As electronic technology has moved towards miniaturization, new manufacturing technologies have needed to be developed. Smaller components cannot always be connected or soldered by older conventional methods, such as flow soldering. At the same time, one of the technologies which has become more commonly used is flexible circuits, which can become damaged by the high temperatures associated with flow soldering.
Anisotropic Conductive Film Bonding (ACF) was developed to provide a method of mechanically and electrically attaching flexible circuits to the glass substrate of liquid crystal displays. Since then, ACF technology has been expanded, using it for chip on board and chip on film assembly as well.
The Anisotropic Conductive Film itself is made of a thermoset biphenyl epoxy resin with miniature metal spheres suspended in the epoxy. Earlier versions of ACF were made of various materials, including rubber, acrylic and other adhesives. However, thermoset epoxy has risen as the preferred material. The metal spheres are not added into the ACF with enough density to cause electrical conductivity in the film, avoiding the possibility of the film creating a short circuit between adjacent contacts.
When installed, the epoxy is compressed, migrating from the contacts to the voids between them. This allows the metal spheres to come into contact with the opposing contacts in the two components, creating the electrical connection. The natural spring action of compressing the spheres and contacts ensures a continued electrical connection, even when faced with vibration.
Anisotropic Conductive Film is installed using a combination of heat and pressure in what is known as a hot-bar bonding technique. The hot-bar is made of either metal or ceramic, with an electrically actuated heating element contained within the bar. These bars are made to fit the dimensions of the ACF contact area.
The hot-bar bonding technique is a three step process. The first step is to laminate the ACF to the primary substrate. Both pieces are then placed in fixtures to ensure precise alignment of the electrical contacts. Sufficient pressure and heat is applied to cause the ACF to become tacky, bonding the two substrates together. This only bonds the two pieces together mechanically, but not electrically. Finally, heat and pressure are applied via the hot-bar to cause the thermoset epoxy to flow away from the contacts and into the voids, creating a solid mechanical and electrical connection.
The amount of heat and pressure applied to bond the two materials together depends upon the type of ACF that is used, along with the thickness and material of the substrates. Early versions of ACF required temperatures of 170o F and pressure for 10 to 12 seconds. Modern versions can bond in as little as one second, with temperatures under 150 degrees. This minimizes risk of damage to the electrical components and helps in maintaining process flow in high-volume manufacturing operations.
This technology of using anisotropic conductive film installed with a hot-bar bonding technique is widely used in the electronics industry, especially for cellular telephones, the automotive industry, mobile computers and anywhere that LCD or Plasma displays are used. The ability to easily and reliably connect components together without the use of connectors and the space that they take has been a major advance in the ability to miniaturize electronics.
