CPO Has Changed the Reliability Question

The appeal of Co-Packaged Optics is easy to understand. As AI systems continue to move larger volumes of data between processors, switches and memory, the electrical interconnects around high-performance ASICs become increasingly difficult to scale. CPO brings optical engines closer to the ASIC, shortening the electrical path and creating a more tightly integrated optical-electrical package.

From a system perspective, that is a major advantage.

From a packaging perspective, however, things become considerably less forgiving.

A CPO assembly may bring together silicon photonics, optical fibers, glass or optical components, substrates, metals, solder joints, adhesives, underfill and thermal interface materials in a relatively small physical area. These materials do not all respond to temperature in the same way. When the package heats up, each material wants to expand according to its own thermal characteristics. When it cools, the process happens in reverse.

The package does not allow every component to move independently.

That is where thermo-mechanical stress comes into the picture.

For reliability engineers, this means that looking only at the highest temperature reached by a CPO device may leave part of the problem unexplored. The temperature transition itself can become a source of mechanical loading, particularly when it is repeated over a large number of cycles.

This is the reason thermal cycling has become an important consideration in CPO reliability testing.

The Important Part of the Test May Happen Between the Setpoints

Consider a CPO package sitting at a low temperature and then being heated to a high-temperature condition.

At first glance, the test appears simple. The chamber changes temperature, the DUT reaches the required condition, the temperature is held, and the process is repeated.

Inside the package, the situation is more complicated.

Silicon, metals, glass, polymers, and other packaging materials have different coefficients of thermal expansion. When they are connected together, their different dimensional changes cannot occur freely. Mechanical stresses develop around the areas where these materials interact.

The same thing happens during cooling, but in the opposite direction.

One cycle may not cause a visible problem. Reliability concerns often arise from the repetition. The package experiences the same general expansion and contraction process again and again, and interfaces that are repeatedly stressed can gradually accumulate damage or change their mechanical behavior.

This does not mean that every CPO package will fail because of thermal cycling. Package design, material selection, geometry, bonding methods and thermal management all influence the result.

It does mean that thermal cycling provides a way to investigate a class of reliability mechanisms that a constant-temperature exposure cannot reproduce in the same manner.

For CPO, that distinction matters because the optical path can be sensitive to relatively small changes in component position and package geometry.

Optical Alignment Is Closely Connected to Mechanical Stability

One of the defining differences between photonic packaging and conventional electronic packaging is the sensitivity of optical coupling to physical alignment.

A CPO package may contain optical interfaces that need to maintain a very specific relationship with one another. Depending on the architecture, this can involve fiber arrays, photonic dies, lenses, couplers, or other optical structures.

When the package experiences thermal expansion and contraction, those relationships can change.

The resulting effect is not necessarily an immediate catastrophic failure. A small change may first appear as a change in optical power, coupling efficiency or insertion loss. Whether a particular displacement produces a significant optical effect depends on the optical design, coupling method, wavelength and alignment tolerance.

That is exactly why reliability engineers need to look at the behavior of the complete package rather than assuming that mechanical integrity automatically means optical integrity.

A package can remain physically intact and continue operating while its optical performance is gradually moving away from its original condition.

For CPO development, that performance trend can be more useful than a simple statement that the device “passed” a thermal test.

Package Warpage Can Become an Optical Problem

Warpage is another issue that becomes more interesting when optical and electronic components are integrated into the same package.

As different materials respond differently to temperature, the package can experience deformation. The amount and direction of that deformation depend on the package structure, material stack-up, geometry, and manufacturing process.

For an electronic assembly, some deformation may have limited immediate impact. For a photonic package, even a small geometric change can potentially affect the relative position of optical components.

This does not mean that every amount of package warpage will cause coupling loss. The relationship between deformation and optical performance is highly dependent on the specific CPO architecture.

What matters from a reliability-testing perspective is being able to detect whether thermal exposure is changing the package in a way that eventually affects its optical or electrical function.

This is one reason why thermal cycling is often more useful when combined with measurements taken during or immediately after the environmental exposure.

Solder Joints and Interfaces Experience the Same Temperature History

The optical path is not the only area that needs attention.

Electrical interconnects, solder joints, underfill materials, and other package interfaces are also exposed to the same repeated temperature changes. Differences in thermal expansion can create mechanical strain in these structures, and repeated strain can contribute to fatigue over time.

The same principle applies to bonded interfaces and thermal management materials.

A thermal interface material may continue to function normally during an initial test, but changes in its physical condition over repeated thermal exposure can eventually influence heat transfer. An adhesive interface can experience stress without immediately separating. A package can accumulate fatigue without producing an obvious visual defect.

These mechanisms are not unique to CPO, but CPO makes them more important because many of them exist within a tightly integrated optical-electrical structure.

A reliability problem in one part of the package can also influence another part.

For example, package deformation can affect optical alignment, while changes in thermal resistance can influence the temperature distribution around the optical engine. The electrical and photonic portions of the package therefore cannot always be treated as independent reliability problems.

Why a 125°C Soak Does Not Answer the Same Question

High-temperature exposure remains a useful reliability test, but it should not be confused with thermal cycling.

A high-temperature soak places the DUT under a sustained thermal condition. This can be useful for understanding high-temperature operation, material stability and degradation mechanisms associated with prolonged exposure.

Thermal cycling introduces repeated movement between temperatures.

That movement creates a different type of mechanical history for the package.

If the reliability concern is primarily long-duration exposure to elevated temperature, a soak may be appropriate. If the concern involves differential expansion, package fatigue, alignment stability, or repeated thermal stress, cycling provides information that a steady-state exposure cannot provide by itself.

In practice, a CPO reliability program may need several environmental tests rather than one universal test.

The mistake is assuming that the maximum temperature specification tells the entire story.

It does not.

A test that reaches 125°C is not necessarily equivalent to a test that repeatedly moves a CPO package through a defined low-to-high temperature profile. The package experiences these two conditions differently, even though the same upper temperature may appear in both test plans.

Ramp Rate Is More Than a Chamber Specification

Rapid temperature change is becoming increasingly relevant to advanced packaging, but ramp rate needs to be considered carefully.

A chamber might be specified at 10°C/min or 15°C/min. That tells you how quickly the controlled chamber environment can change under the specified conditions. It does not necessarily mean that every part of the CPO assembly is changing temperature at exactly the same rate.

The DUT has its own thermal mass. The fixture has thermal mass. Cables, connectors and monitoring hardware also influence heat transfer. The internal structure of the package can create temperature differences between components that are physically close to one another.

This is particularly important when engineers are trying to reproduce a defined thermal stress condition.

A very aggressive chamber ramp may create a large temperature difference between the external environment and the internal package. That can be useful if it represents the intended reliability condition, but it should not automatically be assumed to be better simply because it is faster.

For CPO testing, the more useful question is whether the chamber can reproduce the required thermal profile consistently and provide a meaningful thermal condition at the DUT.

This is also why test engineers should consider actual DUT temperature measurements when the test objective requires them.

Airflow and Thermal Load Can Change the Test Result

A chamber can perform very differently once a real test setup is installed.

An empty chamber provides one set of conditions. A chamber containing CPO assemblies, fixtures, optical fibers, electrical cables and measurement equipment provides another.

Airflow affects how heat reaches the DUT. Fixture geometry can block or redirect airflow. Multiple devices can alter the local thermal environment. Heat-generating components can increase the thermal load inside the chamber.

These effects become particularly relevant when the test involves a large number of cycles and the objective is to produce repeatable results.

A test engineer should therefore pay attention to the complete arrangement inside the chamber instead of evaluating only the controller display.

The location of the DUT, the position of fixtures, cable routing, and available working space can all influence the actual thermal conditions experienced by the sample.

This is one reason chamber selection should begin with the test setup and DUT, not simply with a preferred chamber size or a headline ramp-rate number.

Monitoring CPO Performance During Thermal Cycling

There is another limitation to a simple environmental test: the chamber tells you what the environment is doing, but it does not automatically tell you how the optical device is responding.

For CPO reliability testing, this distinction is important.

If an optical parameter is measured only before and after a thermal cycling test, an engineer may know that the device changed, but not exactly when the change occurred.

Connecting the CPO test setup to appropriate optical and electrical measurement equipment can provide a much clearer picture.

Depending on the device and test objective, engineers may monitor optical power, insertion loss, coupling performance, wavelength characteristics, electrical parameters or other application-specific indicators.

The value comes from correlating the measurement with the temperature profile.

Suppose a performance change appears during heating and then largely recovers during cooling. That tells a different story from a change that remains after the temperature returns to the original condition. Likewise, a gradual drift that becomes larger after repeated cycles may indicate a different mechanism from a short-lived temperature-dependent response.

The environmental test therefore becomes more than a pass/fail exercise.

It becomes a way to understand how the CPO package behaves while thermal stress is being applied.

What a CPO Thermal Cycling Test Chamber Should Provide

There is no universal chamber specification that is suitable for every CPO reliability program. The equipment should be selected according to the required temperature profile, DUT characteristics, and measurement setup.

Temperature range is obviously important, but ramp rate should be considered alongside thermal load and DUT response. Temperature uniformity matters because large gradients can introduce unintended stress. Airflow matters because it determines how effectively the chamber transfers heat to the test article.

Working volume is another practical consideration. A chamber that is physically large enough for the samples may still be unsuitable if fixtures, optical connections and measurement hardware leave insufficient space for proper airflow.

Cable access is also worth considering. CPO testing can require optical fibers and electrical connections to remain attached during cycling, so suitable cable ports or customized access arrangements may be necessary.

Finally, the control system needs to handle the actual test profile. A reliability engineer may need multiple temperature levels, defined dwell periods, controlled transitions and a large number of repeated cycles rather than a simple high-temperature hold.

The chamber should support the test plan instead of forcing the test plan to fit the chamber.

KOMEG Rapid-Rate Thermal Cycling Chambers for CPO Applications

KOMEG’s rapid-rate thermal cycling chambers are designed for applications that require repeated and controlled temperature transitions.

The KST series covers working volumes from approximately 150 L to 1000 L, with configurations reaching -70°C to +150°C and optional higher-temperature capability. Depending on the model and configuration, heating and cooling rates of 5°C/min, 10°C/min, and 15°C/min are available. Linear and non-linear temperature-change profiles can also be configured.

For CPO and photonic packaging development, these capabilities allow engineers to build a thermal cycling test around the required temperature range and transition profile rather than relying on a conventional steady-state temperature exposure.

The equipment can also be configured around practical test requirements such as cable access, fixture arrangements, DUT quantity and thermal loading.

This flexibility becomes useful when the CPO test setup includes optical monitoring or when the package configuration changes during product development. The chamber is then treated as part of the overall test system rather than as a standalone environmental enclosure.

KOMEG can also provide customized environmental test chamber configurations when the standard chamber arrangement does not fully match the application.

A Better Way to Think About CPO Reliability Testing

The most useful CPO reliability tests are not necessarily the tests with the most extreme temperature or the fastest possible ramp.

They are the tests that answer a specific reliability question.

If an engineer is concerned about high-temperature operation, sustained exposure may be appropriate. If the concern is CTE mismatch and repeated mechanical loading, thermal cycling becomes more relevant. If optical alignment is a concern, optical performance should be monitored. If package deformation is suspected, the thermal test may need to be combined with dimensional or structural analysis.

In other words, the environmental chamber is only one part of the reliability investigation.

The more complete picture looks something like this:

Thermal profile → package temperature response → mechanical behavior → optical/electrical performance → degradation trend

That connection is particularly valuable for CPO because the optical and electronic functions are physically close enough that a change in one part of the package can influence the behavior of another.

CPO Reliability Is Moving From Survival to Understanding

CPO technology is being developed to support increasingly demanding AI and data-center architectures. As integration density increases, packaging reliability becomes harder to separate into individual mechanical, thermal, optical, and electrical problems.

Thermal cycling is one of the tools that can help engineers investigate this interaction.

The important point is not that every CPO failure is caused by temperature cycling, nor that a rapid ramp automatically produces a better reliability test. The actual behavior depends on package design, materials, optical architecture, thermal management, and the conditions of the intended application.

What thermal cycling provides is a controlled way to repeatedly expose the package to temperature transitions and observe what happens as that process continues.

When the environmental profile is combined with appropriate optical and electrical measurements, the result can be considerably more informative than a simple end-of-test inspection.

For CPO reliability engineers, the question is increasingly shifting from:

“Did the package survive the temperature test?”

to:

“What changed inside the package while the temperature was changing?”

That is where thermal cycling becomes particularly valuable.

And as optical engines move closer to high-power ASICs and CPO architectures continue to evolve, understanding that thermal-mechanical behavior will become an increasingly important part of photonic packaging reliability.

Frequently Asked Questions

What is CPO reliability testing?

CPO reliability testing evaluates whether Co-Packaged Optics can maintain the required optical, electrical and mechanical performance under defined environmental and operating conditions. Thermal cycling is one method used to investigate the effects of repeated temperature changes on the package.

Why is thermal cycling important for CPO?

CPO packages combine materials with different thermal expansion characteristics. Repeated heating and cooling can therefore create mechanical stress within the package and at material interfaces. Depending on the design, this may affect optical alignment, package deformation, interconnect reliability or interface stability.

Is a 125°C soak enough for CPO reliability testing?

A 125°C soak and a thermal cycling test address different conditions. A soak evaluates sustained exposure to elevated temperature, while thermal cycling introduces repeated heating and cooling. If the reliability concern involves CTE mismatch or repeated thermo-mechanical stress, temperature cycling may provide additional information.

Does a faster ramp rate always produce a better CPO test?

No. The appropriate ramp rate depends on the intended test condition, DUT thermal response and reliability objective. A chamber’s rated ramp rate describes the chamber environment under specified conditions and should not automatically be interpreted as the temperature ramp experienced throughout the CPO package.

What parameters can be monitored during CPO thermal cycling?

Depending on the test objective, engineers may monitor optical power, insertion loss, coupling performance, wavelength characteristics, electrical parameters and other device-specific indicators. Monitoring these parameters during cycling can help correlate performance changes with the thermal profile.

What should be considered when selecting a CPO thermal cycling chamber?

Engineers should consider temperature range, heating and cooling rate, temperature uniformity, airflow, thermal load, working volume, DUT arrangement, cable access, controller capability, and compatibility with optical or electrical monitoring equipment.

Can KOMEG thermal cycling chambers be used for photonic packaging?

KOMEG rapid-rate thermal cycling chambers can be configured for demanding temperature-change applications, including photonic and semiconductor packaging development. The appropriate chamber configuration depends on the DUT, thermal profile, loading conditions, fixture arrangement and monitoring requirements.

The thermal reliability challenge in CPO is not simply about how hot an optical engine becomes.

It is about what happens when a highly integrated package repeatedly moves from one thermal condition to another.

Silicon, glass, metals, substrates, and other materials respond differently to those changes. The resulting thermo-mechanical stress can influence optical alignment, package geometry, interconnects and interfaces, sometimes producing gradual performance changes before a conventional functional test identifies an obvious failure.

That makes thermal cycling a useful part of CPO reliability testing, particularly when the test profile is carefully controlled and the actual DUT response is understood.

The most meaningful test is not necessarily the fastest or the most severe one. It is the one that produces a thermal condition relevant to the reliability question and provides enough information to understand how the package responds.

For next-generation CPO and silicon photonics systems, that shift—from simply testing whether a device survives to understanding how its performance changes under thermal stress—is becoming increasingly important.