CPO Testing at Scale Hinges on Fiber Alignment, Durable Connectors and Parallel Tests

Scaling co-packaged optics (CPO) testing depends on solving a practical bottleneck: connecting optical fibers to devices reliably, and doing so across multiple sites. Production testing is largely single-site today, according to engineers from Teradyne, Advantest America, ISE Labs and Keysight Technologies. Multi-site probe cards could help increase throughput, but connector durability, radio-frequency access and long-running measurements also shape how quickly that approach can mature.

The engineers discussed the shift from single-site testing in a roundtable with Semiconductor Engineering. Their comments point to a broader challenge than simply adding more test positions: optical and electrical interfaces vary across devices and package designs, so a test setup that works for one architecture may not scale easily to another.

More test sites depend on the device and insertion

Whether a device can be tested alongside others depends in part on its power and interface demands. Dave Armstrong of ISE Labs said a large GPU with connectors on multiple sides can be difficult to test beyond one die. The tester must supply and remove substantial power while making contact with those interfaces. Network optical engines, by contrast, are typically single-connector devices, which may make multi-site testing more achievable as production volumes rise.

Armstrong sees later test insertions as promising places to increase the number of devices tested at once. These insertions tend to involve longer, more complex testing, where parallel operation could make a meaningful difference to throughput. Photonic integrated circuit wafer testing, however, is generally single-site today. He pointed to probe-card developments, including work from Technoprobe, as a possible route to testing more sites in parallel.

Matt Griffin, Teradyne’s senior product manager for optoelectronics test, identified fiber alignment as the main bottleneck for optical-engine testing. Companies are working on multi-site solutions, he said, but the technology needs further development before it can support a wider production ramp. The prospect is useful precisely because a single aligned fiber connection takes time; repeating that process across more devices could improve throughput if the connections remain dependable.

Connector lifetime has two sides

Repeated testing need not mean repeated contact between an optical probe or fiber array unit (FAU) and the device itself. Griffin said a well-designed setup should allow repeated testing without that contact. Early systems can still suffer collisions while teams bring up a new test process, and contamination on the FAU remains a more persistent concern. Cleaning is therefore part of maintaining the test interface.

At module level, or on an optical engine with receptacles attached, the device-side connector raises a different lifetime question. These connectors are designed to be attached once and ideally left connected. Each new connector generation can also require changes to both the connector design and the automation used to handle it.

Ira Leventhal of Advantest America compared the wear problem with repeatedly socketing a ball grid array: too many connections can damage the device interface. He said test-side connectors may need hardened metal to survive tens of thousands or even 100,000 insertions, while early versions may wear after only 20 to 50. But a hard test connector pressed against a less hardened device receptacle can shift the wear onto the device. Connector designers will need to balance optical performance with mechanical durability, including the ability of the device-side receptacle to withstand test contact.

Simultaneous contact is an RF problem too

Making optical and electrical connections at the same time brings mechanical and measurement challenges. Kazuo Yamaguchi of Keysight Technologies said the harder task is characterizing electro-optic devices at radio frequencies while maintaining both optical and high-frequency electrical contact. Modulators and photodetectors are validated through S-parameters, and probe cards have limits on how many RF lines they can carry and the bandwidth they can support. Devices are characterized beyond 200 GHz today, making RF probing a significant constraint at wafer and die test stages.

Package warpage adds further difficulty, Leventhal said. Test equipment must align along multiple axes and establish reliable contact with a large number of BGA pins. The interfaces may sit on the top, bottom or both sides of a device; at module level, optical connections can occupy two to four package sides across several engines. Griffin said alignment becomes especially difficult when test systems must accommodate this range of architectures at once.

Combining operations could reduce the time spent moving through separate tests, but it won’t remove the work needed to align each interface. The field still needs test systems that can handle different package layouts and connector arrangements without sacrificing contact quality.

Test time depends on what teams choose to measure

Armstrong said insertion loss can attract attention because it is measured often, but he considers eye quality, receiver sensitivity and bit error rate more important measures of system performance. Griffin added that wavelength sweeps can take significant time, depending on customer requirements. Those measurements compete for test time with the need to check optical and electrical behavior across the assembled device.

Design for test (DFT) could help. Armstrong described a possible approach that runs optical and electrical loopback tests concurrently. Today, teams may perform loopback on one side and a bit error rate test on the other, requiring a costly setup and potentially doubling effective test time. Griffin expects customers targeting volume to use some form of electrical loopback, optical loopback or both, though which configuration becomes common remains open.

Yamaguchi cautioned that a device passing tests before assembly isn’t necessarily a reliable device after integration. Fiber-to-FAU coupling loss can emerge after attachment; resonance can drift relative to laser tunability; and thermal or packaging effects can alter performance. Transmitter and receiver quality is assessed using metrics such as TDECQ and receiver sensitivity, but pre-assembly measurements need to correlate with performance through integration and service life.

Leventhal’s closing concern was diagnostic: when data transmission fails, teams need enough information to identify whether the problem lies in the electrical-to-optical path, the optical-to-electrical path or an interaction between them. The path to higher-volume CPO testing will depend on collecting that information early, shortening test sequences through concurrent operations, and building interfaces that tolerate repeated handling. Multi-site testing could lift throughput, but fiber alignment and connector lifetime will determine how much of that potential production lines can capture.