforms, such as tablets, portable gaming devices, smartphones and devices requiring SSDs. Its extensible design provides scalability for multiple technologies and host interfaces, including Wi-Fi®, Bluetooth®, NFC, 2G-5G, SSD and WWAN.

PCI-SIG has designed the PCI Express M.2 connector to deliver the smallest footprint of PCIe connectors, and the form factor is available in varying lengths including 42mm, 80mm and 110mm. PCI Express M.2 connectors support both single- and double-sided module cards and are available in connectorized or soldered-down forms. The connectorized forms allow single-sided modules for low profile solutions, or dual-sided modules for increased integration within the platform. All soldered-down module cards are single-sided and are intended for use in low profile applications.

Like all PCI-SIG solutions, the PCI Express M.2 connector was designed to support forward and backward compatibility. Given its focus on mobile applications, PCI Express M.2 connectors support low power options (3.3V and 1.8V using four power pins) as well as 14 vendor-defined pins to enable customized solutions.

PCI Express U.2™ Connector
The PCI Express U.2 connector is a 2.5” form factor that is primarily used in SSDs and has seen prolific adoption by the industry. As a cased solution, PCI Express U.2 connectors deliver ease of deployment and hot plug capabilities which is very important for the storage market. Its pinout allows for up to four PCIe lanes, with PCIe 4.0 solutions delivering up to 16 GT/s. Physically larger than PCI Express M.2 drives, PCI Express U.2 drives also deliver higher storage capacities. As a high-capacity, high-density NVMe form factor, it can support up to 24 modules in a 2U enclosure or up to 32 14x18mm packages per module. 

By supporting multiple form factors for multiple applications, PCI-SIG has enabled PCIe technology to be cost-effective and easy to implement. PCI-SIG members have access to the form factor specifications and can help develop the next-generation of PCI Express architecture. Learn more about PCI-SIG membership here and visit our Events page for information about our upcoming members-only Developers Conferences in Israel and the Asia-Pacific region.