Understanding the advantages and challenges of Electrically Conductive Adhesives
09 September 2016
Electrically Conductive Adhesives (ECAs) are used to perform the flex-to-board or flex-to-flex connections used in handheld electronic devices such as mobile phones, MP3 players, or in the assembly of CMOS camera modules.
Since the start of the 1990s, Anisotropic Conductive Pastes have been adopted for use in various mobile devices, including pen-based electronic notepads, portable game units, cell phones, portable audio devices, and digital cameras.
Consumer electronics are smaller and more mobile than ever before, especially with the trend towards wearable electronics. They are expected to work in increasingly demanding environments. They need materials for grounding, shielding, mechanical attachment, heat transfer and electrical conductivity for quality and to comply with regulations. But designers don’t want to include bulky mechanical connectors. To save space and weight, companies are developing multi-functional tapes, which can be as thin as 50 microns.
ECAs consist of a polymeric resin (such as an epoxy, a silicone or a polyimide) that provides physical and mechanical properties such as adhesion, mechanical strength and impact strength, and a metal filler (such as silver, gold or nickel) that conducts electricity.
There are a number of advantages of ECAs including:
• No lead or other toxic metals
• Mild processing conditions enable use with heat-sensitive, low-cost components and substrates
• Fewer processing steps reduce processing cost
• Low stress on the substrates allows flexible substrates without warpage
• Low thermomechanical fatigue and good temperature cycling performance
• Fine pitch interconnect capability enables the miniaturisation of electronic devices
• Compatibility with a large range of materials
• No fluxing or cleaning is required so no CFCs or washing equipment are needed
• Low or no significant VOCs
One of the biggest challenges is reliability. The conductive adhesive must have a long lifetime even in challenging conditions. This includes corrosion resistance, consistent mechanical properties, and stability during temperature cycling. No currently commercialised ECAs can replace tin–lead metal solders in all applications due to lower electrical conductivity, conductivity fatigue, limited current-carrying capability, and poor impact strength.
The heat applied during soldering can damage electronics. Scientists at Boston's Northeastern University created MesoGlue, which bonds metal to metal, or to other materials, and sets at room temperature. MesoGlue is made up of microscopic nanorods that have a metal core. Some of them are coated with the element indium, and some with gallium. A layer of the indium-coated rods is applied to one surface, while a layer of the gallium-coated rods goes on the other. In both cases the rods stand up from the surface. The nanorods interlace. When the indium and gallium on the rods come into contact, they form a liquid. The metal cores of the rods then react with that liquid, causing it to harden into a cohesive solid. This results in a bond that reportedly matches the strength of a traditional weld or solder.
MesoGlue bonds are thermally and electrically conductive, they aren't adversely affected by heat, they're highly resistant to air/moisture leaks, and they require little pressure when being formed.
The metallic glue has multiple applications, including in solar cells, pipe fittings and many applications in the electronics industry for computers and mobile devices. As a heat conductor, it may replace the thermal grease currently being used, and as an electrical conductor, it may replace today's solders.
Luna has developed conductive structural adhesives for commercial and military aerospace applications. These adhesives combine the mechanical strength of industry standard structural adhesives with the electrical conductivity of common conductive adhesives. With an increased use of composites in the aerospace industry, maintaining conductivity between structural elements, even after repair, has become a significant challenge. Current manufacturing techniques involve the use of a non-conductive structural adhesive for component bonding, and often a second conductive adhesive is used to maintain conductivity. The use of multiple adhesives and additional grounding strategies results in additional weight and a complex, costly, and time consuming process.
UV curing of ECAs allows for low cure temperature (<110°C) to enable low cost PET and paper substrates. PET is a commonly used substrate for flexible, bendable electronics but it has a glass transition temperature of only around 80ºC. Shrinkage caused by raising the PET film above its glass transition temperature distorts any electronics on the surface. The stiffness of a PET film decreases as temperature increases so it is more likely to buckle or warp. Any Electrically Conductive Adhesive (ECA) used on PET film will need to cure below 80ºC in order to avoid damaging or warping the substrate during the curing process. There is much research into developing ECAs with a cure temperature of below 80ºC so they can be used with a wider range of substrates. It is possible to use UV curable ECAs but UV exposure can also damage PET films, lowering the ultimate tensile strength and elongation to break. It also lowers the PET film’s transparency, which is important in optoelectronic and display applications. PET films are also affected by humidity so the wetness of the adhesive must be considered
Fraunhofer IZM has attempted to use ECAs to connect components such as LEDs and sensors to textiles. The ECA alone is not robust enough to withstand the mechanical stress and bending so must be used in conjunction with an off-the-shelf structural adhesive. The ECAs are costly, and the long cure time at high temperature makes them unfavourable to manufacture with. With the currently available systems, ECAs are not the best way to attach components to e-textiles, with textiles manufacturers still favouring robust snap fasteners. Silicone-based adhesives are less hard than thermosetting epoxies, so are a possibility for flexible electronics.
Anisotropic Conductive Films are widely used in LCD TVs. Future flexible display technologies will need very flexible, high performance interconnect materials. By 2017, it is likely the driver IC will be bonded directly on the plastic substrate. In general, bending or physical deformation of ECAs can degrade the electrical conductivity. ECAs are based on elastomeric materials but do not have true flexible capabilities. Therefore, if the display becomes rollable or foldable, as expected by 2020, novel, flexible, electrically conductive materials will need to respond to repeated bending or high elongations.
For flexible versions of OLED lighting arrays, expected from 2017, adhesives will need low temperature bonding, and flexibility. The degree of flexibility depends very much on the end-product requirements. Flexible OLED lighting will allow new curved designs, which will not necessary be flexible once the lighting module is integrated into a fixture. Adhesives have to allow bending to obtain the final shape, but do not need to withstand stress cycling.
The particle size in ECAs is becoming smaller, in an effort to get better performance from less material. However, there is serious implication from the point of view of the manufacturing process. Large, compliant conductive particles ease the manufacturing process because these particles can accommodate the warpage and non-coplanarity, to ensure all the metal pads on the mating surfaces are making contact to the conductive spheres. With nanometre-sized conductive particles, the manufacturing will become even more challenging.
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