PROC ESS FOR CONDITIONING TH E SURFAC E OF PLASTIC SUBSTRATES P RIOR TO M ETAL PLATING

Technical Field

This inv ention relates to the metal plating of plastics and, in particular, to enhancing the adhesion of metal plating to polyetherimide plastics, such as, the plastic sub st rate of elect ronic circu it boards, by conditioning the board p rior to metal plating of the board.

Backg round Art

The metal plating of plastic parts is well-known to be of considerable commercial importance because the desirable characteristics of both the plastic and the metal are combined to offer the technical and aesthetic advantag es of each. Thus, a part plated with a b right, metallic finish tak es advantag e of the economies in cost and weight afforded by substituting mold ed plastic parts for metal and, additionally, the plated finishes are not as susceptible to pitting and corrosion because there is no galvanic reaction between a plastic substrate and a plated metal. Such metal plating of plastics is used for a wide range of applications f rom decorative plating to radio f requency shielding.

An important process is the preparation of electronic circuit boards which requires the electroless plating of a conductive metal layer, usually copper, onto the plastic substrate of the board. At present, boards are predominately made of epoxy resins but there is a constant search for different materials which will offer improved economies and other benefits. One such material is a olyetherimide resin which has a higher temperature stability than epoxy and which can be injection molded with through-holes, thus eliminating the costly drilling step now required when preparing epoxy boards. Resins of this type are made by General Electric Company under the trademark ULTEM. For convenience the following description will relate specifically to the process for conditioning polyetherimi e resins although it will be understood that the process may be suitably employed for other resin materials.

These boards vary in design and may have a copper layer on each surface face of the polyetherimide resin (two-sided boards) or they can be multi-layer boards which have a plurality of inter-leaved parallel planar copper and resin layers. In both type boards through-holes in the boards are metal ' plated to facilitate connection between the circuits on the copper layers. The problems in plating either the through-holes or other plastic parts of the board are well-known in the art and a number of methods have been developed to improve the adhesion of the metal plating to the plastic substrate.

While different uses require different adhesive strengths, in general, it has been established that a minimum peel strength of about 8 lbs. per linear inch (approximately 1.4 kilograms per centimeter) as measured by the Jacquet Peel Test is required to prevent a metal coating from blistering or peeling from a plastic surface during manufacture and use. According to ASEP

Guidelines, the test measures the force needed to peel a one-inch (2.5 centimeters) wide plate strip 37.5 micrometers ± 0.51 thick acting at 90° ± 5° to the substrate surface. The measurement is done with an Instron Tensometer programmed to peel the plate strip from the substrate. at the rate of 1.0 + 0.1 inch (2.5 + 0.2 centimeters) per minute.

The present invention treats the plastic surface before plating to enhance the adhesiveness of the metal plating using a procedure generally known as a swell and etch technique and employs solvents which condition or swell the plastic and oxidants to etch the plastic. U.S. Patent No. 3,758,332 discloses the use of chemicals such as methyl ethyl ketone, tetrahydrofuran, dioxane, pyridine, dimethylf orma ide, and an alcohol mixture comprising methyl ethyl ketone, ethanol and methanol as _SW ellants for epoxy resin. The swelled plastic is then exposed to an etchant for a sufficient period of time to etch the swelled surface without unduly weakening or otherwise adversely affecting the physical characteristics of the plastic. Etchants are oxidizing materials and are generally aqueous solutions containing materials such as sulfuric acid, phosphoric acid, permanganate ions, Cr+6 ions and the like. U.S. Patent No. 4,086,128 shows pretreatment of an epoxy resin with an organic solvent comprising alcohols, acids, esters, ketone, nitriles, nitro compounds, and polyhydric

compounds such as ethylene glycol, gylcerine and 1, 2-p ropylene gylcol p rior to etching with hydrogen peroxide and sulfu ric acid. U.S. Patent No. 3,865,623 shows immersion of expoxy resin in an organic solvent 5 such as dimethylformamide to render the epoxy receptive to an acid etch. The disclosu res of the above publications are hereby incorporated by reference.

To be commercially useful, however, it is important 10 that the swell and etch process provide a consistent and reproducible resin conditioning effect which results in the minimum peel strength being achieved practically for g reater than 99% of the parts being conditioned and plated. Without this reproducibility the costs to the ---5 printed circuit board industry would be staggering and result in highly increased manufactu ring expenses.

Disclosu re of Invention 20

It has now been discovered that the adhesiveness of metal plating and, in particular, electroless metal plating, to plastics such as polyetherimide can be enhanced by a swell and etch process using specially

25 formulated compositions und er controlled op erating conditions. The method includes first exposing the plastic for a suitable time to a swellant composition comprising a solution of a polar material, preferably having a dielectric constant g reater than about 15 and a

30 dipole moment g reater than about 3 Debye Units (D) and an org anic solvent such as a carboxylic acid, ketone, hyd rocarbon, ether, ester, alcohol, p olyhyd ric, e.g., glycol and gylcol ethers and esters. The swelled plastic is then exposed to an etchant comprising C r+6 ions,

35 an d , p r ef erably, also including an acid such as

H2S04 , at an elevated temperatu re, e.g., above about 160°F., for a sufficient period to etch the swelled su rface without unduly weakening or otherwise adversely affecting the physical characteristics of the plastic.

The resultant etched su rface is now in a condition of improved adhesion for any species such as p rinting inks, paints, coatings, and, in particular, for metal coatings, which may be deposited thereon. As will be app reciated by those skilled in the art, rinsing of the plastic may be performed at any stage of the process and removal of residual etchant or etchant-plastic material may require fu rther treatment if desired.

In general, the process of using the compositions of the invention in manufactu ring printed circuit boards and,- in particular, boards containing through-holes, is a sequ ence of steps comm encing with a laminate or multi-lamin ate m ade f rom, e.g., filled polyetherimide material. A predesigned series of through-holes may be formed in the board by injection molding or d rilling. The board is then contacted with the swellant composition of the invention and, after water rinsing, the board is etched at an elevated temperatu re with an oxidant such as a solution of H2 S04 and C r03 and w ater rin sed. Treatment with a reductant to lower the oxidation state of the chrome residues is then typically performed and the board rinsed providing a board ready for electroless metal plating using conventional p rocedu res. A p referred step is to pretreat the board with a composition such as ENPLATE PC-4459 sold by Enthone, Incorporated to remove hyd rocarbon soils and promote the catalyst and then to contact the board with a composition such as ENPLATE PC-236 to eliminate d rag-in. The board is now immersed in a catalyst, such as a tin-palladium solution, which

conditions the surface of the resin for electroless copper plating. ENPLATE Activator 444 is exemplary of this type of catalyst. The board is then immersed in a post activator such as ENPLATE PA-493 to activate the Scatalyst by removing the excess tin and freeing the metal palladium ions on the board, rinsed and immersed in an electroless copper plating solution for a period of time sufficient to plate copper to the desired thickness on the surfaces and to plate the surfaces of the holes to lOform through-hole connections between the laminate surfaces. ENPLATE CU-700 and other similar plating compositions may be employed. The boards may then be immersed in dilute sulfuric acid followed by electroplating using conventional techniques if a thicker

15coating is desired. Rinsing of the board between steps may be conventionally employed as is well-known in the art.

20Modes for Carrying Out The Invention

It is preferred that the polyetherimide resin contain other materials, preferably a filler or reinforcing material, e.g., glass fibers. Other fillers include

2 >aper, synthetic fibers, carbon black, alumina powders, silica powders, wax, etc. Pigments, mold release agents, and other conventional additives may also be employed in the resin for specific purposes.

30 It has been found that a swellant pretreat ent is necessary to enable the etching process to roughen, or to otherwise condition, the surface and to provide the enhanced adhesion of the invention. The swellant composition comprises a solution of a polar material and

35an organic solvent, which composition is essential to

provide a swelled plastic capable of being etched and plated with the desired adhesiveness at a high degree of reproducability as will be demonstrated in the examples.

The polar materials useful herein are those materials having dielectric constants greater than about 15, preferably greater than about 20, most preferably greater than about 30 and dipole moments greater than about 3 D and preferably greater than about 3.5 D. Exemplary of these materials are dimethylformamide, dimethylacet amide, dimethyl sulfoxide, tetrahydrothiophene dioxide, N-methylpyrrolidone, hexa ethylphosphoric triamide, tetramethylurea, and acetonitrile. Other materials of similar structures may also be utilized as the polar materials hereof. Thus, other sulfoxides which are liquid at or near room temperature may be so employed, e.g., those having the formula

"

R-S-Rl

wherein R and Rl may each comprise alkyl groups of from 1 to 4 carbon atoms. Similarly, other sulfones may be so used, e.g., those of the formula

R2-S02-R3

wherein R2 and R3 may be lower alkyl (1 to 4 carbon atoms) or may be linked to form a tetrahydrothiophene ring.

The second component of the swellant composition may be any suitable organic solvent selected from the carboxylic acids, ketones, hydrocarbons, ethers, esters.

alcohols, polyhydrics, e.g., glycols and polyglycols, including ethers and esters thereof. In general, the organic solvent will typically contain less than about 10 carbon atoms, with the proviso that the polar material be soluble in the organic solvent to produce the desired composition. Exemplary solvents include acetic acid, n-pentane, ethyl acetate, ethanol, methoanol, and the like.

The preferred organic solvents because of their demonstrated effectiveness are the polyhydrics such as the glycols, and the ethers and esters thereof. These solvents may be represented by the formula

RlO(AO)nR ² wherein Rl and R2 are independently selected from the group consisting of hydrogen atoms, aryl groups and alkyl and acyl groups of 1-4 carbon atoms, A is a straight or branched chain C2 to C4 alkylene group, and n is an integer of 1 to 4. Examples of these solvents include ethylene glycol, ethylene glycol m on om ethyl ether, propylene glycol, propylene glycol monomethyl ether, ethylene glycol acetyl ester, etc.

It will be appreciated by those skilled in the art that the polar material and organic solvent swellant mixture may comprise one or more compounds of each ingredient. The swellant composition may also be used in the form of an aqueous solution although it has been found that the water is desirably limited, in weight percent, to less than about 20%, and preferably less than 10% and more preferably less than 5% and even 1%. In general, water decreases the swellant effect of the composition and the adhesiveness of the subsequent metal plating.

Two particularly preferred polar materials are dim ethylsu If o ide (DMSO) and N-methylpyrr olid one and the preferred solvents are the polyhydrics represented by the formula RlO(AO)nR2. In general, compositions of the two components contain, by volume, about 25% to 90% DMSO, preferably 50% to 85% and most preferably 70% to 80%, with the balance being the polyhydric component. For compositions containing N-methylpyrrolidone a range of 15% to 70%, preferably 20% to 50% and most preferably 20% to 30% may be employed. A preferred solvent because of its demonstrated effectiveness is propylene glycol onomethyl ether wherein Rl is H, A is C3, n is 1 and R2 is methyl.

Suitable additives can be employed in the swellant composition for specific purposes such as wetting agents to enhance the capability of spreading the composition on the resin surface.

To practice the method of the invention the plastic substrate is contacted with the swellant composition at an elevated temperature for a sufficient time to render the surface receptive to the etching process. Contacting procedures may vary widely, e.g., 1 to 60 minutes at temperatures up to about 180°F. (82°C), and preferably 2 to 20 minutes at 140° to 160°F. (60° to 71°C). Satisfactory results for glass-filled polyetherimide resin are provided by immersing the part in the composition for about 5 minutes at 150°F. (65°C). The time and temperature will, in general, vary inversely as will be appreciated by those skilled in the art. Other means such as spraying, may be used for treating the plastic part.

The treated plastic part is then ready for oxidative

etching using a chrome containing solution and basically comprises contacting the treated plastic part with the etchant at an elevated temperature for a sufficient time to promote adhesion to the surface. It is preferred to then rinse the etched plastic part to remove excess solution and to remove any chrome residues by neutralization with a material such as sodium bisulfite or chemical reduction using reductants such as hydrazine and oxalic acid.

The aqueous chrome containing etchant solutions are well-known in the art and preferably comprise Cr+6 ions, e.g., Cr207 ⁼ and Crθ3. An acid such as

H2S04 is preferably included in the etchant solution and other acids like H3P04 may also be employed. A preferred etchant composition is an aqueous solution of H2S04 and Cr03. Suitable additives may also be used such as surfactants, e.g., perf luorinated sulfonates, to insure uniform etching of the conditioned resin surface. The concentration of the etchant solution may vary widely with the chromium component added as Cr03 being, by weight, about 100 grams/liter (g/1) to saturation, preferably about 200 to 600 g/1, and most preferably about 300 to 500 g/1. The acid component is about 100 to 500 g/1, and more preferably about 200 to 400 g/1. A preferred composition contains about 400-450 g/1 Cr03, e.g., 420 g/1, and 250-350 g/1 H2S04, e.g., 300 g/1.

An important feature of the invention is the temperature of the etching process. Conventional etching procedures utilize a temperature which may vary over a broad spectrum from as low as room temperature to the boiling point of the etching solution. It has been found for the conditioned polyetherimide resins however, that

temperatu res above about 160°F. (71°C) and p referably above about 170°F. (77°C) are n ecessary and that unexpected adhesive p roperties to the metal coating are p rovided. While the etching time will vary depending on the concentration of the etching solution and temperatu re of the etching p rocess as will be app reciated by those skilled in the art, for the p referred composition of 420 g/1 C r03 and 300 g/1 H2 S04 , an etching time of about 5-15 minutes at 170°F. (77°C) p rovides excellent results. In general, the etching time may be up to about 60 minutes but is typically less than 30 minutes.

It is to be understood and stressed for both the s w ellant and etchant p rocedu res, that the above concentrations, temperatu res and time parameters are all interdependent and that variations in temperatu re will p roduce variations in the other parameters whereby optimum results will be attained. In this regard, the variou s parameters and their interdepend ency are well known in the art and their interaction between one another is also well known or can be easily ascertained experimentally by one skilled in the art.

An optional step of removing traces of deposits on the etched resin may now be performed. Rinsing will remove deposits but a p referred p rocedu re is to contact the resin with a suitable reducing ag ent for chromium ions such as NaHS03 , NaOH, and the lik e. Immersion of the etched resin in a solution of 150 g/1 NaHS03 for 5 minutes has p roduced satisfactory results. Typically, exposu re to the reducing agent rang es f rom 30 seconds to 10 minutes at a temperatu re ranging f rom room temperatu re to 160°F. (71°C).

If a smoother metal plated su rface is desired, another

optional step is to treat the etched resin to dissolve part of the filler material. Materials such as hydrogen fluoride and ammonium bifluoride may be suitably employed. ACTANE 70 sold by Enthone, Incorporated has proven very satisfactory for this purpose.

The etched resin is now prepared for metal plating by known means to render the surface catalytic. Among them are cleaning, applying catalyst promoters, sensitizing using an aqueous tin chloride solution and then activating by means of palladium chloride. On the other hand, unitary baths may be employed for such purposes, such as the dispersions of colloidal palladium and tin ions described in Shipley, U.S. Patent No. 3,011,920 or the soluble complexes of noble metals, stannous ion and anions as described in Zeblisky, U.S. Patent No. 3,672,938. The surface may now be plated, with a film of metal by ^' electroless ^' plating. Activating and plating compositions and methods for copper metal electroless deposition are described in U.S. Patent Nos. 2,874,072; 3,011,920; 3,075,855; 3,095,309; 3,672,938; and 3,736,156; the disclosures of said patents being hereby incorporated reference. Other methods of deposition may also be used such as vacuum vapor deposition, electrolytic plating or a combination of electroless plating and electorlytic plating.

The present invention will now be described in detail by reference to the following examples.

EXAMPLE I

The following example illustrates the process of using the swellant and etchant compositions to enhance the adhesiveness of electroless plated copper to glass-filled polyetherimide resin.

An injection molded plaqu e of ULTEM glass-filled polyetherimide resin was metallized using the following p rocedu re:

(a) immerse the plaqu e for 5 minutes at 150°F. (65°C) with mild agitation in a solution comp rising, by volume, 75 % DMSO and 25 % propylene glycol monomethyl ether (PGMM E);

(b) rinse for 5 minutes in running water;

(c) etch for 10 minutes at 170°F. (77°C) with mild agitation in a solution comp rising 420 g/1 C r03 and 3 00 g/1 H2S04 ;

(d) rinse for 5 minutes in running water;

(e) neutralize for 5 minutes at room temperatu re in a solution comp rising 150 g/1 NaHS03 ; (f) rinse in running water for 5 minutes;

(g) immerse in conditioning cleaner ENPLATE PC-4459 for 5 minutes at 150°F. (65°C);

(h) rinse in running water for 5 minutes: (i) immerse in ENPLATE PC-236 for 2 minutes at room temperatu re;

(j) immerse in one-step palladium catalyst ENPLATE Activator 44 for 5 minutes at 75°F. (24°C); (k) rinse with running water;

(1) immerse in post activator solution ENPLAT E PA-493 for 5 minutes at room temperatu re;

(m) rinse in running water for 5 minutes; (n) plate in an electroless copper solution ENPLATE CU-700 for 30 minutes at 118°F. (48°C); (o) rinse with running water; (p) immerse in 3 % sulfu ric acid for 1 minute;

(q) electroplate in an acid copper electrolyte to about 0.001 inch (25 microns);

(r) rinse with running water for 2 minutes; and (s) air dry.

The metal layer was tested for adhesion using the Jacquet Peel Test and an ultimate peel strength of greater than 11 lbs/in (2 kilograms/cm) was obtained. The peel strength generally increases with time to a maximum (ultimate) value and the ultimate peel strength as used herein represents the adhesion value obtained for the metal layer greater than 3 days after metallization.

EXAMPLE II

EXAMPLE I was repeated except that the etched board was treated with ACTANE 70 prior to step (g) and a smooth adhesive coating was obtained.

EXAMPLE III

The procedure of EXAMPLE I was repeated except that a swellant composition containing, by volume, 25% N-methylpyrrolidone and 75% PGMME was used in step (a) instead of the DMSO composition and immersed for 10 minutes instead of 5. An ultimate peel strength of greater than 10 lbs/in was obtained.

EXAMPLE IV

Step (a) of EXAMPLE I was repeated using 100% DMSO and the plaque's surface was attacked and unsuitable for further processing. Similar results were obtained when 100% N-methylpyrrolidone was used in Step (a).

It will be apparent that many changes and modifications of the several features described herein

may be made without departing from the spirit and scope of the invention. It is therefore apparent that the foregoing description is by way of illustration of the invention rather than limitation of the invention.