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Chip on Board Assembly: An Effective Solution to Electronics Miniaturization

There is now a new packaging method far different from the SMD package technology. The chip on board assembly offers more advantages. Also known as COB, you can find this package in the smallest of all electronic devices. Also, you can find it in applications where there is limited space.

Chip on Board Assembly – What is it?

Chip on board assembly involves installing bare semiconductor chips on a PCB or substrate. The manufacturer uses non-conductive or conductive epoxy to achieve this. Chips incorporate aluminum wedge bonding or gold ball bonding to achieve electrical connection. There are various semi-sintered epoxy resins for chip-on-board assembly. However, these are for applications requiring superior electrical and thermal conductivity.

Also, manufacturers can connect the chip to the PCB by using flip-chip technology. Chip on board technology directly connects semiconductor to the PCB substrate. This technology incorporates the technique of surface mount technology. However, it is different from SMT. COB assembly involves active devices and high lead count. Also, it doesn’t demand for ceramic external device packaging.

The chip on board assembly mounts the die or microchip to the board. Also, the manufacturer attaches the chip to the substrate interposter. Therefore, a flip chip or wirebonding helps to achieve functional connection.

The flip chip on board employs a chip with bumped bond pads. Therefore, it doesn’t require wirebonding. The chip on the flip-chip on board faces down on the board. Also, it is important to underfill flip-chip to prevent any chemical or thermo-mechanical damage.

Chip on boards with LEDs have made LED lighting more efficient than ever. COB is a method of PCB manufacturing in which integrated circuits (ICs) are directly bonded and wired to a PCB board.

What is the Chip on Board?

This is a bare chip mounted directly on the PCB. After the wires have been connected, a ball of plastic covers the chip to create a connection. The manufacturer wire-bonds the bare chip to the board and pours epoxy resin into it. In addition, chip on board is a great option for miniaturized circuits. It provides a better solution when traditional assembly technology can’t meet the design parameters.

The chip on board assembly comprises three steps. These steps include die mount, wirebonding, and encapsulation of the wires and dies.

Die mount

This involves applying a die attach adhesive to the substrate and installing the die over the die mount. The application of adhesive may include pin transfer or dispensing. Therefore, accurate die placement is important to ensure good die planarity. Also, after applying adhesive, there is a curing process. This process enables the adhesive to achieve its final electrical and thermal properties. Also, the PCB manufacturer must use solvent or plasma cleaning to remove organic contaminants.

Wirebonding

This process involves the use of ultrasonic AI wedge bonding or thermosonic Au ball bonding to connect wires between the substrate and the die. The bond pads of the board and the die must have no defects or contaminants.  Also, this will help to create reliable and good bonds.

Encapsulation of the wires and dies

In this process, the manufacturer encapsulates the bond and die wires to shield them from any chemical and mechanical damage. Also, this process involves dispensing encapsulant material over the wires and die. However, this encapsulants have to undergo curing.

Advantages and Limitations of Chip on Board Assembly

Pros

The main benefit of a COB is that it minimizes the weight of a circuit. Also, when weight is a major factor, the chip on board assembly is an ideal solution. The advantages of chip on board assembly include:

  • Enhanced protection against reverse-engineering
  • Minimized cost
  • Improved performance as a result of minimized interconnection resistances and lengths
  • Minimized space requirements
  • Shorter time-to-market
  • Greater reliability as a result of heat distribution
  • Wide application range
  • Higher reliability due to small number of solder joint

Cons

  • The chip on board led package features a high maintenance cost and low pass rate.
  • High manufacturing costs. The manufacturing cost is more than SMD due to the high defect rate.
  • In addition, the color uniformity is less than the display screen.

Chip on Board Packaging Process

Crystal expansion

For this step, the manufacturer uses the expansion machine to expand LED chip film. Also, this helps the LED die attached to the surface of the film to pull apart to enhance the thorn crystal.

Adhesive

Put the crystal ring on the surface of the backing machine. A dispensing machine can help to locate the right amount of silver paste on the PCB board. After this place the crystal expansion ring into the piercing crystal holder. Also, the operator will use a piercing pen to pierce the LED chip.

Curing

Place the pierced PCB in a thermal cycle oven. After curing the silver paste, take it out for a short while. Don’t take it out for too long as the coating of the LED chip will be oxidized.

Stick the chip

In this step, the manufacturer will put a right amount of red glue on the IC position. Also, a dispenser can help you achieve this. Also, an anti-static device can help to place the IC die on the black glue or red glue.

Drying

This involves placing the glued die in an oven on a flat heating plate. Let the glued die stand at a constant temperature. Also, you can cure it naturally, but it will take a longer time.

Wire Bonding

The aluminum wire bonding machine bridges IC chip with the aluminum wire of the pad on the board. Therefore, this means the operator welds the inner lead of the chip on board.

Pre-test

This step involves using special inspection tools to inspect the COB board. Also, these tools repair the unqualified PCB board.

Dispensing

With the help of a glue dispenser, you can place the right amount of the AB glue on the bonded LED die.

Conclusion

This article discussed the chip on board assembly and its packaging process. Also, it talked about the benefits and limitations of the chip on board assembly.