The principle of anisotropic conductive adhesive in detail

Anisotropic Conductive Adhesive (Anisotropic Conductive Adhesive, ACA) is a special kind of conductive adhesive, its conductivity has a directional, that is, after hot pressing and curing in one direction (usually vertical) with good conductivity, while in another direction (such as the horizontal direction) is manifested as insulation.This characteristic makes ACA have unique application value in the field of electronic encapsulation and connection.

The following is a detailed explanation of the principle of anisotropic conductive adhesive:

1. Conductive particle distribution and directionality control

Vertical Conductivity and Horizontal Insulation: ACA controls the concentration of conductive particles below the percolation threshold to ensure that the particles are spaced sufficiently far apart to form random conductive paths when unpressurised.When vertical pressure is applied, the particles are compressed into contact in the Z-direction, creating localised conduction, while the XY-direction remains insulated due to unchanged particle spacing.

Concentration control: The particle concentration needs to be precisely balanced.Too low a concentration results in poor conductivity, while too high a concentration may cause a horizontal short circuit.The optimal ratio is usually determined experimentally and combined with dispersion techniques (e.g. ultrasonic treatment) to ensure uniform distribution.

2. Pressure control and process optimisation

Pressure parameters: The pressure needs to be sufficient to deform or displace the particles to make contact, but not excessive enough to cause damage to the substrate or components.Typically in the range of a few MPa to several tens of MPa, depending on the hardness of the particles and the elastic modulus of the substrate.

Guarantee of uniformity: Precision alignment equipment and pressure equalisation modules (e.g. elastomer shims) are used to ensure a uniform distribution of pressure and to avoid localised poor conductivity.

3. Curing process and substrate material

Thermoset vs. thermoplastic: thermoset resins (e.g. epoxy) are cured by cross-linking to form a rigid structure that is resistant to high temperatures but difficult to rework; thermoplastic materials (e.g. polyimide) can be heated to soften, which is suitable for scenarios that require flexibility or detachability.

Curing conditions: Temperature, time and pressure combine to affect the cure.For example, epoxy resins may cure at 120-150°C for 5-30 minutes and at 180°C within 1 minute, and some conductive adhesives may achieve 6-8 seconds while maintaining pressure to prevent the particles from springing back.

4. Conductive particles material selection

Metal selection: gold, silver conductivity is the best but the cost is high; nickel cost-effective but easy to oxidise; metal plating (such as silver-plated copper) to balance the cost and performance.Copper is less commonly used due to its oxidisation and the need for surface treatment.

Coating design: core-shell structure (e.g. polymer core metal plating) can reduce density, improve dispersion, and reduce cost.

5. Long-term reliability challenges and countermeasures

Particle migration inhibition: immobilise particles through high adhesion of matrix materials (e.g. modified epoxy resins) or cross-linking structures; add nano-fillers (e.g. silica) to increase matrix viscosity.

Environmental stability: Selection of low hygroscopicity resins (e.g. fluorinated epoxy) or anti-oxidation particles (gold, gold-plated) to resist degradation of performance in hot and humid environments.

6. Application examples and operational details

LCD driver chip connection: Use ACA to align the chip pins with the electrodes of the glass substrate, and then apply local pressure and heat curing through the hot press head.Precise control of alignment accuracy (micron level) and pressure distribution is required.

Flexible circuit board connection: Pre-coated ACA between FPC and PCB, laminated and then cured under pressure as a whole.Flexible substrates (e.g. polyurethane) are adapted to the bending requirements.

RFID electronic tag connection: use ACA to connect the tag RF chip and antenna together, to achieve the electronic tag in the case of medium-high frequency, a certain distance to identify the information of the tag, widely used in logistics, valuables, tickets and other products.

7. Nanoparticles and environmental protection material trends

Advantages of nanoparticles: higher specific surface area and lower percolation threshold, can be achieved at lower concentrations of conduction, reducing the amount of materials.However, surface treatment (e.g. silane coupling agent) is required to prevent agglomeration.

Environmental requirements: Halogen-free flame retardants (e.g., phosphorus-based compounds) to replace traditional brominated flame retardants; bio-based resins (e.g., soy epoxy) to reduce petroleum dependence.

8. Comparison with conventional welding

Advantages: no need for high temperature (to avoid thermal damage), adaptable to flexible substrates, lead-free and environmentally friendly.

Disadvantages: lower electrical conductivity (resistivity ~10-³-10-⁴ Ω-cm vs solder 10-⁵-10-⁶ Ω-cm), lower mechanical strength, need auxiliary structure to fix.

9. Future directions

Multi-functional composite materials: for example, adding boron nitride to enhance thermal conductivity, taking into account the needs of conductive and heat dissipation.

Ultra-fine pitch applications: Develop sub-micron conductive particles (e.g., 100nm gold particles) to fit advanced packages with pitches below 50μm.

Dynamic Adaptability: Research on reversible cured ACA for repeatable disassembly and connection, suitable for modular electronic products.

10. Typical resistivity and performance parameters

ACA resistivity: typically in the range of 10-³-10-⁴ Ω-cm, while isotropic conductive adhesives (ICA) can reach 10-⁵ Ω-cm. Higher resistivity of ACA maylimit its application in high-frequency signal transmission and needs to be improved by optimising the particle contact area (e.g. flattening the particles).

Summary:

Anisotropic conductive adhesive achieves directional conductive properties through precisely designed material system and process control, becoming a key material in the field of microelectronic packaging.The core of the technology lies in the distribution control of conductive particles, pressure-sensitive response and matrix stability.With the development of electronic devices towards miniaturisation and flexibilisation, ACA will continue to iterate and innovate to solve the challenges of reliability and environmental protection, and expand to broader application scenarios.