Chip on Glass Technical Note

The Silver Heat Seal Connector is an easy to use and reliable solution for Chip on Glass applications. A Heat Seal Connector is far more flexible than copper flex circuits and may be hard creased, which is very desirable for tightly stacked components. The cost of the HSC with a Zero Insertion Force (ZIF) stiffener end is about the same as the cost of copper flex with Anisotropic Conductive Film (ACF). ACF with copper flex means a factory has to deal with very short adhesive shelf life and additional handling in assembly. Costs are further reduced if the Heat Seal Connector is bonded to the printed circuit board.

Our silver ink has a resistance of 0.2 ohms per square. This is plenty of conductivity for very small parts, but the conductance of longer parts may be improved by increasing the trace width. This was not an issue with STN or even Active Matrix LCD, but with the advent of Chip on Glass trace resistance is critical. A good example is a HSC that is 75mm long and has a 1mm pitch. If it had 0.5mm wide traces they would have a resistance of 30 ohms. Making the trace width 0.75mm reduces the resistance to 20 ohms.

When the application calls for the tail to be connected to a ZIF connector, Graphite ink is printed over the exposed areas to prevent silver migration, and a 0.3mm plastic stiffener is adhered to the base film. Graphite ink has excellent performance in this type of application. Graphite is a natural lubricant, so multiple re-insertions will not compromise the integrity of the trace. There is a negligible effect to the overall resistance because the Graphite ink is a parallel resistance path on the trace. If we use the 1mm pitch example from the previous paragraph, 15mm of the 0.75mm wide trace becomes Graphite/Silver. That section would have a resistance of:

(15mm 0.75mm) x 80 ohms/square = 1,600 ohms [resistance of Graphite overlay]

(15mm 0.63mm) x 0.2 ohms/square = 4.76 ohm [resistance of Silver trace which is 15% narrower under the Graphite]

(1,600 ohms x 4.76 ohm) (1,600 ohms + 4.76 ohm) = 4.75 ohms

After this 15mm area the traces would be covered with a polyester film lamination, would be silver only, and 0.75mm wide. Just prior to the bond area, the trace would neck down to 0.5mm wide because even trace and space width is better for bonding.