Scaling CPO Testing Beyond a Single Die
Introduction
Co-packaged optics (CPO) is moving from R&D labs into volume manufacturing, driven by the insatiable bandwidth demands of AI accelerators and hyperscale data centers. By 2026, CPO is expected to be a mainstream interconnect technology for 51.2T and 102.4T switches. However, testing CPO modules remains one of the most complex challenges in semiconductor manufacturing. Unlike traditional discrete optical transceivers, CPO integrates photonic integrated circuits (PICs) and electronic integrated circuits (EICs) alongside a switch ASIC in a single package. This tight integration creates a cascade of testability issues that cannot be solved by conventional single-die test methodologies.
The CPO Test Challenge
Known-Good-Die at the Photonic Level
Traditional known-good-die (KGD) test focuses on electrical integrity. For CPO, KGD must also encompass optical functionality. Testing PICs at wafer level requires coupling light into and out of the die—a process that is notoriously difficult to scale. Each optical interface adds insertion loss, alignment sensitivity, and potential contamination. In 2026, wafer-level optical test is still a bottleneck: probe cards that combine electrical and optical probes are expensive, fragile, and have limited touchdown counts. Ensuring that only known-good optical engines are assembled into a multi-die package is critical—a single defective PIC can scrap an entire CPO module worth thousands of dollars.
Multi-Die Integration and Test Access
CPO packages typically use 2.5D or 3D integration with a silicon interposer or bridge. The EIC, PIC, and switch ASIC are flip-chipped or bonded together. Once assembled, test access to individual die is severely restricted. Most test pads are consumed by the assembly interface, and optical ports are often edge-coupled or grating-coupled, requiring precise alignment. Testing the assembled module demands a new class of test equipment that can simultaneously probe high-speed electrical signals (e.g., 224G SerDes) and multi-wavelength optical signals.
Thermal and Mechanical Stress
CPO modules dissipate significant power—often exceeding 500W for a combined switch and optics package. Testing under thermal conditions that mimic real-world operation is essential to catch latent defects. However, thermal cycling during test can induce mechanical stress in optical alignments, leading to false failures or, worse, damaging the module. Test engineers must design thermal chambers and probe systems that maintain optical alignment across temperature extremes.
Strategies for Scaling CPO Test
1. Design-for-Test (DFT) for Photonics
DFT is no longer optional for CPO. In 2026, leading PIC designs incorporate on-die photodetectors, tunable lasers, and loopback paths that enable self-test without external optical equipment. Built-in self-test (BIST) for optical links allows the module to verify its own functionality at speed. This reduces reliance on expensive external optical test setups and enables test at the final assembly stage.
2. Wafer-Level Optical Probing Advances
New probe technologies are emerging to address wafer-level optical test. Vertical grating couplers with relaxed alignment tolerances (e.g., >2µm) are being adopted to enable multi-site optical probing. MEMS-based probe cards that can switch between electrical and optical domains are under development. Additionally, 2026 has seen the rise of “test-ready” photonic wafers that include dedicated optical test structures (e.g., test gratings, on-wafer lasers) that do not interfere with functional die.
3. Known-Good-Engine Test
Instead of testing every die individually, the industry is moving toward testing at the “engine” level—a sub-assembly containing one EIC, one PIC, and passive components. These optical engines are tested before final assembly with the switch ASIC. This approach balances cost and coverage: it catches most defects early while avoiding the prohibitive cost of full module test for every unit.
4. ATE and Optical Test Integration
Automated test equipment (ATE) vendors are integrating optical test capabilities into their platforms. In 2026, commercial ATE systems offer both electrical and optical channels, with software that coordinates the two domains. This enables parallel testing of electrical and optical parameters, reducing test time. However, standardization of optical interfaces (e.g., via the OIF’s CPO test framework) remains an ongoing effort.
Outlook and Conclusion
Scaling CPO testing beyond a single die is a multi-faceted problem that demands innovation in DFT, probe technology, and test methodology. As CPO moves to high-volume manufacturing in 2026 and beyond, the industry must collaborate to standardize test interfaces and develop cost-effective solutions. The companies that master CPO test will have a significant advantage in the race to deliver next-generation AI infrastructure. Testing is no longer a back-end afterthought—it is a critical enabler for CPO commercialization.