KR19990029060A - Copper foil for printed circuit manufacturing and its manufacturing method - Google Patents

Abstract

The surface of the copper foil is protected from oxidation and discoloration by electrodepositing a protective layer comprising at least one compound of metal zinc and trivalent chromium on the surface, the protective layer can be easily removed by dissolution in an aqueous alkali solution and , Preferably having a chromium to zinc weight ratio of at least 1: 1. Electrodeposited copper foil, which is particularly suitable for multilayer printed circuit boards, has a protective layer electrodeposited on its matte side and a copper bonding treatment electrodeposited on its glossy side.

Description

Copper foil for printed circuit manufacturing and its manufacturing method

The production of copper foils for electronic applications, such as copper-clad laminates for printed circuit boards, involves the use of known electrodeposition processes. This process uses a large cylindrical drum cathode that is partially immersed in a copper sulfate-sulfuric acid electrolyte. The drum cathode is adjacent to and directed to a pair of curved anodes which may be formed of lead, lead-antimony, platinum and titanium alloys, iridium oxide or ruthenium. Both the drum and the anode are electrically connected to the DC power supply by heavy buss-bars, and currents of 50,000 A or more are generally used. As the drum rotates in the electrolyte, a deposit of copper forms on the drum surface, and when it leaves the electrolyte, the electrodeposited copper is subsequently stripped from the rotating drum in the form of a thin foil, which breaks up into plates and takes up It is wound on a roll. The top of the drum is usually formed of stainless steel, titanium or chromium.

Foil produced in such a process before being treated is commonly referred to as raw foil. The raw foil is a light pink and has two very different looking faces, the "shiny side" which is plated on the drum surface and then peeled off very smoothly, and has a velvety finish towards the electrolyte and anode. It has a face called a matte face. The matte side can be thought of as a full set of cones with a height of 3 to 10 microns, the height of which depends on independent variables such as foil thickness, current density, solution composition and the like. This provides the basic shape of the foil surface that is embedded in the resin of the substrate to promote the adhesion of the copper-clad laminates used in the manufacture of printed circuit boards (PCBs).

Since the matte side of the foil has some degree of micro-roughness, the surface bonding process is usually performed without the matte finish of the raw foil to ensure proper bonding strength after the copper-clad laminate is formed. Applies to cotton. The term "bonding treatment" is generally used to describe changing one or both sides of the electroformed foil to make it suitable for boarding the laminated resin.

The bonding processing operation is performed in a machine called a "treater", in which the rolls of raw foil are continuously unwound by a drive roller and fed to the treater (a manner of handling a roll of paper in a printing press). And cathodes by contact rollers, facing each of the rectangular anodes in each tank and passing through one or more plating banks in a serpentine shape. Each tank has its own suitable source of electrolyte and a direct current power source. Between tanks, the foil is completely rinsed on both sides. The purpose of this operation is to deposit, on at least one side of the foil, usually the matte side, a complex shaped micro-projection that ensures the foil is firmly bonded to the base polymeric materials used to make the copper-clad laminate. It is.

The mechanical support of the current carrying capability as well as the circuit elements of the PCB is provided by a copper foil-polymer joint, so that the peel strength (copper foil and supporting insulating substrate material) The force required to separate is the most important characteristic. It is essential that the foil be able to withstand all the manufacturing steps in PCB manufacture without degrading the initial adhesion that the foil is very firmly and securely bonded to the substrate and that such an adhesive joint must remain constant throughout the service life of the PCB.

This bonding operation is carried out in the lamination facility and involves heating and cooling cycles. The sheet of copper foil is placed on a sheet of " prepreg " (e.g. glass fiber impregnated with an epoxy resin). Both materials are placed in a hydraulic press with a heated press plate, and both materials are pressed together under high pressure. At elevated temperatures the resin liquefies and flows into the tiny bumps on the foil surface by pressure. Then, in the second cycle, when both materials are cooled while the pressure is maintained, the resin solidifies at the rugged portions of the foil surface, so that both materials are firmly bonded together, which makes it very difficult to separate them. This is due to the matte side of the foil which ensures high peel strength.

The matte side of the finished foil, ie the base foil addition treatment, accounts for the combined effect of the micro-terrain of the matte side of the base foil (electrodeposited on the drum machine) and the bonding treatment plated on that side in the trimer. Both of these are equally important.

Just a few years ago, the main segments of the PCB's total output were represented by single-sided, especially double-sided, substrates. Conventional copper foil has been an ideal material for the production of such substrates.

As shown in FIG. 1, the metallic cross section of the copper base foil 10 shows that both sides of the foil are not identical. The surface formed adjacent to the drum 12, i.e., the glossy surface of the foil is relatively flat and smooth at large magnification, but the surface formed adjacent to the electrolyte 14, i. Have a valley As shown in Figure 2, the matte side comprises a very dense and uniform coating of round micro-projections 16 after the application of the bonding treatment, which greatly improves the surface area useful for bonding to the polymeric substrate. Let's do it.

The glossy side of the foil represents the processing side of the copper-clad laminate after lamination. Thus, it functions as a substrate for image patterning and soldering to ensure the necessary electrical connections between the parts. In the production of multilayer PCBs (MLBs), the glossy side of the foil functions as the side treated by chemical means for bonding (brown oxide or black oxide treatment).

Numerous features of copper foil are important in the manufacture of rigid single-sided or double-sided PCBs, but one of the most important of them is peel strength. It should be remembered that copper cladding constitutes the outer surface of the stack, and thin copper foil lines can be lifted relatively easily from the surface of the insulating base material if the peel strength is not good.

This is because the copper foil manufacturer uses a "natural" micro-roughness of the matte surface of the base foil, so that the base foil already has a potential "bondability" to the polymer at that stage. And further enhanced with bonding treatment to achieve the best possible peel strength. This is not necessary, but for the purpose of producing a multilayer substrate that is dominant in the current PCB market, it is a desirable feature of copper foil. In the case of the inner layer of MLB, the copper foil is enclosed or "sandwiched" between the layers of the prepreg so that the double-sided lamination to the inner layer is considerably thin. This increases the need for "low-profile", "not-too-high peel strength" so that the dielectric properties of the laminate due to excessive bonding treatment are not adversely affected. .

On the other hand, the fact that the upper surface (the glossy surface of the foil) is laminated on the prepreg that separates it from the next inner layer raises the reliability problem of such adhesive joints. The glossy side of the foil is very smooth and provides very little "bonding force". This is because manufacturers of MLB apply so-called oxide treatments on the top of copper tracks to increase their bonding power.

The use of oxidation techniques to increase the adhesion between the copper surface of the inner layer and the prepreg in the manufacture of MLB is widely accepted. Without the oxide treatment, the bond between the copper and the prepreg would be insufficient to withstand the thermal shock of the reflow solder.

During the formation period of the multilayer substrate industry, the pattern between the inner layer circuits was relatively less dense, so the bond between the prepreg and the base layer of the inner layer was not considered important. It was thought that the copper track could be encapsulated in a cured prepreg. On the other hand, today, the internal circuits are very dense and most of the bonding is for copper rather than base stacks. Today, the surface of the copper track must be "adhesion-prone".

Oxide treatment techniques used in MLB manufacturing are problematic, expensive and create their own technical problems. One of them is the so-called "pink fing" due to the chemical attack of the copper oxide layer by the chemicals used for through-hole plating. In contrast to CuO, which is readily soluble in mineral acids, bonding with copper is not affected by the pink ring, so using an additional step of brown oxide treatment with a reduction in copper oxide for metallic copper is now possible. It is customary. This reduction step further complicates the brown oxide process, making them even more expensive.

It has been suggested that special copper foils provided with bonding treatment on the glossy side of the foil are more suitable for the production of MLB. When the bonding treatment is plated onto the foil side of the foil, the peel strength is lower than when the same treatment is plated onto the matte side of the foil (eg about 12 lbs./inch) (eg about 8 lbs./inch).

For copper foils intended for use in MLB production, the brown oxide treatments currently applied to the glossy side of the foil to provide significantly lower peel strength, on its own, as opposed to the glossy side of the foil with virtually no peel strength It has been found that due to its peaks and valley topography and the resulting micro-roughness, it can be favorably applied to the matte side of the base foil with significant peel strength of about 4 lbs / inch. When this is done, very little brown oxide should be applied to the matte side of the foil to achieve a desired level of peel strength, for example 7 lbs / inch. The reduced amount of brown oxide is much weaker for the structure than the higher amount of brown oxide that must be applied to the glossy side of the foil to achieve the same peel strength. Thus, the need to reduce cupric oxide over metallic copper can be eliminated and the overall process becomes simpler and less expensive, while the quality of MLB (particularly the resistance to delamination due to dielectric laterality and solder shock) is reduced. Is improved.

However, a change in the process of making such a special copper foil requires more than the application of a simple bonding treatment to the glossy side rather than the matt side of the base foil, compared to the traditional process.

When the circuit pattern is transferred to the panel, the matte side of the particular foil is first "imaging" and then treated with brown oxide, thus "stainproofing" to protect the matte side from discoloration and oxidation. The general method of "stainproofing" should be reformulated to be more suitable for use in commercial operation.

Brown oxide treatment and micro-etching techniques of MLB have in common the property that sodium chloride or peroxide sulfate micro-etching solutions must reach the surface of copper in order to produce the desired reaction or effect evenly. Therefore, the stainproof layer should be easily removed by the precleaning solution or easily penetrated by the brown oxide or the micro-etch solution. An overly adherent stainproof layer can form an impermeable shield between the surface of the copper and the processing chemicals, delaying the desired reaction or causing apparent nonuniformity.

With the advent of miniaturized electronic components, very tightly packed printed circuit boards are needed. Miniaturization often requires the copper conductors or track lines of today's printed circuit boards to be narrowed down to less than 5 mils. The degree of high resolution of the fine line circuit depends on the quality of the copper foil produced for the electronics industry, in particular the surface quality of both sides of the foil.

In the manufacture of printed circuit boards, it is practical to form an image of a desired printed circuit pattern on a copper surface of the laminate from a copper-clad laminate by a photographic technique that leaves a desired pattern formed of a photoresist material on the surface of copper. In order to sharpen and precise photographic imaging, the photoresist must diffuse well on the surface of the foil and adhere well to it.

It is practical to roughen the surface of the glossy surface of copper foil in order to achieve good resist adhesion in the manufacture of a printed circuit board. This roughening removes the non-sticking stainproof film that the foil manufacturer applies to the foil to protect it from oxidation and discoloration before the foil arrives at the user. The photoresist does not adhere to the stainproof film, so it must be removed. Therefore, the roughing of the foil surface aims to remove the stainproof film as well as change the copper surface topography to smoothness and micro-roughness to promote photoresist adhesion, which is a condition of good resolution of the track line.

Such roughening is carried out by mechanical means (eg brushing, scrubbing by scrubbing) or by chemical means (so-called micro-etching), which chemical means are used for the oxidation of copper on the copper surface of the copper-clad laminate. It is achieved by performing an etching operation of an acid. Such acids attack the smooth side of the foil along the copper grain boundaries, causing pits and holes to change the copper surface from smooth to micro-roughness.

In the production of MLB, copper foil is laminated (bonded twice to the polymerized substrate). First, a thin double sided copper-clad laminate is made. These stacks are then image patterned and the unnecessary copper is etched to create the desired circuit pattern. Several layers of the double-sided substrate prepared in such a manner are stacked together, with a prepreg sheet inserted between them to genetically separate each inner substrate from the other. Thereafter, the stacked circuit boards and prepregs are stacked together to form a monolithic multilayer substrate. Thereafter, holes are punched or drilled at prearranged positions of the substrate and so-called copper through-hole plating is used to ensure electrical interconnection between all layers of the copper-track conductor.

During the second (so-called step B) lamination, good bonding between the top surface of the track line (the surface used for image patterning) and the sheet of prepreg is required.

In the production of MLB it is practical to carry out so-called brown oxide treatments on the inner layer substrates with their circuit patterns to alter the micro-topography of the top surface of the track lines in order to improve their bonding power to the polymerization prepregs. This brown oxide treatment, due to its oxidation, depends on the type and operating conditions of the electrolytic cell, if possible, in a mixture of cupric oxide (Cu ₂ O), the metal copper on the upper surface of the exposed copper tracks (CuO). By dipping the substrate in an alkaline solution of sodium chloride which is converted to This oxide coating grows in the form of dendritic crystals perpendicular to the surface of the copper track. Thus, the surface area useful for bonding to the polymeric substrate is increased and increased bonding force is achieved.

Several patents relating to bonding treatments for copper foil provide a bonding treatment on one or both sides of the foil bonded to the substrate (US Pat. No. 5,207,889), or the surface of the foil on which the treatment of copper foil used for lamination to the substrate is to be laminated to the substrate. US Pat. No. 4,572,768 describes the electrodeposition of a dendritic layer of copper prior to the copper's Gilding layer. In addition, the glossy or gloss of the foil to achieve flexibility with respect to the surface properties of the final copper-clad laminate with a mirror-polished surface or a "copper-clad laminate with a satin-like finish" (matte side). The use of cotton without this is described in US Pat. No. 3,998,601. U.S. Patent No. 3,857,681 describes applying a copper resinous and guild layer to at least one of the surfaces of a copper foil to improve bond strength when laminated to a polymeric substrate and then applying a zinc coating to prevent lamination discoloration or staining. .

As described in US Pat. Nos. 3,625,844 and 3,853,716, it is also known to apply a stainproofing chromic acid layer to the copper foil surface to protect against discoloration and oxidation.

The matte side of the foil with its own micro-roughness and the resulting bonding force, even if it is roughened by micro-etching or mechanical polishing, is better for growing a layer of brown oxide than the conventionally polished side of the foil. to be.

The present invention provides a method of protecting the surface of a copper foil against ternishing and oxidation and electrodeposited copper foil suitable for use in the manufacture of printed circuit boards, in particular multilayer printed circuit boards. (electrodeposited copper foil).

The following description of the invention will be better understood with reference to the accompanying drawings, which form a part thereof. In the drawings,

1 shows a conventional copper base foil.

Fig. 2 is a view showing a conventionally completed copper foil in which the typesetting side thereof is typesetting;

3 is a view showing a copper foil according to the present invention.

It is a general object of the present invention to provide a method for controlling the surface properties of a matt surface of a copper foil to make it particularly suitable for high resolution image patterning and a stainproofing layer that can be easily removed by dissolution in an aqueous solution of alkali during the manufacture of a printed circuit board. It is to provide a method for providing a to a matt surface. Other objects and effects of the invention will be apparent from the following description and practice of the invention.

In order to achieve the above object of the present invention, copper foil, metallic zinc and at least one trivalent chromium compound suitable for the production of a printed circuit board comprising an electrodeposited copper base foil having a matt surface and a glossy surface opposite it And an oxidation-chromic protective layer provided on the matte side, which is dissolved in an alkaline solution, and an electrodeposited copper comprising bonding process provided on the polished surface.

The weight ratio of zinc (calculated as metallic zinc) to chromium (calculated as metallic chromium) to at least 1: 1 in the protective layer on the matte surface is preferably about 2: 1.

In another embodiment of the present invention, the method of protecting the surface of the copper foil against discoloration and oxidation forms an electrolyte comprising an acidic solution containing a hexavalent chromium ion-containing anion and zinc, and injects the copper foil into the electrolyte. A copper foil cathode is formed as a component thereof in a protective layer containing zinc and at least one chromate during its introduction into, and has a zinc to chromium ratio of at least 1: 1. The electrolyte preferably has 3 to 4.5 pH.

According to the present invention, the matte side of the foil does not require brushing, scrubbing or micro-etching while protecting the foil from pre-use oxidation of the foil and at low temperatures (eg about 8 lbs./inch Rough atmosphere) Stainproofing layer (derived from stainproofing electrolyte and electrolytically applied to foil surface) that can be easily removed from the surface of copper-clad laminates by simply immersing in dilute alkaline solution such as aqueous sodium hydroxide or potassium Is provided).

The purpose of the stainproofing process in the production of copper foil is to protect the foil from a protective coating that extends the shelf life of the foil by protecting it from oxidation in air as well as oxidation at elevated temperatures used during the lamination process in which copper-clad laminates are manufactured. To form on the surface of the.

The stainproofing layer that protects the copper foil against oxidation has a function other than merely extending the shelf life of the foil. Once the copper-clad laminate is ready for the next process, it is required that complete removal of the stainproofing compound ensures good adhesion of the photoresist, unobstructed response to etchants and good tolerability of the brown oxide treatment. Therefore, the protective layer should be easily removed in terms of image patterning of the foil by rapid and complete dissolution in alkali. Therefore, the type, structure, chemical composition and thickness of the stainproofing layer protecting the "processing" side of the foil (the side on which the imaging is performed) are very important.

The present invention takes advantage of the fact that the copper foil produced by electrodeposition on the rotating drum cathode has two unequal top surfaces. The surface adjacent to the drum, i.e. the glossy side of the foil, is relatively flat and smooth even when viewed at very large magnification, but the matte side of the foil is already micro-rough (as seen with high-resolution electron microscopy, the surface is micro- Seems to consist of peaks and micro-valleys). In addition, the degree of micro-roughness can in this case be controlled by the manufacturer of the copper foil better than when mechanical or chemical roughening is performed by the manufacturer of the printed circuit board.

Thus, the lamination produced by bonding the matte side of the foil to the polymeric material ensures excellent photoresist adhesive force, thus ensuring fine-line precision of high definition sharpness. Bonding treatments applied to the glossy side of the foil (or drum side) ensure good adhesion of the track lines to the polymeric substrate.

Another effect of the present invention arises from the production of copper foil which uses a matt surface for image patterning and a bonding treatment is applied to the glossy surface of the foil. Since this effect can achieve the highest functional density of the circuit in electronic packaging, such foils are particularly suitable for use in the manufacture of MLBs, which are currently dominant in the printed circuit board market.

Referring to Figure 3, the copper foil completed according to the present invention is a matte surface on which a protective layer 28 containing zinc and one or more compounds of trivalent chromium, hereafter referred to as chromate, is deposited. Electrodeposited copper base foil 20 having 24. The foil 20 has a smooth or polished face 22 with electrodeposited bonding treatment 26. It is preferred that the matte surface of the raw foil has a surface roughness or R ₂ (defined below) of about 3 to about 10, more preferably about 5 microns.

The base foil is any known method for producing a copper foil such as a method in which a thin foil is electrodeposited on a smooth surface of a rotating drum cathode partially immersed in an electrolyte from an electrolyte solution containing copper ions, and then peeled off from the surface of the drum. Can be formed by technology. The copper foil thus produced has one surface that is smooth or glossy on the drum side and the other surface that is not glossy on the electrolyte side opposite thereto.

In the manufacture of copper-clad laminates for printed circuits, copper foil is subjected to mechanical interlocking at the interface between two materials on a polymeric substrate (synthetic material such as epoxy, polyimide or resin reinforced with glass fiber tissue). By bonding to the polymerization substrate.

Bonding treatment is provided on the bonding surface of the foil so that a high degree of engagement is achieved. Such treatment consists of a very dense coating of round micro-projections of copper electrodeposited on the shiny or smooth (drum) side of the base copper foil.

The peel strength of the copper foil (the force required to separate or flake the foil from the polymer substrate) depends on the respective micro-projections, their mechanical strength and hardness, density per surface area and their distribution on the drum side smooth surface of the base foil. In addition, all the above-mentioned factors depend on the conditions in which the bonding process is electrodeposited.

Preferred bonding treatments are carried out by carrying out four successive electrodeposition steps on the glossy side of the base or " raw " foil. The first is to deposit micro-dendritic copper, which improves the bonding ability of the foil by improving the actual surface area of the matte side to a very high degree. Thereafter, the bag or the guild layer whose function is to reinforce the dendritic layer is electrodeposited so that it is not affected by the transverse shear force of the liquid resin in the lamination step of the PCB. Thereafter, after the stainproofing layer is applied, a so-called barrier layer is deposited on the two-layer copper treatment.

The purpose of dendritic deposits is to increase the "true" surface area of the polished side, since its properties are ultimately responsible for the bonding characteristics of the foil. The shape, height mechanical strength and number per surface area of the dendritic micro-projections constituting the dendritic deposit are helpful factors in achieving the proper bond strength of the foil after all processing steps have been completed. Dendritic deposits, the first treatment step, impart relatively mechanically weak and unacceptable treatment delivery properties.

The encapsulation step of the treatment is very important, as it eliminates the tendency of the foil to "treatment transfer" and thus eliminates "laminate staining" which can lead to degradation of the dielectric properties of the laminate. The role of this second treatment step is to mechanically reinforce the brittle dendritic layer by overplate the brittle dendritic layer and a thin layer of healthy and strong metallic copper that adheres the dendritic crystals to the base foil. Such dendritic-encapsulated composite structures suffer from high bond strengths and lack of process delivery. The processing parameters that guarantee this are relatively narrow. If the amount of guild deposits is too small, the foil is given a process delivery. On the other hand, if the guild layer is too thick, a partial loss of peel strength can be expected. The two layers of these first treatments consist of pure copper in the form of microscopic and round micro-projections.

The two-layer copper bonding treatment can electrodeposit a very thin layer of zinc or zinc alloy called a barrier layer. During the manufacture of copper-clad laminates for PCBs, the zinc-containing layer alloys all underlying copper bonding treatments by a heat diffusion process of the metal in the solid state. As a result, a layer of chemically suitable alpha brass is formed on the surface of all copper treatments. Its purpose is to prevent direct copper-epoxy vertical contact, because the layer containing zinc (converted to alpha brass in the stack) is called the barrier layer. When the bonding treatment composed only of copper is laminated with the epoxy resin system, it tends to react with the amino groups of the resin at high lamination temperatures. In addition, this can generate moisture at the foil-resin interface, causing the detrimental effect of "mealsing" and possibly delamination. Barrier layers plated on all copper bonding treatments prevent these deleterious effects.

All three treatment steps described above are carried out by electrodeposition, as is well known in the art, to alter the topography and shape of the smooth side of the foil to ensure the mechanical strength of the surface area.

The foil treated as described above may then be subjected to an electrochemical stainproof which alters the chemical properties of the surface. As a result of this step, the bonding surface is chemically stable. This stainproofing action removes the soft surface film that can greatly reduce the adhesion of the foil to the substrate, resulting in a stable film of controlled thickness responsible for giving the treated surface a "durability" of its properties. to provide.

The bonding treatment, barrier layer and stainproofing are based on the method described in US Pat. No. 4,572,768 (Wolsik et al.), US Pat. No. 5,207,889 (Wolski et al.), US Pat. Re 30.180 and / or US Pat. No. 3,857,681 (Yates et al.). It can be applied to the glossy surface of the foil.

The proper chemical composition and thickness of the stainproof layer is of great importance in achieving a good and easily removable stainproof layer without reducing its protective function.

Providing a layer of stainproofing on the matte side of the foil according to the invention involves the simultaneous deposition of chromium ions and metal zinc, in which one component of the electrolyte, ie chromic acid, is in a metallic state at the foil surface (cathode) Is reduced for the trivalent state, which is also a very rare case of alloy plating, since it also results in the formation of a chromate stainproofing layer on the matt surface 24.

Since the stainproofing electrolyte used in the present invention has a double function of chromate and zincate, it is a double protection role that provides both mechanical protection of the conversion coating as well as electrochemical (sacrifice) protection of the zinc coating. A protective layer of stainproofing of the present invention is formed.

The factor that enables co-deposition of chromate and metal zinc is the pH of the electrolyte. At very low pH values, for example pH 2 (value of 3 g / l CrO ₃ ), hexavalent chromium compounds are very strong oxidants, thus neutralizing the reduction of the cathode of zinc. At such Ph, the standard electrode potential E ₀ has a value of +1.33 V for the following reaction,

Cr ₂ O ₇ ^2- + 14H ⁺ + 6e = 2Cr ³⁺ + 7H ₂ O

Under such conditions, co-deposition of zinc is not possible. In the basic solution, chromate is more widely used than dichromate, and there is very little oxidation.

reaction

CrO ₄ ^2- + 4H ₂ O + 3e = Cr (OH) ₃ + 50H ^- E ₀ = 0.13 V

It is very close to the standard electrode potential E ₀ = 0.76 of silver zinc, allowing the deposition of chromate and metal zinc.

According to the present invention, most electrolytes are moderately acidic, preferably have a pH of about 3 to about 4.5, more preferably about 3.5 to evil 4, and usually have a pH of about 4 away from alkaline as well as electrolytes, See most electrolytes. However, the pH at the foil-solution interface is above 7. Every time there is a flow of current, there is necessarily a reduction of species at the cathode (foil). In the current process, the reduction of cathode is

Reduction of Cr ⁺⁶ (see above)

Reduction of zinc Zn ²⁺ + 2e = Zn

Reduced water ^{_{2H 2 O + 2e = 2OH -}} + H 2

In the last reaction, ie by the release of hydrogen at the foil surface responsible for the local increase in pH described above, the deposition of zinc and simultaneous precipitation of the chromate layer are allowed.

By studying the chemical composition of the experimental stainproof layer using a means of surface analysis (scanning auger microprobe and electron spectroscopy of chemical analysis), the layer for stainproof layer is dipped in alkali. It is easily removable and can provide good protection and usually contains about 10-20% chromium (calculated as metal chromium) and 20-40% zinc (calculated as metal zinc), with a balance of water , It has been found that it has a thickness of less than 100 mm 3. The ratio of chromium to zinc is very important. The relatively high zinc content in the layer ensures that the layer is easily dissolved by alkali and dissolves in sodium hydroxide due to the positive nature of this metal to form sodium citrate while releasing a large amount of hydrogen. Thus, stainproofing has a chromium to zinc ratio (calculated as metal) of at least 1: 1, preferably 2: 1, by weight.

The atoms of zinc are uniformly dispersed in the lattice of chromates such as the chromium hydroxide component of the protective layer, and the alkali cleaner attacks the atoms of zinc to dissolve to form hydrogen, which combines alkali attack and "fizzing". The effect is that the chromium compound lifts the surface of the foil, making it pure and clean after rinsing for the next PCB treatment.

The following electrolyte and electroplating conditions can be used to form the stainproofing layer described above.

Electrolyte

CrO ₃ - 0.75 grams / liter (g / l) - 2 g / l; Preferably 1.25 g / l

Zn (calculated as Zn)-0.3 g / l-1.0 g.l; Preferably 0.5 g / l

_{H 3 PO 4 - 0 g /} l - 2 g / l; Preferably 0.5 g / l

H ₂ O-Balance

Electroplating Condition

pH 3.5-4.0

T-90 ℉

Current density ^{- 0.5A / ft 2 - 20 A} / ft 2; Preferably 10 A / ft ²

Plating time-1 second to 5 seconds; Preferably 3 seconds. Copper foil becomes a cathode with respect to an anode, is immersed in an electrolytic cell, and faces a foil, and electrodeposition of a stainproofing layer is achieved. The current stainproofing method is an improvement on the conventional stainproofing methods described in US Pat. No. 3,625,844 (Mckean) and US Pat. No. 3,853,716 (Yates et al.).

The ability of the stainproof layer to protect the "processing" side of a copper foil or copper-clad laminate from various oxidation forms, while maintaining a layer that is easily removable later by chemical means, is to deposit the stainproof layer on the same side of the foil. Previously, it could be further improved by plating a very thin zinc layer on the matte side of the foil.

The description of this improvement is as follows. Of the two components of the stainproof layer, zinc ensures the resistivity of the copper surface, protecting it against direct oxidation due to the heat of the lamination process and post-bake. In addition, due to its positive nature, zinc can be easily dissolved in both mineral acid and alkali, thus contributing to the easy removal of the protective layer by chemical means.

The trivalent chromium component in the layer is responsible for protecting the copper surface against atmospheric or "wet" corrosion, thus providing a good shelf life for the copper foil. However, chromium compounds chemically bonded to the copper surface are much less soluble than zinc in acids and alkalis, and are therefore much more difficult to remove by chemical means than the zinc component of the protective layer.

Eventually, a compromise between protective action and cleanability is reached by carefully selecting the thickness of the protective film and the ratio of chromium mixture and zinc in the stainproof layer.

The nature of the process ensures that the distribution of both elements and their proportions throughout the layer thickness is uniform.

Obviously, the best way to address the conflicting demands on the protection and removability of the stainproof layer is if the depth profile of this layer favors zinc, then the first 20Å of the total 100 Å of the stainproof layer will be applied to the surface of the metallic copper. Is very close. The remaining 80 mm 3 towards its outer diameter of the layer should consist of a mixture of trivalent chromium and zinc composite in the above-mentioned proportions.

If a thin metal zinc coating free of chromium compounds is very close to the surface of the metal copper, the zinc's ability to sacrificially protect copper against direct oxidation is better than that of the stainproof film alone.

Likewise, having a pure zinc coating adjacent to the copper surface facilitates easy and complete removal of the protective film by the chemical cleaner.

The deposition of the stainproof layer is carried out in a separate plating tank of a trimer by a pre-deposition cathode process of a very thin metal zinc coating. Foil becomes the cathode in the tank. The anode faces the processing surface of the foil. Thus, the electrical circuit is completed, and by controlling the amount of current flowing, a zinc coating of desired thickness can be deposited on the copper surface. After this coating, deposition of the stainproof layer is carried out in the plating tank next to the trimmer.

If necessary, the same stain proofing can be applied to the glossy side of the foil having the above-described bonding treatment.

Bonding treatments applied on the drum's drum or polished side have lower peel strength (about 8 lbs / inch than 12 lbs / inch) than the same bonding treatments applied on the polished side of the foil. Of course, such low peel strength is more appropriate for MLB. On the other hand, brown oxide treatment, which is currently carried on the glossy side of the foil to provide very low peel strength, substantially improves the peel strength of the base foil due to its peaks, valley topography and micro-roughness. When applied at about 6 lbs / inch, the desired level of about 6 lbs / inch, a relatively small amount of brown oxide foil is applied when applied to the untreated polished surface of the foil that has no foil at all. Should be applied on matte surfaces. This reduced amount of brown oxide is structurally much weaker than the higher amount of brown oxide currently applied to the glossy surface of the foil to achieve the same peel strength. When brown oxide treatment is performed on the matte surface of the foil, the need to reduce cupric oxide to metallic copper is eliminated, and the quality of MLB (particularly resistant to delamination due to dielectric properties and solder shock) As it improves, the whole process becomes simpler and cheaper.

When the glossy surface of the copper foil is used as the processing surface of the foil, cleaning and roughening before application of the resist (etch resist and plating resist) is important. Since the surface area to which the resist adheres is small, the surface must be in optimal condition in order for the resist to adhere and provide a successful etch. Areas where the resist lifts the edges of the circuit traces or areas with deep gouges that the resist does not completely cover are etched-through traces that may require expensive repairs or even scraping the entire substrate. It may mean. Such cleaning and roughening of the process surface of the copper foil is carried out by known mechanical scrubbing and micro-etching techniques in which the need is prevented by the copper foil in accordance with the present invention.

The following example describes a preferred embodiment of the present invention and illustrates some of its effects.

Yes

A web of 35 micron thick (so-called 1 ounce foil by weight per surface area) of a base (or raw) foil is disclosed in US Pat. No. 5,215,646 (Wolski et al.) Except that only the primary anode is used, not the secondary anode. Prepared by electrodeposition of copper on a rotating drum cathode, using the electrolyte, grain-refining agents and plating parameters described in column 17.

This base foil has a smooth or shiny one top surface and its half-time top surface that is matt due to its complex micro-topography. The second surface consists of micro-peaks and micro-valges that together form the micro-roughness of the matte side. The matte side of the foil was examined for micro-roughness (by stylus-type instrument) and found to be 210 microinches.

The above-described base foil is further processed to provide a multi-layer (copper dendritic layer, copper gilding layer and barrier layer) bonding treatment on the glossy side of the foil and to provide a stainproofing layer that is easily removable on the glossy side of the foil. Passed through the fire.

This multilayer bonding process was applied to the glossy side of the foil employing the technique, plating parameters and electrolyte solution described in the referenced US Pat. No. 4,572,768 (Wolsik et al.) To produce a treated side.

On the matte side of the foil was provided a stainproofing film that was easily removable (by dissolution in a 5% solution of sodium hydroxide or potassium). The technique of the electrolytic copper stainproofing process used was based on the referenced US Pat. No. 3,853,716 (Yates et al.) Using an electrolyte comprising:

CrO ₃ - 1.0 g / l

Zn (added as ZnSO ₄ )-0.4 g / l

_{H 3 PO 4 - 0.5 g /} l

H ₂ O-Balance

pH-3.9

T-90 ° F.

The stainproofing layer was electrolytically deposited on the matte side of the foil (used as cathode) using a current density of ² A / ft ² and a plating time of 1.5 seconds. The final stainproofing layer was examined to find that it contained metal zinc and chromate and had a chromium to zinc ratio of 1.85: 1.0.

The following test was done to the copper foil manufactured by the above-mentioned method.

Two variants

1.treated-side-down,

2. matte-side-down

And laminated (adhesive) onto prepregs (composite materials of glass fiber tissue and epoxy resin) designated FR4 by the National Electrical Manufacturer's Association (NEMA).

The peel strength of each of the treated and matte sides of the prepreg was then measured. The peeling strength of the glossy side of the foil provided with the bonding treatment was 9.8 lbs / inch with respect to the width of the lamination, and the peeling strength of the matte side of the foil was 4.2 lbs / inch.

As described above, the laminated stack was prepared on the matte side and the "cleanability" of the matte surface was examined. The laminate was first immersed in 5% sodium hydroxide solution at room temperature for 30 seconds and then washed thoroughly. The laminate was then immersed in a commercial brown oxide solution, such as 9804/9805 bronze oxide, a solution made by the Mac Dermid Company. The pink matte side of copper quickly became dark brown due to the reaction of copper with sodium chlorite, the main component of the brown oxide solution. This indicates that the stainproofing film is completely removed by immersing in the sodium hydroxide solution so that the stainproofing film can be easily removed. If the stainproofing layer was not removed, the pink matte side would not react with the brown oxide solution and the dark brown color of the copper oxide would not appear on the copper surface.

Claims (16)

a. Electrodeposited copper base foil having a matt surface and an opposite polished surface; b. An oxidation resistant discoloration protective layer on the matte surface comprising zinc and at least one trivalent chromium and soluble in a diluted alkali solution; And c. Electrodeposited multilayer bonding on the glossy surface Copper foil used in the manufacture of a printed circuit board comprising a. The method of claim 1, Copper foil, characterized in that the weight ratio of zinc to chromium of the protective layer is at least 1: 1. The method according to claim 1 or 2, The protective layer is copper foil, characterized in that it comprises 20 to 40wt.% Of zinc and 10 to 20wt.% Of chromium. The method according to any one of claims 1 to 3, Copper foil, characterized in that the weight ratio of zinc to chromium in the protective insect is about 2: 1. The method according to any one of claims 1 to 4, Zinc in the protective layer is copper foil, characterized in that the zinc is not glossy. The method according to any one of claims 1 to 5, The alkali solution is a copper foil, characterized in that the dilute aqueous alkali solution at room temperature. The method according to any one of claims 1 to 6, And said bonding treatment comprises a dendritic copper layer electrodeposited on said gloss surface and a gilding copper layer electrodeposited on said dendritic copper layer. The method according to any one of claims 1 to 7, Copper foil, characterized in that the protective layer is electrodeposited. The method according to any one of claims 1 to 8, The copper foil which further contains the intermediate | middle layer of zinc between a base foil and a protective layer. The method of claim 9, Zinc in the intermediate layer is copper foil, characterized in that the zinc is not glossy. In the method for protecting the copper foil surface against discoloration and oxidation, a. Forming an electrolyte solution containing an aqueous solution containing hexavalent chromium and zinc containing anions; b) dipping the copper foil in the electrolyte solution; And c) dipping the copper foil into the cathode while immersing the copper foil in the electrolyte so that an oxidation discoloration protective layer comprising at least one compound of zinc and trivalent chromium and having a chromium to zinc ratio of at least 1: 1 by weight is formed. Steps to Method comprising a. The method of claim 11, The electrolyte solution is characterized in that it has a pH of 3 to 4.5 pH preferably 3.5 to 4. The method according to claim 11 or 12, The electrolyte solution comprises mineral acid. The method of claim 13, The mineral acid is characterized in that the phosphoric acid (phosphoric acid). The method of claim 11, The zinc is included in the acidic electrolyte in the form of zinc sulfate, and the aqueous solution comprises mineral acid to dissolve the zinc sulfate. The method according to any one of claims 11 to 15, And applying an intermediate layer of zinc to the base foil before applying the protective layer.