THROUGH-HOLE PLATING

Abstract: Compositions and process for the plating of through-­ holes in an epoxy printed circuit board base material in the manufacture of printed circuit boards. The treatment process is designed to increase adhesion between the ep­ oxy board material and a subsequently deposited metal plate. The process is characterized, in part, by the use of an activated permanganate solution.

Description: Background of the Invention 1. Introduction This invention relates to the formation of through-hole and multilayer printed circuit boards using an epoxy printed circuit board base material, and more particularly, to an electroless metal plating process for the fabrication of such boards employing a pretreatment sequence including contact with an activated permanganate solution. 2. The prior art When preparing printed circuit boards having more than one circuit pattern, conductive holes, known in the art as through-holes, pass through the board to accomodate insertion and soldering of electrical component leads and to make electrical connections between two or more circuit patterns. Conductive through-holes are conventionally introduced by drilling or punching holes through a copper clad, rigid board followed by a plating procedure. The holes are usually plated by a copper reduction procedure known in the art as electroless copper plating such as disclosed by Coombs, Jr., "Printed Circuits Handbook", McGraw-Hill Book Co., N.Y., N.Y., 1967, chapter 5 and by Draper, "Printed Circuits and Electronic Assemblies", Robert Draper, Ltd. Teddington, 1969, Chapter 6, both incorporated herein by reference. The copper clad board with plated through-holes can be processed into printed circuit boards using resists and processes such as those disclosed in the aforementioned "Printed Circuit Handbook" or, for example, in any of U.S. Patent Nos. 3,469,982; 3,526,504; 3,547,730; 3,622,344 and 3,837,860, all incorporated herein by reference. As is known in the art, the copper reduction process involves pretreatment with a catalytic solution catalytic to the deposition of an electroless metal such as a tin-palladium colloid, followed by electroless metal plating, typically an electroless copper deposition solution. Processes of this nature are disclosed in U.S. Patent No. 3,011,920 incorporated herein by reference. Printed circuit board failure often occurs as a consequence of poor adhesion of the metal plated onto the sidewalls of the through holes. There are several causes of poor adhesion. For example, when a through-hole is drilled through the board, a substantial temperature increase occurs at the interface between the drill bit and the wall of the through hole as a consequence of friction. This results in melting of the plastic substrate at its interface with the drill bit. The melted plastic on the surface of the hole wall forms in a smooth smear over the surface of the hole which is poorly adsorptive of plating catalyst. Another cause of poor adhesion is the presence of minute particles of the plastic formed during the drilling or punching operation. These particles can interfere with the bond between the metal plate and the wall of the through hole. When a circuit board base material comprising glass filled epoxy is used as the circuit board base material, the fibrils of the glass fiber often protrude into the hole and interfere with the bond between the subsequently deposited metal and the side wall of the hole. An advance in the art of printed board manufacture was the introduction of the multilayer board where multiple circuits are stacked in layers on a circuit board and pressed into a multilayer board. Each circuit is separated from another by a layer of a dielectric material. In a manner similar to the double sided through-hole circuit board, holes are formed in the multilayer stack to form interconnections between the same. Adhesion of copper to sidewalls of through-holes is more critical in the manufacture of multilayer boards than in the manufacture of the two sided through-hole board. However, because there are a stack of multiple circuits, through-hole plating is more difficult than through-hole plating of double sided boards and good and consistent adhesion of the copper to the sidewall is more difficult to achieve. Another problem in the manufacture of multilayers is epoxy smear over the copper of the copper circuit protruding into the through-hole. For electrical contact between the inner layer circuit and the metal on the hole wall, there must be contact between the two. The epoxy smear often acts as a dielectric preventing this contact. Consequently, treatments such as sulfuric acid or chromic acid etching or plasma etching have been used to remove this smear. However, these treatments smooth the hole walls further increasing the problem of adhesion of a metal plate to the sidewall of the hole. FR-A-2 290 822 discloses a method of a removing smear from the walls of through-holes of printed circuit boards comprising treatment of the board base material with an aqueous alkaline solution of permanganate maintained at an elevated temperature during a maximum period of 50 minutes at a pH of at least 13 and preferably within a pH range of from 13.4 to 13.5. Summary of The Invention The invention described herein is a process for metal plating the side-walls of through-holes in the manufacture of printed circuit boards using epoxy resin printed circuit board base materials. The process results in reproducible copper to epoxy bonds that are sufficiently strong to minimize rejects in printed circuit board manufacture. The invention is applicable to the manufacture of both double-sided printed circuit boards and multilayer circuit boards. The improvement of the process of the invention comprises pretreatment of the epoxy circuit board base material between the steps of forming the holes and metal plating with a pretreatment sequence comprising the steps of contact with a first solution of an oxygenated epoxy solvent at an elevated temperature for at least one minute in order to remove epoxy particles and to open the surface of the epoxy to penetration by subsequent treatment solutions, contact with a second solution comprising an aqueous alkaline solution of permanganate ions maintained at an elevated temperature for at least two minutes so as to activate the walls of the holes, and contact with a third solution of a reducing agent for permanganate ions at an elevated temperature for at least one minute. The solvent pretreatment uses oxygenated water soluble solvent for epoxy resin, preferably diluted with water. The permanganate solution is one that is activated by high pH. The reducing step preferably uses an amine reducing solution that effectively removes permanganate ions from the surface of the treated part. The combination of steps described, together with the conventional steps involved in metal plating, reproducibly produce an epoxy to metal bond strength of at least 3.5 N/cm (2 lb/in) following the procedures of IPC Test Method No. 2.4.8, Peel Strength, Metal Foil and leaves essentially all metal retained on the surface of an epoxy part following plating as determined by the procedures of IPC Test Method No. 2.4.26 Tape Test for Additive Printed Circuits. Description Of The Preferred Embodiments In accordance with the invention, the substrate used for the fabrication of a printed circuit board is any epoxy substrate conventionally used for such purpose. Typically, the epoxy substrate is filled with glass fibers and copper clad on at least one of its surfaces. As is known in the art, the epoxy can be substituted or mixed with other resins for specific purposes. In the manufacture of a double-sided printed circuit board, a first step comprises the formation of through-holes by drilling, punching or any other method known to the art. Following formation of the holes, it is desirable to employ the conventional steps of desmearing the holes by sulfuric acid or plasma etching followed by glass etching. Thereafter, the processing sequence of the subject invention is employed. The first step, in accordance with the invention, comprises treatment of the walls of the holes with an oxygenated epoxy solvent selected from the group of ketones, ethers and ether alcohols. Preferred solvents include, by way of example, acetone, methylethyl ketone, Methyl Cellosolve®, butyl carbitol, butyl carbitol acetate, Butyl Cellosolve® and Butyl Cellosolve® acetate. The epoxy solvent may be used as a 100% solution, but is preferably an alkaline aqueous solution to decrease the attack on the surface of the epoxy. Preferably, the solvent is diluted with water whereby the solvent comprises from 10 to 70% of the solution and more preferably, from 15 to 40% of the solution. As above, the solution is preferably an alkaline solution having an alkaline normality of from 0.05 to 0.80 N and more preferably, from 0.10 to 0.5 N. Any source of alkalinity is suitable, hydroxides and amines being preferred. Regardless of the strength of the solvent solution, to obtain peel strengths consistently over 3.5 N/cm (2 lb/in), the solvent solution is desirably used at an elevated temperature of at least 43°C and preferably, at a temperature ranging between about 54°C and 74°C. The treatment may be by either immersion of the part in a solution of the solvent or by spraying. The time of contact with the solvent solution is dependent upon the strength of the solvent used and its temperature and may vary from about 1 minute to 10 minutes, but more conventionally, varies from about 3 to 8 minutes. Following contact of the epoxy circuit board base material with the solvent, the part is water rinsed and treated with a permanganate solution. The permanganate solution used is one preferably having a pH higher than that conventionally used for the treatment of plastics for decorative plating. Though a pH within the range of 11 to 13 is operative, when combined with the other steps of the pretreatment process of the invention a reproducible peel strength exceeding 3.5 N/cm (2 lb/in) is not obtainable from part to part unless the permanganate solution is activated by an elevated pH exceeding 13. Accordingly, in accordance with the subject invention, the pH should exceed 13 and should range between about 13.1 and 13.6. Moreover, harsh treatment conditions are desirable with the temperature of the permanganate solution being maintained elevated, preferably in excess of 60°C and preferably ranging between about 74°C and 93°C. The immersion time in the permanganate solution may vary from about 2 to 20 minutes and preferably varies betewen about 3 and 10 minutes. It should be noted that the preferred operating pH may result in a decreased permanganate stability. However, the concomittant increase in peel strength and the consistency in the results obtained by the process of the subject invention more than offset the disadvantage of reduced permanganate stability. Any metal salt of permanganate that is stable and soluble to the extent of at least 2 grams per liter in water can be used to form the permanganate solution of the invention, but it is preferred to use an alkali metal salt, e.g., a salt of sodium, potassium, lithium or cesium, etc., or an alkaline earth metal salt, e.g., a salt of calcium, etc. as the permanganate salt. Especially preferred because of ready availability at reasonable cost and good solubility are sodium and potassium permanganates. The concentration of the permanganate in solution preferably varies between 10 to 100 grams per liter and more preferably, varies between 20 and 50 grams per liter. The pH of the permanganate solution can be controlled by the addition of a base. A suitable base is a hydroxide having a common ion with the permanganate salt. Sodium and potassium hydroxides are preferred. A buffer can be added to the permanganate solution, and it has been found that peel strength is increased if a buffer is included in the formulation. Those buffers known to the art are suitable, phosphates, borates and carbonates being examples of suitable buffering compounds. In addition to buffers, the permanganate solutions can also contain conventional wetting agents to improve the wettability of the treatment solution. Following treatment with the permanganate solution, the epoxy circuit board base material is water rinsed and preferably treated with an aqueous acidic solution of a reducing agent. This agent reduces any manganese residues left on the surface of the part following water rinsing. Typical reducing agents include agueous solutions of stannous ions, bisulfite ions, hydroxylamine, sugars, etc. The solution is acidified to dissolve the manganese residues which are insoluble in alkaline solution. The part is immersed in an aqueous solution containing from about 0.5 to 25 grams of the reducing agent per liter of solution and then water rinsed to remove residual manganese residues left on the surfaces of the hole sidewalls. Treatment of the reducing agent is preferable for from about 1 to 10 minutes at a temperature of from 54°C to 82°C. A suitable reducing solution is disclosed in U.S. Patents Nos. 4,592,929 and 4,610,895 incorporated herein by reference. The reducing solution, in a preferred embodiment, comprises an aqueous acid solution of a hydroxyl substituted lower molecular weight amine and an alkylene perfluoroalkyl sulphonate. The concentration of the compound varies between about 0.1 and 20 grams per liter of solution. Following the pretreatment sequence described above, the through-holes of the epoxy printed circuit board base material are ready for metallization and may be processed into circuit boards in accordance with conventional art recognized procedures. For example, the next step in the conventional process would involve catalysis with a known plating catalyst. The most commonly used catalyst is that formed by the reduction of palladium with tin in a strongly acidic solution. Such catalysts are disclosed in U.S. Patent No. 3,011,920. Following catalysis, it is conventional to apply a step of acceleration which comprises treating the circuit board base material with a mild acid solution believed to dissolve stannic acid colloids on the surface of the catalyst to activate the same as described in the aforesaid U.S. Patent No. 3,011,920. Following catalysis, the part is metal plated with a conventional electroless copper plating solution. Typical electroless copper plating solutions suitable for purposes of the subject invention include those disclosed in U.S. Patents Nos. 3,765,936; 3,728,137; and 3,661,597, all incorporated herein by reference. Other electroless copper plating solutions known to those skilled in the art would also be suitable for purposes of the subject invention. The process of the subject invention is especially useful for the manufacture of through-hole and multilayer printed circuit boards by both additive and subtractive techniques. Procedures for the formation of printed circuit boards are described in numerous publications, for example, in Coombs, Printed Circuit Handbook, referenced above. In a typical process for the manufacture of a printed circuit board, a suitable base material would be copper clad epoxy filled with glass fibers. Holes are drilled or punched at appropriate locations on the board and the walls of the holes are metallized with copper to provide electrical contact between the two surfaces of the circuit board base material. The method for metalizing the holes in accordance with this invention would include the steps of solvent treatment with the oxygenated epoxy solvent described above for a period of about five minutes at a temperature varying between 54°C and 74°C, oxidation with the permanganate solution described above by a treatment for about five minutes at a temperature from about 77°C to 88°C and reduction of the permanganate with the reducing solution described above for about five minutes at a temperature from about 66°C to 77°C. Thereafter, the epoxy board may be treated with a glass etchant to remove fibrils extending into the holes, catalyzed with a tin/palladium catalyst, treated with an accelerator and metal plated with a conventional electroless copper plating solution. Following metalization of the holes, utilizing a process known as pattern plating, a copper pattern would be formed on the copper by application of an organic resist material which may be either a photoresist or a screen resist, dependent upon the desired design and definition. The resist coats the copper that is not part of the conductor pattern and leaves the copper bare in a conductor pattern. Thickness of the copper pattern may be increased using electroless copper plating procedures optionally followed by electrolytic copper plating. Following copper plating, a dissimilar metal etch resist such as solder may be applied over the copper in the conductor pattern to protect the same from subsequently applied etchant, the organic resist is removed exposing unwanted copper (not part of the conductor pattern) and the unwanted copper would be dissolved with a suitable etchant for the copper such as a peroxide/sulfuric acid etchant. Typically, the dissimilar metal etch resist is selected not only for its ability to withstand the etchant, but also for its solderability so that electrical connections can be soldered directly to the etch resist. A multilayer board is made by a similar process except that pre-formed circuits are stacked on top of each other and coated with a dielectric layer. The stack is pressed and heated and holes are then drilled and plated as described above. The invention will be better understood by reference to the examples which follow: Example 1 Preferred treatment solutions in accordance with the invention are as follow: The formulations set forth above constitute the preferred embodiments of the invention. Example 2. A series of 6 epoxy circuit board base test parts were cut to a size measuring 5 x 10 cm. The parts were treated by by the following process sequence: a. drill through-holes at desired locations; b. desmear and etch by immersion in a 98% sulfuric acid solution maintained at 21°C for 20 seconds and water rinse; c. immerse in the solvent solution of example 1 maintained at between 60°C and 66°C for five minutes and water rinse; d. immerse in the oxidizing solution of example 1 maintained at between 80°C and 85°C for five minutes and water rinse; e. immerse in the reducing solution of example 1 maintained at a temperature of between 71°C and 77°C for five minutes and water rinse; f. immerse in cleaner conditioner 1175 of Shipley Company for 5 minutes at a temperature of from 66°C to 71°C and water rinse; g. immerse in a solution of a micro etch identified as Preposit® 746 Etch for 2 minutes at 43°C and water rinse; h. immerse in a pre-drip solution identified as Cataposit® 404 for 1 minute and water rinse; i. immerse in catalyst identified as Cataposit® 44, 3% solution, for 4 minutes at 46°C and water rinse; j. immerse in accelerator identified as Accelerator 19 for 7 minutes at room temperature and water rinse; and k. immerse in an electroless copper plating solution identified as "CP-78 Electroless Copper" (unregistered Trade Name) for 20 minutes at 43°C and water rinse. The copper deposit on the side walls of the through-holes was approximately 2 µm (80·10⁻⁶in) in thickness. The bond strength between the epoxy circuit board base material and the copper plate on the surface of the part was determined by the above identified IPC Tape test. For each of the 6 epoxy parts plated, it was found that in excess of 99% of the copper adhered to the epoxy substrate establishing that the bond strength exceeded 3.5 N/cm (2 lb/in). Examples 3 and 4 The procedure of example 2 was repeated. For one series of parts, the pH of the permanganate solution was maintained at 12.8 and for a second series of parts, the pH was maintained at 13.4. The pH used and the bond strength obtained, as determined by the percentage of copper retained on the epoxy surface, are set forth in the following table: Example 5 The procedure of example 2 was repeated, but the step of solvent treatment (step c above) was omitted. The percentage of copper retained on the surface of the epoxy part following the adhesion test was 5%. Example 6 The procedure of example 2 was repeated, but the step of treatment with the permanganate solution and with the reducing solution (step d and e above) were omitted. The percentage of copper retained on the surface of the epoxy part was 5%.

Claims: In a process for the manufacture of a printed circuit board having a plurality of metal plated through-holes interconnecting at least two circuits comprising in either order the initial steps of forming through-holes in an epoxy printed circuit board base material and forming the circuits, and thereafter metallizing the holes; characterised in that the epoxy circuit board base material is pretreated between the steps of forming the holes and metal plating, with a pretreatment sequence comprising the steps of contact with a first solution of an oxygenated epoxy solvent at an elevated temperature for at least one minute, contact with a second solution comprising an aqueous alkaline solution of permanganate ions maintained at an elevated temperature for at least two minutes, and contact with a third solution of a reducing agent for permanganate ions at an elevated temperature for at least one minute. The process of claim 1 where the pH of the permanganate solution exceeds 11. The process of claim 2 where the pH of the permanganate solution varies between 13.1. and 13.6. The process of claim 3 where the concentration of the permanganate ions in solution varies from 10 to 100 grams per liter. The process of claim 3 where the permanganate ion is derived from an alkali or alkaline earth metal permanganate salt. The process of claim 5 where the permanganate ion is derived from potassium permanganate. The process of claim 6 where the permanganate solution is at a temperature exceeding 60°C. The process of claim 7 where the temperature varies between 71°C and 93°C. The process of claim 1 where the permanganate solution contains a buffer selected from the group of compounds of phosphate, carbonate and borate. The process of claim 9 where the buffer is a phosphate compound. The process of claim 3 where the oxygenated solvent is a solvent selected from the group of ketones, ethers and ether alcohols. The process of claim 11 where the solvent is an aqueous alkaline solution of an aliphatic ether. The process of claim 11 where the solvent is an aqueous alkaline solution of butyl carbitol. The process of claim 11 where the treatment temperature with the solvent varies between 54°C and 74°C. The process of claim 11 where the reducing agent is an aqueous solution of a member from the group of bisulphites, stannous ions, hydroxylamine and sugar. The process of claim 15 where the reducing agent is an acid solution of hydroxylamine sulfate. The process of claim 15 where the treatment temperature of the reducing solution varies between 54°C and 74°C. The process of claim 16 where the solution of the reducing agent contains a surfactant. The process of claim 3 used for the manufacture of a double sided through-hole printed circuit board. The process of claim 3 used for the manufacture of a multi-layered printed circuit board. The process of any one of claims 1 to 20 where the concentration of the oxygenated epoxy solvent in solution varies between 10 and 70% by volume. The process of claim 21 where the concentration varies between 15 and 40%.

Inventors: BONETTI, WILLIAM F., COUBLE, EDWARD C., DECKERT, CHERYL A.
Applicants: SHIPLEY COMPANY INC.
Application year: 1984
Main classification: H05K3
IPC codes: H05K3, C23C18