Most polymers are good insulators, with resistivities in the range 1010Ω.cm for PVC to 1018Ω.cm for PTFE (Figure 1), and this insulating property is a major reason why these materials are widely used throughout the electrical and electronic industries in applications such as insulators, dielectrics, and resists.
Research began during the 1970s on the development of a new class of polymer materials that exhibited intrinsic conductivity, but these are only now beginning to be used for specialist applications.
For board-to-board connection, it is possible to create through connections in an otherwise insulating material by using conductive columns (see Conductive inserts), but an approach with more general application is to employ a composite Anisotropic Conductive Adhesive (ACA) material that will conduct only through its thickness, usually a partly-cured adhesive film or paste with large conductive particles.
The contrasting, but confusing-named, Isotropic Conductive Adhesive (ACA) conducts electricity equally in all directions, and is typically made by adding metallic fillers to polymers. ICAs are very commonly used within semiconductor packages for die attach, where silver-loaded resins have a 35+ years history of making highly-reliable joints between non-fusible surfaces.
More recently ICAs have attracted much attention as a potential solder substitute, both because of increased environmental concern about lead contamination and in response to the drive toward materials that are more fatigue resistant than solder.
The concept of ‘anisotropic conductive adhesives’ has been around since the 1950s, when Barrows developed dielectric coatings which contained randomly-dispersed particles to provide electrical connections only in the vertical (‘Z-axis’) direction. There were other patents, but no real products until the 1980s, when Z-axis bonding films were found to provide the answer to the problem of connecting to Liquid Crystal Displays, temperature-sensitive glass parts whose conductors are thin vacuum-deposited coatings.
These ACAs disperse conductive particles in a non-conductive film or paste, keeping the particle loading low enough so that the individual particles do not contact one another to produce unwanted electrical paths in the X and Y plane. When the ACA is sandwiched between opposing conductors, the non-conductive polymer is squeezed out, allowing a monolayer of conductive particles to bridge the gap, as shown in Figure 2.
With particles typically less than 25 µm in diameter, non-coplanarity must be very tightly controlled to ensure good contact. For this reason, ACAs generally perform best when one adherend is compliant, allowing the bonding pads to be forced into coplanarity with the opposing surface, and for this reason conventional ACAs have found favour with users of flexible printed circuits (Figure 3).
after Gilleo 1996
The coplanarity problem can be reduced by using compressible conductive particles such as metal-coated elastomeric spheres which deform under bonding pressure, an action which also helps maintain force on the pressure contacts forming the junctions.
Many types of polymer matrix have been used: thermoplastics process faster, require less pressure, and can be repaired; thermosets usually require a post-bake after bonding for maximum performance, but are available as pastes for dispensing. A key factor is the polymer’s coefficient of expansion relative to that of the particles: if the polymer expands more than the conductor, the pressure contact connection can open.
Ongoing developments in ACAs include:
The main problem with ACAs lies in their method of use, because pressure must be applied until curing is complete. For this reason, ongoing developments in ACAs include replacing pressure contacts by particles which bond to the substrate, as well as placing particles in an array, rather than at random, to enhance electrical performance.
Isotropic Conductive Adhesives (ICAs) are so called because they conduct equally in all directions, and are mixtures of metal in fine powder form in a polymer base. The components are thoroughly mixed, so that metal is spread evenly throughout the mixture, and the conductive path is formed after curing by contact between the particles of metal powder (Figure 4).
Compared with unloaded resin, ICAs exhibit significantly increased thermal conductivity, but the electrical conductivity values for silver and copper-loaded resins are several orders of magnitude below those for the metals themselves.
Typical ICAs are silver-loaded one-part epoxy compounds supplied uncured. However, the principle can be applied both to other thermoset materials and to thermoplastics.
The properties of metal-filled polymers vary with the percentage of metal, but this is limited by the amount that can be incorporated before the compound becomes unworkably viscous. As with solder paste, this will depend on the particle shape and size distribution and not just on the metal loading.
Over the past ten years ICAs have been evaluated as potential replacements for lead-containing solders, intended to fit into a normal SM assembly process as an alternative to solder paste. The ICA is printed onto the component pad sites on the board, components are placed into the soft adhesive, and then the assembly passes into an oven to cure the adhesive and fix the components to the board. But this is cure, and not reflow, and there is no behaviour similar to solder reflow, because the surface tension forces are entirely different.
Adhesives have been shown to perform well for SM component assembly and are capable of use even at the finest of pitches. However, whilst nearly every conductive adhesive forms a stable junction with surfaces coated with precious metal, early materials were incompatible with the fusible metal finishes used on SMDs and boards. Most electrical failures occured in less than a week during the standard 85% RH/85°C humidity test. Mechanical failures also occured during temperature cycling, but researchers at DuPont suggested that mechanical and electrical performance were substantially independent.
Alpha Metals have developed materials that reportedly exhibit greatly increased stability under heat and humidity ageing using an adhesive formulated to encourage oxide penetration by small conductive particles as the polymer hardens and shrinks, as shown in Figure 5. You can read more about this material at this link.
after Gilleo 1996
The main problem with any kind of conductive adhesive is stability, both of the bonds between particles and between particles and contact surfaces, and of the resin itself. This is particularly the case at elevated temperatures, where most of the resins used will creep and lose adhesion. Much work has been carried out to improve performance by stabilising the resin and, by including anti-oxidants, to reduce the rate of build-up of oxides on the contact surfaces within the joint structure.
Apart from stability and reliability concerns, rework is a major drawback– the polymer which bonds the metal powder to the PCB pads and component will not melt at the same low temperature at which it was cured. Even at relatively high temperatures, close to the maximum temperature which packaged components can withstand, the polymer softens but does not melt. Although removing the components is possible, cleaning the component legs or pad sites is a time-consuming and imperfect process.
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