Copper laminates made from transparent polyimide films are now available
Significant progress has been made over the last few years in building transparent and heat resistant flexible circuits. Several transparent materials were developed along with special manufacturing processes. A copper laminate made from a thin transparent polyimide file plated with a copper layer on both sides for use in traditional manufacturing processes is now available. DKN Research, a leading engineering firm specializing in microelectronics and packaging technology, developed a new copper metalizing process using transparent polyimide films. The new laminate will provide manufacturing capabilities using transparent flexible circuits for traditional printed circuit manufacturers.
Polyimide films are the primary substrate material for heat resistant flexible circuits because of the balanced performances between heat resistance and physical properties. The problem is they tend to have an orange or brown color, so the film’s transparency is very low. This is a huge barrier for any optical applications. PET films are another popular substrate material for transparent flexible circuits, but their heat resistance is to low and will not work in standard assembling processes such as soldering and wire bonding. A couple of new heat resistant resins such as LCP have been synthesized and are considered the next generation material for flexible circuits; however, they may not be transparent either. Many chemists use to question the availability of a product that is both transparent and heat resistant. That has changed. Several chemists were successful in synthesizing transparent polyimide resins, and few companies have brought to market some thin transparent polyimide films that serve as the substrate for heat resistant flexible circuits.
There are still a few technical hurdles in building transparent flexible circuits. The biggest one is the availability of an appropriate copper laminate for the standard photolithography-chemical etching process. Some of the copper laminates had problems with the surface treatment and gluing of the copper. The transparency of the circuits was remarkably reduced by absorption and scattering.
DKN Research developed a total solutions package for transparent flexible circuits based on the each application. During the R&D process, DKN Research has established electroless and electro plating process of nickel, copper and gold on the transparent polyimide films, that provides thin copper laminates available for the standard photolithography-etching process of high density flexible circuits. The new copper laminate could be valuable for traditional flexible circuit manufacturers, because the new laminate does not need any additional equipment or chemicals to build transparent flexible circuits. The transparency of the circuits can be over 85%.
The standard construction of the laminate uses double sided 3 micron thick copper layers on a 25 micron thick transparent polyimide film. The copper thickness can be thinner at 0.2 microns, and wider thickness ranges will be available for circuit manufacturers. The thin copper conductor could be valuable for semi-additive processes for ultra-fine flexible circuits with micro via holes in the transparent flexible circuits. DKN Research will provide free laminate samples along with a processing guide to circuit manufacturers.
DKN Research will upgrade the laminates and process capabilities to build the leading edge flexible circuits. DKN Research is happy to share the materials and information with circuit manufacturers and device manufacturers.
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