Engineers who need very high performance or extremely small form factors may soon be able to find semiconductors in smaller form factors. Flip chip and unpackaged bare die are gaining acceptance after years of failing to meet marketing productions.
The two array packaging techniques eliminate wire bonds by putting solder bumps on the bottom of the device, lowering the part's height a bit while offering far more space than with perimeter wire bonding. Manufacturing techniques for both chips and boards are reaching critical mass, offering handling equipment and finer line widths.
Though the technologies have been limited to niches, the production advances are sparking an upturn in usage. "I expect solder bump devices, flip chip and bare die to see greater than 40 percent growth in unit volume from 2004 to 2005," says Jan Vardaman, TechSearch International of Austin, TX.
In some applications, a key factor driving the acceptance of flip chip is that the distance between the solder balls and the substrate is far shorter than the length of bond wires, shortening delay times. "Performance has now gotten to the point that it demands flip chip instead of wire bond," says Bill Stone, manager of advanced packaging development at Freescale Semiconductor Inc. of Austin, TX.
In addition, more power and ground lines can be placed on the underside of the package, providing more functionality than wire bonds, which are limited by the number of attachment pads that can be placed around the edges of the chip.
That makes flip chip attractive in large, high-speed parts such as the PowerPCs used in network processors. "High-performance DSPs are starting to use it, too," Stone says.
He notes that flip chip is a necessity when chips have more than 600 I/O while it's rarely used for devices with less than 300 contacts. "In between, it's kind of a gray area," Stone says.
Bare die, or known good die, offer even smaller package sizes and faster speeds by eliminating the housing used for flip chips. Many chips aren't available in unpackaged form, but many industries make extensive use of those that are available.
Great Expectations: Flip chip
devices from Freescale and others offer speed, more I/O
"It's used for high reliability by military and aerospace, and in wireless
products where form factor is critical," says Jackie Collard, marketing manager
of National Semiconductor's die products business unit in Portland, ME. He notes
that the auto industry, known for its reliability and low-cost requirements, has
used bare die in hybrids for years, partly as a way to get chips closer to the
unit they are controlling.
As in other fields, this is where the digital and analog worlds meet. "One of our key markets for bare die is analog technology," says Mark McClintick, a process engineer for National Semiconductor. National has made the majority of its line available in bare die form since 1994.
Power is another area where bare chips are broadly available. "We offer most of our parts as bare die," says Rick Pierson, manager of die sales support at International Rectifier of El Segundo, CA. Though most bare die are housed in modules with ceramic substrates or other advanced materials, there are chip-on-board applications. COB is used mainly when cost is a driver, McClintick says.