Silver plating procedure

A silver electroplating procedure is disclosed which permits rapid and efficient plating and yields ductile, adherent silver films. The electroplating bath comprises silver complexed with an aliphatic polyamine compound with 3 to 20 carbon atoms. Particularly useful are such polyamines as diaminopropane (particularly 1,3-diaminopropane), diethylenetriamine, 1,4-diaminobutane, 1,6-diaminohexane, etc. The procedure is also useful for electroplating a variety of silver alloys. In addition, the bath is highly stable, does not adversely affect the base material being plated and does not contain hazardous materials which require special disposal procedures.

TECHNICAL FIELD 
The invention is a process for electroplating silver from an aqueous 
plating bath. 
BACKGROUND OF THE INVENTION 
Precious metals are used as protective films on surfaces for a variety of 
reasons. In the jewelry trade, it is used to improve the appearance of an 
article as in gold plated jewelry. In other applications, it is used to 
protect against corrosion of metals and other surface materials. In the 
electrical arts, protective films made of precious metals are used as 
conduction paths in electrical circuits and as contact surfaces in devices 
with electrical contacts. Gold is used extensively in these applications 
with great success. However, the increased price of gold makes it 
attractive to look at other precious metals as protective films on various 
surfaces. 
Silver and silver alloys are used extensively in a variety of industrial 
applications. Typical examples are the jewelry trade where such films are 
used to protect surfaces against corrosion and to improve appearance and 
the electrical arts in various electrical devices and electronic circuits. 
Silver is used as conducting paths in various types of printed circuits 
and integrated circuits. 
Because of relative chemical inertness and reasonable hardness, silver is 
especially attractive as an electrical contact material in electrical 
connectors, relay contacts, switches, etc. Indeed, because of the 
increasing cost of gold, silver and silver alloys become more and more 
attractive economically as a contact material, surface material and in 
other applications. In many applications where gold is now used, it is 
often economically attractive to use silver, provided an inexpensive and 
efficient method of plating ductile and adherent silver is available. 
Silver is also used as a brazing material and is sometimes applied by 
electroplating in the manufacturing of devices. Examples are planar 
triodes and surge protectors. 
Highly desirable is a process for plating silver and silver alloys from an 
aqueous solution which is operable at high rates of deposition and yields 
silver and silver-alloy films which are ductile and adherent. 
Conventionally, silver is electroplated from a cyanide-type bath in basic 
solution. Such processes have been described in a number of references, 
including: U.S. Pat. No. 2,777,810, issued to B. D. Ostrow on Jan. 5, 
1957; U.S. Pat. No. 2,735,808, issued to Lawrence Greenspan on Feb. 21, 
1956 and U.S. Pat. No. 2,666,738, issued to Otto Kardos on Jan. 19, 1954. 
Although such processes yield satisfactory results from a pollution, 
disposal and environmental point of view, it is often desirable to avoid 
cyanide baths. Also, replenishment of the baths often involves 
introduction of additional anions already present in the electroplating 
bath or different anions not already present in the bath. The introduction 
of such ions often complicates the silver electroplating process and 
often, because of buildup of the anion concentration, limits the lifetime 
of the electroplating bath. A cyanide-free silver electroplating process 
where anion concentration remains relatively constant is highly desirable. 
K. Hosokawa et al in a paper published in Proceedings of the 8th Congress 
of the International Union for Electrodeposition and Surface Finishing, 
Forster-Verlag AG, Zurich, 1973, pp. 180-186, described some experiments 
in which silver is plated from an aqueous solution containing 
silver-ethylenediamine complex ion. Although the paper contains much 
discussion on the mechanism for electroplating from such a solution, the 
commercial value of the process is not apparent. 
SUMMARY OF THE INVENTION 
The invention is a process for electroplating silver (both pure metal and 
alloys with various metals) from an aqueous plating solution in which the 
plating solution comprises silver in the form of a complex ion and the 
complexing agent is one or more organic aliphatic polyamines with from 3 
to 20 carbon atoms. Both straight chain and branch chain aliphatic groups 
may be attached to the amine groups. Typical complexing agents are 
diaminopropane (particularly 1,3-diaminopropane), diethylenetriamine, 
1,4-diaminobutane, 1,6-diaminohexane, etc. Secondary polyamines such as 
N,N' dimethyl-1,3-propanediamine and tertiary polyamines such as 
N,N,N'N'tetramethylethylenediamine are also useful provided the total 
number of carbon atoms does not exceed 20. A limited number of 
substituents are also useful, such as hydroxy groups (i.e., 
2-hydroxy-1,3-diaminopropane) and halogen groups such as chloride and 
bromide. It is preferred that the aqueous electroplating bath be alkaline 
(pH greater than 7.0) to avoid possible protonation of the silver complex 
and precipitation of silver. Additional substances may be added to the 
silver plating bath to control and adjust the pH (such as a buffer), to 
increase conductivity and to improve the properties of the plated metal. 
Typical substances used to improve the plated metal are lactones (i.e., 
phenolphthalein, phenolsulfone-phthalein, etc.), lactams, cyclic sulfate 
esters, cyclic imides and cyclic oxazolinones. Certain polyalkoxylated 
alkylphenols may also be useful. The process is useful for plating a 
variety of silver alloys including 10 mole percent silver, remainder 
copper, nickel and/or gold. Additional advantages are that the bath is not 
highly toxic and does not present a disposal problem or environmental 
problem. Replenishment can be accomplished by the addition of Ag.sub.2 O 
which avoids the introduction of extraneous anions and maintains the pH of 
the electroplating bath and the bath chemistry is simple and highly stable 
which ensures long bath lifetimes.

DETAILED DESCRIPTION 
The invention is a process for electroplating silver metal and silver alloy 
in which a certain class of organic aliphatic polyamines is used as 
complexing agent in the silver-plating bath. Most useful are aliphatic 
polyamines with from 3 to 20 carbon atoms. Complexing agents with less 
than three carbon atoms yield useful results but tend to evaporate and 
limit the lifetime of the bath. Complexing agents with more than 20 carbon 
atoms, although useful, usually have limited solubility in aqueous 
solutions. Aromatic polyamines are also useful but often are difficult to 
work with (often poisonous with undesirable odor). Most preferred are the 
complexing agents 1,3-diaminopropane and diethylenetriamine because of the 
excellent quality of the silver plating obtained, especially at high 
plating current density (above 50 ASF). The preferred polyamine compounds 
have high solubility without the tendency to evaporate under the 
conditions of the electroplating process. Also, the composition of the 
bath is such that chemical attack on the surface being plated is minimal 
or insignificant. Replenishment of the bath is particularly convenient 
with the addition of Ag.sub.2 O. This replenishment procedure is very 
convenient since it does not add foreign anions to the solution and 
stabilizes the pH of the bath. 
Within the limitations set forth above, the structure of the complexing 
agent may vary considerably. In particular, these complexing agents may 
contain certain substituents which do not significantly alter their 
complexing properties but may increase solubility, stability, 
electrochemical reduction (or oxidation) potential, etc. Typical 
substituents are hydroxyl groups, chloride and bromide. The complexing 
agents should be stable to the conditions of the electroplating process 
and in particular not undergo oxidation or reduction under the conditions 
of the electroplating process. For example, carboxylic acid groups should 
be avoided because such substituted aliphatic polyamines are generally not 
electrochemically stable. 
Alloy plating may also be carried out using the polyamine complexing agent. 
Typical elements alloyed with silver are copper, nickel, cobalt, iron, 
gold, chromium, manganese, ruthenium, rhodium, palladium, platinum and 
iridium. Particularly useful are copper, nickel and gold. Preferred are 
alloys comprising at least 10 mole percent silver, remainder copper, gold 
and/or nickel. Other useful alloys are 40 mole percent silver, remainder 
gold, copper and/or nickel, 60 mole percent silver, remainder gold, copper 
and/or nickel, etc. 
In addition, the silver alloy or essentially pure silver may contain small 
amounts of substances (generally other metals) which change or improve the 
properties of the plated silver. Up to 10 mole percent of such substances 
may be present in the electroplated material. 
A large variety of counter ions (anions) may be used in the electroplating 
bath provided the anions are stable (chemically and electrochemically) and 
in particular are not subject to oxidation or reduction under conditions 
of the electroplating process. In addition, the anion should not interfere 
with the plating process by either chemical attack on the surface being 
plated or on the metal complex system. Typical anions are halides, 
nitrate, sulfate and phosphates. Chloride ion is preferred because of the 
low cost of silver chloride and the stability of the chloride ion under 
conditions of the electroplating process. Also, certain ions, including 
those set forth above, may be used as supporting electrolyte to increase 
conductivity of the electroplating bath. The cation used for the 
supporting electrolyte may be any soluble ion which does not interfere 
with the electroplating process. Alkali-metal ions (Na, K, Li) are 
particularly preferred because of solubility and stability. Generally, a 
separate supporting electrolyte is not used and conductivity is obtained 
from the silver-complex ions and any anions present including hydroxyl 
ions. Also, conductivity is obtained from any buffer system present such 
as phosphate ions, hydrogen-phosphate ions and/or dihydrogen-phosphate 
ions. 
It is most preferred to use a silver bath in which the anions (for the most 
part) are hydroxyl ions. This is done by charging the bath with silver 
oxide (i.e., Ag.sub.2 O) together with the polyamine for complexing the 
silver ions. This procedure avoids introducing unnecessary anions to the 
electroplating bath. Optionally, a buffer system may be used to stabilize 
pH and the buffer substance introduces some ions to the electroplating 
bath. 
Various compounds may be used as a source of silver. Silver oxide is 
preferred because of availability and stability and for the reasons set 
forth above. Also useful are various silver compounds such as silver 
chloride, silver bromide and silver nitrate. These compounds may be used 
initially to make the bath and to replenish the bath. 
Generally, the pH of the bath may vary over large limits provided the 
silver-polyamine complex remains stable. Typically, good results are 
obtained in the pH range from 7.5 to 13.5. In this range, the 
silver-polyamine complex is most stable and the bath is not inconveniently 
corrosive. However, it should be recognized that the plating process may 
be used outside this pH range. The most preferred pH range is 9 to 12.5. 
The preference particularly applies when the preferred polyamines are 
used, namely, 1,3-diaminopropane and diethylenetriamine. Within this pH 
range, very rapid plating can be carried out with excellent plating 
results. Generally, a bath composition which permits rapid plating with 
more alkaline solution is preferred because of decreased attack on the 
surface being plated. 
The plating process may be carried out with or without a buffer system. A 
buffer system is often preferred because it maintains constant pH and adds 
to the conductivity of the bath. Typical buffer systems are the phosphate 
system, borax, bicarbonate, etc. Preferred is the HPO.sub.4.sup.-2 
/PO.sub.4.sup.-3 system often made by adding an alkali-metal hydroxide 
(KOH, NaOH, etc.) to an aqueous solution of the hydrogen phosphate ion. 
Generally, the concentration of buffer varies from about 0.1 molar to 2 
molar (about 1.0.+-.0.2 molar preferred) and the mole ratio of hydrogen 
phosphate to phosphate varies from 5/1 to 1/5 (with equal mole amounts 
within .+-.50 percent preferred). These mole ratios often depend on the 
particular pH desired for the plating bath. 
Various additives may be used to improve the performance of the bath and/or 
the properties of the silver plate. Various additives are described in a 
number of references including U.S. Pat. No. 4,265,715 issued to D. R. 
Rosegren et al on May 5, 1981. 
The bath temperature may vary over large limits, typically from the 
freezing point to the boiling point of the electroplating bath. Often, the 
preferred plating temperature range depends on bath composition and 
concentration, plating cell design, pH and plating rate. Preferred 
temperatures for typical conditions are from room temperature to about 80 
degrees C. with 30 to 65 degrees C. most preferred. 
Various surfaces may be plated using the disclosed process. Usually, the 
plating would be carried out on a metal surface or alloy surface, but any 
conducting surface is sufficient. Also, electrolessly plated surfaces are 
useful. Typical metal and alloy surfaces are copper, nickel, gold, 
platinum, palladium, silver (as, for example, a surface electrolessly 
plated with silver and then electroplated with silver in accordance with 
the invention). Various alloy surfaces may also be used such as 
copper-nickel-tin alloys, beryllium-copper alloys, etc. 
Typically, conventional anodes are used including platinum and platinized 
titanium. Also, consumable anodes may be used in which the anode comprises 
silver. With consumable anodes, silver is replenished from the anode. 
The composition of the bath may vary over large limits provided it contains 
a source of silver and significant amounts of one or more polyamines of 
the class set forth above. In general, sufficient polyamine should be 
present to complex with the silver. Usually, it is advantageous if excess 
polyamine is present in the bath solution. 
The silver concentration in the bath typically varies from 0.01 molar to 
saturation. Preferred concentrations often depend on plating rate, cell 
geometry, agitation, etc. Typical preferred silver concentration ranges 
for high-speed plating (50 to 1000 ASF) are higher than for low-speed 
plating (up to 50 ASF). Preferred silver concentration ranges for 
high-speed plating vary from 0.05 to 1.0 molar. For low-speed plating, the 
preferred range is from 0.02 to 0.2 molar. Where silver alloy plating is 
included, the alloy metal (usually copper, gold or nickel) replaces part 
of the silver in the composition of the plating bath. Up to 90 mole 
percent of silver may be replaced by alloy metal. 
The amount of complexing agent (polyamine) may vary over large limits, 
typically from 0.5 times (on the basis of moles) the concentration of the 
silver species to saturation of the complexing agent. Generally, it is 
preferred to have excess complexing agent, typically from 2 times to 12 
times the mole concentration of the silver species. Most preferred is 
about three times the mole concentration of silver. The preferred ranges 
of complexing agent in terms of silver species are the same for high-speed 
and low-speed baths. 
The concentration of buffer may vary over large limits. Such concentrations 
often depend on cell design, plating rates, etc. Typically, the buffer 
concentration varies from 0.1 molar to saturation with from 0.2 to 2.0 
molar preferred. 
The bath is prepared in a variety of ways well known in the art. A typical 
preparation procedure which yields excellent results is set forth below: 
Equal amounts (11.3 gms) of 1,3-diaminopropane and water are mixed in a 
beaker. Heat of solution is sufficient to heat the resulting solution to 
about 60 degrees C. To this solution with vigorous stirring are added 11.6 
gms of Ag.sub.2 O in portions of about 0.5 gms approximately every two 
minutes. Since the resulting reaction is exothermic, the solution can be 
maintained at 60 degrees C. by adjusting the rate of addition of Ag.sub.2 
O. The solution is filtered to remove solid matter (generally undissolved 
Ag.sub.2 O) and diluted to one liter. 
To this solution is added 50 gms of K.sub.2 HPO.sub.4. The pH is 11.3 at 25 
degrees C. and can be adjusted upward by the addition of KOH and downward 
by the addition of H.sub.3 PO.sub.4. This bath produces excellent silver 
platings at low plating rates such as about 50 ASF. 
Another bath composition with 116 gm/l Ag.sub.2 O, 113 gm/l 
1,3-diaminopropane and 173 gm/l potassium hydrogen phosphate yields 
excellent results at high plating rates, typically about 500 ASF. 
The electroplating bath may be prepared with silver compounds other than 
Ag.sub.2 O. A typical bath, made as described above is 17 gm/l AgNO.sub.3, 
22 gm/l 1,3-diaminopropane and 101 gm/l KNO.sub.5. The potassium nitrate 
is used as the supporting electrolyte and the pH is about 11. 
Electroplating is typically carried out at about 50 degrees C. 
Electroplating experiments are carried out in an electroplating cell 
provided with means for high agitation. Temperature is maintained between 
30 and 65 degrees C. with 55 degrees preferred. Current is passed through 
anode, electroplating bath and cathode. The electrical energy is supplied 
by a conventional power supply. Typical current densities are from 50 to 
500 ASF. Typical thicknesses in these experiments are 40 to 150 
microinches. The deposit is crack-free as determined by a scanning 
electron micrograph at 10,000 magnification. Both adherence and ductility 
are excellent. Plating rate is often determined by the thickness desired 
after a predetermined period of plating. For example, in a strip line 
plating apparatus (see, for example, U.S. Pat. No. 4,153,523, issued to D. 
E. Koontz and D. R. Turner on May 8, 1979 and U.S. Pat. No. 4,230,538, 
issued to D. R. Turner on Oct. 28, 1980) the strip line being plated is 
exposed to the plating solution for a set period of time (depending on the 
speed the strip is moving down the line and the length of the plating 
cell) and the plating rate is adjusted to give the desired thickness in 
this period of time. The advantage of high plating rates is that the strip 
can move down the line faster yielding greater output. Similar results are 
obtained with diethylenetriamine. Experiments carried out with 2 
hydroxypropanediamine, 1,4-diaminobutane, 1,5-diaminopentane and 
1,6-diaminohexane yield similar results. 
The apparatus described in the above-cited patents are particularly 
advantageous for carrying out the process. They permit good control of the 
bath conditions, the rate of plating and permit rapid silver plating. The 
silver plating process is highly advantageous for plating electrical 
contact pins for electrical connectors such as described in the above 
references. 
The FIGURE shows apparatus 10 useful in the practice of the invention. The 
surface to be plated 11 is made the cathode in the electrolytic process. 
The anode 12 is conveniently made of platinized titanium or may be made of 
various other materials such as oxides of platinum group metals, binder 
metal oxides, etc. Both anode and cathode are partially immersed in the 
electroplating bath 13 containing source of silver complex with an organic 
aliphatic polyamine. A container is used to hold the silver plating 
solution and the anode 12 and cathode 11 are electrically connected to 
source of electrical energy 15. An ammeter 16 and voltmeter 17 are used to 
monitor current and voltage. The voltage and current are controlled inside 
the source of electrical energy 15.