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Adhesives Ingredients

Conductive Adhesives: Technology & Innovation Update

SpecialChem – Mar 11, 2020

We would like to acknowledge Industry Expert Edward M. Petrie for providing technical information needed to develop this article

Electrically and Thermally Conductive AdhesivesCertain applications, like electrical and electronic industry, batteries, solar modules etc., demand adhesive systems to have a certain degree of electrical and / or thermal conductivity. Also, technological advancements have resulted in an increase in the demand for advanced adhesive materials that offer:

  • Strength
  • Conductivity and,
  • Compatibility with components

Most adhesives systems inherently have very low electrical and thermal conductivity due to their organic nature. And, to increase the conductivity of adhesive along with better adhesion, durability and combined resistance to high humidity and temperature remains the biggest challenge.

These challenges can continue for a longer time because of growing applications such as:

  1. Miniaturizing is increasing
  2. The amount of heat being generated within the device is increasing and must be managed 
  3. There is increasing use of thermally sensitive substrates in electronic design

Hence, to develop a suitable electrical or thermally conductive adhesive, one must understand the degree of conductivity that is necessary for the application.

Now, let’s understand importance of conductivity in adhesives in detail.

Conductivity in Adhesives

Electrical conductivity is, of course, important in adhesives that must make an electrical interconnection between components and in adhesives that must provide electromagnetic or radio frequency interference (EMI and RFI) functions. Electrical conductivity in adhesive systems can be used to provide:

  • Antistatic behavior
  • Dissipate or shield against charge
  • Connect circuits, or
  • Any number of functions

The two main types of electrically conductive adhesives are isotropic and anisotropic. Each is formulated to provide specific benefits where an electrical interconnect is desired. The characteristics of these two types of adhesives are summarized in below table.

Class Description Applications
Isotropic Conducts electricity along all axes
Alternative to solder, ground paths, etc. on thermally sensitive substrates

Anisotropic Allows electrical current to flow only along a single axis
LCD assembly
Smart cards
Flip chips, etc.

Thermal conductivity is also important in highly integrated electronic applications where the heat generated by components must be transferred outside of the electronic package. Thermal conductivity within adhesive systems is also a means of reducing temperature build-up from exothermic reactions and stresses that could develop during the curing cycle or during excursions between different service temperatures.

Role of Fillers

Electrical or thermal conductivity of an adhesive can be increased by filling it with conductive particles.

Important properties while choosing a filler material for conductive adhesives include:

  • 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

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), and
  • 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.

Fillers for Electrical Conductivity

Fillers such as silver, gold, nickel, aluminum, and carbon have been used to produce adhesives with varying degrees of electrical conductivity and adhesion. Electrically conductive adhesives owe their conductivity as well as their high cost to the incorporation of high loadings of metal powders or other special fillers of the types. If enough metal particles are added to form a network within the polymer matrix, electrons can flow across the particle contact points making the mixture electrically conductive.

Fillers in Conductive Adhesives

  • 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

Fillers for Thermal Conductivity

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. Some applications, however, must conduct heat but not electricity. In these applications, the adhesive must permit high transfer of heat plus a degree of electrical insulation. Fillers used for achieving thermal conductivity alone include aluminum oxide, beryllium oxide, boron nitride, and silica.

  • Theoretically, boron nitride is an optimum filler for thermally conductive adhesives. However, it is difficult to fill systems greater than 40% by weight with boron nitride
  • Beryllium oxide is high in cost and toxic, and its thermal conductivity drops drastically when mixed with organic resins
  • Aluminum, aluminum oxide, and copper fillers are commonly used in thermally conductive adhesive systems
  • Titanium or beryllium oxides also provide a degree of improvement in thermal conductivity to epoxy systems
  • Magnesium oxide and aluminum oxide have also commonly been used for this purpose, although the degree of improvement is not as great as with the fillers discussed above
  • The incorporation of metal fibers with metal powders has shown to provide synergistic improvement to the thermal conductivity of adhesive systems probably due to the effect of the fibers physically connecting the particulate filler in the system

Importance of Resin Matrix

Resins in Conductive Adhesives
The resin provides a mechanical bond between the two substrates and between the conducting particles. The resin matrix that is used in conductive adhesive formulations includes 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 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 have a different wettability for the filler, which effects the ultimate concentration of filler that can be used in a practical 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 tim
  • Rigid resins will be prone to cracking due to fatigue stress or thermal cycling. Microcracks will reduce the conductivity of the cured adhesive.

Electrically conductive adhesives are finding their way in many applications where they are replacing solder. Solders require oxide free metal substrates and high temperatures can be a safety issue and limit the use to certain (temperature resistant) substrates.

Conductive adhesives are generally lower in cost when one compares the total cost related to each joint. One gram of adhesive is equal to about six grams of solder (adhesive is 1/3 the density and 1/2 of volume). Considering waste, the adhesive assembly requires approximately half as much as does solder for the same application.

The processing costs of conductive adhesive can be substantially lower
than those of solder as there are fewer steps involved in adhesive bonding.

Recent Developments in Conductive Adhesives

Conductive Solutions Supporting EV Industry

Evolution of electric and hybrid vehicles development increasingly demand for light weight material, crash resistant components, effective design for batteries packs, etc. Therefore, apart from maintaining structural integrity, adhesives offer safe and efficient thermal management and electrically conductive capabilities to optimize battery safety and performance. We see several developments for adhesive suppliers to support these needs. They include:

  • Lord Corp. has developed CoolTherm® thermal management adhesive solutions to help EVs go longer, charge faster and have higher reliability by managing heat in batteries, chargers, motors and power electronics. CoolTherm® thermally conductive adhesives couple high-bond strength with thermal conductivity for ease of use and cost-savings.

  • Henkel has developed line of silicone-free, automation-friendly liquid gap fillers offering a thermal conductivity of 3.0 W/m-K.

    Henkel offers Gap Pad and Sil Pad materials for batteries, as well as thermal adhesive solutions which provide structural shear strength of >10 MPa, allowing different coefficients of thermal expansion to be overcome through high elongation.

  • Henkel Thermal Management Solutions
    (Source: Henkel)

  • Chomerics (Division of Parker Hannifin Corporation) has developed a new silicone free, one component, thermal cure-in-place material - THERM-A-FORM CIP30-U. It:
    • Is a single-component product
    • Has thermal conductivity of 3.0W/m-K
    • Requires no mixing or on-site formulation by the user
    • Contains no silicone and is RoHS-compliant

  • Copaltec has developed polyurethane-based thermally conductive potting resins with high thermal conductivity whilst maintaining relatively low viscosity and hence exhibit good flow behavior. These potting resins are used to cast battery packs and it was shown that they increase the operating time by 10% whilst also increasing the capacity by up to 30%.

    Through the efficient thermal management of the battery pack it was also possible to increase the service time of the battery pack significantly. Further, the combination of an innovative filler matrix with a highly optimized production process, Copaltec has developed resins with up to 3.5 W/m*K whilst maintaining good processability.

  • Dow has developed DOWSIL EC-6601 – an electrically conductive adhesive which is a flexible silicone adhesive that combines reliable performance with EMI shielding. This material is used in demanding applications such as radar, LiDAR, sensors and batteries.

    Dow Flexible Silicone Adhesive for EMI Shielding
    (Source: Dow)

    DOWSIL EC-6601 Electrically Conductive Adhesive is uniquely formulated to form strong bonds to many substrates and has greater than 150% elongation to enable flexibility at the joints. This innovative new adhesive has its longer shelf life, better material strength, increased flexibility, stronger adhesion. Hence, according to the company, it provides key advantages over other conductive elastomers widely in today’s printed circuit board and advanced systems assembly market.

  • DuPont has developed BETASEAL™ TC thermal-conductive interface material. It is an effective solution HEV/EV battery assembly. Through its AHEAD™ (Accelerating Hybrid-Electric Autonomous Driving) initiative, DuPont is now offering material solutions for lightweighting, thermal management, safety, NVH, vehicle structure/durability, sensing, control and connectivity etc. for reliable and efficient e-mobility for today and tomorrow.

Graphene as a Filler for Exceptional Conductivity

Graphene - a nanomaterial derived from graphite - has outstanding thermal and electrical conductivity. These properties of this wonder material are combined with other properties like strength and barrier, which make it an exciting material in development for conductive applications.

When mixed into resins, graphene can turn them into conductors of electricity while making them more heat resistant and mechanically robust. Some of the recently seen conductive solutions featuring graphene as filler include:

In 2019, AGM has commercialized very low density, graphene-enhanced adhesives with high thermal conductivity for space applications. New formulations are recommended for a range of satellite and general space applications where thermal management is critical in structural bonding or gap filling. According to the company, these epoxy adhesive systems exhibit high levels of thermal conductivity (between 3 and 6 W/mK), combined with excellent mechanical, adhesive and outgassing performance.

Other interesting conductive adhesive systems featuring graphene as filler include:

  • Team of researchers at Graphene Laboratories has developed G6-Epoxy™, a range of Electrically Conductive Adhesives (ECA). According to the company, the line includes graphene-enhanced conductive epoxies with exceptional electrical characteristics and are more cost effective in comparison to silver-based counterparts. They have improved resistance to stress, corrosion, cracking, and fatigue because of their lower silver load requirements.

  • Graphene-enhanced Conductive Adhesives

  • Master Bond EP30NG has formulated a two-part epoxy adhesive system for applications that require high thermal conductivity. It contains specialty graphene filler which contributes to its high compressive strength as well as enhanced dimensional stability. EP30NG bonds well to a variety of substrates including metals, composites, ceramics, glass and many plastics.

To further maximize the performance of conductive adhesives, adopt the best formulation practices by joining the online course: online course: Electrically and Thermally Conductive Adhesives Formulation Strategies and know more about:

  • Key requirements for conductive adhesives
  • Review applicable materials & processing properties
  • Practical strategies (flexible device assembly, reduced stress and shrinkage, miniaturized parts...)

Electrically and Thermally Conductive Adhesives Formulation Strategies

This article was first published on Jun 23, 2017 and is revised on March 11, 2020

1 Comments on "Conductive Adhesives: Technology & Innovation Update"
abdoljalil m Sep 14, 2017
truly informative. Thanks

Omya: Paints & Coatings
For stronger adhesives stick to Nynas
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