Section: DESIGN

Components

Sunnyvale, Calif -- Cellular phones, compact consumer electronics and other equipment seeking lighter, smaller, thinner and conformable interconnections are fueling growth in flexible circuits. The latest developments in flex circuits for advanced microelectronics will be highlighted at Flexcon'96, the third international conference on the subject, to be held Oct. 23-25 at the Sunnyvale Hilton.

As manufacturing activity surges around the world, flexible circuits are making cost and performance strides and gaining acceptance in myriad electronic products. Besides cell phones and consumer devices, those include personal computers and peripherals, flat-panel displays, office equipment and automotive electronics.

A paper by packaging-technology consultants TechSearch International (Austin, Texas) shows expanding markets and solid revenue growth. From under $2 billion last year, the flex-circuit market is projected to climb to more than $5 billion worldwide by 2000, according to TechSearch.

The paper identifies Japan as the major producer of flex circuits, with 60 percent share of the worldwide production in 1995. The United States and Europe together accounted for 30 percent of production in 1995, with such Asian countries as Korea, Taiwan and Singapore making up the remaining 10 percent, TechSearch said.

Cost has been a major impediment to wider market acceptance, an issue that suppliers continue to address even as they advance the technology. Chip-on-flex (COF) is one area seeing ongoing refinements to make solutions cost-effective, and such packages as chip-scale and ball-grid arrays are rapidly tapping those advances.

In a joint paper by Lockheed-Martin's Government Electronics Systems (Moorestown, N.J.) and General Electric's Corporate Research & Development Center (Schenectady, N.Y.), researchers describe a COF process that is being used to package multiple ICs on flexible circuits, as well as a single chip in a chip-scale-package (CSP) configuration. The process addresses many of the concerns that users have with conventional multichip modules (MCMs) and CSPs, said Ray A. Fillion, manager of aerospace packaging programs at the GE facility.

Developed specifically to address cost issues, the Lockheed-Martin/GE COF process is comparable in price to any technology, according to Fillion. He said the technology can support a range of chips, from low-pin-count devices to ASICs requiring 400 I/Os.

Lockheed Martin has used the COF technique to implement several flex-circuit solutions in aircraft engine-control applications. Working jointly under a contract awarded from the Advanced Research Projects Agency, GE and Lockheed-Mar-tin researchers are developing a family of building blocks using the COF process for control applications. The developers are also working with telecom companies and merchant fabricators to commercialize the technology.

The COF process involves direct mounting and interconnection of bare chips on a prepatterned flex circuit without the need for solder bumps, wire bonds or tape-automated bonding, according to the Lockheed-Martin/GE paper. The flex circuit provides all the chip-to-chip interconnection and interface to the I/O pins or pads of the module.

Laser-drilled via holes are formed through the flex circuit-which is a 1-mil-thick poly-imide--to the chip's I/O pads and to selected traces on the flex circuit. The vias and interconnect traces are then metallized with a barrier metal and electroplated copper. A screen-printed solder mask passivates the surface with openings for I/O solder pads. Next, solder balls are formed using solder paste.

A paper from Electro Scientific Industries (Portland, Ore.) will discuss the use of high-speed ultraviolet lasers in via formation for chip-on-flex and CSPs. The paper will describe the process parameters and techniques for forming vias down to 1 mil in diameter and will present an application of those vias for routing flip-chip on flex at 10-mil pitch and CSP on printed-wiring board at 20-mil pitch.

Like CSPs, ball-grid arrays (BGAs) are exploiting the benefits of flexible circuits. Papers from Amkor/Anam and a team of CTS, Texas Instruments and Sheldahl will shed light on BGA packages based on flex circuitry.

Amkor Electronics (Chandler, Ariz.) has developed a high-density BGA package that combines the attributes of flex-circuit substrates with wire bonding and an overmolded body. The result, the fleXBGA, is said to offer 1-mm ball pitch with improved thermal performance and moisture sensitivity. Amkor promises to present both analytical and experimental results obtained on the package jointly with Anam Industrial Co. (Seoul, South Korea).

Similarly, the joint paper by CTS Corp., Texas Instruments Inc. and Sheldahl will describe the development of a tape-BGA package that leverages advances in adhesiveless flex circuitry and low-temperature sinterable materials.

To produce its tape BGA, CTS engineers colaminate Sheldahl's three pairs of poly-imide/copper-based flex circuits with bond-ply layers and a metal stiffener to form a six-metal-lay-er substrate. TI's high-pin-count ASIC is attached to the substrate as a flip-chip to attain a 7514/O tape-BGA package.

In another paper from CTS, the company discusses the use of its colaminated multilayer flex circuit in magnetoresitive disk-drive applications. Here, the CTS engineers report preliminary results for a colaminated flex-circuit solution that works as a drop-in replacement for cofired ceramic design.

For flip-chip on flex, special aluminum bond-pad metallization and wafer-bumping processes are needed to provide wafers with solder balls. But many semiconductor suppliers are not ready for that service.

To eliminate wafer bumping, researchers at Dresden University of Technology's Institute of Semiconductor Technology and Microsystems have developed a direct metallization process that connects Al bond pads to the flex circuit's interconnection traces.

The process will be described by researcher Mathias Bottcher.

DIAGRAM: CHIP-SCALE PACKAGES IS FULLY ENCAPSULATED -- SOURCE: GE CORPORATE R&D

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By ASHOK BINDRA