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.
Most of the devices we saw in the bonding wire assembly room were for flash memories, more specifically, USB flash memories. It makes sense since the memories are huge and dense while the form factor is simultaneously decreasing. In the pictures you below, you can see how big the flash memory die are compared to a standard logic die. Since they deal with so much flash memory, they also let us see their bare die flash memory binning setup. This is the process where they sort the bare flash memory die by the number of good blocks the memory has. If a memory die has 34 GB of good blocks, it would get binned in the 32 GB category. If it has 20 GB of good blocks, it would get binned in the 16 GB category and so on. The binning process is amazing. They use test jigs with tiny needle like points that are aligned to the bonding pads on the flash memory die, ie: they're super duper tiny. You can only align them and see them on a microscope. Once the probes make contact, a software program will run them through test software to count good blocks and mark bad blocks.
We spent quite a bit of time checking out the whole bare die attach operation and I was suddenly thinking of the possibilities of taking AVR bare die and repackaging them into pastel colored QFP packages. Hmmm...it's something that I would seriously consider if I had a bit of money to burn. Unfortunately, that has rarely happened in my life.
Next up, we were taken downstairs to a different area where they do silkscreening and painting. They seem to do a lot of USB memories so there were all types of USB flash memory enclosures being painted. They also had a pad printer set up to do lettering on the curved enclosures as well as multiple silkscreening machines for lettering on flat surfaces. I was a bit surprised when I saw that they also had a full plastic injection area. It seems like everyone has plastic injection molding capability in Shenzhen. They had quite a nice tooling shop with CNCs, mills, and EDM machines. They also had a about eight injection molding machines in a line.
I was talking to the owner of the factory and he said that if we ever needed to do a USB flash memory device, we can just talk to them and they can handle all aspects including getting the memory die. For other products, we need to get the bare die ourselves, but they can attach them for us at approximately $0.01 per pad. Of course, like anything, this is negotiable. The surprising thing is that he told us he wouldn't hold us to a minimum order quantity. So if we get a few bare die, then its possible to do a custom bare die PCB attached design. It was quite interesting and I'd like to try a bare die design in the future just to go through the process.
As everything else I've seen, each factory I go to seems to expand my design options a little bit. It's really amazing.