Selecting Conductive Fillers for Adhesives & Sealants

Need for Conductive Adhesives

Certain applications, such as electrical and electronic industry, batteries, solar modules etc. demand adhesives or sealants to have a degree of electrical and/or thermal conductivity. Also, technological advancements demand advanced adhesive materials that offer strength, conductivity and, compatibility with components.

Most adhesives inherently have very low electrical and thermal conductivity due to their organic nature. There are only two ways of producing polymers that have moderate degrees of conductivity:

Synthesizing conductive molecules with intrinsic conductive behavior, or
Adding specific fillers to more common polymeric adhesive formulations.

The first method is extremely difficult. The filler route is a more direct and commercial approach in obtaining high conductivity adhesives.

Many different fillers are used with conductive adhesives and sealants. The type of filler used, and its loading level can have a profound effect on conductivity, cost, formulating characteristics, and final mechanical properties. And, for a specific filler, there can be several variances, such as size (mean and distribution), aspect ratio, surface area, and surface chemistry.

Therefore, among different filler chemistries, it is important that you find the right filler to develop a suitable electrical or thermally conductive adhesive to match end-user requirements.

Conductive Fillers for Adhesives and Sealants

The advantages and disadvantages of conductive adhesives are outlined in the table below. Most of the disadvantages are the result of the high filler loadings necessary to reach conductivity levels similar to solder.

Advantages	Disadvantages
Low temperature processing (cure from RT to 150°C) Low toxicity Higher fatigue resistance Low volatiles Fine pitch applications Low environmental impact Improved processing characteristics (no flux, no cleaning after bond is made)	High materials and energy (cure) cost Long cure times Poor impact strength Adhesion problems with high loadings of filler Voids (nanoscale) at fine interconnects Fails at high current density applications Rework issues (unless a hot melt adhesive base is used) Not ink-jetable because of high viscosity

Let’s begin by understanding the conductivity mechanisms followed by role of resins and fillers.

Electrical Conductivity Mechanism

Electrical conductivity is important in adhesives that must make an electrical interconnection between energized components and in adhesives or sealants that must provide electromagnetic interference (EMI) or electrostatic dissipation (ESD) functions.

Generally, the electrical conductivity resulting from an applied adhesive varies according to the direction that the conductivity is measured. Often, applied thin films show isotropic characteristics due to the shearing forces that occur during the application process. This tends to align the filler particles and create greater conductivity in the direction of the applied stress.

However, certain adhesives require directional conductivity. For example, conductive pressure-sensitive adhesive tapes in certain applications may require conductivity in the z-axis, perpendicular to the substrates. Bulk interconnection and EMI/ESD applications generally require the same degree of conductivity in all directions.

The two main types of electrically conductive adhesives are:

Isotropic
Anisotropic

Common configurations are illustrated in the figure below.

Isotropic & Anisotropic Electrically Conductive Adhesives

Isotropic (T) and Anisotropic (B) Electrically Conductive Adhesives

Each is formulated to provide specific benefits depending on the required direction of conductivity. The characteristics of these two types of adhesives are summarized in the table below.

Class	Description	Example Applications
Isotropic	Will conduct electricity along all axis	Alternative to solder, ground paths, etc. on thermally sensitive substrate
Anisotropic	Allows electrical current to flow only along a single axis	LCD assembly, smart cards, flip chips, etc.

Major Classifications of Electrically Conductive Adhesives

Anisotropic adhesives are used to provide structural strength without an electrical connection on all areas of the device. In this way, they provide electrical interconnection without a sacrifice in bond strength or other adhesive performance properties. Ninety percent of anisotropic adhesives are currently sold as films used for LCD production.

Fillers for Electrical Conductivity

Fillers, such as silver, gold, nickel, aluminum, carbon, of differing size and shape, have been used to produce adhesives with varying degrees of electrical conductivity, adhesion, mechanical strength, and durability.

Silver provides high conductivity and greater stability than other conductive fillers. Silver's oxide layer is thin and relatively conductive. But in very critical applications gold is preferred to silver because of "silver migration"

Conductive carbon (amorphous carbon or fine graphite) can also be used in conductive adhesive formulations if the degree of conductivity can be sacrificed for a lower cost adhesive

Nickel, antimony oxide and coated fibers are also used in low conductivity applications

Aluminum powder cannot be used because of its insulating oxide film

Thermal Conductivity Mechanism

Thermal conductivity is important in highly integrated electronic applications where the heat generated by components must be transferred outside of the electronic package.

Many of today’s electronic products feature miniaturization. In these applications, higher thermal conductivity is required from adhesive systems. Thermal management of electronic devices has become a significant area of development, and thermally conductive adhesives provide a way to transfer heat away from sensitive electronic components.

Thermal conductivity also helps to reduce heat evolution during cure. This reduces exothermic temperatures and extends pot life, particularly at high filler loadings. Stresses related to shrinkage during cure and mismatch in thermal expansion coefficients are also reduced because of the thermal conductivity of the adhesive.

Most unmodified polymeric resins have a very low thermal conductivity. There are certain applications where high levels of thermal conductivity are required. For example, power electronic devices and other heat-generating components are bonded to heat sinks and other metal sources. Metal powder-filled adhesives, such as those described above for electrically conductive adhesives, can conduct both heat and electricity. However, some fillers are either electrically conductive or thermally conductive.

The principles leading to high electrically conductive adhesive formulations also apply to high thermally conductive adhesives. Thermal fillers show a percolation-type behavior. Thermal energy must traverse across the material thickness by hopping from one conductive particle to another. The binder matrix coats each filler particle, so the thermal energy must go through a high-resistance layer before finding another highly conductive particle.

Most commercial thermally conductive adhesives/sealants have thermal conductivities in the range of 0.4 to 10 W/m-K. A true vacuum (thermal conductivity=0) and a diamond (thermal conductivity=2300) are considered the limits of the thermal conductivity spectrum.

Fillers for Thermal Conductivity

Conventional thermal conductive adhesives are prepared by filling the resins with one or several kinds of common fillers, such as:

Graphite powders
Carbon black
Aluminum oxide
Aluminum nitride
Zinc oxide
Boron nitride

Metal powders, such as copper, nickel, aluminum, and silver are also often used in thermally conductive adhesives especially when electrical conductivity is also needed.

Role of Resins & Fillers in Conductive Adhesives

Typically, adhesive and sealant formulations can be made conductive by filling with conductive particles. This is true for electrically conducting as well as thermally conducting formulations. The resin provides a mechanical bond between the two substrates and between the conducting particles. However, it is the conducting filler particles that provide the desired electrical or thermally conductive path as shown in the figure below.

Conductivity is Dependent on Close Contact Between Filler Particles within the Adhesive

The particles must come into close physical contact with one another. Conductivity increases abruptly when a threshold level of well-dispersed conductive filler is achieved (shown in the figure below). This threshold level is termed the percolation level.

Conductivity Increases Resistivity Decreases Abruptly When a Threshold Level of Filler is Used

Alternatively, the resistivity increases abruptly when the conductive path is broken. There can be several reasons why a conductive path is interrupted. These include:

Loss of adhesion to the substrate
Loss of adhesion to the particulate filler
Oxide growth on the substrate or the filler (before or after the bond is made)
Separation of particles due to thermal expansion differences between resin matrix and filler
Mechanical or environmental stress causing fracture of the resin matrix

The oxide layers formed on metal particles rule out most metals for use in electrically conductive adhesives. For example, the aluminum powder cannot be used because of its insulating oxide film. Only noble metals form both thin and relatively conductive oxides.

Importance of Resin Matrix

The base resins that are used in conductive adhesive formulations include the most popular resins. Epoxy resin is most common, but urethane, silicone, acrylic, and polyimide resins are also employed for specific end properties. In addition to its adhesion and binding properties, the nature of the resin matrix is important for the following reasons.

The resistance of the cured adhesive varies according to the resistance of the resin used; however, when compared to the metal fillers the binders have relatively similar high resistivity.
Each resin matrix will shrink at a different rate and to a different degree while curing. High shrinkage results in poor conductive paths.
Each resin will provide different wetting characteristics for the filler, which affects the ultimate concentration of filler that can be used at a practical viscosity in an adhesive formulation.
The barrier properties of the resin will vary by type. Resins that have high permeability to oxygen or moisture will cause changes on the encapsulated filler’s surface, which could reduce conductivity over time.
Rigid resins will be prone to cracking due to fatigue stress or thermal cycling. Microcracks will reduce the conductivity of the cured adhesive.

Fillers That Enhance Conductivity

Common fillers that are used to enhance electrical and thermal conductivity are indicated in the table below. Some fillers provide good electrical conductivity and are thermal insulating; others provide good thermal conductivity and are electrically insulating. Yet others are both electrically and thermally conductive.

	Electrically Conductive Material*	Specific Gravity g/cm³	Volume Resistivity ohm-cm	Thermal Conductivity W/m-K
	Fillers
Principally Electrical and Thermal Conductors	Silver	10.5	1.6 x 10^-6	429
	Copper	8.9	1.8 x 10^-6	401
	Gold	19.3	2.3 x 10^-6	318
	Aluminum	2.7	2.9 x 10^-6	237
Principally Electrical Conductors Only	Silver-plated glass beads	-	< 1.5 x 10^-5	60 - >90
	Nickel	8.9	10 x 10^-6	91
	Platinum	21.5	21.5 x 10^-6	72
Principally Thermal Conductors Only	Diamond powder	3.51	1 x 10¹³	800 - 2000
	Boron nitride	2.25	1 x 10¹⁵	100 - >300
	Aluminum oxide	3.45	1 x 10¹⁴	17 - 40
	Aluminum nitride	3.26	1 x 10¹⁴	170 - 260
	Adhesives:
	Unfilled epoxy	1.1	10¹⁴ - 10¹⁵	0.2 - 0.3
Principally Electrical and Thermal Conductors	Silver-filled epoxy	-	1 x 10^-4	2.4 - 8
	Gold-filled epoxy	5.2	8 x 10^-4	2 - 3
	Silver-filled polyamide	-	3 x 10^-4	2
Principally Thermal Conductors Only	Boron nitride-filled epoxy	-	10¹⁴- 10¹⁵	3 - 4
Principally Thermal Conductors Only	Oxide-filled epoxy	1.5 - 2.5	10¹⁴- 10¹⁵	1 - 2

Electrical and Thermal Properties of Fillers and Filled Adhesives¹
As the filler route is a more direct and commercial approach in obtaining high conductivity adhesives, there still exist significant formulation challenges - to increase the conductivity of adhesive along with better adhesion, durability and combined resistance to high humidity and temperature. If you already have the suitable filler(s) for your conductive formulation, you might be interested in renewing your formulation strategy for the optimal performance. Take our course discussing practical strategies.

Electrically and Thermally Conductive Adhesives Formulation Strategies

Now that you've learned about conductivity & what role each component plays, let's walk through the tips to select the best suitable filler for your product’s success.

Types of Filler Particles for Conductive Adhesives

Silver-Based Conductors

Silver is by far the most used filler for electrically conductive adhesives. Virtually, all high-performance conductive products today are based on flake or powdered silver.

Advantages: Silver offers an advantage in conductivity stability that cannot be matched by other lower-cost metal powders. The most important feature of silver particles is the high conductivity of the oxide. There is almost no change in conductivity as silver particles oxidize, unlike copper oxide that becomes non-conductive after exposure to heat and humidity.

Disadvantages: The primary disadvantages of silver filler in conductive adhesives are their high cost and electrochemical activity. Under conditions of high humidity and DC voltage, however, silver is reported to undergo electrolytic migration. Silver ions are leached out of the filled resin and can be re-deposited elsewhere in the circuit.

Silver particles are also easy to form and can be manufactured in a wide range of controllable sizes and shapes. This allows dense packing to achieve high conductivity. To minimize cost, silver particles with high aspect ratio, such as flakes and fibers, are used to give a higher number of contact points and higher conductivity than would spheres. The flakes, however, may orient parallel to the adherend surfaces during the bonding process.

Conductive adhesives are generally compared to tin-lead solders as they provide:

Relatively better environmental properties
Fatigue and impact resistance
Finer pitch, and
Lower processing temperatures

The table below provides a general comparison between tin-lead solder and a generic commercial electrically conductive adhesive.²

Property	Sn/Pb Solder	Conductive Adhesive (Isotropic)
Volume electrical resistivity, ohm-cm	0.000015	0.00035
Thermal conductivity, W/m-K	30	3.5
Shear strength, psi	2200	2000
Finest pitch, µm	300	<150-200
Minimum processing temperature, °C	215	150-170
Environmental impact	Negative	Minor
Thermal fatigue	Yes	Minimal

Gold Particles

Gold is another non-oxidizing metal like silver. However, its cost is generally prohibitive for most applications. In very critical applications where moisture and heat are prevalent, gold is sometimes preferred to silver because of silver migration. Therefore, gold particles are generally used in adhesives for military and aerospace applications requiring long-term reliability in aggressive environments.

Copper Particles

The challenge for copper-based conductive adhesives is that of inhibiting oxidation under heat and humidity conditions. Copper-filled adhesives do not retain stable electrical conductivity after exposure to elevated temperatures because surface copper oxide is easily formed. A copper-filled epoxy will typically increase in resistivity by a factor of 100 or more after 24 hrs in the air at moderate temperatures.

Since copper readily forms stable complexes with nitrogen bases, like benzotriazole and imidazole, the complexing approach has been widely used to reduce oxidation. Over-plating with metals, like silver, has given some improvement in stability, but not enough for many applications.

Nickel Particles

Nickel oxidizes slowly so that it can be used to make somewhat stable conductive adhesives and inks. Nickel is a hard, poorly malleable metal which limits the ability to make optimized shapes.

Isotropic nickel adhesives have a much higher resistance than silver-based products, up to 2 orders of magnitude higher.
Spherical nickel particles are commercially available with a variety of tightly controlled diameters (up to 50 μm), and narrow size distribution. These spherical particles are mainly used in the fabrication of low-cost anisotropic adhesive films.

Nickel can also be easily plated with electroless gold to provide even more oxidation resistance.

Coated Particles

Coated or plated conductive particles are available. These can be divided into two broad categories:

Metal core
Non-conductive core

The various types of plated particles are often designed for specific anisotropic characteristics and end uses although the original intent was to reduce cost.

Low-cost conductive adhesives include those based on nickel or other metal powders (e.g., copper) where the oxide layer is removed or coated with a thin surface of silver, nickel, or gold. Silver is easy to plate and the plating remains conductive in an oxidizing environment.

Non-metals, like glass or plastic spheres, can be silver-plated and have been commercially available for some time. Silver-coated glass beads, although more stable than silver-coated copper, tend to be fragile and have both rheological and conductivity limitations.

Some plastic spheres can deform under pressure to make better contact with bonding surfaces. Plated plastic particles have lower density and, therefore, are less prone to settling.

Coated Particles in Conductive Adhesives

Carbon Fillers

Conductive carbon (amorphous carbon or fine graphite) can also be used in low-cost but less conductive adhesive formulations. These fillers are often used for EMI, ESD, and conductive inks. The amount of carbon black needed to obtain the required conductivity depends on the type of carbon black.

Carbon is an extremely inert element occurring in several forms, including:

Diamond is the most thermally conductive filler. Due to the high cost of the diamond, the carbon forms of most interest are graphite and carbon black. Both are electrically conductive and are used in making conductive materials.
Graphite is used as an electrically conductive filler because the cost is moderate, and it also has good conductivity. It has a crystal, platelet form resembling mica sheets. Graphite has strongly linked atoms with a very weak bond between the sheets.

Carbon black can be characterized as a substance with over 97% amorphous carbon content. Carbon black is spherical in shape and arranged into aggregates and agglomerates. Carbon black's purity and composition are practically free of inorganic pollutants and extractable organic substances.

Short carbon fibers have also been used as conductive fillers in adhesive formulations. However, carbon-based conductive adhesives show much lower electrical conductivity than silver-filled ones.

Carbon Nanotubes (CNT) and Nanofibers

Nanomaterials are on the same size scale as the polymer chains in the adhesive’s resin matrix. Due to their similarity in size, seamless structure, and extensive overlap, significantly high levels of conductivity, adhesion, and mechanical properties are expected from these materials.

Carbon nanotubes are sheets of graphite rolled into a seamless cylinder. The carbon nanotubes can be 1-50 nm in diameter and 10-100 μm or greater in length. They can be single-wall nanotubes (SWNTs) or multiple wall (MWNTs) as illustrated in the figure below. To date, multiwall carbon nanotubes have been predominantly used as conductive fillers due to their lower cost, better availability, and relatively easy dispersibility.

Structure of a single (left) and multi-wall (right) carbon nanotubes

CNTs have an electrical resistivity(10^-2-10^-3 Ω cm) that is higher than most conventional conductive fillers. As a result, it is generally used in applications where its other properties are important (e.g., strength, toughness, reinforcement). Compared to conventional conductive flakes and fibers, carbon nanotubes have larger aspect ratios, high strength and flexibility, and lower density. A significant additional benefit of carbon nanotubes is that conducting structures may be developed at low loadings of filler due to a lower percolation threshold resulting from their high aspect ratio.

Low-Melt Fillers

To improve electrical and mechanical properties, low-melting-point alloy fillers have been used in conductive adhesive formulations. With this approach, a conductive filler powder is coated with a low-melting-point metal. The conductive powder is selected from the group consisting of Au, Cu, Ag, Al, Pd, and Pt. The low-melting-point metal is selected from the group of fusible metals, such as Bi, In, Sn, Sb, and Zn. The filler particles are coated with the low-melting-point metal, which can be fused to achieve metallurgical bonding between adjacent particles and substrates.

Boron Nitride Fillers

Boron nitride has very high thermally conductive adhesives (about 300 W/m-K) and relatively low cost. When formulated into an adhesive, the thermal conductivity will depend on factors such as size and shape.

Boron nitride fillers can be very large, and as a result, it is difficult to compound high volume into an adhesive. It is difficult to fill systems greater than 40% by weight with boron nitride. In contrast, silver particles are very fine, flake-like particles that can result in higher thermal conductivity and a smooth, creamy adhesive consistency.

3M™ Boron Nitride Cooling Fillers Thermal Materials

Other Metal Oxides

Titanium oxide or beryllium oxides can also provide a degree of improvement in thermal conductivity to adhesive systems. Beryllium oxide has a very high thermal conductivity, and it is often used by itself as a heat sink. However, it is generally not used in adhesives because of its toxicity when ground and high cost.

Magnesium oxide and aluminum oxide have also commonly been used for enhancing thermal conductivity, although the degree of improvement is not as great as with the fillers discussed above. The thermal conductivity increases as the filler volume increases, but this is generally limited due to practical viscosity concerns.

Aluminum oxide, as a thermally conductive filler, is inexpensive, provides excellent strength, and can be added to epoxy and silicone resin in high concentrations before the viscosity becomes excessive. It is, however, abrasive and may cause wear and other damage to dispense equipment.

Particle Morphology & Surface Chemistry of Fillers

In addition to the conductivity of the filler, size and shape are important parameters. The typical shapes that are used are:

Conductive fillers - Particle Shape and size are important

Morphology of Particles: Flakes (L), Granular particles (spheres) (C), and Fibers (R)

For most adhesives, conductive fillers of different sizes and shapes are blended to provide for the densest packing.

Flakes vs Granular Particles (Spheres)

Flake fillers provide lower resistance because they provide facial contacts, compared to more ball-shaped fillers that provide point contacts. However, when only flake fillers are used, the viscosity of the system rises significantly and reduces the ease of handling.

The most common morphology of conductive fillers used for ICAs is flake because flakes tend to have a large surface area, and more contact spots, and thus more electrical paths than spherical fillers. The particle size of ICA fillers generally ranges from 1 to 20 mm. Larger particles tend to provide the material with higher electrical conductivity and lower viscosity.

A new class of silver particles, porous nano-sized silver particles, has been introduced in both ICA and ACA formulations. Conductive adhesives made with this type of particle exhibit improved mechanical properties, but the electrical conductivity is less than ICAs filled with silver flakes.

Fibers

The incorporation of metal fibers with metal powders has shown to provide synergistic improvements to the thermal conductivity of adhesive systems probably because of the fibers physically connecting the particulate filler in the system.

Carbon fibers are used mainly for ESD or EMI compounds with loading beginning at about 10% by weight.
Nickel-coated graphite fibers are used almost exclusively in EMI shielding compounds. They have much higher conductivity than carbon fibers.
Stainless steel fibers are not as brittle as carbon fibers and their advantage is used in low concentrations: 3-5% by weight for ESD and 5-10% by weight for EMI.⁴

Surface Chemistry of Filler Particles

As described above, the chemical stability of the filler’s surface is an important factor in maintaining a high degree of conductivity throughout the adhesive’s service life. Adsorbed organic molecules and oxide films, for example, will impede the passage of electrons or heat across contact points.

Filler particles may be treated with surface coupling agents to optimize their performance. These agents can prevent phase separation of the filler from the binder and may help provide for higher loading of the particles in the binder at defined viscosity levels.

Formulators also often use wetting and dispersing agents in the adhesive formulation to help achieve good filler dispersion and higher filler loadings at a given viscosity. However, the effect of the surface treatment or wetting agent on inter-particle conductivity must be determined and weighed against lower viscosity and other compounding parameters.

Your Final Checklist

Now that you've learned about the conductivity mechanisms and various available fillers, you can easily increase the electrical or thermal conductivity of your formulation by filling it with the suitable conductive particles. Here's the final checklist of the properties that are important in choosing a filler material for conductive adhesives.

✔ Particle conductivity (electrical or thermal)
✔ Loading level
✔ Surface morphology and chemistry
✔ Oxidation potential and reactivity with other adhesive components
✔ Aspect ratio, particle size, and particle size distribution

Important Properties for Choosing Conductive Fillers

In addition to the conductivity of the filler, the size and shape are important parameters. The typical shapes that are used are:

Flakes
Balls (granular)
Aboroid

Flake fillers provide lower resistance after the adhesive is cured because they provide facial contacts, compared to more ball-shaped fillers that provide point contacts. However, when only flake fillers are used, the viscosity of the system rises significantly and may reduce ease of handling.

For most conductive adhesives, conductive fillers of different size and shapes are blended to provide for the densest packing.

The chemical stability of the filler surface is perhaps one of the most important factors in maintaining a high degree of conductivity throughout the adhesive's service life. Adsorbed organic molecules and oxide films, for example, will impede passage of electrons or heat across contact points.

Available Fillers for Thermally & Electrically Conductive Adhesives

View a wide range of fillers available today for formulating thermally & electrically conductive adhesives, analyze technical data of each product, get technical assistance or request samples.

Formulation Strategies for Electrically & Thermally Conductive Adhesives

Talk to Edward M. Petrie and learn how to increase electrical & thermal conductivity of your adhesives formulation, while maintaining good adhesion, durability, combined resistance to high humidity & temperature by renewing your formulation strategy.

References:

Licari, J.J. and Swanson, D.W., Adhesives Technology for Electronic Applications 2nd ed., Elsevier Publishing Co., New York, 2011, p. 104.
Liu J., (Ed.), Conductive Adhesives for Electronics Packaging, Electrochemical Publications Ltd., Port Erin, Isle of Man, 1999, Chapter 1.
Klason, C., et. al., “Electrical Properties of Filled Polymers and Some Examples of Their Applications”, Macronid. Synip. Vol. 108, 1996, pp. 247-260.