Increased use of additive technology in the electronics and other industries brings challenges in dispensing electrically conductive adhesives.
As part of the increase in the use of 3D printing for electronics, electrically conductive materials are being used in a wide variety of industries. The automotive, aerospace and especially the electronics industries are driving the demand for these materials.
Electrically conductive materials for printing are generally dispersions and are referred to as electrically conductive adhesives, silicones or inks.
Types of Ink
The materials are divided into epoxy resins, polyurethanes, silicones and acrylates. Epoxy resins are widely used in industry due to their processing and material properties. The solids they contain correspond to the dispersion phase, they are electrically conductive and common particle materials are silver, copper, carbon, nickel or gold.
Another conductive material is graphene, a modification of carbon with a two-dimensional structure. In addition to its high thermal and electrical conductivity, this solid is characterised by its low density and high tensile strength. Dispensing tests are taking place to assess their suitability for industrial applications, but preliminary results show they should be well suited for the wearable and electronics industries.
3D printing applications
Applications for electrically conductive adhesives include the protection of electronic components against electrostatic discharges (ESD) and for providing printed shielding against RFI. Applications also include direct chip mounting on printed circuit boards, where the chip is glued directly to the substrate. These are isotropic materials that allow electricity to flow in all directions.
They are also used in the assembly of surface mount devices (SMD). Anisotropic materials are also available, which allow electricity to flow in only one direction. They are used in the manufacture of LCD connections and RFID antennas.
A challenging material
Handling the adhesives in fine printing equipment can be a challenge as the particles in electrically conductive materials often have a diameter of 25 µm and more. The solid content varies depending on the manufacturer and application but is usually between 75% and 90%, so they can be considered as highly filled and abrasive materials.
Also, the difference in density between the dispersion phase and the dispersion material can lead to sedimentation.
However, industrial applications require a homogeneous product and so suitable dispersion agents are selected that prevent or reduce sedimentation during storage and processing. This results in high viscosities for electrically conductive materials. When dispensing these materials, users are faced with the challenge of dispensing abrasive material with a high degree of accuracy, independent of viscosity.
According to Steffen Garbe of additive manufacturing material handling specialist, ViscoTec, due to the properties of abrasive fillers, pump types such as peristaltic or rotary lobe pumps aren’t fully suitable for dispensing highly filled materials.
As an alternative, ViscoTec offers its “Preeflow” microdispenser, a progressive cavity pump in which a stainless steel rotor moves eccentrically in an elastomer stator. The interaction of rotor and stator creates chambers. The size of the alternating opening chambers is constant during rotation, so that there is no compression of the transferred product. Due to this dispensing geometry, a constant volume is always transferred per revolution, independent of the viscosity of the dispensing material and with pulsation-free product flow.
Garbe explains that due to the compression-less transfer, highly viscous abrasive and shear sensitive materials can be pumped this way.
One advantage of the pump in printing applications is its retraction capability. This is a reversal of the direction of rotation of the dispenser, which results in a controlled thread breakage so that dripping is prevented. Thread pulling gels, resins or adhesives can therefore be dispensed more precisely.