Section: DESIGN: COMPONENTS

San Jose, Calif. -- The winds of change are blowing through circuit-board production sites as OEMs ask for thinner, lighter boards. Techniques aimed at meeting those demands were demonstrated at the IPC Printed Circuits Expo here earlier this month.

Board producers are moving new technologies into production in order to meet the requirements of portable equipment. The supply of flexible circuitry is increasing, and thin-film technologies that extend the lifetime of FR-4 are also being launched.

Eventually, conventional FR-4 won't handle the necessary line widths, but that doesn't mean the technology is facing death row. IBM (Endicott, N.Y.) is moving forward with surface-laminar circuits (SLC), in which high-density thin,film areas are screened onto laminate boards. That allows IBM boards to allot high-density sections for flip-chip, chip-scale packages and other high-I/O packages, while maintaining cheaper laminate substrates for the bulk of the board.

IBM currently produces the technology at three plants, and Amp-Akzo (Chadds Ford, Pa.) is gearing up production capabilities acquired under a licensing agreement. To underscore both the viability and capability of SLC, IBM described the technology's application in all versions of its Thinkpad notebook computer. SLC lets designers reduce the board layer count from 10 to six while reducing by 40 percent the number of costly plated through holes. Trimming the layer count also provides a slight reduction in height and weight.

Reducing thickness and weight is among the goals of flexible circuits, which are far thinner than their rigid counterparts.

Litchfield Precision Components Inc. (Litchfield, Minn.) contends that fine-line flex circuits are on the verge of moving beyond their niche markets into more mainstream applications. Flex circuits are becoming common in printers, medical equipment and liquid-crystal displays, among other applications. One obvious reason is their thinness and flexibility.

Fine-line circuits

But beyond that is the fine-line capability of many flex-circuit providers. Most can produce circuits with 75-p line widths, which is ahead of the schedule put forth in the IPC technology road map last year. The road map maintains that 75-mu lines on flex will be mainstream in three years, while 50 mu will be common at the turn of the century. There is already a fair number of leading-edge companies that can provide 50-mu line widths, though that technical level is far from mainstream, according to Litchfield.

Below 50 mu, it's no longer feasible to manually check for quality. However, technologies such as chip-scale packaging will require line width at that level, which will put pressure on companies both technically and economically. However, solid growth rates for flex circuits, particularly fine-line flex circuits, should make the necessary investment worthwhile, Litchfield predicts.

Prismark Partners (Cold Spring Harbor, N.Y.) noted that pricing and density will continue to be intertwined, particularly in small-form-factor products. The research firm examined three applications using ceramic substrates, various multi-chip-module (MCM) substrates and small circuit boards. Ceramics and MCMs have largely the same density and costs, which isn't surprising since MCMs use ceramics and thin-film or laminate substrates. Thin films offer higher densities, while laminates have lower densities and lower costs. Line widths for ceramics are 25 p, while the aggregate MCM capability is 50 p. Circuit boards achieve 75-p line widths.

Prismark also highlights the trade-offs engineers have to make to get the higher densities. Ceramics and MCMs cost $21 and $20 per square inch, while circuit boards cost $1/square inch. Target prices for the start of the next decade are much closer, with ceramics at $4/square inch and MCMs at $3.70. Circuit-board costs will drop to 50 cents/square inch, according to Prismark.

                                             Limited
Feature                 Typical  Available  availability  Future

Line width                 3[a]      3           2          1
Line spacing               5         3           3          2
Photo via diameter         4         4           3          2
Photo via pad             12        10           6          5

a All numbers in mils

SOURCE: IBM

~~~~~~~~

By TERRY COSTLOW