WO2000050230A1

WO2000050230A1 - Board with nickel-plated thru-holes and/or blind vias

Info

Publication number: WO2000050230A1
Application number: PCT/US2000/004685
Authority: WO
Inventors: Michael K. Walsh; Chris A. Hunrath; Henry E. Lajoie, Jr.; Mark G. Rodgers
Original assignee: Omg Fidelity
Priority date: 1999-02-24
Filing date: 2000-02-24
Publication date: 2000-08-31
Also published as: JP2000252630A; AU3603900A

Abstract

Boards prepared to receive printed circuits comprised of a dielectric substrate optionally containing at least one inner layer having a metal cladding, e.g., a copper layer on at least one face of the substrate and containing at least one electrolessly-nickel plated thru-hole extending between the surfaces of the two faces of the substrate and/or at least one electrolessly-nickel plated via extending between the surface of the metal-clad face of the substrate and an electroconductive face of an inner layer. The nickel is deposited in a manner such that there will be no continuous nickel film deposited on the surface of the metal cladding. Optionally, the surface of the metal cladding and the surface of the nickel-containing thru-hole and/or via may contain a layer of electroplated copper.

Description

BOARD WITH NICKEL-PLATED THRU-HOLES AND/OR BLIND VIAS

Field of the Invention

The invention pertains to a board prepared to receive a printed circuit comprising a dielectric substrate having a metal cladding on at least one face thereof and containing one or more thru holes and/or one or more blind vias which are electrolessly plated with nickel without the deposition of any continuous nickel film on the metal cladding.

Background of the Invention

In double-sided and multilayer printed circuit boards, it is necessary to provide conductive interconnection between and among the various layers of the board containing conductive circuitry. This is achieved by providing at least one metallized thru-hole and/or blind via in the board requiring electrical connection. The predominant method for providing conductive thru-holes and blind vias is by electroless deposition of a metal, typically nickel, on the non-conductive thru-holes and/or blind vias drilled or punched through the board. A "thru-hole" is understood in the art to be a hole which extends from the surface of, and is in communication with, an electrically-conductive surface of one face of a dielectric substrate to an electrically-conductive surface of the opposite face of the dielectric substrate. A "blind via" is understood in the art to be a hole which extends from the surface of, and is in communication with, an electrically-conductive surface of one face of a dielectric substrate to an electrically-conductive surface of an inner layer interposed between the two faces of the dielectric substrate.

There are many prior art methods for the application of a metal coating to an electrically non-conductive, i.e., dielectric, surface in order to produce printed circuit boards which will conduct an electrical current in accordance with the patterns of conductive metal coated on their surface(s). These methods have involved the following three basic steps: (1) treating the surface of the substrate with an agent to make it catalytically receptive to electrolessly- formed metal deposits; (2) electrolessly depositing a metal on the surface of the treated substrate; and (3) electrodepositing a plating metal over the electrolessly-formed metal deposits. The pattern of the printed circuit is achieved through the use of screen or photoresist imaging.

The substrate may initially be metal-clad or not; however, most boards have a metal cladding, e.g., a copper foil or plate, at the beginning of the process, which is subsequently removed in the nonpattern areas. The latter processes are referred to as subtractive.

In typical processes associated with printed circuit board manufacture wherein thru-hole and/or blind via metallization is employed, the catalytic material most often comprises palladium metal. The process of applying the catalytic material to the substrate surfaces typically involves contact of the surfaces with a true or colloidal solution of palladium and tin compounds, see, e.g., U.S. 3,011,920 and 3,532,518. In most cases, catalysis of the substrate surface is followed by an acceleration step which exposes or increases exposure of the active catalytic species.

Following deposition of the catalytic material on the substrate surface, the surface is electrolessly plated by contact with an aqueous metal solution in which plating by chemical reduction leads to the deposit of metal from the bath onto the catalyzed surface.

The thru-holes and/or blind vias are usually plated with a nickel or copper reduction procedure known to the art as electroless nickel or copper plating, such as that described by Clyde F. Coombs, Jr. in Printed Circuit Handbook , 3rd Edition, McGraw-Hill Book Co., N.Y., N.Y., 1988, Chapter 12.5, which is incorporated herein by reference in its entirety.

Methods of the type described above have proven to be expensive and demanding of strict process controls. Further limitations on the use of these processes result from the chemical susceptibility of the electroless metal layer, and by the required use of hazardous and toxic chemical agents. Efforts to overcome these disadvantages have met with only partial success in the past, and have brought with them their own disadvantages. Description of the Prior Art

The prior art is replete with a variety of processes for the preparation of two-sided and multilayer circuit boards as well as process for forming and metallizing thru-holes and blind vias. Examples of prior art which describe processes for preparing two-sided circuit boards containing metallized thru-holes include U.S. Patents 5,770,032 and 5,648,125 assigned to the present assignee, as well as U.S. Patents 4,782,007; 4,806,200; 4,931,148 and 5,474,798 assigned to Macdermid, Incorporated. Examples of patents which describe processes for preparing multilayer circuit boards containing metallized blind vias include U.S. Patent 4, 211, 603 assigned to Tektronix, Inc. and U.S. Patent 4,756,930 assigned to Macdermid, Incorporated.

In a typical process for the manufacture of a single- or double-sided printed circuit board, suitable substrates generally comprise laminates consisting of two or more plates or foils of copper which are separated from each other by a layer of non-conductive material. The non-conductive layer or layers are preferably comprised of a ceramic or an organic material such as an epoxy resin impregnated with glass fibers. Holes are drilled or punched at appropriate locations on the board, providing side-to-side connections when metallized. Thereafter, the board is treated with a cleaning composition, typically alkaline, which removes soils and conditions the through-holes, followed by a slow acid etching treatment which is used for removal of copper surface pretreatments, oxidation, and presentation of uniformly active copper. Typical compositions for this microetching step are persulfates and sulfuric acid-hydrogen peroxide solutions. The board is next catalyzed with a neutral or acid solution of tin/palladium catalyst, which deposits a thin layer of surface-active palladium in the through-holes and on the surface of the board. Any residual tin on the board surfaces and through-holes is removed by treatment with an accelerator composition. The board is then ready for electroless copper plating, which is typically carried out with an alkaline chelated copper reducing solution that deposits a thin copper layer in the through-holes and on the surfaces of the board. After acid-dipping, commonly with sulfuric acid, the board is metal plated with a conventional copper plating solution. It is more usual, however, to precede this metallization step with an imaging step. In a process known as pattern plating, a dry film photoresist is applied to the board and then exposed to transfer the negative image of the circuit, after which it is developed to remove the unexposed portions. The resist coats the copper that is not part of the conductor pattern. Thickness of the copper pattern is increased by electrolytic copper plating. The imaged dry film resist is then removed, exposing unwanted copper, i.e., copper which is not part of the conductor pattern; the unwanted copper is dissolved with a suitable etchant, e.g., cupric chloride, ferric chloride, ammoniacal copper, sulfuric acid/hydrogen peroxide, etc.

A multilayered printed circuit board is made by a similar process, except that performed circuit boards are stacked on top of each other and coated with a dielectric layer. The stack is pressed and bonded together under heat and pressure, after which desired thru-holes and/or blind vias are drilled and plated in the above-described manner. However, one problem present with the manufacture of multilayer printed circuit board containing blind vias is that the drilling of the holes causes resin "smear" on the exposed conductive copper metal interlayers, due to heating during the drilling operation. The resin smear may act as an insulator between the later plated-on metal in the blind vias and these copper interlayers. Thus, this smear may result in poor electrical connections and must be removed before the plating-on operation.

Various alkaline permanganate treatments have been used as standard methods for desmearing surfaces of printed circuit boards, including the thru-holes and blind vias. Such permanganate treatments have been employed for reliably removing smear and drilling debris, as well as for texturing or microroughening the exposed epoxy resin surfaces. The latter effect significantly improves through-hole metallization by facilitating adhesion to epoxy resins. Other conventional smear removal methods have included treatment with sulfuric acid, chromic acid, and plasma desmear, which is a dry chemical method in which boards are exposed to oxygen and fluorocarbon gases, e.g., CF₄.

Generally, permanganate treatments involve three different solution treatments used sequentially. They are (1) a solvent swell solution, (2) a permanganate desmear solution, and (3) a neutralization solution. Typically, a printed circuit board is dipped or otherwise exposed to each solution, with a deionized water rinse between each of the three treatment solutions. When the desmearing process is continued, it results in a cleaned and resin-free surface of the inner-layer copper, promoting better adhesion to the latter-applied metallized layer. All of the prior art processes pertaining to the preparation of two-sided and multilayer circuit boards described above are generally satisfactory and produce an acceptable product. Nevertheless, they all suffer from a common problem, i.e., how to metallize the surfaces of the thru-holes and/or blind vias without forming a continuous metal film on the surfaces of the metal cladding on the dielectric substrate. All of these prior art processes will, in fact, form such a continuous metal film on the metal cladding, thereby necessitating the use of special plating masks and/or solder masks on the surface of the metal cladding to prevent the formation of a continuous metal film thereon. Accordingly, additional steps resulting in increased labor and material costs are associated with such prior art processes.

Summary of the Invention

It is an object of the invention to provide a board prepared to receive a printed circuit containing a metallized thru-hole and/or via.

It is a further object of the invention to provide a process for metallizing the surfaces of thru-holes and/or blind vias which reduces the number of steps and the variety of chemicals currently necessary to produce circuit boards containing such metallized thru-holes and/or blind vias.

It is yet a further object of the invention to provide a process which will cause the uniform metallization of only the thru-holes and/or blind vias to the desired metal thickness without any continuous metal film being deposited anywhere else on or in the circuit boards.

It is still a further object of the invention to provide a process which will cause the uniform metallization of only the thru-holes and/or blind vias to the desired metal thickness without the need for any special plating masks and/or solder masks to prevent concurrent metallization of conductive layers present on the surfaces of two-sided and multilayered circuit boards.

Additional objects and advantages of the present invention will be apparent from the description set forth below, all of which may be realized and attained by means of the features set forth in the appended claims.

In accordance with the present invention, there is provided a board prepared to receive a printed circuit and a process for preparing same. The board comprises:

(a) a dielectric substrate having two faces and optionally at least one inner layer having an electrically conductive surface interposed between the faces;

(b) a metal cladding on at least one of the faces of the substrate; and

(c) nickel electrolessly deposited on the surface of the thru-hole and/or the blind via, with the proviso that there is no continuous layer of nickel present on the surface of the metal cladding. Typically, the dielectric substrate will have a thickness of about 1 to about 500 mil and may be comprised of a ceramic or a resin such as an epoxy, a polyimide, teflon (i.e., polytetrafluoroethylene), a polyacrylate (or polymethacrylate), a silicone or a polycyanate. The dielectric substrate which is predominantly used to prepare printed circuit boards is a fiberglass-reinforced epoxy resin. Preferably, the metal cladding will comprise a copper layer, e.g., a foil or plate, having a thickness of about 0.3 to about 2.0 mil bonded to at least one face of the substrate. Most printed circuit boards will also contain a layer of electroplated copper having a thickness of about 0.8 to about 1.5 mil on the surface of the bonded copper layer. Many printed circuit boards will have a copper layer having a thickness of about 0.3 to about 2.0 mil bonded to both faces of the substrate and an electroplated copper layer having a thickness of about 0.8 to about 1.5 mil on the surface of each bonded copper layer.

Typically, a thru-hole formed in the substrate will have a diameter in the range of about 10 to about 40 mil, while a blind via formed in the substrate will have a diameter in the range of about 5 to about 20 mil. In general, the thickness of the electrolessly deposited nickel on the surface of the thru-hole and/or blind via will be in the range of about lto about 500 microinches. For those boards which will not receive a layer of electroplated copper on the surface of the metal cladding and on the surface of the electrolessly deposited nickel, the thickness of the electrolessly deposited nickel will typically be at least 10 microinches. If the board will contain an electroplated copper layer on the surface of the metal cladding and on the surface of the electrolessly deposited nickel, the thickness of the nickel deposit is typically about 1 to about 15 microinches.

The process for preparing the board to receive printed circuitry comprises the following steps:

A. forming at least one thru-hole extending through both faces of the substrate and through the metal cladding and/or forming at least one blind via extending through the metal-clad face and through the metal cladding thereof to an electrically-conductive surface of said inner layer , such that the substrate is exposed within the thru-hole and /or the blind via;

B. to the extent necessary, deburring the thru-hole and/or the blind via;

C. to the extent necessary, desmearing the substrate containing the metal cladding, the thru-hole and/or the blind via; and

D. electrolessly-depositing nickel on the surface of the thru-hole and/or the blind via in a manner such that there will be no continuous nickel film deposited on the surface of the metal cladding. Steps A, B and C are well known in the prior art and numerous techniques exist for carrying out these steps. Such techniques may be exemplified as follows:

The thru-holes and/or blind vias are typically formed by drilling, punching, high intensity laser beam ablation, of the metal-clad substrate at the desired locations. The thru-holes will penetrate the entire board, i.e., through both faces, while the blind vias will penetrate a metal-clad face of the board and penetrate into the interior of the board so as to contact an electroconductive surface of an inner board, but will not penetrate to the other side of the board. Procedures for forming the thru-holes and/or blind vias often result in the creation of burrs or other irregularities surrounding the holes which, if present, will seriously interfere with the ultimate plating process and must therefore be removed by deburring. Typical deburring steps involve sand , vapor blasting, mechanical abrasion, etc.. Typically, the resultant board is desmeared to the extent necessary by conventional alkaline permanganate treatment or by treatment with sulfuric acid, chromic acid or by plasma desmear, which is a dry chemical method in which the board is exposed to oxygen and a fluorocarbon gas such as CF₄. Generally, permanganate treatments involve three different treatments used sequentially: (1) a solvent swell solution, (2) a permanganate desmear solution and (3) a neutralization solution. Typically, the board is immersed in each solution, with a water rinse between each of the three treatment solutions.

Step D may be carried out by the following sequential steps: (i) conditioning the dielectric substrate subsequent to optional steps B and/or C with one or more wetting agents and/or surfactants ;

(ii) to the extent necessary, rinsing the conditioned dielectric substrate resulting from step (i) with water; (iii) optionally microetching the metal clad surface(s), typically bonded copper foil layer(s), of the dielectric substrate resulting from step (ii); (iv) contacting the conditioned dielectric substrate subsequent to optional step (iii) with an aqueous sensitizing solution comprising (a) ions of a metal selected from the group consisting of Group NIII and IB transition metals, (b) stannous ions, and (c) an acid and/or a buffering salt; (v) to the extent necessary, rinsing the sensitized substrate resulting from step (iv) with water;

(vi) activating the sensitized substrate resulting from step (v) with an aqueous solution of a salt of a metal that is autocatalytic to nickel; (vii) to the extent necessary, rinsing the activated substrate resulting from step (vi) with water; (viii) contacting the activated substrate resulting from step (vii) with an aqueous acidic electroless nickel deposition solution comprising (a) nickel ions, (b) a precipitation-preventing complexing agent, (c) a reducing agent capable of reducing the nickel ions to nickel metal in an acidic state said reducing agent not containing any formaldehyde or any formaldehyde-generating compositions; (d) one or more stabilizers; and (e) one or more copper complexing agents; and (ix) to the extent necessary, rinsing the substrate resulting from step (viii) with water. In step (i), the conditioning is generally carried out by contacting the board with an aqueous neutralizing and conditioning solution which typically comprises water, hydrogen peroxide, at least one acid sulfate compound such as sodium bisulfate (neutralizer) and one or more organic amines and wetting agents, e.g., alkyl glycol ethers and/or anionic, cationic, nonionic and/or amphoteric surfactants, e.g., polyoxyethylene octyl phenol condensates, ethylene oxide-acetylenic glycol adducts, etc. Steps (ii), (v), (vii) and (ix) pertaining to rinsing with water is optional. However, rinsing with water, preferably deionized water, is recommended between each step in order to remove materials or compositions which have come into contact with the surface of the board during the preceding step. Step (iii) pertaining to microetching of the metal clad surface is well known in the prior art and is optional in nature. The purpose of this step is to provide a microtoothed surface structure to the metal-clad surface to facilitate bonding to a later- applied layer such as a layer of copper applied by electroplating as well as to provide anchoring sites for the later-applied sensitizer and activator. Step (iv) pertaining to sensitizing of the metal clad surface is well known in the prior art. Typically, the board is immersed in the sensitizer for about 10 seconds to 10 minutes at a temperature of about 20 to about 50° C. The sensitizer will typically consist of an aqueous solution comprising (a) ions of a metal selected from the group consisting of Group NIII and IB transition metals, (b) stannous ions, and (c) an acid and/or a buffering salt. The metal may be one such as manganese, iron, cobalt, nickel, copper, platinum, silver, gold or palladium (which is preferred) in the form of a salt such as a chloride or nitrate. The stannous ions will typically be present in the form of stannous chloride. The buffering salt is usually sodium chloride.

The next step, i.e., step (vi), pertains to activation of the sensitized surface. This step is well known in the prior art and is sometimes referred to as the seeding step. This step involves contacting the sensitized board with an aqueous solution of a salt of a metal that is autocatalytic to nickel. The activation step is typically carried out by immersing the sensitized board in the activator solution at a temperature of about 20 to about 50° C for about 10 seconds to about 5 minutes. The activating solution typically contains a salt of a Group VIII or IB noble metal such as a chloride of platinum, silver, gold or palladium, the latter being preferred, together with an acid such as hydrochloric acid which is conveniently utilized in a concentration of about 0.1 to 2 normal.

The key feature of this invention is that of step (viii) relating to the electroless deposition of nickel only on the surfaces of the thru-holes and/or the blind vias, i.e., such that there will be no continuous nickel film deposited on the metal clad surface. This result is achieved by immersing the board, preferably with agitation, in a bath of an aqueous acidic electroless nickel metal deposition solution comprising (a) nickel ions, (b) a precipitation-preventing complexing agent, (c) a reducing agent capable of reducing the nickel ions to nickel metal in an acidic state, said reducing agent not containing any formaldehyde or any formaldehyde-generating compositions; (d) one or more stabilizers; and (e) one or more copper complexing agents.

The electroless nickel deposition step is carried out at a temperature of about 20 to about 50°C, for a period of time of about 15 seconds to about 5 minutes. Preferably, the board is contacted with the electroless nickel solution in a bath accompanied by agitation of about 2to about 20, preferably 5 to 15, bath turnovers per hour. Alternatively, the board may be placed on a conveyor belt which will move through the bath at a speed corresponding to the contact time and bath turnover rate indicated above. Such conditions will insure that the nickel will be deposited on the surfaces of the thru- holes and/or blind vias in a thickness of about 2 to about 15 microinches. If it is intended that the boards receive a later-applied layer of electroplated copper, the nickel thickness may be somewhat less, e.g., about 1 to about 10 microinches. Thicker nickel deposits may be readily achieved by carrying out step (viii) at the higher end of the contact times and temperatures within the ranges indicated above (conversely, thinner nickel deposits may be achieved by utilizing contact times and temperatures at the lower ends of the ranges indicated above).

The nickel in the aqueous acidic electroless nickel deposition solution will be typically be present in the form of a salt such as nickel sulfate.

The precipitation-preventing complexing agent may be a compound such as ethylenediaminetetraacetic acid or its sodium salts, succinic acid, sodium acetate, sodium citrate, potassium sodium tartrate, nitrilotetraacetic acid or its alkali metal salts, gluconic acid or esters thereof triethanolamine, glucono γ-lactone, ethylenediamine acetates and the like.

The reducing agent capable of reducing the nickel ions to nickel metal in an acidic state will be of the type which does not contain any formaldehyde or any formaldehyde- generating compositions such as an alkali metal borohydride, an alkali metal hypophosphite, dimethylamineborane, isopropylamineborane, sodium trimethoxy - borohydride and the like.

The stabilizer is present to prevent spontaneous decomposition of the nickel- deposition solution; useful stabilizers include lead acetate, chromium acetate, thiodiglycolic acid and the like.

The nickel-depositing solution must contain one or more metal complexing agents to insure that nickel will not be deposited as a continuous film on the surface of the metal cladding on the dielectric substrate during nickel deposition of the thru-holes and/or blind vias. In a typical board, the metal cladding will comprise a layer of copper (usually a copper foil) bonded to the surface of the dielectric substrate and, in such case, the metal complexing agent will be a copper complexing agent continuously maintained throughout the course of step (viii) at a level of about 0.5 to about 4 ppm, preferably 1 to 2 ppm. Suitable copper complexing agents include thiourea, benzotriazole, sodium thiocyanate and mixtures thereof. Typically, the board will receive a layer of electroplated copper subsequent to the nickel deposition in the thru-holes and/or blind vias. The electroplated copper will, of course, deposit not only on the surface of the metal cladding on the dielectric substrate, but also on the surface of the nickel deposited on the surfaces of the thru-holes and/or blind vias. Subsequently the board may receive the desired image (e.g., by photolithography in the form of a photo polymer) and the unwanted portions of the copper dissolved away, leaving the desired circuit pattern on the surface of the metal cladding. Alternatively, the board may be pattern plated by depositing a metal such as copper in the form of the finished pattern onto the metallized and imaged surface of the board. These finishing steps are conventional in nature and do not form part of this invention which is directed to a board prepared to receive printed circuitry and a process for preparing such board.

The following nonlimiting example shall serve to illustrate the invention. Example Fifty test panels were employed in this example. Each panel consisted of a glass fiber-reinforced epoxy board to which copper foil of 1 oz. had been laminated on both sides of the board. Approximately 15,000 thru-holes of about 20 mil diameter were drilled through each board in accordance with industry standard and thereafter deburred using a conventional horizontal deburring machine. Debris resulting from the drilling operation was then removed from the panels by means of the following sequential steps:

1) Hole Swell - The panels were immersed for 5 minutes at 140°F in an aqueous solution ("OMG Fidelity UP310") containing al glycol ether.

2) Rinse - The panels were rinsed in running tap water at 3 gpm into a 40 gallon tank with an immersion time for 4 minutes at

58°F.

3) Permanganate - The panels were immersed for 12 minutes at 175°F in an aqueous solution ("OMG Fidelity UP 315") containing sodium hydroxide and potassium permanganate. 4) Rinse - The panels were rinsed twice in running tap water at 1 gpm into a 40 gallon tank with an immersion time of 2 minutes at 57°F for each rinse.

5) Neutralizer - The panels were immersed into an aqueous solution

("OMG Fidelity UP320") containing sulfuric acid, hydrogen peroxide and sodium bisulfate.

6) Rinse - The panels were rinsed in running tap water at 2 gpm into a 40 gallon tank with an immersion time for 4 minutes at 58°F. The panels resulting from step 6) were then conditioned, activated, sensitized and contacted with an electroless nickel deposition solution as set forth in the following sequential steps: 7) Condition The panels were immersed for four minutes at 115°F in an aqueous solution ("OMG Fidelity UP330") consisting of deionized water, monoethanolamine, diethanolamine, nonionic surfactants and a quaternary amine.

8) Rinse The panels were rinsed in running tap water at 3 gpm into a 40 gallon tank with an immersion time for 4 minutes at

58°F.

9) Microetch The panels were microetched by immersion for 2 minutes at 100°F in an aqueous solution ("OMG Fidelity UP335") consisting of hydrogen peroxide, sulfuric acid and sodium phenolsulfonate. 10) Rinse The panels were rinsed in running tap water at 3 gpm into a 40 gallon tank with an immersion time for 4 minutes at 58°F.

11) Predip The panels were immersed for 1 minute at 84°F in an aqueous solution ("OMG Fidelity UP340A")containing sodium chloride and sodium bisulfate. 12) Sensitize - The panels were immersed for 3 minutes at 95°F in solutions of "OMG Fidelity UP340A" and "OMG Fidelity

UP340B" consisting of deionized water, HC1, stannous chloride, resorcinol and palladium chloride. 13) Rinse - The panels were rinsed twice in running tap water at 1 gpm into a 40 gallon tank with an immersion time of 1 minute at 57°F for each rinse.

14) Activate - The panels were immersed for 1 minute at 87°F in a solution of "OMG Fidelity UP350" consisting of deionized water, HC1 and palladium chloride.

15) Rinse - The panels were rinsed in running tap water at 3 gpm into a 40 gallon tank with an immersion time for 4 minutes at

58°F.

Twenty-five of the panels resulting from step 15) were subjected to electroless nickel deposition by immersion in a "normal" nickel bath for 4 minutes at 125°F and the remaining twenty-five panels resulting from step 15 were subjected to electroless nickel deposition by immersion in a "controlled" nickel bath for 4 minutes at 125°F. At the outset, the composition of each bath was the same, i.e., the "normal" as well as the

"controlled" nickel bath each consisted of 2.5 vol% "OMG Fidelity UP385A" and 15 vol% "OMG Fidelity UP 385B". At the outset, the pH of each bath was 6.2 and it had a nickel content of 3.0 g/1 and a reducer content of 32 g/1 together with copper complexing agents being present in a total amount of 1 ppm.

The "OMG Fidelity UP385A" formulation consisted of nickel sulfate and distilled water. The "OMG Fidelity UP385B" formulation consisted of deionized water, organic acids, borax, sodium hypophosphite, sodium hydroxide, lead acetate, sodium ethylenediaminetetraacetic acid and two copper complexing agents consisting of sodium thiocyanate and thiourea.

The first ten panels went through the "normal" nickel bath without any nickel plating out on the surface of the copper foil and nickel was deposited on the surfaces of the thru-holes in an average thickness of about 5 microinches. At this point, the level of copper complexing agents had dropped to 0.6 ppm. As additional panels went through the "normal" bath, nickel began to plate out on the surface of the copper foil and nickel was continued to be deposited on the surfaces of the thru-holes in an average thickness of about 5 microinches and the level of the copper complexing agents continued to drop. At the point at which the level of the copper complexing agents had fallen to nearly 0 ppm, all of the panels emerging from the "normal" nickel bath, had all of their copper foil surfaces covered with a nickel deposit averaging 12 microinches, which was unacceptable.

In the "controlled" nickel bath, the same procedure was used, except that after the first ten panels went through the bath, additional equal amounts of the two copper complexing agents were added to the "controlled" nickel bath such that the total level of the copper complexing agents was continued to be maintained at a level of 1 to 1.5 ppm. It was noted that all of the panels going through the "controlled" nickel bath had nickel deposited on the surfaces of the thru-holes averaging about 5 microinches, with no discernable nickel deposits on the surface of the copper foils.

Claims

WHAT IS CLAIMED IS:

1. A board prepared to receive printed circuitry comprising:

(b) a metal cladding on at least one of the faces of the substrate; and

(c) at least one thru-hole extending through both faces of the substrate and through the metal cladding and/or at least one blind via extending through the metal-clad face and through the metal cladding thereof to an electrically-conductive surface of said inner layer, said thru-hole and/or blind via containing nickel electrolessly deposited on the surface of the thru-hole and/or the blind via, with the proviso that there is no continuous layer of nickel present on the surface of the metal cladding.

2. The board of claim 1 wherein the metal cladding comprises a layer of copper bonded to at least one of the faces of the substrate.

3. The board of claim 2 wherein a layer of copper is electroplated on the surface of the bonded copper layer and on the surface of the nickel electrolessly deposited on the surface of the thru-hole and/or the blind via.

4. The board of claim 1 wherein the substrate has a thickness of about 1 to about 500 mil.

5. The board of claim 1 wherein the substrate is comprised of a ceramic or a resin selected from the group consisting of epoxy resins, polyimides, teflon, acrylic resins, silicone resins and cyanate resins.

6. The board of claim 5 wherein the substrate is comprised of a fiberglass- reinforced epoxy resin.

7. The board of claim 2 wherein the bonded copper layer has a thickness of about 0.3 to about 2.0 mil.

8. The board of claim 3 wherein the electroplated copper layer has a thickness of about 0.8 to about 1.5 mil.

9. The board of claim 3 wherein each face contains a copper layer bonded thereto, each such layer having a thickness of about 0.3 to about 2.0 mil and an electroplated copper layer having a thickness of about 0.8 to about 1.5 mil on the surface of each bonded copper layer.

10. The board of claim 1 wherein the thru-hole has a diameter in the range of about 10 to about 40 mil.

11. The board of claim 1 wherein the blind via has a diameter in the range of about 5 to about 20 mil.

12. The board of claim 2 wherein the thickness of the electrolessly deposited nickel on the surface of the thru-hole and/or the blind via is in the range of about 1 to about 500 microinches.

13 The board of claim 3 wherein the thickness of the electrolessly deposited nickel on the surface of the thru-hole and/or the blind via is in the range of about 1 to about 15 microinches.

14. A process for preparing a board to receive printed circuitry, said board comprising a dielectric substrate having two faces with a metal cladding on at least one of the faces and optionally at least one inner layer having an electrically conductive surface interposed between the faces, comprising the steps of: A. forming at least one thru-hole extending through both faces of the substrate and through the metal cladding and/or forming at least one blind via extending through the metal-clad face and through the metal cladding thereof to an electrically-conductive surface of said inner layer, such that the substrate is exposed within the thru-hole and /or the blind via;

B. to the extent necessary, deburring the thru-hole and/or the blind via;

D. electrolessly depositing nickel on the surface of the thru-hole and/or the blind via in a manner such that there will be no continuous nickel film deposited on the surface of the metal cladding.

15. The process of claim 14 wherein the metal cladding comprises a layer of copper bonded to at least one of the faces of the substrate.

16. The process of claim 15 wherein a layer of copper is electroplated on the surface of the bonded copper layer and on the surface of the nickel electrolessly deposited on the surface of the thru-hole and/or the blind via.

17. The process of claim 14 wherein the substrate has a thickness of about 1 to about 500 mil.

18. The process of claim 14 wherein the substrate is comprised of a ceramic or a resin selected from the group consisting of epoxy resins, polyimides, teflon, acrylic resins, silicone resins and cyanate resins.

19. The process of claim 18 wherein the substrate is comprised of a fiberglass- reinforced epoxy resin.

20. The process of claim 15 wherein the bonded copper layer has a thickness of about 0.3 to about 2.0 mil.

21. The process of claim 16 wherein the electroplated copper layer has a thickness of about 0.8 to about 1.5 mil.

22. The process of claim 16 wherein each face of the dielectric substrate contains a copper layer having a thickness of about 0.3 to about 2.0 mil bonded thereto and an electroplated copper layer having a thickness of about 0.8 to about 1.5 mil on the surface of each bonded copper layer.

23. The process of claim 14 wherein the thru-hole has a diameter in the range of about 10 to about 40 mil.

24. The process of claim 14 wherein the blind via has a diameter in the range of about 5 to about 20 mil.

25. The process of claim 15 wherein the thickness of the electrolessly deposited nickel on the surface of the thru-hole and/or the blind via is in the range of about 1 to about 500 microinches.

26. The process of claim 16 wherein the thickness of the electrolessly deposited nickel on the surface of the thru-hole and/or the blind via is in the range of about 1 to about 15 microinches.

27. The process of claim 14 wherein step D is carried out by the following sequential steps:

(i) conditioning the dielectric substrate subsequent to optional steps B and/or C with one or more wetting agents and/or surfactants ; (ii) to the extent necessary, rinsing the conditioned dielectric substrate resulting from step (i) with water; (iii) optionally microetching the metal clad surface of the dielectric substrate resulting from step (ii); (iv) contacting the conditioned substrate subsequent to optional step (iii) with an aqueous sensitizing solution comprising (a) ions of a metal selected from the group consisting of Group VIII and IB transition metals, (b) stannous ions, and (c) an acid and/or a buffering salt; (v) to the extent necessary, rinsing the sensitized substrate resulting from step (iv) with water; (vi) activating the sensitized substrate resulting from step (v) with an aqueous solution of a salt of a metal that is autocatalytic to nickel; (vii) to the extent necessary, rinsing the activated substrate resulting from step (vi) with water;

(viii) contacting the activated substrate resulting from step (vii) with an aqueous acidic electroless nickel metal-depositing solution comprising

(a) nickel ions, (b) a precipitation-preventing complexing agent, (c) a reducing agent capable of reducing the nickel ions to nickel metal in an acidic state said reducing agent not containing any formaldehyde or any formaldehyde-generating compositions; (d) one or more stabilizers; and (e) one or more metal complexing agents; and

(ix) to the extent necessary, rinsing the substrate resulting from step (viii) with water.

28. The process of claim 27 wherein the metal employed in the aqueous solution in step (iv) comprises palladium.

29. The process of claim 27 wherein the metal employed in the aqueous solution in step (vi) comprises palladium.

30. The process of claim 27 wherein the metal cladding on the surface of the dielectric substrate comprises a layer of copper and the metal complexing agent employed in the aqueous solution in step (viii) comprises a copper complexing agent selected from the group consisting of thiourea, benzotriazole, sodium thiocyanate and mixtures thereof.

31. The process of claim 29 wherein the copper complexing agent is continuously maintained throughout step (viii) at a level of about 0.8 to about 4.0 ppm.

32. The process of claim 31 wherein the copper complexing agent is continuously maintained throughout step (viii) at a level of 1.0 to 2.0 ppm.

33. The process of claim 27 wherein the contacting of the activated dielectric substrate with the aqueous acidic electroless nickel metal-depositing solution in step (viii) occurs at a temperature of about 20 to about 50°C for a period of time of about 15 seconds to about 5 minutes while such solution is agitated at a rate in the range of about 2 to about 20 bath turnovers per hour.

34. The process of claim 33 wherein the solution is agitated at the rate of 5 to 15 bath turnovers per hour.