ENEPIG Or Plating Gold For COB Bonding PCB

COB wire bonding is a wire bonding method in the chip production process. It is generally used to connect the internal circuit of the chip and a package pin or a gold-plated copper foil of the circuit board before packaging with a gold wire or aluminum wire.COB wire bonding is generate high frequency vibrations through ultrasound under the influence of pressure and vibration,metal surfaces to be welded rub against each other and oxide film is destroyed and plastic deformation occurs, the two pure metal surfaces come in close contact to achieve an atomic distance, resulting in a strong mechanical connection. After bonding, the chip is encapsulated with vinyl.

COB wire bonding process

1.Expanding crystal.

The expansion machine is used to evenly expand the entire LED wafer film provided by the manufacturer, so that the LED crystal grains that are closely arranged on the surface of the film are pulled apart to facilitate the thorn crystal.

2.Adhesive.

The expanded crystal expansion ring is placed on the surface of the back-glued machine where the silver paste layer has been scraped, and the silver paste is backed. Point silver paste. Suitable for bulk LED chips. Use a dispenser to spot the appropriate amount of silver paste on the PCB.

3.The expanded crystal ring prepared with the silver paste is put into the gill frame, and the LED chip is stabbed on the PCB circuit board by the operator under a microscope.

4. Put the PCB of Crystal into a thermal cycle oven and stand for a while at constant temperature. After the silver paste is cured, it shall be taken out (not for a long time, otherwise, the LED chip coating will bake yellow, ie, it will be oxidized and cause difficulties for bonding). If there is a LED chip bond, the above steps are required; if only the IC chip is bound, the above steps are cancelled.

5.Sticky chip.

Use the dispensing machine to place a suitable amount of red glue (or black glue) on the PCB’s IC board position, and then use an antistatic device (vacuum suction pen or sub) to place the bare IC chip correctly on the red or black glue.

6.Drying.

Place the sticky die in a heat-circulating oven and place it on a large flat heating plate for a while at a constant temperature. It can also be cured naturally (a long time).

7.Wire Bonding (threading).

Using an aluminum/gold wire bonder to bridge the wafer (LED die or IC chip) to the corresponding pad aluminum wire on the PCB, ie the inner lead of the COB.

8.Pretest.

Use dedicated inspection tools (different equipment for different uses of COB, simple high-precision regulated power supply) to detect the COB board and rework the failed board.

9.Dispensing.

The dispensing machine is used to place a good amount of AB rubber to the well-established LED die. The IC is encapsulated with vinyl and then packaged according to customer requirements.

10.Curing

Put the sealed PCB printed circuit board into a thermocycling oven and keep it at a constant temperature. Different drying time can be set according to the requirements.

11.After the test.

The packaged PCBs are then tested for electrical performance using dedicated inspection tools to distinguish between good and bad.

What kind of PCB surface fit for COB wire bonding?

Thick gold plating can provide excellent bonding performance, but due to the high cost of plating thick gold and poor soldering performance, many projects are hindered. In the deposition reaction of nickel-palladium-gold ENEPIG in the replacement of gold, the electroless palladium layer protects the nickel layer from excessive corrosion by the replacement gold. Excellent resistance to storage time, solder joint reliability, good wire bonding ability, low cost, etc. Gradually replaced electroplating thick gold process. Ordinary gold plating and gold layers are required to be very thick, basically 0.3 microns or more, to meet the requirements of bonding. The ENEPIG PCB can be satisfied with only 0.1 micron palladium and 0.1 micron gold.

The ENEPIG PCB process is similar to ENIG process in that a chemical palladium bath (reduced palladium) is added between chemical nickel and chemical gold. ENEPIG process: Degreasing-àMicro-etching-à Pickling-àPre-immersion-àActivated palladium-àChemical nickel (reduction)-àChemical palladium (reduction)-àChemical gold (substitution). Chemical Immersion Gold currently has many problems with black nickel, as well as diffusion after heating. The addition of a dense layer of palladium in the middle can effectively prevent the diffusion to black nickel and nickel.

Comparatively multiple carrier board used on the BGA carrier ,One side is the bonding wire, and the other side is the need for soldering. Both of the two side gold plating thickness requirements are not the same. Bonding requires a thick gold layer, approximately 0.3 microns or more, The solder only needs about 0.05 microns. The gold layer is thick and good but the solder strength is problematic, thin gold layer solder is OK but wire bonding is not OK. Therefore, the previous processes are covered with dry film, and two different gold plating specifications are required to meet the requirements. Now with the same thickness specifications on both sides of ENEPIG, it can meet bonding and meet the requirements of solder. At present, the palladium and gold film thicknesses of about 0.08 μm or more are sufficient to meet the bonding and soldering requirements.