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Heat sink compound. Thermally conductive grease. Flexible heat transfer compound.

Image Credit: Allied Electronics | OMEGA Engineering | Thermon Manufacturing

 

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.

 

Electrically conductive compounds

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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.

 

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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.

conductive compounds

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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:

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.

how to select conductive compounds

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  • 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.

Thermoplastic / Hot Melt 

Thermoplastics can be repeatedly softened by heat and then hardened, or set by cooling, which allows parts to be injection-molded or thermoformed and scrap to be reprocessed. Thermoplastic or hot melt adhesives can also be removed or repositioned during assembly. Most hot melt adhesives are solvent-free thermoplastics that melt or drop in viscosity above 180°F, and then rapidly set upon cooling. They are used in a variety of manufacturing processes, including bookbinding, woodworking, construction, product assembly, and box and carton heat sealing. Hot melt adhesive technology stemmed from the previous use of molten wax for bonding. Thermoplastic systems were introduced to satisfy performance needs. Typically, a pure hot melt system will not have the heat resistance of two-part, catalyst or thermoset adhesives. Hybrid hot melt systems are available that exhibit a degree of reactive curing. Polyethylenes, polyamides and ethylene-vinyl acetates are common types of hot melt adhesives. Heat-activated adhesives become sticky or tacky when warmed, and are used in contact or PSA-type applications. 

Thermosetting / Crosslinking 

Thermoset plastics and thermoset resins are crosslinked polymeric resins that are cured using heat or heat and pressure. Cured thermoset resins generally have higher resistance to heat than thermoplastics, but cannot be melted down and reprocessed. Cured thermoset resins do not melt and flow when heated, but they may soften. Phenolic, melamine and urea formaldehyde resins are thermosetting adhesives that offer strong bonds and good resistance to high temperatures. Vulcanization is a thermosetting reaction involving the use of heat and/or pressure in conjunction with a vulcanizing agent, resulting in greatly increased strength, stability and elasticity in rubber-like materials. RTV silicone rubbers are room temperature vulcanizing materials. The vulcanizing agent is a crosslinking compound or catalyst. Sulfur is the traditional vulcanizing agent used with natural rubber. Silicones use moisture, acetic acid and other compounds as curing agents. 

Room Temp. Cure / Vulcanizing 

Room temperature curing or vulcanizing products are polymer resins or compounds that either cure or vulcanize at room temperature. 

UV / Radiation Cured (also EB, Light) 

UV or radiation-cured adhesives use ultraviolet light, visible light, or electron beam (EB) irradiation to initiate curing, which forms a permanent bond without heating or excessive heat generation. One disadvantage of UV curing adhesives is the requirement that one substrate is UV transparent. Some UV resin systems employ a secondary curing mechanism to complete curing of adhesive regions shielded from the UV light. EB curable adhesives use electron beam radiation to cure or initiate curing. The electron beam can penetrate through material that is opaque to UV light. 

Reactive / Moisture Cured 

Reactive resins are single-component adhesives that are applied in the same way as hot-melt adhesives. The resins react with moisture to crosslink and polymerize, thus resulting in a cured material. Polyurethane reactives (PUR) are examples of this type of technology. Certain silicones and cyanoacrylates also use a reaction with moisture or water to cure the adhesive or sealant. 

Single Component System 

Single component adhesives or sealant systems consist of one resin that hardens by reaction with surface moisture, a surface-applied activator-primer, or through the application of heat. 

Two Component System 

Two or multi-component adhesive or sealant systems consist of two or more resins or a resin and a hardener, crosslinker, activator or catalyst that, when combined, react and cure into a polymerized compound or bond. Two component systems are mixed and then applied. 

 

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:

 

how to select conductive compounds

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  • 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.

 

Electrical / Electronics Applications:    

  

Electronics (PCB / SMT Assembly) 

Designed for use in electronics applications. For example, they can be used in potting or encapsulating compounds, conductive adhesives, and dielectric sealants. 

Electrical Power / HV (Coils, Motors) 

Resins, compounds, and plastic composites that are suitable for electrical power or high voltage (HV) applications such as generator or motor assemblies, coil or transformer manufacturing, and switch or circuit breaker insulation. 

Optoelectronics / Photonics 

Designed for optoelectronics or photonics applications. Examples include cements for bonding simple lenses into compound structures. 

Semiconductors / IC Packaging 

Designed or are suitable for semiconductor or semiconductor packaging applications. 

Industry Applications

  

Aerospace 

Designed for aerospace applications. For example, they can be used to bond composite structures to other composite or metallic frame components. 

Automotive 

Designed for automotive applications. For example, they can be used to bond panels and seal windows. 

Military / Government (MIL-SPEC / GG) 

Products adhere to U.S. military specifications (MIL-SPEC). 

OEM / Industrial 

Designed for use by original equipment manufacturers (OEMs) for the assembly, sealing, or fabrication of products. 

Optical Grade / Material 

Polymers or elastomers are designed for optical or photonics applications. Examples include transparent polycarbonate or acrylic lens materials. 

 

References

Flip-Chip Technology

Conductive & Anti-Static Plastic Compounds

The Danger of ESD


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