More from Akiba on Bunnie’s MIT Media Lab tour of Shenzen manufacturing. On this leg of the trip, they visited a factory that does Chip-on-Board die attachment. He writes:
Today we went on one of the most interesting tours of this trip. It’s something that I’ve always been interested in but didn’t really know how to approach. The tour was of a chip-on-board bare die bonding assembly house. For those that don’t know, one interesting technique used for very low cost, high volume products is bare die bonding. In this process, the bare die is used rather than a die packaged in a lead frame and epoxy resin. This has two benefits. The first is that the form factor is decreased since only the bare die is used. The second benefit is that its possible to save cost since packaging materials usually add cost to a chip.
There are headaches with doing a bare die process. You’ll have to negotiate with a vendor to purchase bare die rather than packaged die and you’ll also usually have a rather high minimum order quantity. The minimum order quantity can vary depending on whether the manufacturer is set up to do bare die sales, but the general rule of thumb I’ve heard is that the MOQ would be one wafer, which for something like a simple ARM or AVR microcontroller would be in the thousands.
In terms of form factor, bare die is an excellent method to cut down on size, especially if the manufacturer only offers large packages like QFPs or even through hole packages. Actually, the main reason we went to this assembly house is because earlier in the trip, Jie was explaining her project to Bunnie and mentioned that she wanted to figure out ways to decrease size and attach chips to paper for papercraft designs. At the assembly house, I was looking closely at the bare die and comparing the form factor to a packaged die. I had always thought bare die would shrink things by at least half or more. After seeing it live, I could see that although the die was smaller, the bonding pads on the PCB also require room. If you count for the bare die and the PCB bonding pads, the size is comparable to a QFN. Hence if a chip is available in a QFN package, it’s possible to assume that there might not be much size savings by going bare die. Of course this needs to be taken with a grain of salt, since some chips are put into much larger packages than the actual die, especially if a lot of pins are needed.
It was really great to see this whole process in real life though. It started out by going up to the room that does the bare die attach. The interesting thing is that since everything is done on such a small scale, all of the equipment fits into a small room. It starts with an operator hand placing bare die on to a panelized PCB substrate with a bit of glue. The finished panels are handed off to another operator that readies it for the bonding wire attach machine. The bonding wire attachment is fully automated. After the attachment, the panels are checked and another operator will repair any errors made by the machine. The amazing thing is that the repairs are done by hand using a pair of tweezers. Amazing! Imagine manipulating thread about half the thickness of human hair and trying to insert it into a single pore. After the bonding wire attachment process is finished, the die gets a blob of epoxy resin stenciled on to it which provides mechanical strength and protects the device and fragile bonding wires.