What is COB (Chip on Board) Assembly?

COB (Chip on Board) assembly is a PCB manufacturing process where bare semiconductor chips are mounted directly onto a printed circuit board or substrate using epoxy, while the electrical interconnections are made through flip-chip technology or wire-bonding process. COB process eliminates the need for packaging of individual semiconductor devices, which makes the completed product to be cheaper, lighter, and more compact. In some cases, COB construction improves the efficiency of circuits by reducing the inductance and capacitance of integrated circuit leads. Hence, it is popularly used for applications requiring excellent electrical and thermal conductivity, like LED lighting.

COB Assembly Process:

Chip-on-board assembly includes three steps - die mounting, wire-bonding or flip-chip technology, and encapsulation.

• Die Mounting: The die is mounted onto the substrate using an adhesive, which is then subjected to a curing process for proper solidification.
• Wire Bonding: It involves the use of aluminum wedge bonding or gold ball bonding to connect wires between the substrate and the die. This process helps to create reliable and good bonds and mandatorily requires clean bond pads.
• Flip-Chip Technology: The chip can also be connected to the PCB by using flip-chip technology. The chip and board are passed through a reflow soldering process to make the electrical connections. This process is used to connect chips with bumped bond pads. Although it incorporates the technique of surface mount technology, it is different from SMT as it involves active devices and a high lead count. This process does not require ceramic external device packaging either.
• Encapsulation: Wires and dies are encapsulated using an encapsulant material to shield them from chemical and mechanical damage.

The assembled PCB boards undergo curing to solidify the adhesives and encapsulants used in the manufacturing process and ensure that the components are securely attached and protected. Following this, the COB board is checked for any defect by inspection tools. A different set of tools can also be used to repair unqualified boards detected during pre-testing.

Advantages 

• Reverse Engineering Protection: PCBs assembled using the COB process have enhanced protection against reverse engineering. 
• Improved Electrical Performance: This is a result of a reduction in interconnection lengths and the resultant resistances.
• Space Efficiency: Minimized space requirements enable more compact designs within the same area.
• Faster Time-to-Market in production.  
• High Reliability: This process is exceptionally reliable as it involves fewer solder joints and heat being distributed more evenly.

Disadvantages of Chip on Board Assembly  

Chip-on-board packages are high maintenance and the manufacturing process has a low pass rate. The high defect rate also results in increased manufacturing costs.

Applications

As Chip-on-board (COB) PCB assembly results in assembled boards that are compact in size, highly reliable and have efficient heat dissipation, this manufacturing process is extensively used in high-power LED lighting, consumer electronics like wearables & smartphones, and automotive lighting & dashboards. Medical devices such as wearable health monitors and portable diagnostics benefit from COB's miniaturization ability. Telecommunications devices are assembled utilizing the COB process because of the compact form factor of optoelectronics and RF modules, while industrial sensors and automation systems leverage the robustness of this process. Rugged electronics and avionics for military and aerospace applications are also made using COB assembly. Hearing aids, IoT devices, and security systems, including surveillance cameras and biometric devices, also rely on COB technology for their performance and space efficiency.