Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – From phenol – phenol ether – or inorganic phenolate
Reexamination Certificate
2001-05-04
2003-11-25
Hampton-Hightower, P. (Department: 1711)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
From phenol, phenol ether, or inorganic phenolate
C528S125000, C528S126000, C528S128000, C528S171000, C528S172000, C528S173000, C528S174000, C528S176000, C528S183000, C528S185000, C528S188000, C528S220000, C528S229000, C528S350000, C528S351000, C528S353000, C428S411100, C428S457000, C428S458000, C525S420000, C525S432000, C525S436000, C524S600000, C524S602000
Reexamination Certificate
active
06653433
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates generally to polyimide compositions and more particularly to metal clad laminates made therefrom wherein the laminates have improved peel strength.
BACKGROUND OF THE INVENTION
Polyimides constitute a class of valuable polymers characterized by high thermal stability, inert character, insolubility in strong solvents, and high glass transition temperatures, among other properties. Polyimide precursors are polyamic acids, which are then imidized by either chemical or thermal processing to form a polyimide.
Polyimide films are found in a wide variety of applications in the flexible circuit industry. These films are used primarily as the base dielectric material in the construction of the flexible circuit. In the manufacture of circuit laminates, the polyimide films are generally clad with a metal layer, usually copper. As used herein the term “metal layer” means a layer made from a single metal such as copper, tin, chromium, nickel, silver or gold or a metal alloy. The metal layer may be in the form of a prefabricated metal foil, which is subsequently bonded to the surface of the polyimide film substrate. Bonding is accomplished by well-known means, such as the use of numerous types of adhesives. It is also known to cast an imidizable polyamic acid solution directly onto a metal foil, thereafter imidize the polyamic acid, and drive off the solvents which ultimately accomplishes the bonding.
Another method of bonding a metal layer to a polyimide film involves sputtering or vapor depositing metal onto the surface of the film. This step is typically followed by an electroplating or electroless-plating step, which increases the metal layer to the desired thickness. Those skilled in the art will appreciate that sputtering and plating in general may be used to deposit and bond, a plurality of different metallic foils to the surface of the polyimide substrate. For example, it is known to first sputter a chromium layer, followed by a copper layer, followed by copper electroplating to produce a copper foil laminated polyimide film. Thus, the term “metal layer” as used herein includes a single layer of a single metal, or may include an alloy, and may include multiple layers of differing metals and alloys.
Circuitry is fabricated by known etching techniques applied to the polyimide/metal laminate.
It is believed that a common location for failure in the circuit laminates is in the outer surface of the polyimide film. It is theorized that a weak boundary layer may exist at the surface of the polyimide, which ultimately becomes the ‘weak link’ in the laminate construction. It is believed that the failures may occur at a depth of approximately 10 nm into the polyimide film.
It has been known in the art of polyimide film processing to use tin additives in the polyimide composition to improve the peel strength of metal layer clad polyimide films. However, these additives have been found to cause a color change in the film, which is undesirable in the circuitry industry. Thus, a need exists for a technique that improves the peel strength of the metal clad polyimide film laminate without changing the color of the polyimide film.
SUMMARY OF THE INVENTION
In accordance with the present invention there is provided a polyimide composition which, when made in film form and clad with a metal layer, exhibits improved peel strength between the film and metal layer. The polyimide composition of the invention comprises the reaction product of components comprising:
(a) a polyamic acid, said polyamic acid being dissolved in a solvent so as to form a solution;
said polyamic acid having a minimum gel-film formation temperature and a minimum green film formation temperature associated therewith;
(b) an esterified polyamic acid oligomer having from two to twenty repeating units;
said oligomer having at least two crosslinkable groups selected from the group consisting of carbonyl, cyano, hydroxy, alkyne, maleimide, norbornene and sulfonyl groups;
said oligomer having an imidization temperature associated therewith; and
said imidization temperature of said oligomer being greater than the minimum gel-film formation temperature or the minimum green film formation temperature; and
said esterified polyamic acid oligomer being soluble in said polyamic acid solution and present in an amount of 0.5 to 10 weight percent of the combined weights of components (a) and (b).
The peel strength of laminates made by cladding the polyimide film of the present invention with a copper metal layer have been determined using the IPC Peel Strength, Flexible Printed Wiring Materials method 2.4.9, Revision C, Method B when an acrylic adhesive is used. Peel strength of a laminate, when sputtering and electroplating methods are used in the fabrication, is measured by Method 2.4.9A of IPC-TM-650. Both test results are reported in pounds per linear inch, pli (N/cm). The test results reported herein reveal that the laminates, made using either an adhesive or sputtering and electroplating, exhibit peel strengths of at least 8 pli (14 N/cm). Peel strengths as high as 12-14 pli (21-24 N/cm) have been observed. The present invention also has the desired feature that no color change in the polyimide composition, such as the color change associated with tin additives, is observed in the polyimide film.
DETAILED DESCRIPTION OF THE INVENTION
The polyimide composition of the invention comprises the reaction product of a polyamic acid (a) and an esterified polyamic acid oligomer (b).
As is known in the art, the polyamic acid (a) is a reaction product of one or more dianhydride monomers and one or more diamine monomers. The polyamic acid (a) is capable of imidization, either by chemical or thermal conversion, thereby forming a polyimide.
In accordance with the present invention, the polyamic acid is dissolved in a solvent so as to form a polyamic acid solution.
When operating a chemical conversion process, the polyamic acid solution (a) has a “minimum gel-film formation temperature” associated therewith. As used herein the term “minimum gel-film formation temperature” means that temperature at which imidization of the polyamic acid occurs, in a chemical conversion process, to such an extent that a self-supporting gel-film is formed within twenty minutes. The minimum gel-film formation temperature may be as low as 15° C. It is understood by those skilled in the art however, that gel-film formation temperatures well in excess of the minimum are preferably employed so that a self-supporting gel-film is formed in a much shorter time. In a continuous film casting operation for example, the formation of the self-supporting gel-film preferably occurs in less than two minutes. This corresponds to gel-film formation temperatures between about 60° and 125° C. During the gel-film formation step, solids content of the gel film is typically about 20 weight percent.
When utilizing a thermal conversion process, the polyamic acid solution (a) has a “minimum green film formation temperature” associated therewith. As used herein the term “minimum green film formation temperature” means that temperature at which solvent loss and imidization of the polyamic acid occurs, in a thermal conversion process, to such an extent that a self-supporting green film is formed in sixty minutes or less. The minimum green film formation temperature may be as low as 50° C. Higher green film formation temperatures are employed to form self-supporting films in shorter times. As a practical matter however, green film formation temperatures in excess of 200° C. are not generally used because poor film quality results. The green film has a solids content that is typically about 75 weight percent and the level of imidization is generally only 25 to 30% of full imidization.
The esterified polyamic acid oligomer (b) is soluble in the polyamic acid solution (a) and has an imidization temperature associated therewith. As used herein in connection with the esterified polyamic acid oligomer (b) the term “imidization temperature” means that temperatu
Edman James Richard
White Meredith Lynn
E. I. du Pont de Nemours and Company
Hampton-Hightower P.
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