Conductive compounds provide an electrically and/or thermally conductive path between components. There are two main types of conductive compounds, electrically conductive and thermally conductive.

Electrically conductive compounds provide low resistivity and are used to prevent electrostatic discharge (ESD), electromagnetic interference (EMI), and radio frequency interference (RFI). They are also used in thick film metallization and electrical interconnections.

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To improve heat dissipation, thermally conductive compounds are applied between a heat-generating electrical device and a heat sink. These products are designed to form a thermally conductive layer, either between components or within a finished product. Thermally conductive compounds can include resins, potting compounds, and encapsulates. Both electrically conductive and thermally conductive compounds are available in many forms.

Conductive compounds are important in many industries becausethey help prevent static electricity. This natural phenomenon can lead to a disaster if left to build up on fuel pumps, polyethylene bags, or tools used in confined spaces. One spark can causea fire in spaces such as mines or tunnels. It can also cause damage to sensitive electronics and erase or alter magnetic media. Conductive compounds have decay rates that measure in nanoseconds, which is instant enough to provide a ground pathway and bleed off stronger electrical charges.

Conductive Compounds Specifications

Material Form

Types of conductive compound material forms include:

Adhesive and casting resins are used for joining electrical or electronic components such as die bonding compounds. They require clean surfaces that are compatible with the adhesive. Adhesives are an alternative to tin-lead solders. There are many advantages including ease of processing, low cure temperatures, and eliminating the need to clean after the bonding process. Adhesives are typically chosen for their high adhesive strength, low humidity, stress relief properties, and low coefficient of thermal expansion.

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Greases and pastes have a high enough viscosity to seal joints, openings, or fittings without mitigation or leakage from the gap. Vacuum sealants have low vapor pressures to prevent gases from leaking into the system. Greases don't cure or polymerize under normal end-use conditions. Pastes may cure or set depending on the composition.

Encapsulates, also known as conformal coatings, are compounds that are used to seal or cover an element or circuit from mechanical and environmental ingresses. Typically, the encapsulated layers are less than 100 mils thick.

Gap filling compound or under fill compounds are used to fill gaps or spaces between two surfaces that are to be bonded or sealed. Flexible sheet materials, as well as sealants or form-in-place (FIP) compounds are used to fill gaps between seams or on surfaces to contain fluids, prevent leaks, and prevent infiltration of unwanted materials.

Material Systems

There are many chemical systems and filler materials for conductive compounds. Some chemical systems contain:

• Acrylics
• Elastomers
• Natural or synthetic rubbers
• Epoxy resins
• Ceramic or inorganic cements
• Silicone compounds
• Vinyl systems
• Polyamides, or polyimides
• Thermoset bismaleimide (BMI) resins
• Cellulosic or cellulose compounds
• Fluoropolymers
• Liquid crystal polymers (LCPs)
• Polybutadiene
• Polycarbonate (PC)
• Polyethylene (PE)
• Polyester
• Polyolefin
• Polypropylene (PP)
• Polysulphide
• Polyurethane (PU, PUR)
• Phenolics/formaldehyde resins

Material Specifications

Conductive plastics have many advantages over metals or surfactant coatings. They are lighter in weight, easier to handle and fabricate, will not dent or chip, and plastics cost less to ship.

Rubber-based- Elastomers and rubber materials are characterized by their high degree of flexibility and elasticity (high reversible elongation). Natural rubber, synthetic rubber, or elastomer sealants and adhesives can be based on a variety of systems such silicone, polyurethane, chloroprene, butyl, polybutadiene, isoprene, or neoprene. 

Water-based - Water-based or water-borne adhesive resins are water soluble or water emulsion-based resin systems that typically do not contain any VOC solvents. Water-based adhesive resins usually present fewer environmental or regulatory control problems. 

Solvent-based - Solvent-based adhesive resins use a volatile organic solvent (VOC) to thin or alter viscosity. Typically, solvent-based adhesive resins result in greater environmental or regulatory control problems. Solvents can also present a fire hazard or a risk of explosion, depending on the plant or job site.

Composition

Conductive compounds can have filler materials added to the composition. For filler materials, some conductive adhesives and compounds contain aramid fiber, chopped fiber, carbon powder, or graphite powder. Other products contain glass fillers, metal fillers, or inorganic compounds. Unfilled products are also available.

Typically, these raw materials are used as starting components for the production of finished compounds.

Curing Technologies

There are several curing technologies for conductive compounds.

Conductive Compound Specifications

Selecting conductive compounds requires an analysis of physical, mechanical, thermal, electrical, and optical properties.

Thermal Properties

Thermal properties include:

Use temperature is the range of temperature a product can be exposed to without the degradation of structural or other required end-use properties.

Thermal conductivity is the linear heat transfer per unit area through a material for a given applied temperature gradient. This is represented by the equation:

heat flux

thermal conductivity

temperature gradient

There are two types of thermal conductivity: anisotropic and isotropic. Anisotropic describes when a temperature gradient is present in a material; heat will always flow from the hotter to the colder region to achieve thermal equilibrium. Anisotropic means the thermal conductivity of the material is dependent on the direction in the heat flows through the material. Isotropic describes when the thermal conductivity in all directions is equivalent for heat flow. Isotropic materials are more commonly found.

Coefficient of thermal expansion (CTE) is the amount of linear expansion or shrinkage that occurs in a material with a change in temperature.

Physical Properties

Physical properties include:

• Viscosity is a measurement of a fluid's resistance to flow. Water is lower in viscosity than motor oil or honey. Oil is lower in viscosity than tar or molasses. Depending on the application method, viscosity determines how well a resin fills the cavities or voids in a mold.
• Gap fill describes the area where products are applied at a particular functional surface thickness, or between two surfaces (the 'gap fill' thickness) to ensure performance criteria, such as strength, electrical resistance, etc. are met.
• Melt flow index (MFI) is the output flow occurring in a 10 minute period through a standard die while a fixed pressure is applied via a piston to a 190° C melt.
• Water absorption is the amount of water absorbed by the material.

Mechanical Properties

Mechanical properties for conductive compounds include:

• Tensile strength at break is the maximum amount of stress required to break the material under tension-loading test conditions, or to case failure. Typically, tensile tests are performed according to test procedure standards such as ASTM D-638. Additional standards are listed below.
• Tensile modulus and elongation is the amount of deformation as a percentage that occurs during a tensile test or other mechanical test.

Electrical and Optical Properties

Electrical and optical properties for conductive compounds include:

Electrical Properties

• Dielectric strength is the maximum voltage field that the material can withstand before electrical breakdown occurs.
• Dielectric constant is an important electrical property. This constant is the relative permittivity of a material compared to a vacuum or free space. As an equation, it is expressed as:

absolute permittivity

absolute permittivity of a vacuum 8.88 x 10^-2 F/m

Optical properties

• Index of refraction is a measure of the speed of light in a material.
• Transmission is the amount of light transmitted through a material.

Features

Conductive compounds are available with a variety of features that enhance performance for a particular application.

EMI/RFI shielding material - Polymers or elastomers are designed to provide shielding from electromagnetic interference (EMI) or radio frequency interference (RFI). Typically, these compounds have a high degree of electrical conductivity.

ESD control - Anti-static materials have relatively high electrical conductivity or low electrical resistivity. They are used in electronic, anti-static or electrostatic discharge (ESD) applications.

Flame retardant - The material is flame retardant in accordance with industry standards from Underwriters Laboratories, Inc. (UL), Flame Class 94, or other ISO standards. Flame-retardant materials are designed to reduce the spread of flame or resist ignition when exposed to high temperatures. They also insulate the substrate and delay damage to it.

Flexible or dampening - Products are designed to provide flexibility or dampening of sound, vibration, or shock in suitable applications. Flexible adhesives or sealants form a layer that can bend or flex without cracking or delaminating. 

Phase change - Thermal interface materials use a phase change to enhance thermal characteristics or heat absorption from electronic devices or electrical components. 

UL approved - The material is approved to or recognized under one or more requirements of Underwriters Laboratories, Inc. (UL).

Applications

Conductive compounds are used in many industries and applications. Some products are used in the manufacture of printed circuit boards (PCBs). Others are designed for electrical power and high voltage products such as generators, transformers, circuit breakers, and motor assemblies. Specialized conductive compounds meet military specifications (MIL-SPEC) or are suitable for aerospace applications. Products for automotive and optoelectronic applications are also available. Flame retardant materials resist ignition or reduce the spread of flames when exposed to high temperatures. Flexible or dampening materials form a layer that can bend without cracking or delaminating. Below is a table that describes some of the many places in which conductive compounds are used.

References

Flip-Chip Technology

Conductive & Anti-Static Plastic Compounds

The Danger of ESD