Understanding Chip on Board (COB) Technology

Chip on Board (COB) is an integrated circuit (IC) packaging technology where bare semiconductor dies are directly mounted onto a printed circuit board (PCB), electrically connected via wire bonding, and then encapsulated as a single module. Unlike traditional packaged components (e.g., DIP, QFP, BGA), COB eliminates the intermediate package (lead frame, plastic encapsulation) and bonds the die directly to the PCB, enabling higher integration, smaller form factors, and lower costs for high-volume, space-constrained applications.

Core Principles of COB

COB merges “Level 1” (die packaging) and “Level 2” (PCB assembly) into a single step, simplifying the supply chain and reducing parasitics. Key traits:

Direct Die Attachment: Bare dies are glued to the PCB with conductive or non-conductive adhesive (e.g., silver-filled epoxy for thermal/electrical conductivity).
Wire Bonding: Ultra-fine gold, aluminum, or copper wires (25–50 μm diameter) connect die bond pads to PCB traces via thermosonic or ultrasonic bonding.
Encapsulation: A blob of epoxy or silicone covers the die and wires to protect against moisture, dust, and mechanical damage.
Thermal Path: Heat from the die flows directly through the PCB substrate, lowering thermal resistance by 30–50% vs. packaged components.

COB Assembly Process

COB assembly is highly automated for high-volume production. Key steps:

PCB Preparation: Clean the PCB and apply adhesive to die sites.
Die Attach: Use a pick-and-place machine to position dies on the adhesive, then cure at high temperature to secure them.
Wire Bonding: Bond wires between die pads and PCB traces (ball bonding for gold, wedge bonding for aluminum).
Encapsulation: Dispense epoxy/silicone over the die and wires, then cure to form a protective “black blob”.
Inspection & Testing: Check for bond integrity (X-ray, AOI), electrical continuity, and functional performance.

COB Variants

Variant	Description	Use Case
Wire-Bond COB	Standard gold/aluminum wire bonding; most common	Consumer electronics, LED modules, touch ICs
Flip-Chip COB	Die flipped to face the PCB, with solder bumps for connections; no wires	High-frequency, high-power, or fine-pitch applications (e.g., RF modules)
COB LED	Multiple LED dies on a reflective PCB; integrated as a single light source	LED lighting (downlights, flashlights), display backlights
Multi-Die COB	Multiple dies (e.g., MCU + memory + sensor) on one PCB area	System-in-Package (SiP) for wearables, IoT devices

Advantages of COB

Miniaturization: Reduces footprint by 30–50% vs. packaged components; thickness as low as 0.5 mm.
Cost Efficiency: Eliminates package materials (lead frame, plastic) and cuts assembly steps, lowering costs by 20–40% for high volumes.
Electrical Performance: Shorter wire bonds reduce parasitic inductance/capacitance, improving signal integrity and high-frequency performance (up to GHz).
Thermal Efficiency: Direct die-to-PCB thermal path improves heat dissipation, critical for power LEDs and high-current ICs.
Mechanical Robustness: Encapsulation protects against vibration and shock; ideal for industrial and automotive environments.

Limitations of COB

Non-Repairable: Encapsulation makes die replacement impossible; a faulty COB requires replacing the entire PCB.
Process Complexity: Wire bonding and die placement demand high precision; tooling costs are higher for low volumes.
Testing Challenges: Probing bare dies before encapsulation is difficult; defects are often found post-encapsulation, increasing scrap rates.
High-Frequency Limits: Fine-pitch wire bonds still introduce parasitics; flip-chip or BGA is better for >10 GHz applications.
Supply Chain Dependency: Requires direct access to bare dies, which may not be available for all ICs.

COB vs. Traditional Packaging (DIP, SMT, BGA)

Characteristic	COB	DIP/SMT (Packaged)	BGA
Form Factor	Smallest (no package)	Larger (package + leads)	Compact (ball grid)
Cost (High Volume)	Lowest (no package)	Higher (package + assembly)	Highest (complex process)
Thermal Resistance	Low (direct PCB path)	Medium (package + leads)	Low (thermal pad)
Signal Integrity	Excellent (short wires)	Good (leads add parasitics)	Excellent (short balls)
Repairability	None (encapsulated)	Easy (replace package)	Difficult (rework needed)
Pin Count	Moderate (limited by wire bonding)	Low–Medium (leads/balls)	High (up to 50k balls)
Use Case	LEDs, consumer electronics, IoT	Prototyping, low-volume, replaceable parts	High-performance CPUs, GPUs, high-pin-count ICs

Applications of COB

COB is widely used in cost-sensitive, space-constrained, and high-volume products:

Consumer Electronics: Calculators, toys, remote controls, and low-end wearables (e.g., smartwatches) — known for the “black blob” 封装.
LED Lighting: COB LED modules for downlights, spotlights, and automotive headlights, offering high luminous efficacy and uniform light output.
Display Technology: Touch ICs for POS terminals, ATMs, and large-format touchscreens (e.g., meeting boards).
Industrial & Automotive: Sensors, motor controllers, and dashboard electronics, where vibration resistance and thermal efficiency are critical.
IoT & Wearables: Small-form-factor devices (e.g., TWS earbuds, fitness trackers) requiring high integration and low power dissipation.

When to Choose COB

Opt for COB if:

Space is Critical: You need the smallest possible footprint (e.g., wearables, IoT sensors).
Cost is a Driver: High-volume production (100k+ units) where package costs are significant.
Thermal Performance Matters: Power LEDs or high-current ICs need efficient heat dissipation.
Repairability is Not Required: The product is disposable or designed for replacement rather than repair.

Avoid COB if:

Repairability is Key: You need to replace ICs in the field (e.g., industrial control systems).
High Pin Count: The IC requires thousands of pins (e.g., modern GPUs, FPGAs) — use BGA instead.
Low-Volume Production: Tooling costs for COB are not justified for small batches (use packaged SMT components).

Summary

COB is a mature, cost-effective packaging technology that excels in miniaturization, thermal efficiency, and high-volume production. While it lacks repairability and is less suitable for high-pin-count, high-frequency ICs, COB remains a cornerstone of consumer electronics, LED lighting, and IoT devices. As demand for smaller, cheaper, and more integrated electronics grows, COB will continue to play a vital role alongside advanced packaging (e.g., fan-out wafer-level packaging, 2.5D/3D ICs).