How Chip On Board (COB) Circuits are Assembled
If you like to open electronic devices when they get damaged, either to try to fix it, or to use it for spare electronic parts or just to take a look inside, out of curiosity, to see how it is built, you might see in some devices, a black chip which seems to be part of the circuit board. Some call it the “black dot”, some others call it “epoxy chip” and there are several other names, but the name is “Chip On Board” or COB.
These cob chips are in most cases a Micro Controller Units (MCU), but it can also be any other complex circuit, either digital, analog or a combination of both. You can find it in electronic toys or electronic tools, like multimeters. You may find it anywhere, where a complex circuit is needed, but space is limited.
But my question was always how are they made? How are they assembled within the circuit? I could possibly imagine the process, but nothing like looking at what is exactly done.
Well, I found an article in the sparkfun.com website about someone who was given permission to take pictures and video from the “Victor” factory in China. This factory produces among other things, electronic test equipment, like multi-meters.
The first picture you will see is the assembly line, where workers are installing some parts in the circuit boards. I’m not sure if this is done before or after installing the COB chip in the circuit.
In the picture below you will see a container where the chips to be used as COB are received in the factory. As you can see, there are 100 chips in a single container. These are outsourced from another Company with the specifications that Victory requests. They do not manufacture the chip, but it is their own design.
The chips are completely flat, as you can see in the picture above. There are no leads coming out of it. So how does this connect to the rest of the circuit in the circuit board? They glue the chip in place on the circuit board first and when the glue sets, they use a machine to spot weld each one of the leads from the chip surface, down to the circuit board contacts.
Below you can see a circuit boards with the COB chip already glued to it.
Spot welding (or bonding) can be seen in the two videos below. As they explained, it seems to be that it is not a conventional spot welding process nor soldering, but a bonding process, done by using different types of energy combined, like sound, heat and mechanical pressure.
Below is the COB chip already glued and with its leads bonded to the circuit board.
The final step is to put “The Black Dot”! They manually cover the COB chip with some substance similar to epoxy resin, to protect the chip and its leads. I have worked with epoxy resin and I have cracked open some of those chips and I can tell that what they use is something similar to epoxy, but definitively harder.
You can access the original article on Spakfun.com by clicking on the link below: