High-Speed Optical Transceiver COB Packaging in Data Centers

The rise of artificial intelligence, big data, blockchain, cloud computing, the Internet of Things, and 5G has led to a rapid growth in the demand for optical transceivers. The main application markets for optical transceivers currently include the data communication market, telecommunications market, and emerging markets, with the data communication market experiencing the fastest growth.

As the cost of optical sub-assemblies (OSA) accounts for over 60% of the cost of modules, and the cost reduction potential of optical chips has become increasingly limited, it has become crucial to drive packaging technology from expensive hermetic packaging towards low-cost non-hermetic packaging while ensuring the performance and reliability of transceivers.

With the continuous development of high-speed optical transceivers towards miniaturization and high-density, the application of non-hermetic COB packaging technology has become imperative.

COB Meaning

COB, also known as Chip-on-Board, refers to the packaging of chips or optical components by first attaching them to a PCB using epoxy die bonding, then electrically connecting them with wire bonding, and finally encapsulating them with a top dispensing adhesive.

Currently, COB packaging technology has been widely adopted, especially in cases where VCSEL arrays are used for short-distance data communication. Additionally, high-integration silicon photonics can also be packaged using COB technology.

Data Center Optical Transceiver COB Technology

Based on different application scenarios and requirements, optical transceivers can be roughly divided into telecom-grade and data center-grade. The former is used in harsh environmental conditions and is difficult to replace and maintain, while the latter is deployed in relatively mild environments and is easier to maintain.

For example, telecom-grade optical transceivers used in outdoor base stations may face high-temperature working conditions of up to 80°C during intense sunlight, while in the cold winter nights of the northern regions, the environmental temperature may drop to as low as -40°C.

Moreover, to ensure signal coverage, these base stations may be located in remote areas such as mountains and forests, making regular maintenance challenging. These characteristics determine the high reliability requirements for telecom-grade optical transceivers.

In contrast, in data center applications, transceivers are typically housed in temperature-controlled rooms with air conditioning, and maintenance personnel are present to perform repairs at any time. Therefore, the reliability requirements are relatively lower. Taking into account various factors such as application scenarios, requirements, and costs, different packaging technologies have evolved for opticals. Currently, telecom-grade transceivers mostly adopt hermetic To-can or BOX packaging technologies, while data center-grade transceivers mostly use non-hermetic COB packaging technology.

Technical Advantages of COB Packaging

Improved High-speed Signal Connectivity

In telecom-grade optics with hermetic packaging, the connection between the laser diode and the PCB requires FPC (flexible printed circuit) and high-frequency ceramics, and then the laser diode is connected to the PCB through wire bonding. It is difficult to ensure impedance continuity at multiple connection points, leading to inevitable signal integrity loss. In COB packaging, the laser diode can be directly bonded to the PCB using wire bonding, greatly reducing impedance discontinuities and ensuring better high-speed signal connectivity from the PCB to the LD (laser diode). This results in larger eye diagram margins and higher sensitivity performance.

Space and Cost Savings

COB packaging saves space by eliminating components such as high-frequency ceramic packages and flexible cables. This advantage becomes more apparent as optics continue to pursue smaller form factors.

For example, in a 400G QSFP-DD optical transceiver that uses EML lasers, numerous electrical chips such as DML bias, EA bias, EA modulation, and DSP are required, along with optical components like EML, isolators, and lenses.

If hermetic packaging is used, the optical components would occupy a significant amount of space, greatly compressing the layout space for electrical components and presenting significant challenges in module design.

On the other hand, using COB packaging allows for space savings, which can be utilized for additional redundancy designs in the electrical domain. This includes adding more filtering capacitors and implementing larger high-frequency signal isolation layouts, thereby enhancing module performance.

From a cost perspective, COB packaging reduces the need for high-frequency ceramic packages, flexible cables, and other components. It also eliminates processes such as nitrogen sealing, leak testing of boxes, FPC soldering, and individual testing of optical components, resulting in lower material and production costs.

Disadvantages of COB Packaging

Decreased Lifespan of Sensitive Components

In COB packaging, optical components and certain electrical chips such as drivers and TIAs are directly exposed to the environment, which adversely affects their lifespan. In hermetic packaging, the laser diode (LD) is sealed in a nitrogen-filled box, isolating it from the external environment and better ensuring stable LD operation.

In recent years, module manufacturers have developed limited hermetic technologies to enhance the lifespan of LDs in COB transceivers. For example, LDs are mounted in partially open metal boxes, allowing the PCB to directly connect to the LD through openings. The metal box can be sealed with adhesive, providing a certain level of sealing.

Challenging for Repairing Defective Products

In BOX packaging, optical components can be manufactured separately from the PCB and individually tested. If any part has an issue, it can be replaced and repaired individually.

In COB transceivers, since the optical components are directly connected to the PCB, performance testing can only be conducted after the entire assembly is completed. Troubleshooting whether the issue lies with the electrical chips or the optical chips becomes more difficult, and replacing faulty components can lead to higher scrap rates. There is a potential risk of scrapping the entire module due to the failure of a single optical chip, thus increasing the scrap rate during the production process. Therefore, in COB packaging, process stability and yield are particularly important.

Despite the drawbacks of COB packaging technology, its advantages will be further realized and its disadvantages may be improved with the continuous development of packaging technology. More precise packaging equipment and advanced packaging techniques will also reduce manufacturing costs and improve production efficiency for COB packaging. Therefore, COB packaging technology still holds significant potential for widespread applications in the future of the electronics manufacturing industry.

Key Technological Steps in COB Packaging

COB packaging involves several crucial steps in the packaging technology of optical transceivers, including die bonding, wire bonding, optical coupling, and testing.

Die Bonding

This step involves using adhesive to securely attach various chips, such as clock recovery chips, laser driver chips, transimpedance amplifier chips, laser chips, and detector chips, to the PCB. Silver adhesive is commonly used for direct attachment to the PCB. During die bonding, attention should be paid to ensure the positional accuracy meets the requirements and that the chip adhesion is strong. For chips with high power consumption and heat generation, such as lasers and drivers, the thermal performance after die bonding should also be considered.

Wire Bonding

Wire bonding refers to electrically connecting the chip’s pads to the solder pads on the PCB using wire. This is typically achieved through wire bonding technology using gold wires. This step requires ensuring good wire connections without any open circuits. Pull testing of the wires is commonly performed to check their integrity.

In high-speed optical transceivers with complex circuitry, there may be a large number of wire crossings, and issues such as wire sagging and overlapping need to be addressed. For high-speed signal pin connections, attention should be given to wire length and quantity. Generally, reducing wire length and increasing the number of wires can enhance signal integrity.

Optical Coupling

This step is the most time-consuming and prone to producing defective products in the packaging of optical transceivers. For multimode optical modules, vertical-cavity surface-emitting lasers (VCSELs) are commonly used, which are coupled into multimode fibers through reflective mirrors.

The optical path is simple, with large tolerances and relatively straightforward processes. On the other hand, coupling for single-mode fibers is more complex. Since the core diameter of single-mode fibers is only 9 μm, which is smaller than that of multimode fibers, lens focusing coupling is required.

In modules that require multiplexing, such as LR4, wavelength division multiplexing (WDM) components need to be added, further increasing the complexity of the optical path. An important material used in coupling is ultraviolet (UV) curing adhesive, which is primarily used for bonding coupling lenses. Its characteristics include rapid curing after exposure to UV light, low shrinkage, and suitability for precise alignment coupling lenses that require high bonding accuracy.

Dual-lens Coupling

Automated Coupling Equipment

Testing

Testing is the final step in the production of optical transceiver, and it mainly involves performance testing and reliability testing. Common performance testing parameters include eye margin, extinction ratio, transmit power, and receiver sensitivity. Reliability testing typically includes high and low-temperature powered aging testing, high and low-temperature cycling testing, vibration testing, and multiple insertion and removal testing.

Summary

The integration of optical engines and chip-on-board (COB) technology provides significant advantages for optical transceivers, including compact size, higher efficiency, improved thermal management, and reduced costs. With the increasing demand for high-speed data transmission, these advantages make COB-packaged optical transceivers an ideal solution for high-speed data transmission in data centers, telecommunications networks, and high-performance computing systems.