Patent Application
20070259159
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The conductive layer 10 will serve as a conductive electric circuit by, for example, formation of a circuit pattern thereon through an etching process. The conductive layer 10 may be made of any conventional electric conductive metal, and a copper foil is preferred. Examples of the copper foil useful in the present invention may include, but are not limited to, an electric deposited copper foil, a roll annealed copper foil, and a thermally treated electric deposited copper foil. The roll annealed copper foil is preferred if flexibility of the finished products is put into a consideration. The thickness of the conductive layer 10 is not particularly limited in the present invention and may be formed as desired.
The metal layer 24 in the support layer 20 may be prepared by any conventional flexible metal material as desired. Examples of the metal material may include, but are not limited to, aluminum, copper, silver, gold, and iron. Aluminum is preferred, in consideration of weight, prices, or flexibility of the finished products. However, copper is preferred, if the substrate is desired to easily formed with a good flatness.
Examples of the adhesive utilized to prepare the adhesive layer 22 in the present invention include, but not limited to, acrylic resins, flexible epoxy resins, and the like. Examples of the flexible epoxy resin useful in the present invention may include, but are not limited to, carboxylated acetonitrile-butadiene rubbers and dimer acid-modified thermosetting epoxy resins. The dimer acid-modified thermosetting epoxy resins have better properties of flexibility, anti-aging, and resistance to molten solder and are preferably used as the flexible epoxy resin. The term, “dimer acid”, herein means an unsaturated fatty acid having two or more carboxyl groups. The dimer acid-modified thermosetting epoxy resins are also commercially available under the trade name of NPER-172 (dimer acid-modified DGEBA) from Nan Ya Plastics Corporation, Taiwan, HyPox DA323 (dimer acid adducted to an epoxidized bisphenol A resin; CAS NO. 67989-52-0) and ERYSYS GS-120 (dimer acid glycidyl ester; CAS NO. 68475-94-5) from CVC Specialty Chemicals Inc., but not limited thereto.
When the aforesaid flexible epoxy resin is utilized in the present invention to serve as an adhesive, it may be used alone or in a combination with other type of resin for adjusting reactivity or physical properties. Examples of other type of resin useful in the present invention may include, but are not limited to, bisphenol-A epoxy resins, brominated bisphenol-A epoxy resins, bisphenol-F epoxy resins, long-chain bisphenol-A epoxy resins, long-chain bisphenol-F epoxy resins, CTBN (carboxyl terminated butadiene acrylonitrile) modified epoxy resins, carboxylated acrylonitrile-butadiene rubber, acrylonitrile-butadiene rubber, and carboxylated acrylic rubber.
When the dimer acid-modified thermosetting epoxy resins are used in a combination with other type of resin as the flexible epoxy resin as described above, the dimer acid-modified thermosetting epoxy resin is added in an amount of 40 to 100 parts per hundreds of resin (phr) based on the total weight of resins, and other type of resin is added in an amount of 60 phr or less based on the total weight of resins.
In the thermosetting flexible epoxy resin, a curing agent and a catalyst may be further added. Examples of the curing agent useful in the present invention may include, but are not limited to, dicyandiamide, phenol-formaldehyde resins, melamine-formaldehyde resins, polyamides, polysulfides, amidoamines, aromatic amines, and the like. Examples of the catalyst useful in the present invention may include, but are not limited to, amines, imidazoles, and boron trifluoride-monoethylamine (BF3-MEA).
When the curing agent and the catalyst agent are added to the thermosetting flexible epoxy resin, the curing agent is added in an amount of 1 to 30 phr and the catalyst is added in an amount of 0.1 to 10.0 phr, based on 100 phr by weight of all resins.
The thermosetting flexible epoxy resin may optionally further comprise an appropriate amount of a thixotropic reagent, such as, fumed silica, defoamers, leveling agents, organic solvents, pigments, fire retardants, inorganic fillers, etc.
A resin mixture obtained from mixing a carboxylated acetonitrile-butadiene rubber and an epoxy resin may be also used as the aforesaid flexible epoxy resin, which has been widely used as an adhesive for a flexible printed circuit board substrate and a protective layer. The epoxy resin is a general purpose epoxy resin, such as, but not limited to, bisphenol-A epoxy resins, brominated bisphenol-A epoxy resins, bisphenol-F epoxy resins, long-chain bisphenol-A epoxy resins, long-chain bisphenol-F epoxy resins, and CTBN modified epoxy resins. In the resin mixture, the carboxylated acrylonitrile-butadiene rubber is in an amount of 40 to 120 phr based on 100 phr by weight of the epoxy resin. A curing agent, a catalyst, a thixotropic reagent, etc. as described above also may be added into such flexible epoxy resin in an amount as described above.
Materials useful to prepare the protective layer 26 in the present invention may be in a same category as that for the aforesaid adhesive. The protective layer 26 and the adhesive can be prepared from the same material, or from materials of different compositions.
The support layer in the flexible printed circuit board substrate according to the present invention is a three-layered composite structure comprising flexible resin-metal layer-flexible resin, such that the copper foil can be provided with sufficient support and insulation. Such three-layered composite structure is symmetric from the cross-sectional view. Therefore, even though the flexible resin and the metal layer may individually expand or shrink at different levels upon being heated or cooled due to different coefficients of thermal expansion thereof, warpage or dimension variation of the substrate does not occur, because the two flexible resin layers respectively disposed on two sides of the metal layer are in a symmetric position and the thermal stresses of the two flexible resin layers may cancel out each other.
On the other hand, such symmetric structure in the same time solves the problem of expansion caused by other environmental factors, such as the chemical reaction or moisture absorption during wet processes such as developing, etching, etc. Furthermore, two sides of the metal layer are protected by the flexible resin and, thereby, the metal layer will be not affected by environmental factors and can sufficiently play a role to support the substrate and provide the dimensional stability to it. In addition, when the two flexible resin layers, disposed respectively on two sides of the metal layer, are placed in the aforesaid environmental factors, for example, exposed to a same chemical environment simultaneously, any dimensional shrinkage or expansion of the two flexible resin layers will be canceled out by each other.
The flexible printed circuit board substrate according to the present invention exists an excellent flexibility, and most importantly, it overcomes the annoying problems of significant dimension variation and warpage suffered by the conventional substrate. Therefore, the precision of flexible printed circuit board substrates may be remarkable increased by utilizing the flexible printed circuit board substrates according to the present invention. Besides, there are many additional advantages brought out by the support layer comprising a metal layer. For example, the metal layer may inherently have a function of electromagnetic shielding, which is an important advantage with regard to an electronic device requiring wireless communication. In addition, the heat dissipation for the electronic device may be attained by such substrate because the metal layer has an excellent heat conductivity. On the other hand, the problems of moisture absorption and oxidation of the circuit boards may be also solved, since the metal layer is an excellent barrier to moisture and oxygen and provides good protection for the substrate. Furthermore, since such substrate has an excellent plasticity, the substrate of the present invention can be maintained in any bendy status, such that the circuit board is not limited to being utilized on a plane, and, accordingly, a three-dimension design for the printed circuit board becomes possible.
The flexible printed circuit board substrate according to the present invention may be manufactured by roll production or sheet production.
The present invention will be illustrated further with reference to the following examples, but the invention is not limited thereto.
Formulations of the thermosetting resin formed in examples 1 and 2 are shown in Table 1. They are examples of formation of a dimer acid-modified flexible epoxy resin. The physical properties of the dimer acid-modified flexible epoxy resin after curing are also shown in Table 1.
A ½ oz (ounce) roll annealed copper foil (RA Cu foil) having a thickness of 18 μm was cut into square pieces of 250 mm×250 mm, and the resin obtained in Example 1 was coated on one side of the copper foil by screen print using a #40T screen. After coating, the copper foil was placed in a hot-air oven at 150° C. to bake for 10 minutes and the thermal curing reaction was approximately completed. Thereafter, the same resin was coated on the other side of the copper foil in the same way, and the resultant copper foil was placed in the hot-air oven at 90° C. for 10 minutes to remove the solvent, resulting in a resin layer having a sticky surface without a solvent. Such a sandwich-typed composite thin film, i.e. flexible thermosetting resin-copper foil-flexible thermosetting resin, was formed and served as the support and insulation layer of the flexible substrate, that is, the commonly called “dielectric layer”. Thereafter, another ½ oz of roll annealed copper foil having the same area was laminated with the aforesaid dielectric layer at the sticky side by a roller and the resultant laminate was baked at 105° C. for 10 minutes to complete the thermal curing reaction. Thus, a new-type of four-layered flexible substrate was formed.
An aluminum foil having a thickness of 12 μm was used instead of the ½ oz roll annealed copper foil used in the example 1 to serve as the support layer to be the middle layer of the three-layered dielectric layer. The two sides of the aluminum foil were coated with the resin of Example 1 in the same process as that in Example 3. Thereafter, the resultant dielectric layer was adhered to a ½ oz of roll annealed copper foil by heat lamination followed by curing in the same way as that in Example 3, forming a four-layered flexible substrate.
An aluminum foil having a thickness of 12 μm was used to serve as a support layer to be the middle layer of a three-layered dielectric layer. The two sides of the aluminum foil were coated with the resin of Example 1 in the same process as that in Example 3. Thereafter, the resultant dielectric layer was laminated with an aluminum foil having a thickness of 18 μm by heat lamination followed by curing using the same process as that in Example 3, forming a flexible substrate with an aluminum foil as a conductive layer.
An aluminum foil having a thickness of 12 μm was used to serve as a support layer to be the middle layer of a three-layered dielectric layer. The two sides of the aluminum foil were coated with the resin of Example 2 using the same process as that in Example 3. Thereafter, the resultant dielectric layer was laminated with a ½ oz of roll annealed copper foil by heat lamination followed by curing in the same process as that in Example 3, forming a four-layered flexible substrate.
All combinations and sub-combinations of the above-described features also belong to the present invention. Those skilled in the art will readily observe that numerous modifications and alterations of the device and method may be made while retaining the teachings of the invention. Accordingly, the above disclosure should be construed as limited only by the metes and bounds of the appended claims.
| Number | Date | Country | Kind |
|---|---|---|---|
| 095115572 | May 2006 | TW | national |
