Electrically Conducting Adhesives
07 November 2002
There are two main types of ECAs. They can be highly filled with conductive particles, cured, and conduct in every direction, known as Isotropic ECAs. They can also be lightly filled with conductive particles, cured under pressure, and conduct only in the Z axis, known as Anisotropic ECAs.
Isotropic ECAs are possible alternatives to solders, and with a move towards lead free soldering the interest is growing. In general these adhesives are based on epoxy chemistry with the conductive filler, silver. By formulation they can be made to dispense or screen print. They can be one or two part. There are more than a few advantages of using ECAs when compared to solder.
Isotropic ECAs, contain no lead. They can be cured at lower temperatures than solder reflow. There is no flux residue to worry about. Wetting can be achieved to a wide range of different materials. Can be used at ultra fine pitch.
The downside of using Isotropic ECAs means that they do not bond well to tinned surfaces. They can only be used for working applications below 150°C. Reworking is difficult. They have unstable electrical conductivity under elevated temperature and humidity conditions. And probably the most important aspect is that they have inferior impact resistance.
Thus far they have found application only where solders have been shown to be unable to be used. However, with the higher processing temperatures of lead free solder the use of Isotropic ECAs is likely to increase.
Anisotropic ECAs are already being used for fine pitch applications where solders are not viable, and have been for more than 10 years. They have found applications in flip chip and flexible circuit bonding in many different products.
Anisotropic ECAs come as a liquid, but also as a dispersion of conductive particles on an adhesive tape. It is as a tape that it is generally used. The tape is placed across the substrate, the flexi placed over the tape, a source of heat and pressure is applied, such as a hot bar. In recent years a range of different electrically conductive particles have been experimented with, the most popular are silver and gold.
ECAs may not be the drop in alternative to leaded solders but they still have a big future.
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