G. A. Krulik Borg-Warner Chemicals R. C. lngersoll Research Center Des Plaines, Illinois 60018
I
Electroless Plating of Plastics
Electroless deposition of metals can be defined as the controlled autocatalytic reduction o f a dissolved metal ion by a dissolved reducing agent at an interface to give a uniform film ... . The first electroless solutions were developed around 1840 for use in silvering glass mirrors. Electroless deposition of metals is a process which is unfamiliar in detail to most chemists. It can he defined as the controlled autocatalytic reduction of a dissolved metal ion by a dissolved reducine aeent a t an interface to eive a uniform. coherent film. his-definition excludes immersion plating; where a film of comer forms hv replacement on iron. and uncontrolled precipitation, as in thk reduction of aqueous metal ions by high energy electron beams ( I ). The first electroless solutions were developed around 1840 for use in silvering glass mirrors (2).These solutions of ammoniacal silver nitrate and a reducing agent replaced the centuries-old (pre-1500) process of using a tin-mercury amalgam to make mirrors. Further progress was slow until after 1944. A. Brenner and G. Riddel rediscovered in 1944 that hypophosphite could reduce nickel ions to the metal, and developed and patented the first workable electroless nickel haths (3). The earlier discoveries by Roux in 1916 and by Wurtz in 1845 did not lead to anv" oractical results. Electroless comer . .. haths had been used earlier, hut were first used in large amounts in the 1950's for the erowine.,. minted circuits market. A verv.larze numher of patents and articles on various types of electroless haths were published between 1950 and 1970. The three largest applications for electroless deposition are electroless nickel on metals, electroless copper on printed circuit hoards, and electroless copper or nickel on plastics. This paper will focus on plating.of plastics as it illustrates a . wide iange of phenomena. The plating on plastics (POP) industry coats many tens of millions of square feet each year. The plated parts are used in a wide variety of automotive, plumbing, appliance, and decorative applications. The electroless metal deposit has only two functions-to provide an initial electrically conductive layer, so that the plastic can he electroplated, and to provide sufficient adhesion to anchor the electroplate to the part. A very wide range of non-conductors, including ceramics, glass, and plastics can be coated. Among the usable plastics are acrylonitrile-hutadiene-styrene (ABS) terpolymer, epoxies, phenolics, polyphenylene oxide, polypropylene, polycarhonate, and polyfluorocarhons. Well over 90% of POP is done on ABS, as it is the easiest to process, so it will he the main plastic covered here. ~ u ~ c e s s felectroless ul POP is a very technologically and chemically complex process. Its practical use depends on the optimized interaction of five separate complex chemical solutions. These steps are called
-
1) Etching 2) Neutralization 3) Catalysis 4) 5)
Acceleration Electroless Plating
Each solution is an integral part of the operation, and each has its own complexity. These stages will be more fully discussed separately. In addition to these chemical treatments,
Figure 1 . Scannng electron micrograph of unetched and etched ABS plastic surface. (5000X).
there are a number of ancillary factors which contribute to successful POP. These include proper part and mold design, plastic melt temperature and extrusion rate, and selection of the proper grade and type of plastic for optimized performance. Etching
Etching, the initial processing step, is necessary to give good metal-to-plastic adhesion. The two theories which have been used to explain this adhesion are the chemical bond theory (4) and the lock and key mechanism (5).Figure 1shows a photomicrograph of an unetched, molded ABS surface and the same surface after etching. The tremendous increase in surface area is very evident and gives support to the lock and key theory. However, this is not the whole picture. The etching solution is a hot balanced mixture of chromic acid, sulfuric acid, and water. It has been experimentally determined (Fig. 2) that the most effective etchants have about 41% H z 0 and are near saturation with respect to HzS04 and CrO-. This solution dissolves Dart of the . ~ o .h I I e hv 1 oxidation and chemically modifies the remaining surface. The orieinal surface is hvdroohohic. A cross-section (Fie. 3) shows ihe presence of a modified surface film of polymer Volume 55, Number 6. June 1978
/ 361
CrO, A
complexes may contribute to the reactivity of the surface in the post-etching steps. Other compounds which are formed by partial oxidation during etching can be identified on the surface. They include alcohols, carhoxylic acids, and probably sulfonated compounds. Their presence lends support to the chemical bond theory of adhesion as they provide a mechanism for ionic exchange effects. Additionally, the chemical properties of the surface layer of the plastic change as it becomes highly cross-linked and insolublized. This surface layer can be isolated by dissolving away the unmodified ABS with an organic solvent. Neutralization
[=j INSOLUBLE COMPOSITIONS EFFECTIVE FOR ETCHING
[7 INEFFECTIVE
FOR ETCHING
Figure 2. Solubility diagram for the CrO1-H2SOcH20 System.
The etched part is rinsed with water to remove the adherent viscous chromic-sulfuric acid solution. Much chromium remains on the part, however, and would.interfere with both the catalyst and nickel deposition steps. Hexavalent chromium would oxidize stannous ions in the catalyst solution and shorten its useful life. In the electroless nickel solution only 5 to 15 ppm is sufficient to completely inhibit the electroless nickel reduction. The function of this bath is to cause the excess chromium to desorh from the nlastic and to be reduced to innocuous Cr", A wide variety of acidic and basic solutions are used for this . ournose. . Some neutralizers will also additionallv. nromote . catalyst absorption. Catalysis
A catalyst is necessary to initiate the electroless metal deposition reaction on non-conductive surfaces. All electroless metal reductions are dehvdroeenations. whether hv~onhos. phite, formaldehyde, sodium horohydride, or dimeKylamine horane is the reducing agent. For example, the nickel-hypophosphite reduction is
-
Ni2+
Figure 3 Transmission electron micrograph of as-molded ABS plastic (12500Xl.
produced during molding when the molten ABS hits the cold wall of the mold cavity. This surface layer is highly stressed, due to the rapid cooling rate, as can he seen by the irregular elongated polybutadiene particles. This stressed layer is completely removed during etching to leave a hydrophilic surface. Inspection of Figure 3 shows that ABS polymer itself is not a homogeneous mass, but is a two-phase system. The elonzated and snherical oarticles are mainlv nolvhutadiene. The rest of the blastic i~polystyrene-acry~&ile. The etchant attacks and oxidizes both nhases. hut the hutadiene oxidizes much more quickly. ~ h i helps i account for the resultant hiehlv plastics such as .. . rouehened surface. Sinale .. nhase . lx$ypn,p).l~.ucslwa murh IPS.; nughvning. ARS vlilsric will also R I I B O ~ I I chemirals from rhr ~(llution. The eq;ation of etching, if it goes to completion (for 1:1:1 acrylonitri1e:hutadiene:styrene) is
362 / Journal of Chemical Education
-
Nio
+ HzPOJ- + H z
The most efficient dehydrogenation catalyst is palladium metal. The first catalysts were merely acid palladium chloride solutions. They did not absorb particularly well or uniformly, and usually had to be treated with a reducing agent to give the catalytic palladium metal. I t was soon discovered that stannous chloride was an excellent reducing agent.
Figure 4. ESCA of etched ABS plastic surface showing Cr4+ and Cr5+ peaks.
SnC12
ESCA studies have shown that chrome absorbs into the etched surface layer. Neither Cr" nor Cr6+ are found in appreciable amounts (Fig. 4). Instead, large amounts of CP+ and Cr5+are present. Both Cr4+and Cr5+are very unstable in aqueous solution. Their presence here indicates that they are stabilized within the polymer matrix, probably by ester formation. These
+ HPPOZ-+ HzO
+ Pd CI2 --(Red)
--(Gleen --(Blue)-(Green +Red-Browd4Milky
-- Pd'+
+
I) Ill Brown)
SnC14
Figure 5, intermediate stages in the reduction of W2+by SnZt are seen at high ratios of tin to palladium in hydrochloric acid solution.
A solution of stannous chloride in hydrochloric acid could he applied to the non-conductor either before or after the palladium chloride application to give better catalyst coverage and activity. By 1960 it was recognized that the classical tin-palladium redox equation was incomplete. Experimental work showed that the reaction of the two metals actually gives a series of brightly colored intermediates of variable stability, culminatine in a stabilized. reddish-brown catalvtic solution (6-8). . . Any further reaction gives a milky brown inactive colloidal material and finally a black metallic palladium precipitate. The detailed hehavior of this reaction is dependent upon reaction temoerature. time. aciditv. chloride concentration. and absoluteconcent;atiok of tin-and palladium. The most important factor is, surprisingly, the ratio of stannous ions to palladium ions (Fig. 5). When the ratio is two or less, the black palladium precipitate forms rapidly. As the ratio is increased to the commercially supplied level of 30 or 60 to 1, the various color changes are easily seen. The reddish-brown catalysthas been the subject of a large numher of patents and papers ( 6 9 ) . It has been very difficult to decide whether the catalyst in solution is an ionic complex or a reduced metal colloid. This system is so labile that almost any attempts to modify the system or to crystallize the intermediates will cause complicating changes. However. the analoeous chloride-stannous chlo- olatiuum . ride system has been fairly well studied. The complexes are much less lahile. so the corresnondine color nhases have been isolated by Bell&o and others.(l&llj. The&work shows that the color changes are due to successive substitution and isomerization reactions by SnC13- on the PtC142- ion. A large numher of these com~lexeshave been isolated. 'I'hr plntinllm aorkimplirs that thc pulladium color phases nrr also n,mplews of S d l , and I'dC'I ,' . Spertrosrooic work supports this, along with the fact that all color phases are reversible until the milky-brown colloid is formed (9). ESCA studies of the reddish-hrown catalyst isolated in a matrix of excess stannous chloride show no trace of metallic palladium (Fig. 6). The catalyst must he dissolved in asolution of hydrochloric acid for use, however. Paper chromatographic work has shown that the freshly dissolved reddish-brown catalyst will migrate completely. After aging for as short a time as 30 min., much of the palladium will no longer migrate. Thus, there seems to he a mixture of a t least two materials in the catalyst working bath. The detailed hehavior of the tin-palladium system is extremely complex, perhaps more complex than even the electroless deposition reactions themselves.
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it will not initiate room temperature electroless nickel deposition. A mixture of stannous, stannic, and palladium compounds ahsorbs on and in the surface of the etched plastic during catalysis. During the rinsing step, there occurs a complex series of reactions-redox, hydrolysis, precipitation, and diss'olution-which give a layer of stannous and stannic hydrous oxides and oxychlorides with palladium. The acceleraror soluticm w n he any acidir or alkaline solurigm in which t ~ sultsare n appreriahly wluhlr. Thissolution allows any Pd2+ to react andform Pdo, and it removes excess tin salts from the surface. An excess of soluble tin will inhibit electroless nickel deposition. Electroless Nickel The electroless, or autocatalytic, reduction of nickel salts is a controlled heterogeneous catalytic reaction. Most commercial haths use hypophosphite as the reducing agent. Other reducing agents, such as hydrazine, sodium horohydride, and dimethylamine borane are also usable. The nickel-hypophosphite reduction was rediscovered by A. Brenner and G. Riddell in 1944 (3). Their work led to the first usable plating haths. One drawhack was that the reduction was slow or non-existent below 95°C. Thus, this process could not be used to nlate elastics. which normallv deform a t these high temperatures. These haths were mainlv used as a replacement for hard chromium plating on metai until 1966. The first room temperature electroless nickel haths were introduced in this year and helped cause a rapid growth in commercial plating of plastics. The main plastic plated, ABS, deforms a t temperatures above 65-70°C. Low temperature electroless copper haths had been used for coating plastic, hut these solutions were several orders of magnitude-less stable than room temperature electroless nickels. The main purpose of the electroless nickel solution is to deposit a thin conductive metal film. As soon as the film has sufficient conductivity, it can he electroplated in a conventional manner. Usually only 0.12-0.25 microns of nickel needs to he deposited in 5 to 10 min. The nickel-hypophosphite reduction is very unusual. Pure nickel is not deposited. Instead, a mixture of metallic nickel and zero valent phosphorus is formed. This was completely unexpected, as elemental phosphorus cannot he produced by electrodeposition from aqueous solution. The actual reaction is a combination of several separate processes
+
NiZ+ H2P02-
+ H20
-
NiD+ HzPOa-
+ Hp 3040%
Accelerator The original two-step catalyst-separate acid solutions of PdClz and SnClz-did not need any further treatment to be catalytic. The tinlpalladium ratios and reaction conditions were such that metallic palladium rapidly formed. The one-steo tin-oalladium solution is normallv used todav because of its'advakages in activity and stability. strictly speaking, the catalyst is not a catalyst yet for this system, since
Figure 6.ESCA of reddishbrown Sn-Pd reaction product in SnCI, matrix. Only the Pd2+ doublet is present.
Figure 7. Simplified Pourbaix diagram for complexed nickel. Doned lines are the stability limits for water.
Volume 55,Number 6. June 1978 1 363
Figure 8. Simplified Pourbaix diagram for phosphorus. Hypaphasphite and phosphite are thermodynamically unstable but kinetically sluggish. Figure 10. Transmission electron mtcragraph of in8tIal electroless nickel film on etched ABS (122,000 XI.
Figure 11 ABS plastc plated with elect~olessnckel followed by electrolytic nickel and chromium.
Nickel reduction b, hypmlx&te
Figure 9. Pourbaix diagrams for nickel and phosphorus superimposed. Nickel reduction and hypophosphite oxidation can occur within the shaded area.
+
H z P O z OHHPOs2- + H z H z P O z + H P o + HzO + O H -
-
+
55-6596
1-5%
These reactions apply to room temperature alkaline solution; acid solutions have slightly different reactions. An additional oxidation of the phosphite to phosphate is possihle, hut is kinetically hindered, so no appreciable amount of phosphate is formed. The initial reduction takes place on the metal catalyst. Once the palladium is coated with nickel metal, the autocatalytic reaction continues indefinitely on the freshly deposited nickel. Commercial electroless nickel solutions consist of nickel salts, complexing agents, buffers, and hypophosphite, and are used a t pH 8-10. Figure 7 shows a simplified Pourbaix diagram for nickel. Figure 8 shows the corresponding diagram for phosphorous. When the two figures are superimposed (Fig. 9), the shaded area shows the potential and pH range in which electroless de~ositionshould occur. Electrochemical meawrements during elrctrolrss nwkel plating coniirm that the olatinr surhcr has t h c predicted potential 1/21. w he actual mechanism for the production of elemental phosphorus seems to involve atomic hydrogen as the reducing agent. A nickel-phosphorous alloy deposit consists of an amorphous solid solution of phosphorus in nickel. The theo364 / Journal of Chemical Education
retical maximum is about 15% by weight phosphorus, corresponding to Ni3P, hut Ni3P is not seen by X-ray diffraction unless the alloy is heat treated a t a high enough temperature to cause recrystallization. Room temperature nickels can give deposiL3 of about 1-10% phosphorus, depending on the specific reaction conditions. Nickel-phosphorus is more corrosion resistant than pure nickel. The amount of phosphorus also influences the electrical conductivity, magnetic properties, hardness, and ductility of the resulting deposit, especially after heat treating (13).Thus, if the coating has less than 8% phosphorus, the deposit consists of a nickel matrix with dispersed Ni3P. This is magnetic and relatively ductile. Above 8%phosphorus the deposit is a dispersion of nickel in Ni3P, and the coating is non-magnetic and relatively brittle. An example of one unique property of electroless deposition can he seen in Figure 10. In conventional electroplating it is very difficult to get uniform plating thickness on irregular or roughened parts hecause of variations in current density and voltage. Electroless plating does not depend on an external reducine aeent (i.e.. electrons). so the olatine thickness is verv em. The electroless plating process depends on the successful interaction of a nt~mherof reactions and has been commercially refined to a high art (Fig. 11).At least two hundred million square feet of non-conductors are coated yearly hy
electroless deposition just in the plating-on-plastics industry. Large additional amounts are used in the fields of metal platinp, printed circuit production, and specialty applications such as magnetic memories. Many different electroless alloys are possible, including a numhw of non-equilibrium alloys and many which cannot he otherwise prepared from aqueous solution. These include Ni-Fe-P, Ni-Co-P, Co-P, Ni-W-P, Ni-Re-P, Ni-B, Ni-ReZn-P, Co-Mn-P. Ni-Sn-P, and many others. Reducing agents such as organohoron compounds will give the corresponding boride alloys. Hydrazine and formaldehyde produce essentiallv nure nickel and comer. Nor is electroless . , . resuectivelv. . dr.po.ition lirnitrd f r copper ~ anrl iron r r k p metals. Arn,mg the I I I I C T d
