What is Chip-on-Board (COB) Technology: A Complete Guide

In this tutorial, we will get the detailed concept of "Chip On Board"  or COB. If you have ever thought about how cheaper, durable and compact electronic devices are made, the answer is chip-on-board technology. Chip on board is a solution from chip manufacturing to prototyping and development board.

Today we will provide a deep understanding of COB and profitable insights for the electronic miniaturization future. A finished semiconductor wafer is cut into dies. Each die is then physically bonded to the PCB. Three different methods are used to connect the terminal pads of the integrated circuit (or other semiconductor device) with the conductive traces of the printed circuit board. As electronics have grown, packaging technology has also evolved. We will learn how this innovative packaging technology revolutionises electronic components integration. So let's get started and review the details of  Chip On Board technology!

What is a Chip on Board PCB?

A Chip On Board (COB) printed circuit board is a packaging method used for the assembly of electronic components on the PCB board. In this method, no individual components are configured on the board, but bare integrated circuits are connected on the surface of the board. The use of this technology reduces the use of older packing techniques like ceramic or plastic packaging, and that makes the small size and weight of electronic devices and projects. Chip on board (COB) is a method of circuit board manufacturing in which the integrated circuits are attached (wire-bonded directly) to a printed circuit board and covered by a blob of epoxy.

COB eliminates the packaging of individual semiconductor devices and instead merges two levels of electronic packaging: level 1 (components) and level 2 (wiring boards). Compared with other packaging technologies, pcb chip technology is inexpensive (only about 1/3 of the same chip), saves space, and has mature craftsmanship. However Chip On Board technology also has disadvantages such as the need for additional welding machines and packaging machines. The layout of a certain chip on board can improve IC signal performance because they remove most or all of the packages and remove most or all of the parasitic components.

Chip-on-Board in a PCB Layout:

In the chip-on-board approach, a semiconductor die with exposed contacts is being soldered directly to the PCB. In other words, there is no lead frame (for wire bonding), no ceramic/epoxy packaging, and no interposer/substrate. Once attached, the chip can be packaged directly on the PCB using an epoxy encapsulant, which will protect the chip and any wire bonded pads from damage. The two main type of processes for this are:

1) Flip Chip on Board:

FCOB, the solder is fluxed directly onto the PCB; it is not attached to the die. The chip will then be placed just like any other SMD component, and it will be reflowed along with other components. Therefore, some design for assembly (DFA) in terms of the footprint is needed to ensure reliable assembly, following similar guidelines for BGA pad sizing, but based on the bump size instead of ball size.

2) Wire-Bonding:

In wire bonding the chip is attached to the board with an adhesive. Each pad on the device is connected with a fine wire lead that is welded to the pad and to the circuit board. It’s best to encapsulate with an epoxy in order to protect the wire bonds and die from environmental exposure. This primarily prevents corrosion and protects the wires from mechanical damage. When creating the footprint for the wire bonding pads in the PCB, the pads are typically oversized. The parameters to be considered for the footprint include contact pad size, contact pad pitch, and contact pad shape.

After the bare chip that is mounted onto the printed circuit board (PCB) by using one of these processes the wires are attached. A glob of epoxy or plastic is used to cover the chip and its connections. The tape automated bonding (TAB) process is used to place the chip on the board. It is basically a drop of specially formulated epoxy or resin deposited over a semiconductor chip and its wire bonds to provide mechanical support.

7 Main Steps in Chip-on-boards Manufacturing Process:

Here the process is discussed in detail, we have a total of 7 steps:

1. Substrate Preparation: The PCB board is prepared through a cleaning board surface and applied to the adhesive layer of conductive material where chips are bonded.

2. Die Attach: The bare chips are getting and positioned on the adhesive-coated areas of the board. There are pick-and-place machines or specialized instruments used to do this process.

3. Bonding: When chips are configured, they are bonded with a board with the use of conductive solder bumps. This bonding process makes sure a reliable connection between the contact pads of chips and board conductive traces.

4. Wire Bonding: In some conditions there is wire bonding to connect bonding pads with board traces by using fine wires. This process helps electrical signals to be transmitted between the chip and board.

5. Encapsulation: For the protection of chips and wire bonds from outer components. An encapsulant material can be used on the complete assembly. This material also has a clear epoxy coating.

6. Testing: There are different testing methods used in COB assembly to make sure proper reliability and functionality. Such as temperature cycling, electrical testing, and visual inspection are performed to verify the function of COB.

7. Final Assembly: When the chip on board assembly passes all tests, it is now ready to integrate into the final electronic devices like LED lights, phones, or any other projects

Features and Advantages of Chip on Board:

• High and low-pressure design
• Custom coating
• Multi-layer, double-sided
• Functional board test
• High or low volume
• Wide temperature range
• Cost-competitive solution
• Turnkey application

Choosing the Right Glue for COB Bonding:

After the chip is attached to the PCB, it is typically encapsulated with a thermally or UV-cured epoxy material or conformal coating to protect it and any wire bonded pads from damage. When selecting adhesives for a wafer (or die), the choice depends on the wafer's requirements for grounding or heat dissipation. Two types of glues used are:

Silver glue: It requires high-temperature curing, can be cured at 120°C for two hours or at 150°C for one hour.

Anaerobic glue: It is used when electrical conductivity and heat conduction are not required. This type of adhesive cures naturally without needing high-temperature exposure by blocking contact with air.

Conclusion:

Chip-On-Board (COB) technology has revolutionized the electronic world and provides different advantages than conventional packaging techniques. As there is direct bonding of bare chips on board, the chip on board made the small, less weight, and more effective electronic devices. The removal of bulk packaging and shorter electrical paths provides good electrical performance, fewer signal losses, and good thermal management. This technique is also important for the miniaturization of electronic components and paved the way for cutting-edge and compact technology in different industries.