Introduction
Chip on Board (COB) is a method in which chips are mounted over printed circuit boards. Their connection is made with the board using tiny wires. COB technology helps in saving space, improving performance, and reducing costs. In this article, you will learn what chip on board technology is and what impact it has on a PCB.
What is Chip on Board (COB) Technology?
Chip on Board (COB) is the technology used for directly putting a bare semiconductor chip on PCBs. The chip is put on the circuit board and connected to it using fine gold or aluminum wires. Signals and power pass through these wires. Once connected, the chip is covered with a protective layer. This protective layer, which is usually of epoxy, protects the chip from dust, moisture, and damage.
The COB technology fits well in high volume PCB assembly, where cost and space are important. It is commonly used in products that need to be small and efficient. For example:
- LED lights
- Calculators
- Remote controls
- Compact cameras
- Hearing aids and many more.
What is Chip on BoardChip on Board Assembly Process in PCB
Assembling a chip on printed circuit boards involves certain steps. Here is a series of steps that help from placing the chip on a PCB to protecting it.
Substrate Preparation
The COB process starts with the preparation of the PCB surface. For preparation, the surface of the board is first cleaned of dust, oil, or other particles. After cleaning, a thin adhesive substrate or conductive layer is applied to the place where the chip will sit. This layer of substrate helps with strong chip bonding and good electrical contact.
Die Mounting
After substrate preparation, a bare chip (also called a die) is placed on the board. This step is called die mounting. Proper alignment of the chip is very important at this step. Otherwise, there are chances of faulty connections. To position the chip rightly onto the adhesive-coated area, pick and place machines or other precise tools are used.
Wire Bonding
Once the bare chip is placed on the board, thin wires are used for bonding the chip pads with metal traces on the board. These wires are made of material that can carry electronic signals, such as fine gold or aluminum.
Note: Wire bonding is usually needed in designs where the chip contact points do not line up directly with the board.
Flip Chip Technology
In some printed circuit boards, flip chip technology is used in place of wire bonding. In this method, the chip is flipped upside downwards and the tiny solder bumps are directly connected to the chip pads of the board. This is beneficial in reducing space and improving signal performance.
Note: Flip chip technology is considered good for high-speed and high-density PCB designs.
Encapsulation
After connecting the chip and wire encapsulation is carried out using epoxy material. In encapsulation, a protective layer of the said material covers the chip and wires to protect them from dust and other damage.
Testing
After assembly, the board goes through several tests to check if everything works correctly. It helps in making sure that the chip responds well and all connections are stable. Testing often
includes:
- Visual checks
- Electrical testing
- Temperature cycling
Final Assembly
If the chip on the board passes the testing, it moves to the final assembly. In this step, the COB is placed into the product it is made for, which can be LED light, smart devices, gadgets, etc. In simple terms, the COB at this step is ready for real-world use.
Chip on Board Assembly Process in PCBAdvantages of COB
COB technology is a good pick for electronic products that require being compact and highly efficient. Here are the benefits it brings during the PCB assembly and manufacturing processes.
Saving Space
COB lets you occupy smaller spaces efficiently. It eliminates the need for larger packaging and lets you use the circuit board area better without degrading performance. Using the COB technology, you can design smaller but efficient products easily.
Lightweight
In COB, the outer casing is removed, and only a few components are used. This helps in reducing the overall weight of a printed circuit board. This benefits the manufacturing of portable or wearable electronic devices. In addition, the COB process uses less costly materials, which saves you money.
Heat Management
In COB, the heat is distributed to a larger area. It eases the process of cooling. When the device is protected from overheating, it can run longer and perform well for longer periods. Moreover, less soldering is involved, which also reduces the chance of failures caused by thermal stress.
Better Performance
In COB, chips are placed directly onto the circuit boards. This shortens the signal paths, lowering resistance and improving electrical performance. In addition, the technology also prevents reverse engineering, making designs more secure.
Uniform Output (for LEDs)
When using COB in lighting, you get strong lighting without any dark spots. It is good for products like inspection gadgets or medical devices requiring focused lighting.
Disadvantages of COB
Yet, the chip-on-board technology benefits PCBs in many ways. There are also some disadvantages to it. Below are some key ones among them.
Extra Machines Are Needed
During the COB assembly, you are going to need precise tools and machines other than the ones used during normal processes. You may need welding and packaging machines to finish the process.
Moreover, COB technology costs more to make than standard surface-mount designs (SMD). This can impact your budget and may slow down production.
Hard Maintenance
As mentioned earlier, the chips are placed directly onto boards and are sealed. This makes them difficult to remove and replace, making the maintenance or repair difficult. In case a chip fails, you might need to replace the entire unit instead of just one part.
Tough Environmental Control
You need to follow stricter environmental rules when assembling COB. Multiple things, such as dust or temperature, can affect the bonding and performance. That is why during the COB assembly process, you have to implement tough environment control, i.e., totally clean rooms, which requires a lot of effort.
CTE Issues
In COB, you might also face issues related to CTE (Coefficient of thermal expansion). This happens because different materials are used in the board and chip. Each used material expands at a different rate when it gets heated. This leads to weak bonds or cracks and impacts the overall performance over time.
Color Uniformity
COB may not match the color quality of display screens. You may see uneven brightness or strange color spots. In projects that involve precise color matching, such as display or visual indicators, COB might cause problems.
What are Chip on Board LED lights?
Chip on Board LED lights feature several tiny chips mounted directly onto a PCB. Manufacturers place these chips very close together to help them blend light more effectively. As a result, they
deliver more energy-efficient glow than traditional LEDs.
COB LEDHow to Make COB Packaging: The Process
Below is the complete step-by-step process for COB packaging.
Step 1: Crystal Expansion
Manufacturers supply tightly packed LED chips on a film. Technicians then use an expansion machine to stretch these films. The purpose is to make the LED dies spaced out. Why? Because it makes the picking and placement of chips easy for the next steps.
Step 2: Applying Adhesive
After crystal expansion, technicians spread conductive glue, i.e., a silver plate, on the surface of the board. If they are handling chips in bulk, they use a dispensing machine to apply the paste to the area designated for chip placement.
Step 3: Placing the LED Chip
Once operators apply the adhesive, they use a piercing pen to place the LED chips onto boards. They usually perform this process under a microscope to ensure careful and precise placement.
Step 4: Initial Curing
Next, technicians place the PCBs with chips inside a thermal oven. Heat is applied for curing the silver plate and securing the chips in place.
Note: Overheating at this step can cause yellowing or oxidation on the chips on the board.
Step 5: IC Chip Placement
If your COB includes IC chips, technicians dispense red or black glue onto the board. Then, they place the IC on top of the glue using anti-static tools to prevent damage.
Step 6: Drying
Technicians heat the board again for curing the glue that is holding the IC in place. It helps in ensuring a strong bond before moving to the next step.
Step 7: Wire Bonding
The LED or IC chips are connected to the PCB using tiny wires. They usually perform this process with an aluminum wire bonding machine. They make the connections for the COB to function properly.
Step 8: Pre-Test
After bonding, pre-testing is carried out for each board. The testing is carried out using special tools to assess issues. If there are any faults reported, the board will unqualify and reject before the sealing.
Step 9: Encapsulation
Boards that pass the pre-testing go for encapsulation. During this, a glue dispenser places glue, i.e., AB over LED chips and Black over IC chips. This protects the components and gives the COB its final appearance, shaped as required.
Step 10: Final Curing
After the encapsulation process, the sealed PCBs are heated again in a thermal oven. This final curing step hardens the encapsulating glue.
Step 11: Post-Test
The finished COB board is tested for full electrical performance. Any defective units are removed to ensure only quality products go forward.
E- TestConclusion
Chip-on-Board technology helps in putting chips directly onto a printed circuit board. It is good for designs that are compact and need reliable performance for modern electronic products. Its features make it a popular choice for many purposes in spite of some hurdles. If you are looking to achieve a perfect COB process, have a chat with PCBMay, a reputable PCB assembly company.
FAQs
How to Solder a Chip on a Board?
The first step to solder a chip on a board is to clean it and put adhesive or solder paste on it. After that, place the chip using a reliable tool. Connect it, following the type of design, by either bonding or soldering it. The last step is to place the chip in an encapsulation material to protect against damage.
Can You Use COB Technology with Flexible PCBs?
Yes, you can, but it takes extra care. Since flexible PCBs and move, you have to choose materials that can handle mechanical stress. To prevent any possible damage, encapsulation is recommended.
What Materials Protect COB Chips During Encapsulation?
Manufacturers usually use epoxy resin or silicone-based materials to encapsulate COB chips. These coatings cover the chip and connections on it and protect against dust, moisture, and accidental damage.
Is COB More Expensive Than SMT?
Many times, COB costs more than SMT. This is because COB requires special equipment, tidy conditions, and careful handling. However, it may still be cost-effective in high-volume production.
