Method of processing high-resolution flex circuits with low distortion

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METHOD OF PROCESSING HIGH-RESOLUTION FLEX CIRCUITS WITH LOW DISTORTION

BACKGROUND OF THE INVENTION 5

This invention generally relates to the manufacture of flexible printed circuit boards (hereinafter "flex circuits"). More specifically, the invention relates to the use of frames for processing flex circuits. 10

Current flex circuit manufacturing processes utilize rollto-roll or large-panel fabrication equipment. For extremely high volume, roll-to-roll processing is perceived to be the lowest-cost approach.

However, the current state-of-the-art roll-to-roll process- 15 ing requires 2-mil-fhick polyimide film to maintain dimensional stability through the via formation, metal deposition and patterning steps. For double-sided flex circuitry and high-end flex circuitry, certain applications will require thinner polyimide, ultimately as thin as ¥i mil. Current 20 roll-to-roll production lines cannot handle this thickness of polyimide because of distortion issues during processing, and also because of alignment tolerances required for registration of double-sided flex circuitry with micro vias.

Organic polymeric materials such as polyimides have 25 physical and electrical properties that are extremely susceptible to their environment, whether it be humidity, temperature, or processing history. Polyimide films in general have a large coefficient of thermal expansion (CTE) and a large coefficient of humidity expansion (CHE). Depending on the 30 molecular structure, these values can vary significantly from one polymer to another. The properties of some polyimide films used in flex circuit production are summarized in Table 1 (see Appendix).

The Kapton® polyimides are a product supplied by 35 DuPont Microelectronics, Wilmington, Del., and the Upilex® polyimides are commercially available from Ube Chemicals, Japan. These polymers are manufactured on a roll-to-roll line and are supplied as thin films ranging in thickness from 0.5 to 5 mils. The physical properties may 40 vary slightly from batch to batch depending on the exact nature of the manufacturing history. During flex circuit production, the polyimide film is exposed to varying temperature and humidity conditions that may cause expansion and shrinkage both in the plane of the film and perpendicular 45 to the plane of the film. These temperature and humidity excursions result in pattern distortions that limit the size of features that can be patterned in high yield. To minimize these distortion issues, manufacturers have utilized 2-milfhick polyimide film. Thicker polyimide can be handled in 50 high-volume roll-to-roll manufacturing, whereas thinner polyimide films are more susceptible to heat and mechanical distortion in high-temperature bake and vacuum deposition steps.

Current panel processing relies on mechanical pins over 55 which the flex film is registered during processing. Although it is possible to fabricate high-resolution structures over very small areas using this approach, the tolerance budget for double-sided fine-line flex circuitry makes this approach unmanufacturable. Other high-end flex circuit manufactur- 60 ers resort to bonding the entire film to a rigid substrate during processing, and releasing the film upon completion. This approach requires additional processing, prevents the advantage of double-sided processing, induces stresses in the polyimide, and adds defects from the bonding material. 65

U.S. Pat. No. 6,323,096 discloses a method for fabricating a flexible interconnect film that involves using a rigid frame

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to support a dielectric film during processing. There is a continuing need for improved methods of fabricating flex circuits using rigid frames to support the workpiece and minimize distortion during processing.

BRIEF DESCRIPTION OF THE INVENTION

The invention is directed to a mechanism and a method for framing a low-distortion flexible dielectric substrate during subsequent flex circuit processing. The framing mechanism and method provide a way to handle thin flexible dielectric films in high-volume fine-line production facilities, controlling distortion, maximizing packing density, and allowing for either single- or double-sided processing. The method can also be used to fabricate multilayered flex circuits with laminations on one or both sides.

One aspect of the invention is an assembly comprising a rigid frame in the shape of a polygon with rounded vertices and a flexible film made of dielectric material bonded to and spanning the frame. The frame comprises a plurality of straight sections connected by a plurality of circular arcs. The material of the frame has a coefficient of thermal expansion less than the coefficient of thermal expansion of the dielectric material.

Another aspect of the invention is a method of processing a flex circuit with low distortion, comprising the step of bonding the edges of a flexible film made of dielectric material having a first coefficient of thermal expansion to a frame made of a rigid material having a second coefficient of thermal expansion less than the first coefficient of thermal expansion. The frame is in the shape of a polygon with rounded vertices.

A further aspect of the invention is a supported workpiece comprising a rigid frame having an opening and a flexible dielectric substrate joined to the frame and spanning the opening in a state of substantially uniform tension. Again, the frame is in the shape of a polygon with rounded vertices.

Yet another aspect of the invention is a method of processing a flex circuit comprising the following steps: making a flexible dielectric substrate having a thickness less than 2 mils and having an outer periphery; joining a continuous portion of the flexible dielectric substrate near the outer periphery to a rigid open frame, the flexible dielectric substrate being in a state other than a state of substantially uniform tension as a result of the joinder; causing the joined flexible dielectric substrate to undergo a change to the state of substantially uniform tension; and printing an electrical conductor on the flexible dielectric substrate while the flexible dielectric substrate is joined to the frame and in the state of substantially uniform tension.

Other aspects of the invention are disclosed and claimed below.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of the interface between a frame and a membrane.

FIG. 2 is a graph showing the bonding force per unit width as a function of the thickness of an Invar frame for AT=175° C. This is for 1 mil Kapton E membrane with properties given in Table 3. The units for the bonding force are N/m.

FIG. 3 is drawing showing a frame for processing flex film in accordance with one embodiment of the invention. The dimensions are given for one specific example for the sake of illustration.