Circuit protective composites

Electricity: conductors and insulators – Conduits – cables or conductors – Insulated

Reexamination Certificate

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Reexamination Certificate

active

06372992

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates generally to liquid crystal polymer (LCP) film laminates, and more specifically to flex circuits comprising LCP substrate films laminated to a cover-layer of LCP film to encapsulate conductive circuit traces and protect them from environmental contamination.
BACKGROUND OF THE INVENTION
Thermal inkjet printers are fast, quiet devices that produce high quality printing by ejecting ink onto an image-recording medium. An image-recording medium, such as a sheet of paper, is struck only by ink as the printhead moves over the paper surface during image formation.
Thermal inkjet print cartridges operate by ejecting vaporized ink through one or more orifices to produce an ink dot on the recording medium. Ink ejection occurs quickly following rapid heating of a small volume of ink. Orifices may be arranged in arrays on a nozzle plate. As the printhead moves across the recording medium, a sequenced ejection of ink results in a printed portion of the desired image. Typically, after each pass of the printhead, the recording medium moves to allow the next printhead pass to deposit ink onto the adjacent area, until a complete image appears.
A typical inkjet print cartridge includes a printhead, an ink reservoir for containing liquid ink, which is delivered to the printhead, and a flex circuit for transmitting signals to the printhead. The printhead may be any of a variety of conventional structures, including those with nickel nozzle plates, and the like, or may be formed using Tape Automated Bonding (TAB).
Damage to a printhead cartridge may occur in several ways. A common problem involves attack by the ink jet ink composition on the surfaces of the ink jet printhead chamber and nozzles. Attack on the surfaces, by erosion, occurs before and during ink droplet ejection. The process of droplet ejection occurs at temperatures of about 300° C. Such a high temperature may accelerate wear occurring in the printhead. Protective coatings may extend printhead lifetime by retarding the rate of erosion.
It is known to use of organic polymers to protect flex circuitry from environmental agents such as dust and humidity. Although unsuitable for preventing printhead erosion, organic polymers are useful for protecting structures such as printed circuit interconnects in the proximity of an ink jet printhead from environmental agents such as dust and humidity and especially from ink jet ink spray. Ink spray, present during the operation of an inkjet printer, can deposit as ink droplets on interconnects comprised of electronic signal carrying conductive traces, usually made of copper. Droplets of ink contain ionic components. Spacing between individual circuit traces is such that an ionically conducting ink droplet could span the gap between adjacent traces causing an electrical short. In the presence of an electrical short, the inkjet printer may malfunction. Obviously an electrical short is much more likely to develop if circuit traces are exposed without a protective electrically insulating coating. Even with such a protective coating, the deposition of ink droplets in the region of conductive circuit traces may eventually cause electrical shorting between traces. This is possible because ink jet inks contain solvents as well as ionic components. Solvents in the ink will gradually dissolve certain insulative coatings to expose the underlying circuit traces. Solvent attack may be readily facilitated by surfactants also present in ink jet inks. Information on ink jet ink compositions is available by reference to e.g. JP 3097771, U.S. Pat. No. 4,853,037, 4,791,165, 4,786,327, EP 259001, U.S. Pat No. 4,694,302, 5,286,286, 5,169,438, 5,223,026, 5,429,860, 5,439,517, 5,421,871, 5,370,730, 5,165,968, 5,000,786 and 4,990,186.
Problems associated with deterioration of ink jet chambers and nozzles affect the useful service lifetime of an ink jet cartridge. The problem of electrical shorting between circuit traces, of interconnecting flex circuits, poses a more significant problem of premature failure of a printhead when the signal control circuit becomes compromised. Failure of this type primarily involves conductive traces formed on films of polyimide which is the most common substrate polymer used for flex circuits. Desirable properties of polyimide polymers for this application include high dielectric constant and thermal stability. Conductive traces on polyimide typically require a protective coating layer to prevent corrosion by a variety of contaminants including inkjet ink spray. For optimal protection, covercoat materials resist chemical attack, avoid contamination by outgassing and adhere well to conductive circuit traces and the polyimide circuit supporting substrate. A major drawback to firther progress using polyimide film relates to the way in which polyimide absorbs moisture to levels that interfere with high frequency device performance. Higher frequency operation requires the identification or development of electronic packaging materials with less susceptibility to moisture absorption.
Liquid crystal polymer (LCP) films find increasing use as substrates for flexible circuits having improved high frequency performance. Generally they have lower dielectric loss, and absorb less moisture than polyimide films. These beneficial properties of liquid crystal polymers were known previously but difficulties with processing prevented application of liquid crystal polymers to complex electronic assemblies.
The development of multi-axial, e.g. biaxial, film processing techniques expanded the use of liquid crystal polymer film for flexible circuit applications. U.S. Pat. No. 4,975,312 describes a printed wiring board substrate prepared from a multi-axially oriented thermotropic liquid crystalline polymer film having a tailored coefficient of thermal expansion in the X-Y direction and a thickness of not more than about 100 &mgr;. Materials of this type offer several potential advantages over the use of polyimide films for flex circuit substrates. Such potential advantages led to the use of readily available processing techniques for producing single layer or multilayer circuit structures supported by one or more layers of a liquid crystal film substrate. A multilayer flexible circuit is a combination of three or more layers of single or double-sided flexible circuits laminated together and processed with drill and plating to form plated through-holes. This creates conductive paths between the various layers without having to use multiple soldering operations.
U.S. Pat. No. 4,737,398 describes a laminated film comprising a metallic foil and a polymer layer laminated to the metallic foil. Metal foils in the range of 0.005-0.076 mm may be laminated to an anisotropic melt phase forming polymer to provide a polymer laminate 0.01-0.254 mm thick. The resulting laminates provide substrates for a variety of applications including electric and electronic wiring.
It is known that liquid crystal polymer films may be laminated together, or laminated to other materials. According to U.S. Pat. No. 5,248,530, to obtain good adhesion this process involves heating the LCP until it melts and flows to produce a bond between the laminated layers. Unfortunately, when an LCP is heated above its softening or melting point its molecular orientation and overall shape tend to change as the polymer flows. This problem may be overcome, as described in U.S. Pat. No. 5,248,530, by use of a material comprising a coextruded liquid crystal polymer (LCP) film or sheet wherein a higher melting LCP layer is enclosed in or sandwiched between layers or lower melting LCP. The LCP components are coextruded from the same die. This film or sheet may then be laminated on to other materials at a temperature between the melting points of the outer lower-melting LCP and the inner higher melting LCP. Under the conditions indicated above, the inner LCP maintains it shape, orientation and mechanical characteristics while the outer molten LCP component flows and bonds with the other material. Another laminate

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