Technology Update
By Darryl J. Small and Peter Biocca
Conductive adhesives have traditionally been used as die-attach materials that bond integrated circuits to lead frames. They are also used to make laminates for printed circuits, to attach copper foil to boards or flexible substrates, and to bond circuits to heat sinks. As a result of lead-free initiatives, conductive adhesives are now becoming an attractive alternative to solder in many electronics applications.
A number of reasons are behind the effort to eliminate lead from these electronic products. Along with environmental pressures resulting from the element's toxicity, other motivations include hazardous waste disposal concerns, workplace safety considerations, device reliability issues, market competitiveness and environmental corporate image maintenance.
The pressures that electronics manufacturers in North America face are economic rather than regulatory in nature. To eliminate the risk that products will no longer be acceptable for export to Asian and European electronics markets, manufacturers are seeking viable solder substitutes, including lead-free solder materials and conductive adhesives.
Conductive adhesives are good for bonding temperature-sensitive components and for providing electrical connections on nonsolderable substrates like plastic and glass because they cure at room temperature or process quickly with minimal exposure to temperatures between 100° and 150°C. Available in highly flexible formulations, conductive adhesives are also a solution for applications such as assembly and repair of flexible circuits or bonding flexible substrates and connectors.
Lead-free solder and conductive adhesives both provide electrical interconnection and thermal transfer in electronic devices. But depending on the application, one bonding technology may deliver better performance or offer processing or cost advantages over the other.
By nature, solder forms metallurgical junctions between metal substrates, whereas conductive adhesives form mechanical and chemical bonds at the substrate surface. Metallurgical bonds are more conductive and generally stronger than bonds formed with conductive adhesives because adhesives require effective dispersion of the metal filler content to provide good electrical properties. But while filler particles are prone to oxidation, which may degrade adhesive conductivity over time, solder is prone to leaching metals such as gold or copper, which may weaken soldered joints. Conductive adhesives form a strong bond to tarnished and oxidized metal surfaces that are typically nonsolderable.
In the future, electronic designs will continue to use both conductive adhesives and solder. Once lead-free regulations go into affect, both adhesives and solder will deliver environmentally safer assembly options, individually and, at times, symbiotically.
DARRYL J. SMALL is Senior Applications Engineer, Loctite Corp., 1001 Trout Brook Crossing, Rocky Hill, CT 06067; (860) 571-5100; Fax: (860) 571-5358; Web site: www.loctite.com. PETER BIOCCA is Market Development Manager, Multicore Solders, 1751 Jay Ell Dr., Richardson, TX 75081; (972) 238-1224; Fax: (972) 644-9309; Web site: www.multicore.com.
UV-curing Adhesive
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The 3-20556 Adhesive is designed to form durable bonds to polystyrene and other plastics in a few seconds without crazing the plastics. The clear bond is said to resist moisture and thermal cycling and have impact resistance. It has a moderate viscosity of 7,500 cp and is said to form solvent-free bonds on an expanded array of plastic and other substrates, including UV-absorbing clear or tinted plastics and other semitransparent materials. The UV/visible light-curing adhesive cures on exposure to wavelengths from 300 to 500 nm. Dymax Corp., Torrington, Conn.
Silicone Encapsulant
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The 5071 UV-cure Silicone Encapsulant is designed to mask sensitive, fine-pitch leads, protecting them from environmental damage caused by thermal-cycling conditions. This one-part, reworkable, thixotropic silicone paste is formulated to cure into a protective elastomer in seconds on exposure to UV light. The masking material reportedly offers depth of cure and can be removed postcure. The product is said to be suitable for automotive electronics and other applications that require extreme thermal-cycling resistance. Loctite Corp., Rocky Hill, Conn.
Adhesive/Impregnant
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The EP19HT Polymer System is a low-viscosity, one-component epoxy resin system for high-performance bonding, sealing, impregnation and coating applications. The product cures at temperatures above 250°F and has a shelf life at ambient temperatures of as long as 3 months without the need for refrigeration. The cured product is said to feature high physical strength properties and good chemical resistance over a -60° to 400°F temperature range. Master Bond Inc., Hackensack, N.J.
Fiber Optic Adhesive
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Tra-Bond F123LV is a rigid, two-part epoxy system for fiber optic applications. This low-viscosity, fast-curing material can be used for bonding fiber optic bundles, potting glass fibers, and terminating singlemode and multimode connectors. At a cure temperature of 100°C, it cures in 5 minutes, while at 150°C, the cure time is 1 minute. A three-step color change is said to ensure that the material is completely mixed and cured. Tra-Con Inc., a National Starch & Chemical Co., Bedford, Mass.
Connector Adhesive
This Fast-curing Anaerobic Adhesive is for use with standard, low-cost fiber optic connectors. The one-part adhesive is said to be suitable for use with any ceramic ferrule connector. It cures in 3 to 5 minutes, reportedly without the "fiber lockup" that can occur. It does not require primer. The adhesive is available alone or in kits of 100 SC or ST connectors. Fotec Inc., Medford, Mass.
Surface-mount Adhesive
The E 6752 Adhesive is a high-speed-dispense, surface-mount epoxy adhesive for bonding electrical/electronic components in fluorescent red paste. The adhesive bonds a range of components, including chip capacitors, resistors, SOTs, SOICs and PLCCs at speeds up to 50,000 dph. Designed for use in high-speed pneumatic and positive-displacement dispensers, the one-component adhesive is said to cure in 3 to 5 minutes at 120°C in a standard convection oven and prevent component movement during handling and cure. Emerson & Cuming, a National Starch & Chemical Co., Billerica, Mass.
Epoxy System
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The ME-530 Epoxy is described as a 5-minute-cure, one-component, high-purity, semiconductor-grade epoxy system developed for underfill encapsulation of flip chip devices. The material is designed to reduce stress between the substrate and solder bumps by keeping the solder bumps in a constant state of compression. Cured in 5 minutes at 165°C, it does not require two-step curing to minimize stress. The product has a low viscosity and flows under devices with standoffs down to 1 mil. It exhibits adhesion to laminate, ceramic, solder mask and metal surfaces. Thermoset, Lord Chemical Products, Indianapolis, Ind.
Adhesive Films
ARclad 8314, ARclad 8901 and ARclad 7992 are pressure-sensitive adhesive films. The 8314 and 8901 products are both 8 mil nominal, double-sided, high-tack, acrylic, pressure-sensitive bonding tapes. Their heavy adhesive masses provide bond strength to foams, rubbers and low surface-energy materials. The 7992 product is a 5 mil nominal, double-sided, acrylic, pressure-sensitive adhesive supported by a polyester film suitable for adhesion to glass, painted surfaces, metals and most plastics. The films are designed for use in lens gasket, speaker, microphone and battery-pack assemblies for mobile phone handsets. Adhesives Research Inc., Glen Rock, Pa.
Dual-cure Adhesives
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Two High-performance, Dual-cure Ad-hesives are available. The 6051 is a UV-curing, high-Tg adhesive used for structurally mounting components. The 6202 is a UV-curing, thermally conductive adhesive. The fast UV cure of both products is said to make them suitable for limiting the time a component must be fixtured before processing continues. A dual-cure mechanism allows the defixtured material to be fully cured, using heat in areas not exposed to UV light. appli-tec Inc., Haverhill, Mass.
Epoxy Adhesives
After parts A and B of these Scotch-Weld Epoxy Adhesives are weighed and mixed, they are degassed before freezing, which is said to result in fewer bubbles and a cleaner bond for the end user. Three formulations are available: 190-PMF Gray is a high-performance, flexible adhesive that is said to offer good shear and peel adhesion and aging characteristics; 460-PMF Off-White is a high-performance, flexible adhesive that reportedly offers good shear and peel adhesion and high durability; and 2216-PMF Gray is a high-performance, flexible adhesive that is said to offer good shock and vibration resistance. 3M Adhesives Div., St. Paul, Minn.
Connector Adhesive
AngstromBond AB9001MT is a two-part-curing adhesive designed specifically for use in MT and MT-RJ connector assemblies. It can be used at room temperature or heat-cured. The blue color is said to allow for easy polishing and the low-temperature curing capability to produce low stress, eliminating fiber cracking in both single or multimode applications. It reportedly yields high bond strength to glass, ceramic, metals and plastics, and has been successfully used for encapsulating fiber optic adapters into metal housing. Fiber Optic Center Inc., New Bedford, Mass.
Electrically Conductive Adhesive
The 102-32 Adhesive is a flexible, high-temperature-resistant, electrically conductive, silicone-based adhesive that is said to combine flexibility with high-temperature resistance. It bonds to silicone substrates and surfaces, and some types of PTFE surfaces. Applications include EMI/RFI shielding and sealing of electronic enclosures, attachment of surface-mount devices to flexible circuits and electrical attachments for stress-sensitive devices. Creative Materials Inc. (CMI), Tyngsboro, Mass.
Masking Tape
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The 581 Adhesive Product is a polyester-backed plating tape coated with a high-performance silicone adhesive that can withstand an operating temperature up to 350°F. Said to be highly resistant to plating and stripping chemicals, the product can be used in direct immersion applications or for fume/splash protection. It is reportedly suitable for use with automatic taping machines, and removes cleanly after processing without leaving a residue. Scapa Tapes North America, Windsor, Conn.
Flexible Epoxy
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Duralco 4538N Super Flexible Epoxy offers high peel strength and adheres to plastics, metals, ceramics, glass, Zytel, Victrex, polyphenylsufone, polycarbonates, phenolics, nylon and other difficult-to-bond surfaces. It reportedly includes no volatiles, no solvents, no outgassing, and no two- or three-stage curing cycles. It is also said to withstand repeated thermal cycling from -100° to 300°F. Applications include bonding, encapsulating and sealing in electronics, instrumentation and production. Cotronics Corp., Brooklyn, N.Y.
