Chip on Board: Difference Between Flexible and Rigid PCB Processes

Chip on Board (COB) is a direct chip assembly technology that mounts a bare die on a rigid PCB or flex PCB. It connects the chip using wire bonding and encapsulates it with protective resin or silicone.

While the COB concept is the same for rigid and flexible boards, the production details, material behavior, and technical difficulties are very different.

What Is Chip on Board Technology

Chip on Board integrates a bare IC directly onto the circuit board without using a package.The wires from the die are bonded to gold-plated pads, then covered with encapsulant to protect against moisture, dust, and mechanical damage.

This process eliminates the need for a packaged IC, saving space and improving electrical performance by shortening interconnects.

IC Packaging vs. Chip on Board

Aspect	IC Packaging	Chip on Board
Definition	The chip is packaged in a separate plastic or ceramic housing before mounting.	The bare chip is directly attached and bonded to the PCB or FPC.
Connection Method	Soldered through standard SMT assembly.	Gold or aluminum wire bonding on open pads.
Size & Thickness	Larger due to packaging structure.	Thinner and lighter; ideal for compact designs.
Cost	Higher component cost but easier assembly.	Lower material cost, but higher process control requirement.
Application	General electronics, IC sockets, modules.	Smart cards, sensors, cameras, wearables.

IC packaging provides mechanical protection and standardization, while COB focuses on space-saving and direct integration. In many modern products—such as camera modules or flexible sensors—COB is preferred for miniaturization and electrical performance.

Chip on Board on Rigid PCB

Rigid PCB COB is widely used in LED modules, control boards, and power driver circuits.

• The FR4 substrate provides stable surface flatness.

• Standard epoxy materials are used for dam and filling.

• Heat dissipation is a key design factor, especially in high-power applications.

• The process is mature, and many PCB factories have in-house COB lines.

Overall, COB on rigid PCB focuses on thermal stability, mechanical strength, and large-area bonding reliability.

Chip on Board on Flex PCB

COB can also be applied on flex PCBs, commonly used in RFID tags, fingerprint sensors, smart cards, and camera modules.The flexible polyimide substrate allows thinner, lighter, and more compact designs.

Key technical points:

A stiffener is not always required — many flex PCB designs allow chip on board directly on the flexible area when supported by vacuum fixtures.

Encapsulant materials are typically flexible silicone or low-stress black epoxy to allow bending without cracks.

Curing temperature must be carefully controlled to avoid PI deformation or warping.

However, flexible COB demands tighter process control in flatness, adhesion, and wire bonding stability compared to rigid PCB COB.

Why Not Every PCB Factory Can Do Flex COB

Although many PCB manufacturers offer chip-on-board assembly for rigid boards, not all can handle flexible COB production.Here’s why:

Challenge	Description
Base stability	FPC is soft and prone to deformation; requires special vacuum platforms or fixtures.
Bonding precision	Even small movement during wire bonding can cause poor loop formation or lifted pads.
Encapsulation control	Flexible materials need low-stress curing and controlled flow to prevent bending.
Cleanroom environment	COB on FPC often requires higher cleanliness levels, especially for camera or sensor modules.
Process experience	Operators must understand how to manage flexible substrates during alignment, bonding, and curing.

Flex PCB COB is more delicate, labor-intensive, and requires special equipment and experienced technicians. A PCB factory focusing on rigid board COB may not have the right setup or process control to achieve reliable results on flexible substrates.

Material Differences in COB Processes

Material Aspect	Rigid PCB COB	Flexible PCB COB
Substrate	FR4 glass epoxy	Polyimide (PI) film
Encapsulant	Hard black epoxy	Flexible silicone or soft epoxy
Heat dissipation	Good	Limited, but low power consumption
Stiffener use	Not required	Optional, depending on chip size
Customer-specified materials	Sometimes	Common, especially in consumer modules

Advantages of Chip on Board Assembly

1. Enables miniaturization and weight reduction.

2. Reduces electrical resistance and inductance.

3. Improves signal integrity for high-speed circuits.

4. Customizable encapsulation materials for specific applications.

5. Suitable for both rigid and flexible designs.

Summary

Chip on Board technology bridges the gap between IC packaging and direct circuit integration.While COB on rigid PCB emphasizes heat dissipation and mechanical stability, COB on flexible PCB focuses on lightweight, compact, and bendable solutions.

Not every factory equipped for PCB COB can perform flexible COB reliably — the process requires advanced equipment, precise control, and deep experience with flexible substrates.

In modern electronics, chip on board continues to play a vital role in enabling smaller, smarter, and more integrated products.

FAQ

1. What does chip on board mean?

It refers to mounting a bare IC chip directly on a PCB or FPC, wire-bonding it, and encapsulating it for protection, without using a packaged IC.

2. How is chip on board different from IC packaging?

IC packaging encloses the chip in a protective case before mounting, while COB attaches the bare die directly to the circuit board.

3. Why is flexible COB more difficult than rigid PCB COB?

Because flexible boards deform easily, bonding and encapsulation require tighter flatness control, soft encapsulants, and specialized tooling.

4. What are common encapsulation materials in COB?

Epoxy resins for rigid PCBs, and flexible silicone or low-stress black epoxy for flexible PCBs.