Section: DESIGN

Bendable circuitry stretches its reach

Flexible (flex) circuitry is beginning to be used in areas beyond where it's needed to bend, in such applications as a substrate for high-density chips that go into ball-grid-array packages. Observers are predicting the volumes achieved here will help make it possible for flex to have increasing usage as products get smaller and lighter.

Flexible circuit boards have been the neglected stepchild of the packaging world-used only in areas that need flexibility or in compact products where there isn't room for a rigid board. But speakers at the Flexcon '97 conference in Sunnyvale, Calif., last week say flex is getting a boost from the rise of densities and the more common use of portable products.

Flex, typically made from polyimide materials, offers higher density and lighter weight. It can also improve performance by allowing designers to put chips closer together. Flex has fine enough lines to work with flip chip and chip-scale packages, so it is viewed as an enabling technology for those who want to implement these advanced array packages.

"The only way you can reduce cost is to go to flex circuits," said Subash Khadpe, president of the Semiconductor Technology Center (Neffs, Pa.), which sponsored the conference. "You can't drill small holes in rigid boards inexpensively. Flex also has much higher density. You can do in one layer what takes two with rigid boards. You can do 1 mil lines and spaces with flex."

It may take flex marketers some time to convince designers that the pricing might be lower for high-density applications. That's because flex materials are quite a bit more expensive than rigid products. Designers will have to look at the tradeoffs between the increased density of flex and the lower pricing of rigid boards.

"Flex costs around $10 per square foot per layer, and rigid boards cost $2 per square foot per layer," said Jack Balde, president of Interconnection Decision Consulting (Flemington, N.J.). "The move from older, cheaper technologies to newer technologies where assembly costs are cheaper is always driven by density."

Portable applications are driving flex usage in the United States, but semiconductor packagers are starting to turn to the technology as BGAs become popular for high density ICs. IBM has offered flex-based packages, typically called tape BGAs, for some time. But the market is only now starting to move forward, driven by rising I/O counts for advanced semiconductors.

"Chip attached to tape gives us much greater density than plastic BGAs, which have the highest volume," said Kim Blackwell, strategic product marketing manager at IBM Microelectronics (Endicott, N.Y.). "Tape will be used with high I/O chips. If you go to flip-chip in the package, the nice thing about tape is that the polyimide transfers the stress from thermal mismatches, absorbing much of the stress and improving reliability. ASIC guys are the ones who are using tape BGAs, they need the fine pitches it can offer."

As the market for high-density BGAs increases, vendors are finding other benefits and applications. "We have tape BGAs in development and should begin shipping them in the first quarter of 1998," said Marcos Karnezos, vice president of technology at Signetics High Technology Inc. (San Jose, Calif.), a packaging subcontractor. "We see tape used primarily in graphic chip sets and with DSP and microprocessors. It provides high density and high power at low cost, and the cost gets our attention."

Khadpe added that the low profile of flex gives it an edge over laminates in IC packages. "The thinness comes into play in big BGAs. Flex is so thin that you can put a heat sink on the package and it's no taller than a BGA with a laminate substrate," he said.

Other high-density packages are also boosting the market for flexible substrates, while some markets are increasing their use of flex.

"Chip-scale packages are made primarily with flex," said Ken Gilleo, head of Alpha Metals Electronic Polymers Group (Cranston, R.I.). "Tape automated bonding is a specialized flexible circuit that was recently renamed the tape-carrier package by Intel. Beyond that, automotive manufacturers are using it in the dashboard, in anti-lock braking systems, in engine control units. Medical and health products are using flex because it is conformable, thin and in many cases because it's lightweight."

Wanted: U.S. sources

One thing that's still slow in coming is a U.S. source for flexible substrates. After a longer-than-expected development time, Sheldahl is starting to ship substrates in volumes. Sheldahl, which built a new plant and developed advanced technology with the aid of a major consortium, expected to be at this stage about a year ago, but it is now moving quickly to make the most of the opportunity the growing market presents.

"We are now in the production mode, and we have the capacity to do 1,500 square feet per day of substrates that have 1 mil visa, 5-7 mil capture pads and 2-mil line and space features," said Keith Casein, vice president of Sheldahl Micro Products (Longmont, Colo.). "We will have 1-mil lines and spaces within the next few months."

Disk-drive designers are starting to use flex circuitry to replace wires that run from the head to the read-channel circuitry (see Oct. 13, page 49). Seagate Technology (Scotts Valley, Calif.) is among the first to use the technique.

Chip makers also say that the trend may grow quickly. They are involved because drive designers want to mount pre-amp devices on the flex circuit using flip-chip technology. The goal of this move is to get the pre amp closer to the head to improve performance.

That's a key attraction for some potential users. Japanese vendors are putting chip-scale packages on flex circuitry, and several papers at the Flexcon conference explored various facets of this type attachment. Flex circuits have high enough density to permit routing to the tightly packed contacts on these small IC packages.

Researchers are looking to expand this technique to passive components, which comprise a fair number of the components in portable communications products. Researchers at Rensselaer Polytechnic Institute (Troy, N.Y.), working under a Darpa contract, are exploring the potential for attaching thin-film capacitors on polyimide film. This work is proceeding at a fairly brisk pace, researchers said.

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By Terry Costlow