CA1249188A - Process for conditioning the surface of plastic substrates prior to metal plating - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE
A swellant and etch procedure to enhance the adhesive-ness of metal coatings on resins such as polytherimide is pro-vided using specially formulated compositions and operating para-meters.

Description

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THE SURFACE OF PLASTIC SU~STRATES PRIOR
TO METAL PLATING

This invention relates to the metal plating of plastics and, in particular, to enhancing the adhesion of metal plating to polyetherimide plastics, such as, the plastic substrate of elec-tronic circuit boards, by conditioning the board prior to metal plating of the hoard.

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The metal plating of plastic parts is well-known to be of considerable commercial importance because the desirable char-acteristics of both the plastic and the metal are combined to offer the technical and aesthetic advantages of each. Thus, a part plated with a bright, metalllc flnish takes advantage of the economies in cost and~welght afforded by substltuting molded ~;~ plastic parts for metal and,~ additionally, the plated finishes are~not as susceptible to pitting~and corrosion~because there is ~ ~ 25no galvanic reaction between a plastic substrate and a plated ;~ ; metal. Such metal plating of plastics is; used for a wide range of applications from decorative plating to radio frequency shielding~

An lmportant process~ the preparation of electronlc circuit boards which requires the electroless plating of a con-ductive metal layer, usually copper, onto the plastic substrate 360f the board. At present, boards are predominately made of epoxy :: : : :
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resins but there is a constant search for different materials which will offer improved economies and other benefits. ~ne such material is a polyetherimide resin ~7hich has a higher tempera-5 ture stability than epoxy and which can be injection molded withthrough-holes, thus eli~inating the costly drilling step now re-quired when preparing epoxy boards. Resins of this type are made by General Electric Company under the trademark ULTEM. For con-venience the following description will relate specifically to the process for conditioning polyetherimide resins although it will be understood that the process may be suitably employed for lS 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 fa-cilitate connection between the circuits on the copper~layers.
25 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 me-thods have been developed to improve the adhesion of the metal i ~ plating to the plastic substrate.

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While different~uses require~differént adhesive strengths, in general, it has been established that a minimum peel strength of about 8 lbs. per linear inch as measured by the 35 Jacquet Peel Test is required to prevent a metal coating from

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~2~8~3 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 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 0 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 em-p~oys 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, diox-20 ane, pyridine, dimethylformamide, and an alcohol mixture compris- ;
~ ~ ~ing methyl ethyl ketone, ethanol and methanol as swellants for ; epoxy resin. The swelled plastlc is then exposed to an etchant ~; ~ for a sufficlent period of time to etch the swelled surface with-25 out unduly weakening or otherwise adversely affecting the physi-cal charactéristics of the plastic. Etchants are oxidizing , materials dnd are generally aqueous solutions containing materi-als such as sulfuric acid, phosphoric acid, permanganate ions, :
;~ 30 Cr+6 ions and~the like. U.S. Patent No. 4,086,128 shows pretreat-ment of an~epox~ resin with an~organlC solvent comprising al-;~ cohols, acids~, esters, ketones, nltriles, nitro compounds, and polyhydric~compounds such as ethylene glycol, glycerine and lt 2-propylene glycol prior to etching with hydrogen peroxide and

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sulfuric acid. U.S. Patent No. 3,865,623 shows im~ersion of epoxy resin in an organic solvent such as dimethylformamide to render the epoxy receptive to an acid etch.
To be commercially useful, however, it is important that the swell and etch process provide a consistent and repro-ducible resin condi-tioning effect which results in the minimum peel strength being achieved practically for greater than 99~ of the parts being conditioned and plated. Without this reproduc-ibility the costs to the printed circuit board industry would be staggering and result in highly increased manufacturing expenses.

SUMMARY OF THE INVENTION

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 formulated compositions under con-trolled operating conditions. The method includes first expos-1ng the plastic for a suitable time to a swellant composition comprising a solution of a polar material, preferably having a dielectric constant greater than about 15 and a dipole moment greater than about 3 Debye Units~D) and an organic solvent such as a ;carboxylic acid, ketone, hydrocarbon, ether, ester, alcohol, polyhydric, e.g., glycol and~glycol ethers and esters.
The swelled plastic is then exposed to an etchant comprising Cr+6 :

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l ions, and, preferably, also including an acid such as H2S04, at an elevated temperature, e.g., above about 160F., for a suf ficient period to etch the swelled surface without unduly weak-5 ening or otherwise adversely affecting the physical characteris-tics of the plastic.

The resultant etched surface is now in a condition of improved adhesion for any species such as printing inks, paints, coatings, and, in particular, for metal coatlngs, which may be deposited thereon. As will be appreciated by those skilled in the art, rinsing oE the plastic may be performed at any staga of;
15 the process and removal of residual etchant or etchant-plastic material may require further treatment if desired.

In general, the process of using the compositions of 20 the invention in manufacturing pr1nted circuit boards and, in particular, boards containing through-holes, is a sequence of steps commencing with a laminate or multi-laminate made from, e.g., filled polyetherimide material. A predesigned series of 25 through-holes may be formed in the board by injection molding or drilling. The board is then contacted with the swellant composi-tion of the 1nvention aDd, aEter water rins1ng, the board is etched at an elevated temperature with an oxidant such as a 30 so1ution of; H2504 and CrO3 and water rinsed.~ Treatment w1th a reductant to lowar the oxidation state of the chrome resi-; dues is then typically performed and the board rinsed provid-ing a board ready for electroless metal plating using conven-t1onal procedures.~ A preferred step is to pretreat~the board "

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~L2~ 8 1 with a composition such as ENPLATE PC-4459 sold by Enthone, Incorporated to remove hydrocarbon soils and promote the catalyst and then to contact the board with a composition such as ENPLATE
5 PC-236 to eliminate drag-in. The board is now immersed in a - catalyst, such as a tin-palladium solution, which conditions the surfaces of the resin for electroless copper plating. ENPLATE
Activator 444 is exemplary of this type catalyst. The board i5 then immersed in a post activator such as ENPLATE PA-493 to , activate the catalyst by removing the excess tin and freeing the metal palladium ions on the board, rinsed a-nd immersed in an ' 15 electroless copper plating solution for a period of tlme suffi-cient to plate copper to the desired thickness on the surfaces and to plate the surfaces of the holes to form through-hole con-nections between the laminate surfaces. ENPLATE CU-700 and other :
20 similar plating compositions may be employed. The boards may then be immersed in diIute sulfuric acid followed~by electro-plating usl;ng conventional technlques if a thicker coating is desired. Rinsing of the board between~steps may be convention-ally employed as lS well~-k~nown ln~the art.

DETAILED DESCRIPTION OF THE INVENTION

It is preferred that~the polyetherimide resin contain ~ 30 other materials,~prefèrably ~ ller or reinforcing material,''~
e. g., glass flbers.~ Other~fil~lers~lnclude paper~, 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~eor speciflc purposes.

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,: ' , , 1 It has been found that a swellant pretreatment is nec-essary to enable the etching process to roughen, or to otherwise condition, the surface and to provide the enhanced adhesion of 5 the invention~ The swellant composition comprises a solution of a polar material and an 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 lS greater than about 20 r most preferably greater than about 30 and dipole moments greater than about 3 ~ and preferably greater than about 3.5 D. Exemplary of these materials are dimethyl-formamide, dimethylacetamider dimethyl sulfoxide, tetrahydrothio-phene dioxide, N-methylpyrrolidone, hexamethylphosphoric tria-mide, tetramethylurea, and acetonitrile. Other materials of similar structures mdy also be utilized as the polar materials 25 hereof. Thus, other sulfoxides which are liquid at or near room temperature may be so employed, e~g., those having the formula O ~ ':
~ R-S-R
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whérein 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 :
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The second cornponent 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, lncluding ethers and esters thereof. In general, tbe organlc solvent will typlcally contaln 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, methanol, and the like.

The preferred organic solvents because of their dem-onstrated effectiveness are the polyhydrics such as the glycols, and the ethers and esters thereof. These solvents may be repre-sented by the formula RlO(AO)nR2 wherein Rl and R2 are inde-pendently 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 l to 4. Examples of these solvents include ethylene glycol, ethylene ~lycol monomethyl ether, propylene glycol, ; propylene glycol monomethyl ether, ethylene glycol acetyl ester, ~;30 etc~

It will be appreciated by those skilled in the art .
that the polar material and organic solvent swellant mixture ; 35 may comprise one or more compounds of each ingredient. The swel-`~ -8-.... . .. . . .., ~.. .... . . . . . .....

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... ' ', . ' l lant composition may also be used in the form-of an aqueous solu-tion although it has been found that the water is desirably limi-ted, in weight percent, to less than about 20%, and preferably less than lO~ 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.

-lO Two particularly preferred polar materials are di-methylsulfoxide (DMSO) and N-methylpyrrolidone and the preferred .
solvents are the polyhydrics represented by the formula RlO(AO)nR2. In general, compositions of the two components con-15 tain, by volume, about 25% to 90% DMSO, preferably 50~ to 85~ and most preEerably 70% to 80%, with the balance being the polyhydric component. For compositions containing N-methylpyrrolidone a ran~e 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 monomethyl ether wherein Rl is H,~A is C3, n~is l and R2 is methyl.
, Suitable addltives can be employed in the swellant composition for specific purposes such as wetting agents to enhance the capability of spreading the c~mposition on the resin surface.

To pract1ce the method of the lnvention the plastic -substrate is contacted with the swellant composition at an ele-vated temperature for a sufficient time to render the surface re-35 ceptive to the etching process. Contacting procedures~may vary ,:
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1 widely, e.g., 1 to 60 minutes at temperatures up to about 180F., and pre~erably 2 to 20 minutes at 140 to 160F. Satisfactory results for glass filled polyetherimide resin are provided by 5 immersing the part in the composition for about 5 minutes at 150F. The time and temperature will r 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 com-prises contacting the treated plastic part with the etchant at an elevated temperature for a sufflcient 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 ~aterial 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 preEerably comprise Cr+6 ions, e.g., Cr207= and~Cr03. An acid such as H2SO4 is preferably included in the etchant solution and other acids like H3PO4 may also be employed. A preferred etchant composition is an aqueous solution of H25O4~and CrO3. Suitablé~add~ltlves may also be used such as surfactants,~e.g., perfluorinated sulfonatesj to insure uniform etching of the conditioned resin surface. The concentration of the etchant solution may vary widely with the chromium component `

`: ' 18~3 1 added as CrO3 being, by weight, about 100 grams/liter (g/l) to saturation, preferably abou-t 200 to 600 g/l, and most preferably about 300 to 500 g/l. The acid component is about 100 to 500 5 g/l, and more preferably about 200 to 400 g/l. A preferred composition contains about 400-450 g/l CrO3, e.g., 420 g/l, and ?50-350 g/l H2SO4, e.g., 300 g/l.

An important feature of the invention is the tempera-ture 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 15 solution. It has been found for the conditioned polyetherimide resins however, that temperatures above about 160F. and prefer-ably above about 170F. are necessary and that unexpected ad-hesive properties to the metal coating are provided. While the 20etching time will vary depending on the concentration of the etching solution and~temperature of the etching process as will be appreciated by those skilled in the art, for the preferred 25 composition of 420 g/l CrO3 and 300 g/l H2SO4, an etcbing tiwe of about 5-lS minutes at 170F. provldes excellent results. In general, the etching time may be up to about 60 minutes-but is typically less than 30 minutes.

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It is to be understood and stressed for both the swel-~;lant and etchant procedures,~that the above concentrations, ;temperatures and time paraweters are all Interdependent and that variations in temperature will produce variations in the other , ,.,~

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1 parameters ~hereby optimum results will be attained. In this regard, the various parameters and their interdependency are well known in the art and their interaction between one another is 5 also ~ell known or can be easily ascertained experimentally by one skilled in the art.

An optional step of removing traces of deposits on the 10 etched resin may now be performed. Rinsing will remove deposits but a preferred procedure is to contact the resin with a suitable reducing agen~ for chromium ions such as NaHSO3, NaOH, and the like. Immersion of the etched resin in a solution of 150 9/1 1 NaHSO3 for 5 minutes has produced satisfactory results. Typi-cally, exposurc to the reducing agent ranges from 30 seconds to 10 minutes at a temperature ranging from room temperature to 160F.

If a smoother metal plated surface is desired, another optional step is to treat the etched resin to dissolve part of the filler material. Materials such as hydrogen ~luoride and 25 ammonium bifluoride may be suitably employed. ACTANE 70~sold by Enthone,Incorporated has proven very satisfactory for this purpose.

~The etcheù resin is now prepared for metal plating by known means to~render the surface catalytic. Among them are cleaniny, applying catalyst promoters, sensitl2ing using an aque-, ous tin chloride solution and then activating by means of pallad-35 ium chloride. On the other hand, unltary baths may be employed . ..

:. ' , for such purposes, such as the dispersions of colloidal palladium and tin ions described in Shipley, U.S. Pat. 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 rnetal by electroless plating. Activating and plating com-positions 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. Other methods of deposition may also be used such as vacuum vapcr deposition, electrolytic plating or a combination of electroless plating and electrolytic plating.

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

~XAMPLE 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 plaque of ULTEM glass-filled polyetherirnide resin;was metallized using the following proce-dure:

(a) immerse the plaque for 5 minutes at 150F. with mild agitation in a solution comprising, by volume, 75%

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DMSO and 25% propylene glycol monomethyl ether (PGMME~;
(b) rinse for.5 minutes in running water;
- (c) etch for 10 minutes at 170F. with mild agitation in a solution comprising 420 g/l CrO3 and 300 g/l H2SO4;
(d) rinse for 5 minutes in runniny water;
(e) neutralize ~or 5 minutes at ro m temperature in a solution comprising 150 g/l Na~SO3;
(f) rinse in running water for 5 mlnutes;
(g) immerse in conditioning cleaner ENPLATE
: PC-4459 for 5 minutes at 150F.;
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(h) rinse in running water for 5 minutes;
ti) immerse in ENPLATE PC-236 for 2 minutes at room temperature;
(j) immerse in one-step palladium catalyst ENPLATE
Activator 444 for 5 minute.s at 75F.;
(k) rinse with runnlng water; :
: (1) immerse in post activator solution ENPLATE
:: , PA-~93 for 5 minutes at room:tempe~rature;
(m) rinse ln runnin;g water for:5 minutes;
;(n) plate in an~electrolesS copper solution ENPLATE
CU-700 for:30 minutes~at 118F.; : ;
:(o)~rinse with-running water;
(p~ mmerse~in ~g~sulf~urlc~aci~d~for l~minute;~
: (q)~:e~leotroplate in an:ac~i~d~::copper~electrolyte:~to about ~: ~ : 0. 001 inch;~
5 ~ ( r l r ~ n~e w i th ~ ru ~ A; A g~ wa t er 'o r 2 r l nu t os ~a nd -14- ~
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1 (s) air dry.
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The metal layer was tested for adhesion using the Jac~uet Peel Test and an ultimate peel strength of greater than s 11 lbs/in 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 10 for the metal layer greater than 3 days after metallization.

EXAMPLE II -- : - : . ~. ~ :
EXAMPLE I was repeated except that the etched board 15 was treated with ACTANE 70 prior to step (g) and a smooth ad-hesive coating was obtained.

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EXAMPLE III

The procedure of EXAMPLE I was repeated except that a swellant composition containing, by volume, 25% N-methylpyrroli-done and 75~ PGMME was used ln step (~a) instead~of the DMSO
composition and imme~rsed~for 10 minutes lnstead of ~5. An ultim-ate peel~strength of greater than 10 lbs/in was obtained.

EXAMPLE IV

30 ~ Step~(a)~of EXAMPLE~ was repe~ated uslng IOO~ DUSO and the plaque's surface~was attacked and unsuitable for further process~l~ng.~ Slm~ilar results were obtained~whe~n 100% N-methyl-;~
pyrrolidone was used~ in Step (a). ~ ;

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r ~Z~ 8 1 tions of the several features described herein may be made without departing from the spirit and scope of ~he invention. It is therefore apparent that the foregoing description is by way of illustration of the invention rather than limitation of the inventon.

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Claims (8)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A method for improving the adhesive properties of a polyetherimide resin comprising:
(a) conditioning the resin by contacting the resin with a swellant composition comprising a solution of a polar material having a dielectric constant greater than about 15 and a dipole moment greater than about 3 Debye Units and an organic sol-vent at an elevated temperature for a sufficient time to render the resin receptive to an etching process;
and (b) contacting the conditioned resin with an etchant composition comprising chromium ions at an elevated temperature above about 160°F. for a sufficient time to promote adhesion of metal planting on the plastic.

2. The method of claim 1 wherein the plastic is glass-filled polyetherimide.

3. The method of claim 2 wherein the swellant com-position comprises a polar material selected from the group consisting of dimethylsulfoxide; dimethylformamide; N-methyl pyrrolidone; and mixtures thereof and the organic solvent is a polyhydric compound.

4. The method of claim 3 wherein the etchant composi-tion comprises CrO3 or Cr2O7 ions and H2SO4.

5. The method of claim 3 wherein the polar material is N-methylpyrrolidone and the organic solvent is propylene glycol monomethyl ether.

6. The method of claim 3 wherein the polar material is dimethylsulfoxide and the organic solvent is propylene glycol monomethyl ether.

7. A polyetherimide resin product prepared in accord-dance with the method of claim 1.

8. In a method for producing a metal coating on a polyetherimide resin substrate which comprises sensitizing and activating the surface of the resin and electrolessly coating a metal thereon followed, optionally, by electroplating, the improvement for increasing the adhesiveness of the metal coating on the resin substrate comprising:

(a) conditioning the resin by contacting the resin with a swellant composition comprising a mixture of a polar material having a dielectric constant greater than about 15 and a dipole moment greater than about 3D and an organic solvent at an elecated temperature for a sufficient time to render the resin receptive to an etching process; and (b) contacting the conditioned resin with an etchant composition comprising chromium ions at an elevated temperature above about 160°F. for a suf-ficient time to promote adhesion of metal plating on the resin.