Plastic fantastic: conductive epoxies boldly go where no solder has gone before.

With the advent of lead-free legislation in Europe and Japan and the ensuing struggle to find materials that do not require an excessive thermal excursion, thoughts occasionally turn to conductive epoxies. One frequently asked question regards the viability of conductive epoxies as a substitute for lead-bearing solders.

Conductive adhesives command industry attention as a means of attaching surface-mount components to a substrate. The most familiar conductive epoxy is essentially a polymer adhesive material with metal fillers that provide the conductivity of the material. The most common filler is silver, but copper, gold, graphite and other materials have been used. In surface-mount assembly, the epoxy is either stenciled or syringe deposited on the pads like solder paste. After components are placed into the material, a thermal cure occurs. The cure schedule varies with material properties but is typically between 80[degrees] to 100[degrees] C. Some conductive epoxies take 30 to 90 minutes to cure, but snap-cure adhesives can cure in a few minutes. Some conductive epoxies are UV curable, which is also a quick process.

The Pros and Cons 

Conductive epoxies provide good tensile strength, particularly when the application resides at extreme temperatures. The electronic connections on the Lunar Rover, NASA's moon-dune buggy, were joined with a gold-filled conductive epoxy. The thermal excursions required to cure conductive epoxies, even the snap-cure, are a fraction of those required for Sn63/Pb37, not to mention the reflow temperature of around 235[degrees]C required for the tin-silver-copper lead-free alloy. Since conductive epoxies are lead free, they are also no-clean materials with no flux residue to clean.

Conductive epoxies also have their downside. While they are great at steady-state extreme temperatures, they do not do well with a certain degree of thermal cycling; tin-lead and tin-silver-copper solders fare much better. Conductive epoxies are also relatively expensive. Some silver-filled formulations sell for 10 times per gram what tin-lead solder paste costs, so you can imagine the cost of gold-filled epoxies. Also, the properties of the formulation, such as resistance and conductivity, vary from batch to batch due to slight differences in the volume of each of the component materials.

Although conductive epoxies are not a panacea replacement for lead solders, they are worth knowing about for use in the many situations when materials cannot be soldered. A number of years ago, I had an application that required attaching a piezo-electric shock sensor to a disk drive board. The shock sensor could not endure process temperatures in excess of 100[degrees]C, but even the low-temperature alloys required reflow temperatures above 100[degrees]C. The answer to our problems was conductive epoxy. We considered both sides of the assembly with Sn63/Pb37, applied conductive epoxy to the pads for the shock sensor, mounted the component and cured the assembly in a box oven at 80[degrees]C for about 30 minutes.

Years ago, I was doing a seminar on emerging technologies for several Surface Mount Technology Association (SMTA) chapters. As part of the seminar, I discussed conductive epoxies. At the Chicago chapter, I heard many "oohs" and "aahs" concerning the fascinating material. At the Detroit chapter the next night, the part about conductive epoxies was nothing new of interesting; conductive epoxies have been used in automotive applications for many years.

Anisotropic Conductive Epoxies

Interest in anisotropic adhesives, or z-axis conductive epoxies, has recently grown. These adhesives ate designed to conduct electricity in the z-axis only by filling the polymer base with a low concentration of large conductive particles that do not come into contact with one another. The particles are spherical to facilitate uniform contact between the planes of the conductive surfaces of interest (the component lead and printed circuit board pad). Most anisotropic adhesives are available in a film format so that the particle concentration and distribution is consistently regulated.

If you examine a cross section in the x- and y- plane, you observe no consecutive path by the particles in either the x- or y- axis. No electricity is conducted because no continuous path allows for it. However, the electrical path on the z-axis can be completed between the lead and pad.

Electrical continuity usually requires pressure throughout the cure cycle to ensure that the filler particle is in contact with both conductive surfaces. Thus, the tolerance on the dimensions of the filler particles must be strictly specified and controlled. The flatness specification of the surfaces must also have little deviation.

Early anisotropic adhesives used a thermoplastic epoxy and solid metal conductive filler material. Later, they used thermoset plastics with a number of fillers, including tin-lead solder spheres. A number of current z-axis epoxies have gone back to a thermoplastic base because they require about one-tenth the amount of pressure as thermoset materials.

Later generation anisotropic adhesives ate filled with spheres of metal-plated elastomer plastic typically made from nickel-plated styrene to offer greater resilience. Their density is closer to 1, preventing the settling of the filler, which is crucial to the electrical performance of the adhesive. with the advent of tape carrier packages (TCP) and tape automated bonding (TAB), z-axis epoxies have seen widespread use. Casio has used the materials to attach polyester TABs onto polyester circuits in their calculators and electronic organizers since 1987. Typically, localized thermo-compression bonding is applied, making the substitute from tin-lead solder to anisotropic adhesives a simple and easy procedure.

Flip-chip attachment is possible using the film format for the initial attachment and for repair. The need for precision printing is precluded. Z-axis epoxies are particularly attractive for very-fine and ultra-fine pitch component attachment applications involving low current. The materials can also be used for smart card assembly, which requires fast processing. Remember, the material often also acts as an underfill, cushioning the effects of thermal coefficient of expansion (TCE) differentials.

Conclusion

Conductive epoxies were developed back in the 1950s but did not come into use until the 1980s. Nevertheless, we have another industry example that everything old is new again. Remember, we're all in this together.

Phil Zarrow is president and surface-mount process consultant for ITM Consulting, Durham, NH; [phone]; www.ITMconsulting.org; [e-mail].