The Materials Science of Coatings and Substrates Part 9 docx

The function of certain organic compounds is to produce this leveling in plating solutions.. Additives such as sodium allyl sulfonate, labeled by the reaction between allyl bromide and S

Figure 2:

pyrophosphate copper deposits Adapted from reference 22

Effect of plating parameters on the tensile properties of

Figure 3: Tensile properties and morphology of annealed pyrophosphate copper deposits versus additive concentration From reference 28

Reprinted with permission of The Journal of Applied Electrochemistry

levels (3.0 to 4.0 cm3M3) the ductility again decreases, presumably because

of inclusion of excessive additive in the deposit (Fig 3j)

INFLUENCE ON LEVELING

Normal electrodeposition accentuates roughness by putting more deposit on the peaks than in the valleys of a plated surface since the current density is highest at the peaks because the electric field strength is greatest

in this region In order to produce a smooth and shiny surface, more metal has to be deposited in the valleys than on the peaks, which is the opposite

of the normal effect The function of certain organic compounds is to produce this leveling in plating solutions Leveling agents are adsorbed preferentially on the peaks of the substrate and inhibit deposition This inhibiting power is destroyed on the surface by a chemical reaction which releases it, setting up a concentration gradient close to the surface An

example is coumarin which is used in the deposition of nickel It adsorbs

on depositing nickel by the formation of two carbon-nickel bonds and inhibits nickel deposition probably by a simple blocking action It is removed from the surface and destroyed by reduction with the main product which is melilotic acid (29)

Radioactive tracer studies have been particularly effective for studying the behavior of addition agents Additives such as sodium allyl sulfonate, labeled by the reaction between allyl bromide and S labeled sodium sulfite were used in Watts type nickel solutions (30) Grooved brass cathodes were plated with nickel These substrates had been passivated prior to plating so that the foil could be stripped for counting purposes (Figure 4) Results of the counting experiments (Table 2) show that more activity was deposited on the peaks than in the recesses Work of this type supports the theory that the addition agent is preferentially adsorbed on the

high points of an irregular surface where it acts as an insulator This inhibits deposition of metal and diverts current to recessed areas (30)

Radioactive tracer techniques, used in Watts nickel solutions, have revealed that a number of mechanisms are feasible, either diffusion and adsorption, or cathodic reduction (31) When two or more compounds were added, the mechanism of incorporation became more complex Other work

on use of radioactive tracer studies with additives can be found in references

A practical example of the influence of additives on leveling is

shown in Figure 5 (37) A proprietary additive in a copper sulfate solution reduced surface roughness as much as 70 percent with a deposit as thin as

20 um (0 8 mil) Besides producing deposits which level the hills and valleys on a substrate, levelers also inhibit the formation of asperities such 32-36

Figure 4: Cathode foil and shield for radiotracer studies Grooved brass cathodes were plated with nickel which was then passivated to permit stripping of subsequent foils Counting shield had grooves that limited betas activating the counter to those from either one peak or one valley Adapted from reference 30

Figure 5: Leveling power of bright copper deposited in copper sulfate

solution containing a proprietary additive Adapted from reference 37

Table 2: Lead Slit Counting Rates for Foils Shown in Figure 4*

Foil A Topt Foil B Top

Counting was left to right on the top of the foil and right to left on

the bottom, so that the values in the columns are matched

t "Top" refers to the side next to the solution during plating,

"bottom" to the side next to the cathode

* From Reference 30

as nodules This increases the stability of the deposition process, particularly for thick coatings (29)

INFLUENCE ON BRIGHTENING

A bright deposit is one that has a high degree of specular reflection (e.g., a mirror), in the as-plated condition Although brightening and leveling are closely related, many solutions capable of producing bright deposits have no leveling ability (38) If the substrate is bright prior to plating, almost any deposit plated on it will be bright if it is thin enough However, a truly bright deposit will be bright over a matte substrate and it will remain bright even when it is thick enough to hide the substrate completely Plating solutions without addition agents seldom or never produce bright deposits

There is a direct relationship between brightness and surface structure of electrodeposits as shown in Figure 6 (39) The measure of smoothness used in this example is the fraction of the surface area which does not deviate from a plane by more than 0 15 pm, which is of the order

of the wavelength of visible light This value was chosen because it has been found that with specularly bright nickel, there are no hills higher or valleys deeper than 0 15 pm (39,40)

Figure 6: Relationship between quantity of reflected light (brightness) and

fractidn of area with roughness less than 0.15 um Adapted from reference

40

CLASSIFICATION AND TYPES OF ADDITIVES

Additives can be classified into four major categories: 1-grain refiners, 2-dendrite and roughness inhibitors, 3-leveling agents, and 4-wetting agents or surfactants (3) Typical grain refiners are cobalt or nickel codeposited in trace amounts in gold deposits Dendrite and roughness inhibitors adsorb on the surface and cover it with a thin layer which serves to inhibit the growth of dendrite precursors This category includes both organic and inorganic materials with the latter typically being more stable Leveling agents, such as coumarin or butynediol in nickel solutions, improve the throwing power of the plating solution mostly by increasing the slope of the activation potential curve The prevention of pits

or pores in the deposit is the main purpose of wetting agents or surfactants

(3)

Metals differ in their susceptibility to the effect of additives, and the order of this susceptibility is roughly the same as the order of their melting points, hardness and strength; it increases in the order Pb, Sn, Ag, Cd, Zn,

Cu, Fe, Ni (41) Thousands of compounds are known that brighten nickel deposits from the sulfate-chloride solution, while it is only fairly recently that ways of brightening tin deposits from acid solutions have been developed The progression in the series corresponds to: 1) the increasing tendency of metal ions to form complexes and 2) to increasing activation polarization from simple ions This is in the reverse order to the overvoltages observed in the evolution of hydrogen on metal cathodes Lyons suggests that: "An atom which is capable of interacting strongly with other atoms of the same or other kinds tends to form a strong crystal lattice with a relatively high melting point, to coordinate strongly with ligands, to decoordinate water slowly, and to catalyze conversion of atomic to molecular hydrogen" (41)

Additives are often high molecular weight organic compounds or colloids since small ions or molecules are generally not very effective (42) This is shown in Table 3 which relates minimum concentration of organic compounds required to impart appreciable brightness to nickel deposits (43) The size of the molecule can also influence the stress in the deposit Coumarin, which is a small molecule compared to phenosafranine (Figure 7) reduces macrostress in nickel deposits, whereas, phenosafranine increases

tensile macrostress (44)

An open discussion of the components of brightener systems is difficult because many of these systems are proprietary Suppliers guard their formulations from distribution simply because the brightener market

is so competitive However, there are numerous technical publications detailing many of the additives commonly used A listing of the materials that have been used as additives in plating solutions culled from the open

Table 5: Relationship Between Molecular Size and Minimum

Concentration of Organic Compound Required to Cause Brightening in Nickel Plating*

Avg Min Conc

to Brighten

Very large Magenta dye 0.000057

Bicyclic Saccharin o.oooa5

Copper - Some of the materials that have been used with acid copper include glue, dextrose, phenolsulfonic acid, molasses and thiourea Many of the present day commercially available brighteners contain three

components designated as carrier, leveler and brightener Reid suggests that: "Carriers are typically polyalkylene glycol type polymers with a

molecular weight around 2000, levelers are typically alkane surfactants

containing sulfonic acid and amine or amide functionalities, and brighteners are typically propane sulfonic acids which are derivatized with surface active groups containing pendant sulfur atoms" (47)

Additives for cyanide copper systems include compounds having active sulfur groups and/or containing metalloids such as selenium or tellurium Other agents that have worked are organic amines or their reaction products with active sulfur containing compounds; inorganic compounds containing such metals as selenium, tellurium, lead, thallium, antimony, arsenic; and organic nitrogen and sulfur heterocyclic compounds

(48) An extensive listing of additives used in acid and cyanide copper prior to 1959 can be found in reference 48

Figure 7: Schematic representation of coumarin and phenosafranine molecules drawn approximately to scale From reference 44 Reprinted with permission of The Electrochemical Soc

Gold - There are three principal types of additives associated with high purity gold electrolytes: complexing agents, grain refiners, and hardening agents (49) Complexing agents such as pyrophosphate ion, organophosphorus compounds and pol yphosphates are added to reduce the activity of metallic impurities in the solution by forming stable complexes and hence minimizing codeposition Organic chelating agents such as

EDTA and related compounds are also used Relatively small amounts of base metals are used for providing grain refinement These additives also

provide smoothing and semi-brightening of the deposit,while not being codeposited to a significant extent In neutral solutions, arsenic and thallium have been used Some additives such as alums and hydrazine sulfate have been claimed to harden the electrodeposit without being codeposited (49)

Use of heavy metal ions in trace quantities (parts per million) in gold electroplating solutions, induces a marked cathodic depolarization which extends the range of current densiti2s over which smooth, fine

grained deposits can be obtained (SO-5% In slightly alkaline phosphate electrolytes the most effective additives comprise the family of elements Hg, T1, Pb and Bi, which lie immediately adjacent to gold in the periodic table They exhibit a strong tendency to form an adsorbed monolayer on gold and

platinum electrodes This is done at potentials positive to those at which their cathodic deposition as bulk metals would begin, i.e at underpotentials

(52) Deposits obtained with these additives have a very fine and highly

uniform grain size (51) The various heavy metal ions have a brightening effectiveness which is in the order "I > Pb > Bi > Hg This is the inverse order of their electron work functions (the amount of energy required to lift

an electron out of a lattice) The postulated mechanism for this performance

is that the elements form an adsorbed monolayer on the surface of the gold This lowers its work function and thereby lowers its deposition potential so

that deposition then occurs at underpotentials (52) An excellent review on additives for gold plating systems can be found in reference 53

Lead - Common additives for deposition of lead from fluoborate solutions include peptone and resorcinol (54) For plating strip, which requires high current densities of loo0 amp/ft2 or greater, hydroquinone was the best additive out of 230 compounds evaluated on the basis of

performance, stability, cost and lack of industrial hazard (55) Compounds which provided grain refinement and lo00 amp/ft2 or better,limiting current density were the following structural groups listed in decreasing order of effectiveness: aliphatic compounds, benzene derivatives, naphthalene derivatives, anthraquinone derivatives and heterocyclic compounds

Nickel - The key to modem bright nickel plating was the discovery of combining an organic "carrier" brightener with an auxiliary compound to produce brightness and leveling (45) These are referred to

as Class 1 and Class I1 brighteners and materials of each type are listed in Table 4 Brighteners of the first class have two functions: 1-provide bright deposits over a bright substrate and 2-permit the second class brighteners to

be present over an acceptably wide range of concentrations Brighteners of the second class are used to build mirror-like lustre However, most of these lead to excessive brittleness and stress in deposits in the absence of brighteners of the first class (56) Comparisons of the two brightener

'r

Table 5: Comparison of Carriers and Brighteners Used In Nickel

Plating Solutions*

Carriers (Class U Rriahteners (Class lu

Bright or cloudy deposits,

unable to provide high lustre

with continued plating

Brilliant leveling and increasing lustre

Sulfur (0.03%) occluded in

deposit when Class I compounds

are used without Class II

compounds

Introduces carbon in the deposit

No critical upper concentra-

tion, used in high concentra-

tions(1-I0 gll) Cathode poten-

tial increases 15-45 mv in low

concentrations, then very little

change with further additions

Cathode potential continues

to increase sharply with increases in concentration

Do not cause cracking or

peeling

Reduce stress, can result in

compressive stress Lessen

ductility slightly

Deposits crack and peel when the cathode potential increase exceeds approximately 30 mv Have a very deleterious effect

on properties, producing brittle, highly stressed deposits

* From references 43 and 59

classes are provided in Table 5 For more detail on nickel plating brighteners, see references 43, 45, 46, 56-59 and the Corrosion chapter in

this book

Silver - Present silver solutions closely resemble the one described

in the first patent over 140 years ago (8) Carbon disulfide and thiosulfate have been the most widely used addition agents over the years Many other materials have been proposed, including gums, sugars, unsaturated alcohols, sulfonated aliphatic acids, xanothogenates, Turkey Red Oil, Rhodamine Red and compounds of antimony and bismuth (4660) Most of these agents are sulfur bearing organic compounds or reaction products of sulfur and organic compounds (60)