There's a Flip Chip in Your Pocket

In the UK last year, more than 73,000 dogs were picked up as strays. Fortunately, more than 50% of them were eventually returned to their owners. In the United States, less than 15% of stray pets were reunited with their owners. The difference is not one of compassion, but of technology.

In Europe, along with vaccinations, embedded radio frequency identification (RFID) chips are required for all cats and dogs. Once this rice grain-sized transceiver assembly is embedded under the scruff of the neck, there is no lost collar or tags to worry about ever again. Similar embedded RFID modules are replacing bar code ear tags for livestock tracking.

If you drive a Ford less than 8 years old, you use an RFID tag every time you start your vehicle. Embedded in the key itself is a small chip and printed antenna with an encrypted ID number. A sensor in the steering column interrogates the key and reads the 128 bit code, and the on-board computer allows the engine to start if it is one of the approved numbers.

RFID tags are currently employed for tracking and identifying luggage, autos, shipping containers, express mail, trucks, personal access, automated toll collection and library books, to name a few applications. Unlike bar codes, an RFID tag can be read while the part is moving and, in some cases, at a distance of up to 5 ft. An entire pallet of shipping boxes can be read while still on the forklift if each box has an embedded RFID tag.

According to a 1999 Venture Development Corp. study, "Global Markets and Applications for Radio Frequency Identification Equipment and Systems," the market for RFID systems will be about $1.6B in 2002 and is expected to grow 25% annually for the next five years. They predict that end-to-end tracking of materials used in the complete supply chain in all manufacturing industries may be the next killer application for RFID tags.

	Typical RFID tags with epoxy-attached flip-chip connections. (Source: Texas Instruments)

An RFID tag consists of a silicon chip, typically 1-3 mm on a side, attached to a printed or etched antenna on a flexible substrate. The chip costs are in the 25-50 cent range, depending on features such as number of bits stored and whether it is read only or read/write. The assembled cost for an RFID tag can be as low as 60-90 cents.

Since the chip typically operates at <10 µW of power, it can be powered directly from the energy picked up by the antenna. Four frequency ranges are in use: 134 KHz, 13.56 MHz, 900 MHz and 2.5 GHz. The antennas for the low-frequency products are typically wire wound around an air or ferrite core. The modules used for pet and livestock tracking are all of this configuration.

For the highest-volume applications, at the 13.56 MHz range, the antenna is printed or etched. The substrate is typically polyethylene, polyester, Kapton or Mylar, with screen-printed silver ink, etched copper or etched aluminum as the conductor.

There are only three connections to the chip, related to the antenna feeds and read/write enabling. The electrical connection is typically done by wire bond or flip chip with conductive epoxy.

According to Karp, Checkpoint Systems uses a polyester film to support an antenna etched from aluminum foil. The die is epoxied to the foil and then wire bonded to the aluminum antenna leads and encapsulated.

Texas Instruments (Dallas) has shipped more than 50 million RFID tags used specifically in vehicle immobilizer, or "smart key" applications. According to Tony Sabetti, rf systems global business manager, they use a flex substrate with a print and etched copper antenna. Isotropic conductive epoxy bumps are tape transferred to the wafer. An insulating epoxy is also applied to the wafer. Each die is flip-chip mounted to the antenna leads. Electrical connection and underfill is performed at the same time.

In the near future, this simple technology will play a greater and greater role in tracking the location of all high-value assets, not just our pets.

For additional information on assembly and packaging, go to www.semiconductor.net/assembly.