COB Process (Chip On Board)

Hi,

This is a widely discussed subject and since you have asked a brief explanation, I assume, you are a business man or a top executive in the company venturing in to COB.

I waited to see if someone replies to you. May be it is a very old subject that no one wants to repeat. Let me try to give you some information. MANILY BASIC.

Chip on Board (COB) is a process of establishing electrical connections between the IC chip and the printed circuit board.

Die represents the IC chip.

The following are needed:

Printed circuit board, Silver paste, Oven, Dicing saw, Die bonder machine, wire bonder machine,Gold or aluminum wire, Black epoxy, epoxy dispenser,weighing machine, Refrigerator,bond pull tester, ultrasonic cleaner, class 10000 clean room,vaccum line, trays or magazines for transporting the PCBs.

The IC chips can be bought in either wafer / tray form.

If you buy it wafer form then, you require the die bonder machine. This is a fully automatic machine which will pick the IC chip from the wafer and then place it on the Printed circuit board. This is for very high volumes.

If your production quantity is less (3 million) then you can try buying the chips in tray form. The chip manufacturer will place the chips in stackable small size trays and supply. You need to have a vaccum pickup tool to pick the chip and place in printed circuit board.

Silver paste is first dispensed on the PCB and then the IC chip is placed. This is then oven cured so that the chip does not move during wire bonding. A small test should be conducted to check whether the chip is rigid in place. Some chips require this silver paste (electrically conductive) and some other require electrically non conductive paste also. You need to check with your IC chip vendor.

There are many mfrs for dicing saw, wirebonder, die bonder. So you can check yourselves. (K&S and ASM Assembly automation).

There are two types of wire bonders. Gold wire bonders and Ultrasonic aluminum wire bonders. If the loop height required is very less, then you can go in for aluminum wire bonder.

The wire bonder if fully programmable.

The bond pull tester is required to test the bonding strength of the wire to the IC chisp and the PCBs.

After bonding you may need to use the electrical testing jig to check the electrical continuity.

After that test, you need to enclose the wirebonded chip with black epoxy which helps in restricting the effects of atomosphere to the chip. Epoxy dispensers are used here.

After epoxy dispensing, it is oven cured.

There are mfrs who can automate all these operations in one single machine.

Ultrasonic cleaner is either used on the bare PCb or after epoxy is cured.

Weighing machine is used to mix the epoxies (Part A and Part B) in proper propositions. Refrigerator is required to store the epoxy.

You need to check with your PCB supplier whether he gives you bondable surface.

Should you need any assistance in selecting some machines, equipments, epoxies, process, pls. feel free to write. I may be contacted at Ramanandkini@titan.co.in I have some friends who work as consultants and they could suggest you some economically better options.

Regards V.RAMANAND KINI

reply »

After all, you may want to consider a Flip Chip process. The chips are bumped with tiny solder balls while still on the wafer. You may present the bumped chips in Gel Pack, trays or Surftape to your Pick $ Place machine ( if capable for Flip Chip placement ). The chips are usually dip fluxed before placement and then reflow soldered, together with the other surface mount components. After test, the chips are encapsulated with black epoxy.

reply »