Archive of the former Yahoo!Groups mailing list: Homebrew PCBs

Basic ACF or Z-Conductor Testing

I did some basic testing today with making some anisotropic conducting film(ACF) or Z-conductor material today.This is typically used to attach flex cables to LCD's or bond other flex cables together. This was not in anyway a scientifc test, but I will try to descibe my steps as best I can in case someone else wants to experiment with this as well.

The ACF is typically made with a hot melt or thermoplastic adhesive powder which is mixed with
particles of gold, copper or metal coated plastic spheres. The spacing between the metal particles
is far enough that this mixture is not conductive. However if the mixture is placed between two pads and pressure applied, the adhesive is squeezed out and the metal particles get trapped between the pads forming an electrical connection between the pads. The adhesive helps to hold the pads together.


For the test I used Unilon-2159 Hot Melt Adhesive power from advancedscreenprintingsupply.com.
A mesh size was not given for the powder, but it is at least smaller than two thousands of an inch.

I also purchased 325 mesh copper powder from chemicalstore.com. 325 mesh powder has a particle size of 44 microns or about 1.5 thousands of an inch. The metal powder particle size should be kept small to minimize the chances of a short between adjacent pads.

A mixture of the adhesive powder was created of roughly a 40/60 mixture of copper powder to
adhesive powder by volume. I did not make exact measurements of this, but just what
"looked right". The powders were mixed in a covered glass jar. I checked under a microscope to ensure that there enough powder adhesive to ensure that the metal particles would not short together.

A small amount of powder was placed between a folded sheet of release liner. The liner came from
a sheet of laser labels.

Intially the powder was pressed with an iron set to 325F. After the powder was melted, a check was made to make sure that the adhesive would properly release from the liner, which it did. The sample was then passed through a laminator to see how thin it could be. After a few passes, the tape was
went down to about 6 thousands of an inch.

Next the same test sample was reheated with the iron and pressed with a J-Roller to try to get the adhesive into as thin a layer as possible. I was able to get the adhesive down to about 2 thousands of an inch thickness. However the adhesive tape was rather delicate and there was some difficulty getting it to release from the liner without tearing.

A second test was done. This time it was initially pressed with the iron and then passed through
the laminator. The sample produced was approximately 6 thousands of an inch thick.

To test how well the sample would connect to copper layers, two strips of adhesive backed copper
tape was attached to a piece of paper. The tape was about 1/8 of an inch wide and spaced about
1/8 inch apart. With one side exposed from the release liner, the adhesive film was placed over the copper traces and the assembly passed through the laminator a few times. A resistance check was made between the two traces to ensure that they had not shorted together, which they had not.

Next, the release liner was removed from the tape and a second strip of copper tape was placed across to two lower copper strips and over the adhesive film. This was all passed through the laminator again to adhere the the upper strip of copper to the adhesive.

After passing through the laminator a few times, a resistance check was made of the two copper strips. While current did pass through the two copper strips now, it the resistance readings varied
by quite bit. Readings of anywhere from 8 to 50 ohms were typical. A few times, an outright open
was found.

Overall I consider this test successful for a first attempt. Even in commerical anisotropic conductive films (ACF), it is not expected to see resistance readings much below an ohm. I think part of the problems with the higher resistance readings are:
1. The thickness of the second sample was .006 inches. I think trying the .002 would yield better results.
2. Even with commerical ACF's, it is recommended applying pressure until the adhesive has cooled down and set. Not possible with a laminator.
3. The flexible paper/copper tape combination may have not been rigid enough to help maintain good contact.
4. It may also be possible that the hot melt adhesive is not rigid enough. An alternative material
to try might be paint powder coating material, which can be found at Harbor Frieight.

I will be doing some more testing, which I hope will culminate in my attaching a flex cable
of my own making to a bare LCD display that only has ITO pads exposed.