Do you remember the first time you took apart an electronic alarm clock as a child and marveled at what you saw inside? Those shiny metal wires, intricately connecting mysterious little black squares like a spider’s web — at the time, we had no idea that this seemingly chaotic connection was a harbinger of a revolution in electronic packaging. Today, when we open up the latest smartwatch or drone, we discover that the internal electronic components have become more refined and compact. This transformation is the result of the silent evolution of chip-on-board technology.
If this is your first time learning about chip-on-board, don’t worry. This article will take you through an in-depth exploration of all aspects of this technology — not only its working principles and advantages but also the design considerations required to utilize it effectively. Whether you are a hardware engineer, product designer, or simply an enthusiast curious about electronic technology, you will find valuable insights in this comprehensive guide.
What is Chip-on-Board
Chip on board commonly known as COB, is an advanced electronic packaging technology that first emerged in the 1960s and was initially applied in the military and aerospace industries. With the continuous advancement of semiconductor technology and the growing demand for miniaturization and high integration in consumer electronics, COB technology gradually entered the civilian market in the 1980s, finding widespread use in products such as calculators and electronic watches. The rapid growth of the LED lighting industry in the 21st century further accelerated the maturity and application of COB technology, making it an indispensable part of modern electronic manufacturing.
Unlike traditional chip packaging methods, COB technology eliminates the step of packaging the chip into an independent device (such as QFP or BGA). Instead, the bare semiconductor chip is directly bonded to the PCB, followed by electrical connections established through wire bonding or flip-chip technology, and finally protected by an encapsulation layer of epoxy resin. This method acts like a “second skin” that is custom-fitted for the chip, providing effective protection while maintaining its original performance. It avoids the issues of increased volume and higher thermal resistance commonly associated with traditional packaging methods.
Typical COB Structure from Top to Bottom:
- Top Layer: Protective epoxy resin, offering high-temperature resistance, moisture resistance, and insulation properties.
- Middle Layer: “Spider-web-like” bonding wire connections.
- Core Layer: Bare semiconductor chip, fixed to the PCB using conductive adhesive.
- Base Layer: PCB substrate, providing mechanical support and circuit interconnection.
Advantages of COB Packaging Technology
Space Efficiency
The most obvious advantage of COB packaging is its ability to maximize PCB space utilization. Traditional packaging methods typically involve encapsulating chips into independent devices, which are then surface-mounted, increasing PCB area and overall thickness. In contrast, COB technology eliminates the need for encapsulation, significantly reducing the space occupied by the chip.
The overall thickness of COB packaging can be controlled within 0.5mm, which is only one-third the thickness of QFN packaging.
Compared to traditional SMD packaging, COB can save 40-60% of PCB space and allows components to be installed at the edges of the PCB, further enhancing board space utilization.
COB also supports multi-layer chip stacking, enabling three-dimensional integration along the Z-axis, allowing for higher-density chip integration.
Mechanical Reliability
COB packaging excels not only in space utilization but also in mechanical reliability. Since the chip is directly bonded to the PCB, the absence of an encapsulation shell and metal leads minimizes mechanical stress, allowing COB to maintain excellent performance even in environments with vibration, impact, and high temperature and humidity.
COB packaging passes 20G random vibration tests without structural failure and can withstand mechanical shocks up to 1500G.
Flexible COB can endure a 5mm bending radius and maintain stability after 10,000 bending cycles.
COB exhibits high environmental resilience, passing 85°C/85%RH dual 85 tests for 1000 hours without performance degradation.
Cost Efficiency
Another major advantage of COB packaging is its remarkable cost efficiency, which is one of the primary reasons it is widely used in consumer electronics, LED lighting, and low-profit projects.
COB eliminates the step of independently packaging the chip, saving 30-50% of the packaging material cost.
It also reduces the need for SMT placement and reflow soldering processes and allows for modular testing, improving production efficiency by 40%.
Due to its better thermal management and mechanical stability, COB packaging extends product life cycles, reducing long-term maintenance and replacement costs.
COB PCBA Manufacturing Process
- Cleaning and Preparation
First, the PCB must undergo thorough cleaning to remove oxidation and dust from the pads, providing a solid foundation for chip adhesion. Common cleaning methods include ultrasonic cleaning, ion blowers, and alcohol wiping.
- Dispensing and Chip Placement
Accurately apply red glue (conductive or non-conductive glue) to the center of the copper foil on the PCB. The amount and position of the glue must be strictly controlled to ensure uniform adhesion without overflow. Then, carefully place the bare die (chip) onto the red glue.
- Red Glue Curing
Place the PCB with the attached die into an oven or thermal curing furnace, curing the red glue at a set temperature and duration. The typical curing temperature ranges from 150°C to 180°C, with curing times between 30 to 60 minutes, ensuring the glue fully solidifies and the chip is securely bonded to the PCB.
- Die Bonding (Wire Bonding)
Before die bonding, use an ion blower to clean the bonding area again, removing static electricity and dust to ensure optimal bonding.
Use 1-micron diameter aluminum wire for bonding, establishing a stable electrical connection between the die pad and the PCB pad.
- Initial Functional Testing
After die bonding, perform an initial electrical functionality test to check for proper electrical connections between the die and the PCB, ensuring there are no short circuits, open circuits, or defective solder joints. If the test is successful, proceed to the next stage.
- Dispensing and Encapsulation
Place the tested PCB in an aluminum box and place it on a heating platform to preheat, enhancing the flow and adhesion of the dispensing material. Use an automatic dispensing machine to apply black glue (epoxy resin) over the chip and bonding area. The glue should evenly cover the bonding area without bubbles or voids.
- Black Glue Curing
Transfer the dispensed PCB to an oven for curing at 120°C to 150°C for 1 to 2 hours, ensuring the epoxy resin is fully hardened to form a solid protective layer.
- Final Testing and Quality Inspection
After curing, conduct functional tests and reliability checks on the PCB, including electrical performance tests, short-circuit/open-circuit checks, and thermal resistance tests to ensure that the encapsulated chip functions correctly and meets design standards.
COB PCB Design Considerations
Surface Treatment
COB PCB surface treatment typically recommends using gold plating (Gold Plating) or ENIG (Electroless Nickel Immersion Gold). Unlike traditional PCB surface treatments, COB PCBs require a thicker gold plating layer to ensure sufficient bonding energy for chip adhesion, forming reliable Au-Al or Au-Au metallurgical bonds.
Recommended Parameters:
The gold plating layer thickness should be controlled between 1.0 to 1.2 μm.
Insufficient thickness may lead to bonding failure, while excessive thickness may cause brittle solder joints, affecting long-term reliability.
Pad Layout
The layout of the pads on a COB PCB directly affects wire bonding stability and reliability. To prevent wire bonding crossover or short circuits, avoid diagonal pad layouts and prioritize equal spacing and length layouts.
Optimization Recommendations:
Equal Spacing and Length: Maintain consistent pad spacing to ensure equal wire lengths, helping control wire tension and improving soldering quality.
Shortened Pad Spacing: Properly control pad spacing to prevent wire extension or slack, reducing the risk of wire crossover.
Elliptical Pad Design: Elliptical pads help disperse wire positioning and reduce stress concentration, enhancing bonding consistency and mechanical strength.
Cross-Shaped Alignment Points
To improve the positioning accuracy of die bonding and wire bonding, it is recommended to add at least two alignment points for each die region during PCB design.
Considerations:
Since wire bonding equipment prefers linear alignment points, use cross-shaped alignment points instead of SMT round points to minimize chip misalignment or wire bonding errors caused by mismatched alignment.
Die Pad Size
The size of the die pad directly affects the stability and bonding quality of the die. Pad size should be slightly larger than the actual die size to ensure accurate die placement while preventing excessive rotation or displacement during bonding.
Recommended Parameters:
Pad size should be 0.25 to 0.3mm larger than the actual die dimensions to restrict die offset and avoid excessive rotation within the die pad.
Dispensing Area
During COB encapsulation, epoxy resin (Epoxy) is applied to protect the chip and bonding wires from external contamination and mechanical damage. Therefore, special attention must be given to preventing epoxy infiltration and ensuring consistent dispensing quality.
Considerations:
Avoid Vias: The dispensing area should not contain vias to prevent epoxy from penetrating to the other side of the PCB, affecting the encapsulation effect. If vias are unavoidable, it is recommended to perform 100% via filling to ensure no gaps within the vias and prevent leakage during dispensing.
Silkscreen Marking for Dispensing: Add silkscreen markings in the dispensing area to assist operators in accurately positioning and controlling the shape and area of the dispensing, enhancing consistency.
Ending
As the demand for smaller, more powerful, and cost-effective electronic devices continues to grow, COB (Chip on Board) packaging technology is poised to play an increasingly critical role in modern electronics manufacturing. By eliminating the need for traditional packaging and enabling direct chip bonding onto PCBs, COB technology not only enhances space utilization and mechanical reliability but also significantly reduces production costs, making it a preferred choice for industries ranging from consumer electronics and automotive systems to LED lighting and IoT applications.
You Might Be Interested
Discover how POP (Package on Package) technology revolutionizes electronic device design. Learn how this compact, high-performance solution maximizes space, boosts efficiency, and enables advanced features in smartphones, tablets, and more!
Master the complete BGA rework process with our detailed guide. Learn step-by-step techniques for safe desoldering, cleaning, ball placement, and re-soldering. Improve your skills and efficiency while overcoming common challenges
Dual Row QFN or DR-QFN, on the other hand, is an improved version of the traditional QFN, featuring dual-row staggered pad design in the pad area, where pads are arranged
Discover how IPC-7351 revolutionizes surface mount technology (SMT) design with comprehensive guidelines on footprint naming, pad design, and component layout. This article breaks down key standards that ensure precision, reliability,
In the world of electronics, resistors are fundamental components used for current control, voltage reduction, and various other critical applications. They provide resistance to electrical current flow, converting it into
Hard Gold and Soft Gold are PCB finishes; hard gold, an alloy, suits harsh conditions, while soft gold, often pure, offers flexibility. Both use electroplating, with hard gold providing reliability
This article clarifies the concept of a “Blank Circuit Board,” defines it as a PCB without electronic components, and addresses potential terminology variations. It also touches on the challenges of
Discover how Electronic Design Automation (EDA) revolutionizes modern electronics! From automating complex IC and PCB designs to enhancing efficiency and innovation, this article dives into the history, tools, and applications