Stock material or miscellaneous articles – All metal or with adjacent metals – Composite; i.e. – plural – adjacent – spatially distinct metal...
Reexamination Certificate
2002-08-09
2004-10-26
La Villa, Michael (Department: 1775)
Stock material or miscellaneous articles
All metal or with adjacent metals
Composite; i.e., plural, adjacent, spatially distinct metal...
C428S674000, C428S332000, C428S336000, C428S458000, C428S473500
Reexamination Certificate
active
06808825
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a copper alloy foil used in a laminate for a printed wiring board.
Printed wiring boards are used frequently in the electronic circuitry of electronic equipment. Printed wiring boards are classified broadly as either rigid laminates (i.e., rigid boards) or flexible laminates (i.e., flexible boards), according to the type of resin serving as the substrate. Flexible boards, which are characterized by flexibility, are used not only for wiring in flexible regions but also as space-saving wiring material, because they can be housed within electronic equipment in the folded state. Also, because the board itself is thin, it also can be used in semiconductor package interposer applications and as a liquid-crystal display IC tape carrier. In flexible boards, polyimide often is used as the resin that serves as the substrate, and copper generally is used as the conductive material because of its conductivity. Structurally, a flexible board is either a three-layer flexible board or a two-layer flexible board. A three-layer flexible board is structured such that a resin film (e.g., polyimide) and a copper foil, the conductive material, are bonded by means of an adhesive (e.g., epoxy resin, acrylic resin). On the other hand, a two-layer flexible board is structured such that copper, the conductive material, is adhesively bonded directly to a resin (e.g., polyimide). The term “resin” as used throughout the present specification and claims means “synthetic polymer”.
In a printed wiring board, the copper foil of the copper-clad laminate is etched to form various wiring patterns, after which electronic components are connected and mounted by means of solder. However, heat resistance is required because the material of a printed wiring board is subjected repeatedly to such high temperatures. In recent years, lead-free solder has been used more frequently to protect the environment. However, because its melting point is higher than that of conventional lead solder, the heat resistance requirement of flexible boards has become more stringent. As a result, in two-layer flexible boards, only polyimide resin, which has excellent heat resistance, is used as an organic material, so heat resistance can be improved more easily than in three-layer flexible boards, which use adhesives with inferior heat resistance (e.g., epoxy resins, acrylic resins). Thus the usage of polyimide resins has increased.
The principal methods used to produce two-layer flexible boards with a polyimide resin as the substrate are the metallization method, the lamination method, and the casting method. In the metallization method, a method such as sputtering is used to deposit a thin layer of metal (e.g., Cr) on a polyimide film, after which sputtering, plating or the like, is used to form the necessary thickness of copper, the conductive material of the printed wiring board. So, no copper foil is used. In the lamination method, copper foil, which serves as the conductive material of the printed wiring board, is laminated directly onto the polyimide film. In the casting method, varnish containing polyamic acid, the precursor of polyimide resin, the substrate, is applied to a copper foil, the conductive material of the printed wiring board, and the polyimide film formed by thermosetting becomes the resin substrate.
With the miniaturization, weight-saving, and enhanced functionality of electronic equipment in recent years, there has been increased demand for high-density mounting on printed wiring boards, resulting in finer pitches with narrower line or wire widths and line or wire spacing in electronic circuits. If copper foil with high surface roughness or copper foil with irregularities formed by means of a roughening plating process is used as the conductive material, when a circuit is formed by etching, etching residue containing residual copper remains in the resin, so the etching linearity drops, leading to nonuniform circuit widths. As a result, copper foil with low surface roughness is preferable, to enable finer pitches in an electronic circuit. Also, higher-frequency electrical signals are being utilized in electronic equipment (e.g., PCs, mobile telecommunications). However, when the electrical signal frequency exceeds 1 GHz, the skin effect (i.e., the flow of current only on the surface of a conductor) becomes significant, so the effect of variation in the transmission path caused by surface irregularities can no longer be disregarded. Therefore, attempts were made to form a metal film on a flat polyimide film, as in the metallization method, thereby reducing the surface roughness of the copper foil used in the lamination method or the casting method.
The copper foil that serves as the conductive material in a printed wiring board is classified as a rolled copper foil or an electrolytic copper foil, depending on its production process. Electrolytic copper foil is produced by electrolytically depositing copper from a copper sulfate plating bath onto a titanium or stainless steel drum. However, it has become possible to produce so-called low-profile foil, which is copper foil produced by adding additives to the plating bath and then regulating the electrolytic deposition conditions to reduce the surface roughness. Rolled copper foil is produced by using a rolling roll to plastically form it, so the surface pattern of the rolling roll is transferred to the foil surface, thereby yielding a smooth surface. Furthermore, the foil generally is less than 100 &mgr;m thick.
To improve its adhesion to a resin, the copper foil used in a printed wiring board is subjected to a roughening plating process that utilizes electroplating to form copper particles on the surface of the copper foil. This improves the adhesion by means of the so-called anchor effect, which forms irregularities on the copper foil surface and causes the copper foil to bite into the resin, thereby yielding a mechanical adhesive strength. For the aforesaid reasons, however, it is desirable to bond a copper foil with low surface roughness to a resin film, without performing roughening plating processing, so it is necessary to maintain the adhesive strength without performing roughening processing. Also, in a three-layer flexible board, an attempt was made to coat copper foil with a silane coupling agent, and so forth, in order to improve the adhesive strengths of the copper foil, which is a metal, and the adhesive, which is an organic material. However, because the 300-400° C. production temperature used for a two-layer flexible board is higher than the 100-200° C. temperature used for a three-layer flexible board, the coupling agent pyrolyzes readily, so the adhesion has not improved.
A copper alloy containing pure copper and small quantities of additional elements is used as the raw material of the copper foil used as the conductive material. As finer pitches are utilized in electronic circuits, the copper foil (i.e., the conductor) thins and the circuit narrows, so two copper foil properties are desired: low DC resistance loss and high conductivity. Copper is a material with excellent conductivity, so pure copper with a purity above 99.9% generally is used in the aforesaid field, where conductivity is important. However, copper's strength decreases as its purity increases, so if the copper foil is thinned, its handleability deteriorates. Therefore, a high copper foil strength is preferable. Also, for a two-layer flexible board, as in the case of a casting method, it is necessary to perform heat treatment for 10 min. to approximately 1 hour, at an elevated temperature (300° C. to 400° C.), when synthesizing polyimide from polyamic acid, the temperature and time being inversely proportional and at higher temperatures within the aforementioned range the copper foil softening, thereby reducing handleability. Therefore, it is preferable to avoid softening by heat treating at 300° C. for approximately 1 hour.
Under such circumstances, an attempt was made to produce a two-layer flexible board with a poly
Jordan and Hamburg LLP
La Villa Michael
Nikko Metal Manufacturing Co., Ltd.
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