Breaking the solder barrier

01/01/2001

BY MICHAEL HODGIN

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As the electronics manufacturing industry moves into a new century, it finds itself searching to create a more environmentally friendly manufacturing environment. Since 1987 with the adoption of the Montreal Protocol (an international treaty that protects the ozone layer from various substances, such as aerosols, refrigeration products and solvents), there has been a heightened concern for the environment and the industries and practices that negatively impact it. Today, that concern has expanded to include a global interest in eliminating lead from electronics manufacturing.

Lead solder joining has been the dominant method of interconnection for the electronics industry since the birth of the printed circuit board (PCB). Currently, legislation is being implemented in Japan, Europe and North America to reduce the use of lead in manufacturing. This movement, coupled with the drive in the electronics and semiconductor industries toward further miniaturization with increased functionality, has led manufacturers to seek alternatives to traditional solder processes.

The New Industrial Revolution

It is an interesting time of changing technology and industry practices. Solder joining has shown to be a reliable and effective process for the interconnection of electronics for more than 50 years. The challenge, however, has been to develop new materials that emulate the positive properties of solder, such as its thermal and electrical characteristics, as well as mechanical joint strength; at the same time, the industry seeks to eliminate less-desirable factors, such as solvent cleaning and solvent out-gassing. In the past 20 years, manufacturers of adhesives have made headway in breaking through the solder barrier that I believe is worth considering in today's marketplace.

It's All About Chemistry

Technology breakthroughs in chemistry and particle morphology have led to the development of new solder replacement materials. During the past 20 years, adhesive manufacturers have developed electrically conductive adhesives (ECAs) that are lead-free, require no chlorinated solvents for cleaning and are electrically conductive. They also cure at temperatures less than 150°C (compared to the 220°C temperature required for solder reflow joining), which makes them ideal for securing temperature-sensitive components (such as semiconductor chips) and for use with low-temperature substrates and housings (such as plastics). Such properties and manufacturing uses have enabled their acceptance into specific areas of first-level interconnection, including hybrid microelectronics, hermetic packaging, sensor technology and in bare die, direct-chip attachments to flex circuits.

Hybrids, Hermetic Packages and Sensor Technology: Epoxies are widely used in hybrid microelectronics and hermetic packages primarily because these systems have a box package that surrounds the electronics. Such packaging protects the electronics and prevents damage to the components and joining materials. Solder, still traditionally used in second-level connections where damage because of handling is a concern, may not be necessary because the entire electronic package is sealed. Hybrid packages are used mostly in military electronics, but also extensively in the automobile industry for engine controls and timing mechanisms (under the hood) and for some sensors used in under-dashboard applications, like dual-climate control and air bag detonators. Sensor technology also uses conductive adhesives for the packaging of pressure transducers, and motion, light, sound and vibration detectors. Conductive adhesives have shown to be a reliable and effective means of interconnection in these applications.

Flex Circuits: Flex circuitry is another application area that uses conductive adhesives. The substrate materials for flex circuits, such as Mylar¹, require low-temperature processing. Conductive adhesives are ideal because of their low-temperature requirement. Flex circuits are used in consumer electronics, such as cellular phones, computers, keyboards, hard disk-drives, smart cards, office printers and also in medical electronics, such as hearing aids.

The Need for Space

Adhesive manufacturers are making progress in breaking the solder barrier because of space and packaging considerations, lower temperature processing, and decreased use of solvents and lead. As space becomes a premium in designing PCBs and electronic devices, the use of bare chips instead of packaged components becomes increasingly popular. Packaged components typically have pre-tinned connections so they can be solder joined to the PCB. By eliminating these components and using a bare semiconductor chip, space is reduced and epoxies can be substituted as the interconnection means. The temperature of the epoxy cure does not adversely affect the chip and this connection eliminates the use of lead. In situations in which product design can benefit by decreasing overall size (e.g., in hearing aids), eliminating solder joints from surface mount technology packages and replacing them with epoxy would help decrease overall dimensions. Understandably, by decreasing size, manufacturers can often gain a competitive edge that results in increased market share.

In developing electronic devices from inception, packaging and design engineers can take advantage of lower cost plastic components and substrates because conductive adhesives can be used for interconnection. Another emerging trend that eliminates lead is the further use of noble metals, such as gold, silver and palladium, as component electrodes; epoxies can be used as joining materials for these metals.

Additionally, PCBs and electronics manufactured with epoxies do not require solvent rinses or the disposal of solvents so this imparts significant cost savings.

The Future Looks Bright

Conductive adhesives have made significant strides in penetrating the solder market. As the electronics industry continues to grow and develop, I believe that ECAs will play a significant role in interconnect joining - especially as incentives for eliminating lead become more prevalent. And with the trend in electronics manufacturing to use miniature and system-on-chip designs, further investigation into the use of epoxies as first- and second-level interconnects will continue to be explored.

Reference

1. Mylar is a registered trademark of E.I. du Pont de Nemours and Company.

MICHAEL HODGIN, engineering manager, can be contacted at Epoxy Technology, 14 Fortune Drive, Billerica, MA 01821-3972; 978-667-3805; Fax: 978-663-9782; E-mail: [email protected].