Electronic devices are everywhere. They’re the computing workhorses in our offices, factories, and cars; entertainment devices in our homes; and portable communicators in our pockets. The latest high-tech gadgets wouldn’t exist without electronics for diagnostics, data crunching, and control.
To drive further advances, today’s electronic circuits are being housed in ever-shrinking packages and engineered to withstand tough environmental conditions — from underwater labs to space stations, from sterile operating rooms to battered weather stations. And, of course, all while meeting rigid performance requirements and regulatory mandates.
Although proper design, installation, and use of the circuitry are no doubt critical to how a device performs, the assembly and packaging of the electronics are just as important — especially if they’re to work under stressful operating conditions. Increasingly, engineers are turning to electrically conductive adhesives over more-traditional solders to meet these design challenges.
New techniques, new challenges
As electronic circuits become more complex, engineers are finding new ways to assemble and package them. Advances such as flip-chip assemblies, system-in-a-package (SiP) designs (in which chips and other components are stacked on top of one another), and ultrafine-pitch electronics let engineers cram more computing power in smaller devices. This, in turn, presents a new set of challenges.
Shorter leads and interconnects increase the likelihood of thermal damage to temperature-sensitive components, because heat produced during solder processing is conducted quickly along the shortened paths between components. To avoid this damage, temperature-sensitive components must be manually assembled after soldering — increasing manufacturing time and cost.
Lead-free solders exacerbate the problem. They require higher processing temperatures and more-costly circuitboard materials than traditional solder alloys. Ultrafine-pitch technology — packing electronics into closer quarters — can make soldering more difficult and may produce unreliable results. And it increases the need for shielding sensitive components.
Environmental concerns pose another set of problems. Some printed-circuit assemblies must undergo postassembly cleaning with harsh agents that can break down solder bonds. Exposure to extreme temperatures can damage bonds between components with different coefficients of thermal expansion. And excessive shock or vibration may weaken the lead-free solder bonds, which are more brittle than traditional solder joints.
To address these challenges, engineers are turning to environmentally friendly adhesives that offer high bond strength, are electrically conductive, and meet other stringent performance requirements.
Electrically conductive adhesives usually consist of an epoxy or silicone resin filled with randomly distributed metal or conductive-carbon particles. When fully cured, the adhesive provides an electrical pathway between bonding substrates via particle-to-particle contacts. The degree of conductivity depends on the filler material and number and quality of particle-to-particle contacts.
For a given material, a higher proportion of filler particles results in greater conductivity. But it also weakens the bond because more filler means less adhesive. Conductivity may be reduced if the surfaces of the conductive particles contain contaminants — such as metal oxides (which form when metal molecules react with oxygen in air or water) or by-products from the filler manufacturing process. Such contaminants can impede current flow. Thus, both the choice of filler and precise control of the manufacturing process are important in determining the conductivity of these adhesives.
Engineers can compare the conductivity of electrically conductive adhesives by measuring volume resistivity, the electrical resistance of a defined volume of conductive material.
These adhesives are most often filled with one of four conductive materials: silver, silver-coated nickel, nickel, or graphite. Nonreactive metals such as gold and platinum offer superb conductivity with no surface oxides, but their costs are prohibitively high for practical use. Because silver offers excellent conductivity and has relatively conductive oxides, silver-filled adhesives have extremely low volume resistivity and often replace solder in electronics applications.
Economical nickel and graphite-filled adhesives have sufficiently low-volume resistivity for a variety of applications, such as EMI/RFI shielding, radio-frequency ID (RFID) tagging, and bonding and sealing electronic components. The choice of filler ultimately depends on conductivity requirements and budget considerations.
Electrically conductive adhesives can be engineered to combine bond strength and electrical conductivity with other service-critical properties. A wide range of standard, off-the-shelf products can meet the needs of many different jobs. (See the accompanying table for a look at some typical offerings.) Adhesive manufacturers can also tailor formulations to meet one or more specific physical requirements, such as elongation, shock absorption, moisture resistance, chemical resistance, or thermal cycling.
Also, within each adhesive family, thermal and chemical resistance often go hand in hand. Cross-linked adhesives like epoxies and silicones tend to have the best chemical resistance below their glass-transition temperatures (Tg). So grades with higher Tg can often beat the heat and tolerate more chemicals.
Formulations can also be designed to withstand cryogenic or ultrahigh temperatures, to bond specific substrates, or to meet industry standards. In addition, many electrically conductive adhesives are thermally conductive, letting them perform circuit-assembly tasks like bonding boards, providing electrical connections, and cooling components.
To accommodate various production requirements, most formulations offer room-temperature curing and rapid, elevated-temperature curing. Some adhesives can be snap cured at high temperatures within minutes.
Typical temperatures required for rapid adhesive curing range from 250 to 350°F. Cure times range from about 30 min to 2 hr, and most adhesives offer two or more curing schedules to suit different processing needs. Roomtemperature curing generally takes one to two days.
Adhesive curing temperatures are substantially lower than the 450°F minimum temperature required for leadfree soldering, and nearly all electrically conductive adhesives can be cured at temperatures well below the 361°F minimum temperature required for lead-bearing solder.
As mentioned above, engineers can request these products with specific electrical and thermal conductivity, viscosity, cure time, temperature performance, peel and shear strength, moisture and chemical resistance, outgassing, and many other properties. Electrically conductive adhesives can also be formulated to be RoHS compliant, solvent-free, and include grades that are halogen- free, meet NASA low-outgassing specifications, and USP Class VI biocompatibility requirements.
By selecting an electrically conductive adhesive with the appropriate mechanical, electrical, and thermal properties, engineers have a viable solution to the increasing challenges associated with assembling and packaging complex electronic circuitry. With dozens of electrically conductive adhesive grades and custom formulations from which to choose, design and process engineers can satisfy the unique physical, functional, environmental, and regulatory requirements of their target applications, all while staying within budget.