1. The GPU Module Standards Landscape

As AI training clusters scale to tens of thousands of accelerators, the form factor and interconnect standard for GPU modules becomes a critical architectural decision. Two competing standards dominate the landscape: NVIDIA's SXM (Server PCI Express Module) and the OCP OAM (Open Accelerator Module). While both serve the same fundamental purpose—housing a high-power GPU with associated HBM memory and power delivery—their PCB architectures, connector systems, power delivery topologies, and ecosystem implications differ substantially.

This article provides a detailed, PCB-focused comparison of the two standards, drawing on public specifications, reverse engineering analyses, and industry design practices. We examine every aspect: mechanical dimensions, connector pinouts, layer stackups, material selection, power delivery networks, signal routing strategies, and thermal-mechanical integration.

2. SXM: NVIDIA's Proprietary Module Standard

2.1 SXM Evolution

NVIDIA's SXM form factor has evolved through five generations:

2.2 SXM5 PCB Architecture

The SXM5 module PCB for H100 is a marvel of high-density interconnect design. Key characteristics:

• Layer count: 22-26 layers, with 3-4-3 HDI build-up structure

• Dimensions: Approximately 120mm × 175mm (larger than standard PCIe add-in cards)

• Base material: Megtron 7 / EM-891K ultra-low-loss for signal layers, with high-Tg FR-4 core

• Connector: Custom high-density mezzanine connector with 2,000+ pins, combining power, ground, and high-speed differential pairs in a single mate

• GPU attach: Large FCBGA package (55mm+ die size) with 4,000+ solder balls at 0.8mm pitch, requiring via-in-pad and microvia breakout

• HBM integration: 6 HBM3 stacks co-packaged on a silicon interposer, with the interposer mounted on the FCBGA substrate; the PCB provides power delivery and I/O breakout

2.3 SXM Connector Pinout

The SXM5 mezzanine connector integrates multiple signal types within a single connector body:

• NVLink lanes: Up to 18 NVLink 4 ports (each 4 differential pairs TX + 4 RX), totaling 144 differential pairs at 50GB/s per pair

• PCIe lanes: x16 PCIe Gen5 for host communication

• Power delivery: Approximately 40% of connector pins dedicated to power and ground, carrying 700W+ at 12V

• Management: I2C/SMBus for module identification, temperature monitoring, and power telemetry

• Sideband: Reset, clock, presence detect, and fault indication signals

3. OAM: The OCP Open Accelerator Module

3.1 OAM Origins & Philosophy

The Open Accelerator Module (OAM) was introduced by the Open Compute Project (OCP) in 2019 as an open, multi-vendor alternative to proprietary GPU module standards. The specification (OCP Accelerator Module Design Specification v1.0 and v1.5) defines mechanical, thermal, electrical, and management interfaces that any accelerator vendor can adopt.

3.2 OAM 1.0 Specification Highlights

• Module dimensions: 102mm × 165mm (single-wide) or 102mm × 330mm (dual-wide for higher TDP)

• Connector: Two high-density mezzanine connectors (primary + auxiliary) with standardized pinouts

• Host interface: x16 PCIe Gen5 or x16 CXL 2.0 (configurable)

• Inter-accelerator fabric: Up to 7 inter-module links supporting 56Gbps PAM4 each, enabling full-mesh or ring topologies without a central switch

• Power: 48V input (vs. SXM's 12V), delivered through the baseboard

• Management: Standardized OCP management interface with module FRU data, thermal telemetry, and firmware update capability

3.3 OAM PCB Architecture

The OAM module PCB shares many characteristics with SXM but differs in several important ways:

• Layer count: 20-24 layers, typically with 2-3-2 HDI build-up

• Material: Similar ultra-low-loss materials (Megtron 7 class), but OAM's open specification allows broader material qualification

• Two-connector architecture: Separates host-facing (PCIe/CXL) and fabric-facing (inter-accelerator) signals into different connectors, improving signal integrity isolation

• On-module VRM: OAM modules include on-board 48V-to-core-voltage conversion, unlike SXM which relies on the baseboard VRM

4. Mechanical & Connector Comparison

The SXM's single-connector approach simplifies baseboard routing but creates a very dense connector with challenging signal-to-power pin isolation. The OAM's dual-connector architecture requires more baseboard real estate but provides better isolation between high-speed fabric signals and host PCIe lanes—reducing crosstalk and easing signal integrity engineering.

5. PCB Stackup: OAM vs SXM Layer Counts

5.1 Representative SXM5 Stackup (24 layers)

A typical SXM5 module uses a hybrid stackup combining HDI build-up layers on top and bottom with a conventional multilayer core:

• Top build-up (4 layers): Microvia layers for BGA breakout, high-speed differential pairs

• Core (16 layers): Power planes (12V distribution, GPU core, HBM rails), ground planes, additional signal layers for NVLink, PCIe, and management

• Bottom build-up (4 layers): Decoupling capacitor placement, VRM components, test points

5.2 Representative OAM Stackup (22 layers)

OAM modules push more of the power conversion onto the module itself (48V→core voltage), necessitating different layer allocation:

• Top build-up (3 layers): GPU BGA breakout, high-speed fabric signals

• Core (16 layers): 48V distribution, intermediate voltage rails, ground isolation, PCIe/CXL signals, inter-module links

• Bottom build-up (3 layers): On-module 48V VRM components, inductor arrays, decoupling

6. Power Delivery Architectures Compared

6.1 SXM: Baseboard-Regulated 12V

SXM modules receive regulated 12V from the baseboard. This architecture centralizes the high-current VRM on the baseboard, which:

• Simplifies the GPU module PCB by removing the largest, heaviest power components

• Enables better cooling of the VRM (baseboard-mounted heatsinks or direct liquid cooling)

• Requires very low-resistance power distribution through the mezzanine connector (each power pin carrying 2-3A with<5mv drop="">

• Creates I²R losses in the connector and module power planes—at 700W and 12V (58A), even 0.5mΩ total resistance drops 29mV

6.2 OAM: Module-Regulated 48V

OAM modules receive 48V (or optionally 54V) from the baseboard and perform final regulation on-module. This architecture:

• Moves the high-current path to higher voltage, reducing connector current (700W at 48V = 14.6A vs. 58A at 12V)

• Requires fewer connector pins for power (approximately 25% fewer), freeing pins for signals

• Adds complexity and thermal load to the module PCB: the 48V→core VRM must dissipate 15-25W of heat on a space-constrained module

• Enables simpler baseboard designs since only 48V bulk distribution (not multi-phase regulated power) is needed

7. Signal Integrity & High-Speed Routing

7.1 Common Challenges

Both OAM and SXM face identical fundamental signal integrity challenges:

• Channel loss: GPU package trace + PCB trace + connector + baseboard trace + CPU/switch package trace can exceed 30dB at 14GHz (PCIe Gen5 Nyquist)

• Crosstalk: Dense pin fields create opportunities for both near-end (NEXT) and far-end (FEXT) crosstalk between adjacent differential pairs

• Impedance discontinuities: The connector represents the largest impedance discontinuity in the channel; its design is critical

7.2 Key Differences

SXM advantages: NVIDIA controls the entire channel (GPU package, module PCB, connector, recommended baseboard layout), enabling holistic optimization. The SXM5 connector is specifically tuned to match the impedance profile of the NVIDIA SerDes.

OAM advantages: The dual-connector architecture physically separates host and fabric lanes, reducing crosstalk pathways. The OAM connector specification is open, enabling multiple connector vendors to compete on electrical performance.

8. Thermal Design & Cooling Integration

8.1 Thermal Density

Both standards push thermal density to unprecedented levels. At 700W TDP on a ~20,000 mm² module, the heat flux exceeds 3.5 W/cm² averaged across the module—and hotspot heat flux at the GPU die can reach 150 W/cm².

8.2 PCB Thermal Management

The module PCB plays a crucial role in thermal management:

• Thermal vias: Both standards use dense arrays of 0.3mm plated through-holes under the GPU package to conduct heat to the backside of the PCB

• Backside cooling: SXM modules typically rely on a thermal interface material (TIM) between the PCB backside and the baseboard cold plate; OAM modules may include an integrated backside heatsink

• Copper plane utilization: Heavy copper planes (2-4 oz) in the module serve both electrical and thermal functions, spreading heat laterally

8.3 Cooling Solution Compatibility

SXM modules are designed specifically for NVIDIA's recommended thermal solutions (HGX baseboards with integrated vapor chamber cold plates). OAM modules, following the OCP philosophy, are designed to be compatible with a wider range of cooling approaches—air cooling for lower-TDP modules, direct-to-chip liquid cooling, and immersion cooling.

9. Ecosystem, Supply Chain & Cost

9.1 Vendor Lock-in vs. Open Ecosystem

SXM's single-vendor nature means that module PCB specifications, connector pinouts, and even baseboard reference designs are controlled by NVIDIA. This enables tight optimization but limits supply chain diversity. OAM's open specification enables multiple GPU/ASIC vendors (Intel Habana, AMD Instinct, custom ASICs) to produce compatible modules, and multiple connector vendors (Amphenol, Molex, TE) to supply qualified connectors.

9.2 PCB Fabrication Considerations

Both modules push fabrication to the leading edge:

• Minimum trace/space: 2.5/2.5 mils (63/63μm) on outer layers, 3/3 mils on inner layers

• Microvia diameter: 100μm laser-drilled, with 250μm capture pads

• Via-in-pad: Extensive use with copper filling and planarization for BGA breakout

• Panel utilization: Module dimensions must be optimized for standard 18"×24" or 21"×24" panels

9.3 Cost Comparison

SXM module PCBs tend to be more expensive due to NVIDIA's tight specification and single-source connector. OAM PCBs benefit from competitive connector pricing and broader fab qualification. However, OAM modules carry the added cost of on-module 48V VRM components, which can offset the PCB cost advantage.

10. The Road Ahead: OAM 2.0 & SXM6

10.1 OAM 2.0 Proposed Features

The OCP community is developing OAM 2.0, targeting:

• 224Gbps PAM4: Doubling the inter-module link bandwidth for next-generation AI fabrics

• Integrated optics: Provision for co-packaged optical engines on the module

• PCIe Gen6/CXL 3.0: Updated host interface with 64 GT/s per lane

• Wider modules: Up to 150mm module width for higher TDP and more HBM stacks

10.2 SXM6 and Beyond

NVIDIA's SXM6 (B200) pushes power to 1000W+, requiring significant PCB innovations:

• Embedded voltage regulation: Integrating some VRM stages within the PCB or package substrate

• Liquid-cooled connectors: Direct liquid cooling of the mezzanine connector to handle the increased current density

• Advanced substrate materials: Transition toward glass-core or organic interposer substrates for improved power integrity

11. Conclusion

The OAM vs. SXM debate is fundamentally about architecture philosophy: proprietary optimization vs. open ecosystem flexibility. From a PCB designer's perspective, both standards represent the absolute frontier of high-speed, high-power PCB design. SXM's single-connector, baseboard-regulated approach yields a simpler module PCB but demands a more complex baseboard. OAM's dual-connector, on-module regulation approach shifts complexity to the module but simplifies the baseboard and enables multi-vendor sourcing.

For organizations deploying AI infrastructure at scale, the choice should be guided by the broader system architecture: those committed to the NVIDIA ecosystem will find SXM's tight integration compelling; those building heterogeneous accelerator farms or pursuing supply chain diversification will gravitate toward OAM. In either case, the PCB engineering demands are extreme, and success requires world-class signal integrity, power integrity, and DFM expertise.