Metal treatment – Process of modifying or maintaining internal physical... – Processes of coating utilizing a reactive composition which...
Reexamination Certificate
2002-03-05
2004-06-08
Oltmans, Andrew L. (Department: 1742)
Metal treatment
Process of modifying or maintaining internal physical...
Processes of coating utilizing a reactive composition which...
C148S273000, C148S282000, C148S284000, C427S097100, C427S098300, C427S126300
Reexamination Certificate
active
06746547
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to methods and compositions for treatment of Copper surfaces, particularly as used in the production of multilayered printed circuit boards.
BACKGROUND OF THE INVENTION
Printed circuit boards (PCBs) have long been used in the fabrication of electronic components. Printed circuits traditionally provide a support for discrete components, while providing most of the electrical interconnections between components. Today, the printed circuit board can have active electrical functionality, specifically it can have built-in capacitance. For example, innerlayers for multilayer PCBs are produced by first patterning a Copper foil-clad dielectric substrate material with resist in the negative image of the desired circuitry pattern, (e.g., the photoresist is retained on the surface by exposure to light to form a circuitry pattern), and then etching away the exposed, extraneous Copper. The resist is then removed, leaving behind the desired Copper circuitry pattern.
A multilayer circuitry is created by assembling circuitry innerlayers (having a desired circuitry pattern on one (single-sided) or both (double-sided) sides) with one or more partially-cured dielectric substrate material layers (referred to as “pre-preg” layers) between the circuitry innerlayers. The final multilayer product has alternating circuitry innerlayers and cured pre-preg layers. The layers are then laminated together under pressure and heat (which cures the prepreg) to form an integral composite board having several layer of circuitry embedded within to cure the partially cured pre-preg layer(s) and provide for bonding of the circuitry layers to the pre-preg layers. The cured multilayer product is then drilled to provide through holes, which are then plated to provide for conductive interconnection between the circuitry layers. Circuitry patterns are also typically applied to the outermost facing layers of the multilayer product.
Preparation of the innerlayer materials—particularly the Copper layers—is critical to production of the PCB. The smooth Copper surface is not conducive to strong adhesion to other materials, such as a subsequent layer of polymeric material (such as the pre-preg). If the Copper metal of the circuitry innerlayers is not treated, the Copper layer and the cured pre-preg layers (or other non-conductive coatings) are susceptible to delamination in subsequent processing or use This renders the multilayer board non-functional. Thus the industry has focused upon methods to improve adhesion and PCB quality.
A traditional method for improving the adhesive bond between the circuitry layer and the pre-preg layer focuses upon oxidizing the metal surface prior to assembly with the pre-preg layers. One approach has been to grow a crystalline CuO layer (“black oxide” layer) on the top of the Copper surface. The crystalline CuO layer provides for better interaction with the pre-preg layer due to the presence of hook-shaped structures of crystalline CuO, which provide a hook-like function in interacting with the pre-preg layer.
Early attempts in providing an oxidized metal surface involved the use of “black oxide” adhesion promoters, which provides some improvement in the adhesive bond between the circuitry innerlayers and the dielectric substrate layers in the final multilayer circuit as compared to the adhesive bond formed in the absence of oxidation of the metal surface. Later variations on this approach included production of a black oxide coating on the Copper surface, followed by treatment of the black oxide deposit with 15% sulfuric acid to produce a “red oxide” to serve as the adhesion promoter (see, e.g., Osborne, “An Alternate Route To Red Oxide For Inner Layers”, PC Fab. August, 1984).
Later work involved direct formation of red or brown oxide adhesion promoter on the metal surface, with varying degrees of improvement in the adhesive bond (see, e.g., U.S. Pat. Nos. 4,409,037 and 4,844,981, describing oxides formed from relatively high chlorite/relatively low caustic Copper oxidizing compositions, and producing substantially improved results in circuitry inner layer adhesion). The principal difference between these different colors of oxide is the amount, or thickness, of the oxide layer. Within the industry, a black oxide usually means greater than 0.5 mg of CuO per square centimeter, a brown oxide means 0.2-0.5 mg/square centimeter, and a red oxide means less than 0.2 mg/square centimeter. Unexpectedly, the lighter the oxide weight, the better the bond strength.
However, the crystalline CuO “hook” structures are prone to handling damage. Furthermore, metallizing of through-holes in subsequent production steps involves the use of acidic compositions which can dissolve the Copper oxide layer on the coating on the circuitry innerlayer portions exposed at or near the through hole. This localized dissolution of the Copper oxide, which causes formation of a pink ring or halo around the through-hole (due to the pink color of the underlying, and now exposed, Copper metal), can in turn lead to localized delamination.
In response, the industry has attempted many variations to avoid this localized delamination. One suggested approach has been to make the CuO coating thicker, thus avoiding dissolution of the layer in subsequent processing simply by virtue of the sheer volume of Copper oxide present. This approach has proven counter-productive since the thicker oxide coating is inherently less effective as an adhesion promoter per se.
An alternative approach involves post-treatment of the oxidized Copper surface to stabilize or protect the Copper oxide layer prior to assembly of circuitry innerlayers and pre-preg layers into a multilayer composite. For example, U.S. Pat. No. 4,775,444 discloses a process in which the Copper surfaces of the circuitry innerlayers are first provided with a Copper oxide coating and then contacted with an aqueous chromic acid solution before the circuitry innerlayers are incorporated into the multilayer assembly.
In some processes, the Copper oxide layer is reduced to metallic Copper using particular reducing agents and conditions (see, e.g., U.S. Pat. Nos. 4,642,161; 4,902,551; and 4,981,560). The final multilayer assembly employing such circuitry innerlayers does not evidence pink ring formation since there is no Copper oxide present for localized dissolution, and localized exposure of underlying Copper, in subsequent through-hole processing. However, this process complicates the problem in that the metallic Copper is now present in at least two distinct phases: (1) Copper-from-reduction-of-Copper oxide over (2) Copper of the Copper foil. These two phases are prone to separation or delamination along the boundary between the two phases.
A similar approach is described in U.S. Pat. Nos. 4,997,722 and 4,997,516, which describe formation of a Copper oxide coating on the Copper surfaces of circuitry innerlayers, followed by treatment with a specialized reducing solution to reduce the Copper oxide to metallic Copper. Certain portions of the Copper oxide apparently may not be reduced all the way to metallic Copper (being reduced instead to hydrous Cuprous oxide or Cuprous hydroxide), and those species are thereafter dissolved away in a non-oxidizing acid which does not attack or dissolve the portions already reduced to metallic Copper. However, these processes also result in production of two distinct phases of Copper—(1) Copper-from-reduction-of-Copper oxide over (2) Copper of the Copper foil, and thus increases the likelihood of separation and delamination along the phase boundary.
U.S. Pat. No. 5,289,630 describes a process involving treatment of the Copper surface to provide an adhesion-promoting layer of Copper oxide, followed by a controlled dissolution and removal of a substantial amount of the Copper oxide.
U.S. Pat. No. 5,869,130 describes a process for improving the adhesion of polymeric materials to a metal surface by treating the metal surface with an adhesion-promoting composition having adhesion enhancing ions, where treating is
Cole Joseph
Sedlak Rudolf P.
Bozicevic Field & Francis LLP
Francis Carol L.
Oltmans Andrew L.
RD Chemical Company
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