Human inspectors armed with microscopes used to check the die bonds coming out of IBM's Burlington, Vt., facility. Trouble was, it was practical to scrutinize only a few of the 32 lead frames in every die-bonding machine cartridge to see if the die-to-lead frame alignment was out of whack. This sort of cursory sampling was taking a toll on quality. As many as 100 bad devices daily were getting through the screen.
The solution came in the form of a vision system mated onto the 20year-old bonder. "Various aspects of our bonding process are not industry-standard. With the reduced cost of vision processors, it was more plausible to retrofit the old machine with vision rather than buying a new one," says Richard Charlton, an IBM staff engineer.
The fab's ESEC 2005 die bonder got equipped with an MVS-8100 machine vision system from Cognex Corp., Natick, Mass. This PCI frame grabber-based system plugs in to a 233-MHz Pentium industrial PC.
The camera mounts a few inches above the die surface. As a newly bonded device passes under it, a PLC signals the MVS-8100 frame grabber to capture an image. Object location software called PatMax then analyzes the captured image, first looking at the distinct geometric features, then comparing what it sees to a reference image in PC memory. If the die is aligned to the lead fingers within the X-Y tolerance (typically 100 microns at most), the device moves to the wire-bonding stage. Devices falling outside tolerance generate a stop signal to the die bonder. A message on the PC monitor alerts the operator and indicates (in microns) how far the die-to-lead frame alignment is out of kilter. Operators then put the device into another machine that prints out correction values for making the necessary adjustments.
In addition, "If the device is about 75% out of tolerance, the system can temporarily stop the machine and tell the operator that something is wrong," says Charlton. "At that point an operator can take some type of action. This feedback lets us prevent errors before a single device is lost."
Finally, the system can tolerate significant changes in die appearance and optical scale, which can arise during the process. "The image contrast may be high or low depending on how light hits the die surface and the thickness and uniformity of the overcoat used. Reflections off the die can also affect the appearance of the bond pads which the vision system uses as reference points for measurements," explains Charlton. "If a die breaks and leaves debris behind on the heater block, the next die on the block may sit higher, appearing larger to the vision system."
Insensitivity to optical scale lets IBM engineers train a reference image on one system and download it to other machines throughout the fab. "Since the vision system can handle optical variations, we can just train once and use the image on other machines without having to worry about their individual optical settings," says Charlton.