Optoelectronic Integration and CPO: Packaging Challenges & Optical Materials for Future Data Centers

Category:
Semiconductor/Electronic materials

September 14, 2026

As generative AI becomes more widespread, AI data centers continue to expand. To address the rapid increase in communication traffic, optoelectronic integration —a technology concept that combines electrical signal processing with optical signal transmission— is becoming increasingly important.

Optoelectronic integration is already being implemented in various forms, and Co-Packaged Optics (CPO) is attracting attention as one of the most promising approaches. This article explains the basic concept of CPO and its packaging challenges, and introduces optical materials proposed by Resonac, including polymer waveguide materials and optical adhesives.

Challenges in Data Center Networks in the AI Era

As AI clusters scale up, network switches require greater bandwidth and lower power consumption.

Contents

1. Why Is Optoelectronic Integration Important Now? What Is CPO?
2.Basic Configuration of a CPO Module
3.Key Challenges in CPO Packaging
4.Materials That Are Important in CPO and Key Selection Criteria
5.Optical Materials for CPO Proposed by Resonac

Why Is Optoelectronic Integration Important Now? What Is CPO?

AI data centers contain AI clusters that connect thousands to tens of thousands of Graphics Processing Units (GPUs) through high-speed networks. The network switches that connect these clusters require bandwidths of 800 G to 1.6 T per port and tens of Tb/s or more across the entire switch.

Achieving this with today’s electrically focused packaging creates two primary challenges.

Challenge 1: The Bandwidth Wall

In conventional data center networks, a configuration known as Pluggable Optics has been widely used. Signals are transmitted from a switch ASIC through electrical wiring on a printed circuit board to pluggable optical transceivers located at the front of the chassis.

However, as transmission speeds increase, signal loss and noise in electrical wiring also increase, making high-density routing more difficult. At speeds exceeding 800 G per port, electrical wiring on printed circuit boards is becoming a factor that limits the expansion of overall system bandwidth.

Challenge 2: The Power Wall

As bandwidth increases, electrical wiring generates more heat through conductor loss and dielectric loss. In addition, circuits such as digital signal processors (DSPs), retimers, and drivers may be required to compensate for signal loss, further increasing power consumption and heat generation.
As a result, both power and cooling costs rise across the data center, creating a major operational challenge.

Optoelectronic integration is attracting attention as a way to overcome these bandwidth and power limitations. CPO is one of its most promising implementation approaches.

Optoelectronic integration is a technology concept that combines electrical signal processing with optical signal transmission. In data center networking, technology development has progressed from Pluggable Optics to On-Board Optics (OBO), Near-Package Optics (NPO), and CPO. These approaches bring the electro-optical conversion function closer to the Application-Specific Integrated Circuit (ASIC), shortening electrical wiring and reducing signal loss and power consumption.

In CPO, a large semiconductor device such as a switch ASIC and an optical engine—consisting of a Photonic Integrated Circuit (PIC) and an Electronic Integrated Circuit (EIC)—are integrated within the same package or placed in extremely close proximity.

However, CPO is not achieved simply by moving the optical engine closer to the ASIC. Its components—including the ASIC, optical engine, optical wiring, package substrate, and packaging materials—must be designed in an integrated manner from electrical, optical, thermal, and mechanical perspectives. Optimization must also account for actual manufacturing processes, as well as post-assembly inspection and evaluation. In other words, optoelectronic packaging requires not only structural and process design, but also suitable optical and packaging materials.

Shortening Electrical Wiring and Enabling Optical Interconnection with CPO

By bringing the electro-optical conversion function closer to the ASIC, CPO shortens electrical wiring and helps reduce signal loss and power consumption.

Top view

Pluggable

OBO（On-Board Optics）

CPO (Co-Packaged Optics)

Cross-sectional view

Pluggable

CPO

Basic Configuration of a CPO Module

CPO configurations vary by implementation, but generally include the following elements.

Electronic Circuits, Logic, and Control Chips

・Switch ASIC
・EIC（Electronic Integrated Circuit）

Optical Engine and Electro-Optical Conversion Unit

・PIC（Photonic Integrated Circuit）
・Optical functional components such as optical modulators, photodetectors, and MUX/DEMUX
・Laser input section when an external light source is used

Substrates and Interposers

・Package substrate
・Interposer

Optical Connection and Optical Routing Components

・Optical fibers
・FAU（Fiber Array Unit）
・Optical waveguides
・Optical coupling components such as lenses, mirrors, and grating couplers

A key requirement is to appropriately combine the assembly reliability and process compatibility required for electronic packaging materials with the optical properties required for optical materials, according to the CPO structure and application.

The following sections describe the challenges that arise in CPO packaging and examples of material-based solutions.

Basic CPO Module Configuration and Optical, Electrical, and Thermal Interconnection

Example of a basic CPO module configuration

Electrical-to-optical signal conversion flow

Key Challenges in CPO Packaging

Challenge 1: High-Density Optical Routing and Pitch Conversion Between PICs and Fibers

Typical optical fiber array pitch ranges from several tens of micrometers to 250 µm. In contrast, optical port pitches on the PIC side may range from several to several tens of micrometers, depending on the design, or may be set to match the fiber array.

Direct connection can therefore make it difficult to accommodate the pitch difference, creating significant constraints on packaging density and routing flexibility. Inside CPO, optical signals must also be routed with low loss in a limited space while accounting for the placement constraints of the PIC, fibers, ASIC, and package substrate.In such cases, optical waveguides are used to transmit optical signals with low loss, increase the density of optical routing between the PIC and fibers, and convert between different connection pitches. Core materials for optical waveguides include silicon (Si), silicon nitride (SiN), glass-based materials, and polymers, which are selected according to the requirements.

Si and SiN are strong options for integration with the PIC and low-loss transmission. Polymer optical waveguides, on the other hand, offer advantages in pitch conversion for fiber arrays and flexible optical routing on package substrates. They are particularly effective when greater flexibility is needed in positioning PICs and fibers or when relatively long optical paths must be routed across a substrate.

Pitch Mismatch Between a PIC and an Optical Fiber Array and the Challenge of High-Density Optical Routing

Direct connection makes it difficult to accommodate the pitch difference, placing significant constraints on packaging density and routing flexibility.

High-density optical routing and pitch conversion are required.

Challenge 2: Suppressing Misalignment at Optical Coupling Points and Achieving Short Production Cycle Times

In CPO packaging, fiber coupling and FAU–PIC assembly may require submicron-level alignment.

When connecting an FAU to a PIC or fixing optical components such as fibers, lenses, and mirrors, the components must be secured quickly after precise alignment. Slow fixation increases production cycle time. In addition, positional displacement or temperature changes during curing may cause optical-axis misalignment and increase coupling loss.

Alignment, Temporary Fixation, and Final Cure Process at an Optical Coupling Point

Precision alignment

Temporary fixation by UV irradiation using an adhesive

Final cure by heat, UV, or another method

Challenge 3: Reliability of EIC/PIC Assembly

n flip-chip assembly of EICs and PICs, two main types of stress are applied to the interconnects. The first is thermal stress caused by differences in the coefficient of thermal expansion (CTE) between the chip and the substrate or interposer. This stress occurs repeatedly during temperature cycling. The second is mechanical stress caused by warpage of the substrate or interposer. Accumulation of these stresses can lead to cracking and reduced interconnect reliability. Underfills are therefore used to reinforce interconnects and relieve thermal stress. With hybrid bonding and similar approaches, reliability may also be ensured through peripheral protection, encapsulation, and stress-relief materials.

Stress on Interconnects Caused by Temperature Cycling and Substrate Warpage, and the Need for Stress Relief and Reinforcement

Interconnects such as bumps and microbumps are subjected to stress caused by temperature cycling and substrate warpage.

Underfills and similar materials are required to relieve stress and improve interconnect reliability.

Challenge 4: Transparent Materials Near Optical Paths and Long-Term Reliability

As CPO becomes more densely integrated, resin materials may be placed near optical paths involving grating couplers, lenses, mirrors, optical windows, and Arrayed Waveguide Gratings (AWGs). Because these materials are located near regions through which light passes or is reflected, conventional black mold compounds and standard encapsulants can be difficult to use. Optical properties such as transparency are therefore required.

Challenge 5: Thermal Design and System-Level Cooling

Optical components are susceptible to changes in optical characteristics caused by temperature variation. Efficient heat dissipation is therefore essential for stable performance. In particular, ASICs and EICs generate significant heat, requiring system-level design that includes substrate structures and cooling methods.

Challenge 6: Inspection, Failure Analysis, and Rework

In CPO optical connections, even slight positional misalignment or variation in material properties can directly affect optical loss. It is therefore important to identify defects early through in-process inspection and optical characterization, and to feed the results back into high-volume manufacturing processes. In addition, because expensive ASICs and optical components are integrated at high density, rework strategies that enable component reuse wherever possible should also be considered.

Material Selection Criteria in CPO

Addressing the challenges that arise in CPO packaging requires selecting suitable materials for each application.

The table below summarizes the major material groups used in CPO packaging and their respective roles. Because electrical, optical, and thermal functions are closely integrated in CPO, understanding where and how each material functions is the starting point for material selection.

Material	Primary role
Optical waveguide materials	In-package optical routing and pitch conversion
Optical adhesives	Fixation of fibers, FAUs, lenses, and PICs
Underfills	Underfills Reinforcement of flip-chip interconnects
Transparent materials	Protection and encapsulation near optical paths
Thermal management materials	Heat spreading for heat-generating components

The evaluation criteria for selecting these materials are summarized below. It is important to assess not only the physical properties of the materials themselves, but also their compatibility with the package structure, high-volume production processes, and reliability evaluations. The assessment items are classified into four categories: optical design, packaging process, reliability, and thermal/mechanical properties. Material selection criteria can be linked directly to the CPO package structure. For example, optical properties are important near optical paths; low shrinkage and low outgassing are important for bonding and encapsulation areas; and heat resistance and low moisture absorption are important in areas exposed to thermal effects.

Major category	Evaluation item	Key considerations
Optical properties	Transparency and optical loss	Transmittance, absorption, haze, and impact on optical loss at the operating wavelength
Optical properties	Refractive index control	Refractive index matching with optical waveguides, lenses, and adhesive layers
Packaging process	Coating, forming, and curing	Packaging process Coating, forming, and curing Coating performance, curing conditions, cure shrinkage, and impact on positional accuracy
Packaging process	Reflow and downstream process resistance	Heat resistance, low outgassing, and discoloration or deformation during processing
Reliability	High-temperature/high-humidity and thermal cycling	Moisture absorption, delamination, cracking, and changes in optical properties
Reliability	Long-term stability	Yellowing, refractive index changes, and reduced adhesion
Thermal and mechanical properties	Low stress and CTE matching	Warpage, positional misalignment, and stress concentration at interconnects
Thermal and mechanical properties	Compatibility with thermal design	Heat resistance near heat sources and compatibility with thermal management materials

Optical Materials for CPO Proposed by Resonac

Resonac is advancing material development for CPO by leveraging its expertise in semiconductor packaging materials and optical materials.

Dry Film Polymer Waveguide Materials (Under Development)

These materials are intended for high-density optical routing within CPO and pitch conversion between PICs and fibers. Both the core and cladding layers can be formed by photolithography. The dry-film format offers high film-thickness accuracy and process compatibility. By forming optical routing on a substrate, these materials can facilitate optical routing within limited package space and conversion between fiber pitches and PIC-side optical port pitches.

UV-Cure Optical Adhesive “RA Series (Under Development)”

This optical adhesive is suitable for fixing optical components after alignment, including in fiber coupling and FAU–PIC assembly. It is designed to balance high transmittance and a suitable refractive index in the near-infrared region with low cure shrinkage, adhesion strength, and environmental reliability. This helps maintain optical coupling precision while reducing production cycle time in high-volume manufacturing.