Learn What Chip on Board (COB) is in PCB Technology

Introduction

Let’s take a look at how Chip on Board (COB) technology is one of the latest developments in PCB manufacturing. Get a better understanding of how to harness COB to provide improved heat performance, a small footprint design and competitive pricing in the world of electronic applications, which includes LED lighting and automotive. Find out its advantages, its limitations and its varied uses.

What is Chip on Board (COB)?

It is a technology in which semiconductor chips are bare and directly mounted onto the circuit board. Unlike traditional methods, COB does not require individual chip packaging. Rather, the bare chip is put directly on the PCB. The chip then finds its placement, and wires them to other components. Wires are covered with epoxy or plastic to protect these connections. Encapsulation is the process. The chip is then securely bonded in place by a method referred to as Tape Automated Bonding by COB. This direct mounting saves space in the PCB and increases overall performance.

Chip on Board Applications

COB are used in many fields, automotive, electronics, medical devices etc. This supports applications which require strong thermal and electrical conductivity like LED lighting and smartwatches. High performance assemblies can be made with special pressure less epoxies. The die is usually bonded to gold or aluminum. Ribbon bonding enables improved signal quality for RF and mm-wave devices. Encapsulation protects the die, especially for sensors. For low inductance, flip chip technology mounts the die face down making use of solder balls or gold bumps. The die is secured to PCB pads by thermocompression, and underfilled for protection. Advanced testing provides reliable connections, while quick prototyping choices give COB great adaptability.

Advantages of COB

Compact And Space-Saving Design

Provides an extremely compact and small size design. You get more in less space. This design saves space on your PCB, opening allowances for anything else. This is great for a space constrained device. Further, this compact setup enhances device performance. With COB you get more in a smaller space.

Strong Intensity at Close Range

In close setting, you get a powerful, bright output. It makes it perfect for focused lighting needs. Applications such as LED will show you its strength. COB gives you greater luminous power per component than is otherwise possible. This also increases efficiency overall and eliminates the unnecessary need for extra light sources. In small or enclosed spaces COB provides good visibility.

High Uniformity Up Close

It offers high uniformity even at close range. The results are a smooth consistent light output which removes shadows and uneven spots. And this should be ideal for devices that require clear, focused illumination. Light in COB is spread evenly across the beam regardless of the space it fills. It’s perfect for precision tasks, such as in medical or inspection equipment because of its uniformity. With glare or light variation eliminated, you have better visibility.

Simple Single-Circuit Layout

It’s easy to connect multiple LEDs on a single board. This setup makes assembly more simple. More points of failure means that those connections are harder to reach. There’s less wiring, so you get better reliability. Overall performance can also be improved by a simple design.

Great Heat Performance for Stability

You get a better temperature control for your devices. For this reason, the components are always running smoothly and efficiently. The heat is managed well with COB, reducing its risks of damaging. However, improving heat performance increases the lifetime of your products.

Inexpensive

Other packaging methods are quite expensive while COB technology is inexpensive. You’ll only pay around one third of that for the same chip. It is cheap in terms of cost effectiveness and helps you save money on production. COB also saves space on your circuit board. Allows you to put components in a smaller space. In addition, it employs mature craftsmanship to produce reliable quality.

Disadvantages of COB

Additional Machines Required

This technology has some disadvantages. You may require additional welding machines in assembly. Final steps also need packaging machines. That adds to your initial setup costs. Production can be slowed down by the need for more equipment. Before making a choice for COB, you should consider them.

Stricter Environmental Requirements

The PCB patch has stricter environmental requirements. However, if you want to follow these rules, you have to do it closely. It can make it difficult for your manufacturing process. COB assemblies are also difficult to maintain. It’s harder to repair once installed.

Coefficient of Thermal Expansion (CTE) Issues

Most commonly the PCB chip is based on a lead frame (BGA). Doing this may result in no VCC or ground connections to pins. You may have problems with CTE. The materials react to heat differently from what they do when they are not made up of different layers. Substrate connections can be poor and cause performance problems.

Color Uniformity Is Less Than the Display Screen

You will find that the colors are not as well colored as on the display screen. This can result in some bright spots and a hue that is out of place. This can be an issue if your project needs precise color matching. Different colors can confuse your perception of your product. You may need extra intervention to correct these deviations. Perhaps, that makes COB not perfect for applications that have-to-have perfect color accuracy.

Materials for Chip-on-Board Assembly

Opting for the right materials during chip on assembly is equally important. So, you want PCB materials of a low CTE mismatch compared to the semiconductor. Unfortunately, very few standard materials meet such a desire. CTE mismatch as lower as possible helps reduce stress on connections during temperature changes with reducing problems in long term. The main chip material, silicon, has a CTE of ~3 ppm/°C. However, there are some advanced materials that can help:

• PTFE Laminates with Ceramic Filler – RO3003 (CTE = 6 ppm/°C) or RO4835 (CTE = 8 ppm/°C)
• Ceramic Like Aluminum Nitride – CTE = 4-5 ppm/°C (used as an IC substrate material)
• Flex PCBs – Made from polyimide blends, suitable for chip-on-board assemblies
• Silicon interposer – Acts as a buffer layer to reduce stress between the chip and the PCB

Steps in Making a COB Package

COB package is quite simple to make. The semiconductor chip will be directly mounted onto the PCB. Eventually, this method integrates the chip and the substrate into a single unit. You don’t use leads or pads; you don’t use a traditional package. On the contrary it all integrated for better performances. This saves space and makes your design simpler.

• Substrate Preparation

The substrate is where you start to make a COB package. Do well the clean of the substrate. A good connection requires this step. Then, place a conductive layer, slightly thin, on top. This layer is often done in copper. This is done so you can put a chip on the surface. This helps have a better performance if the substrate is clean and coated.

• Chip Placement and Alignment

Then you put the semiconductor chip down on the substrate. Also make sure it lines up with the conductive traces. It is very important to have this alignment. It ensures a good connection. A chip works well only with precise placement. While doing this, take your time.

• Chip Bonding with Conductive Adhesive

You place the chip, then bond it. For this step use a conductive adhesive. Silver die attach epoxy, silicone or acrylate are all common choices. Then apply the adhesive carefully to the chip to which it is to be connected to the substrate. This bonding develops a strong as well as a reliable link. It makes the chip work well in your device. Be sure the bond is in good shape before proceeding.

• Wire Bonding for Electrical Connections

Then you clip on bond wires to the chip. This step should be done using a wire bonding process. This machine puts thin wire between the chip and substrate. These wires allow to create the electrical connection. They secure the chip to the substrate. Make sure the network connection is strong.

• Sealing and Encapsulation

The COB package is sealed after placing the bond wires. A protective material may be used for this. Epoxy molding compound or liquid encapsulant may be used. It protects the chip as well as bond wires. It protects them from damage. Make this seal strong and complete.

Types of Chip on Board

Flip Chip

The device is upside down during this process. The circuit board is on the first (top) layer. On the board small solder balls are placed. These balls go to the chip. The chip and board are then sent through reflow soldering. The result is strong electrical connections. You end up with a small and minimalistic design.

Dam & Fill

Semiconductor chips are well protected by this. Thus, you place the layer on a chip. Then, cover the chip with epoxy. Whether it’s a dam or specifically a barrier by design or natural processes, it creates a barrier at the edges. Then fill with a different material. It is safe from damage, and this keeps the chip safe from damage. You cure both materials with heat or UV light and then. It’s great for chips in tough spots like cars or planes. That’s how the chips work well, even in harsh conditions.

Glob-Top

Applied to the chip, the epoxy is then applied and protected it effectively with a glob or a round form which is a type of encapsulation. This glob protects the chip from damage and dirt. First, you put the chip on a substrate. Then you put the glob top material over the chip and fill it in, making the glob shape. You then cure the material with heat, or UV light. The chip can withstand this type of mechanical stress, and the method isolates it from other parts. It also saves costs as there is no need of extra dam and it reduces electromagnetic interference.

Wire Bonding

This is how it accompanies the board. You first attach the chip with adhesive. Following that, each pad on the chip is connected to a circuit board by fine wires. Both the chip and the board are welded to these wires. It’s similar to connecting an integrated circuit to the frame. But there, the chip is directly wire bonded to the circuit board.

Flexible Circuit Board

The chip itself has metal leads that attach them. Device pads are connected to by these leads. They are then welded to the printed circuit board. It helps to make strong connections. An encapsulant covers the chip and the connections. Protects it from moisture and harmful gases. Additionally, it provides for heat dissipation, while also protecting the wire bonds from damage. For example, pocket calculators, the printed circuit board can be put in final products. For multi-chip modules, the board may be connected to another circuit board. There are some boards with layers to deal with heat, where those boards can handle heat for high power devices, including LEDs. Low-loss others may be designed at microwave frequencies.

COB LED Technology Explained 

With this technology, red, green and blue chips are mounted directly to a printed circuit board (PCB). The result is that you can put more chips into less space using this method. It also produces a flat, even LED surface. With three colors, COB can make stellar images, with vivid colors, just like the real thing. It is possible to fit more LEDs in the same space as other ways like Dual in-line Package (DIP) and Surface Mounted Device (SMD). COB uses less space with no bonding wires. What that means is you get a tighter pixel pitch. The higher the resolution, the closer apart the pixels (the pixel pitch). Image quality, reliability and energy efficiency of COB technology can be improved.

Chip-on-Board Packaging Process (for LED Chips)

High quality assembly and performance are guaranteed in a detailed method of the chip-on-board (COB) packaging process for LED chips. By undertaking these steps, you will create a compact design and also make your products more efficient. Here’s a breakdown of the key steps involve:

Step 1: Crystal Expansion

Take advantage of the crystal expansion machine, it uniformly expanded the LED chip film, and pulled the tightly arranged LED die apart.

Step 2: Adhesive Preparation

The expanded crystal ring is placed on top of a surface coated with a silver paste layer and the paste is applied to help bond.

Step 3: Piercing the LED Chip

Place the crystal expansion ring in the piercing holder and using a piercing pen attach the LED chip to the PCB on a microscope.

Step 4: Thermal Cycle Oven

Because we don’t want to damage the LED chip coating on the PCB, we would place the pierced PCB in a thermal cycle oven to cure the silver paste quickly.

Step 5: Chip Placement

Apply red glue to the IC position and pick the IC die off with an anti-static device, set it on top of the adhesive in the correct position.

Step 6: Drying the Die

Cure the glued die either by putting the put glued die on a flat heating plate in a thermal cycle oven to cure or allow it to cure for a longer time in natural.

Step 7: Wire Bonding

Then you connect the chip (LED die or IC chip) to the PCB with aluminum wires using an aluminum wire bonding machine.

Step 8: Pre-Test Inspection

Specialized inspection tools are used to conduct a pretest on the COB board to see if there are any defects on it and then repair the board if they are unqualified.

Step 9: Dispensing Glue

The bonded LED die is given an appropriate amount of AB glue applied to it with a glue dispenser, and black glue is packaged over top of the IC.

Step 10: Curing the Assembly

Cure PCB at a constant temperature in a thermal cycle oven, seal the PCB, and put into the oven.

Step 11: Post-Test Evaluation

Electrical performance of the packaged PCB boards is tested on special testing tools to test their quality and functionality.

SMD vs. COB Packaging Technology 

Bonding Wires vs. Adhesive

In SMD, bonding wires are used to connect components to PCB. These connections are created by soldering machine, which heats and presses a wire to a material in order to create a connection. These connections can be weak or strong, and if not done correctly they fail. On the other hand, COB technology utilizes adhesive bonding that ensures a strong, reliable interface that will fail less to the effects of environment or soldering error.

COB in Harsh Environments

SMD components are more subject to damage from heat, humidity and impact. Since these are external factors, they sit on the PCB surface. COB technology suffers less from such harsh conditions, however, due to its robust adhesive bonding.

Finer Pixel Pitch with COB

Without bonding wires, this technology doesn’t need to be as fine, because it doesn’t need bonding wires. It creates smaller, high-resolution displays. It is in order to be able to produce sharper images.

Component Size Limits

SMD has size limitation. Usually limited to parts greater than 0.5 mm. However, COB can use even smaller components, only a few micrometers in size. This is an advantage of COB because if one needs to use an advanced display, this flexibility provides the preferred solution.

FAQs About Chip on Board

• Why Is COB Often Used in Automotive Lighting?

It is used in spotlights, downlights and headlights. They make a solid, sharp beam and you’ll see them in cars. It’s perfect for bright directional light. It helps with heat management as they are made like this. Multiple LED chips comprise a single module. Here, the heat is actually spread further out. COB LEDs also offer fine tuning of lighting control. They’re great for automotive uses where accuracy matters.

• How Does COB Differ from Traditional Chip Packaging?

COB is its a special packaging technology. Directly on the circuit board it mounts semiconductor chips. That is different than traditions. On the other hand, chips are packaged one by one in traditional methods. Bare chips go directly onto the board with COB. It adds to performance and saves space. They are also smaller and more efficient. In addition, they are cheaper to produce. COB technology provides better quality and lower costs.

• Is COB Technology Cost-Effective for Large-Scale Production?

Yes. The product creates higher efficiency and reduction of the cost. It is a simple and cheap process. Lower costs go hand-in-hand with fewer manufacturing steps. For good reasons, COB technology attracts high-end markets. COB LED displays can be easily and quickly installed. In the end, it reduces time and money for manufacturers.

• What Materials Are Used in COB For Heat Dissipation?

Copper is used because copper is a good conductor of heat. Besides, thermally conductive adhesives help cool things. Aluminum nitride is a standout when it comes to heat management. Rather, it has better heat conductor than aluminum or alumina. Other effective options are ceramic substrates and direct bonding copper. Heat dissipation in electronic designs is improved. The combination of these materials enables performance and reliability improvement.

Conclusion

In this PCBTok blog, we went deep into Chip on Board technology, where semiconductors are mounted directly on circuit boards. Benefits highlighted include being compact, efficient and yielding strong performance, in LED lighting and automotive applications. But it also showed some downsides, including a need for extra machinery and more stringent environmental rules. COB is a very important part of modern electronics.