Bright palladium electrodeposition solution

A palladium electroplating solution contains the palladium in the form of a soluble organopalladium complex of an inorganic palladium salt and an organic polyamine complexing agent. The solution also preferably contains an imide and free complexing agent.

BACKGROUND OF THE INVENTION 
Numerous formulations for the electrodeposition of palladium have been 
reviewed by Atkinson in Modern Electroplating, 2d Ed., Lowenheim, Ed., 
Wiley, N.Y. 1963, Reid in Plating, 52, 531 (1965) and Wise in Palladium: 
Recovery, Properties and Uses, Academic Press, N.Y., 1968, pp. 97-103. The 
formulations contain various palladium species and are operable at pH 
values throughout virtually the entire 0-14 range. Nevertheless, the most 
commonly employed palladium plating formulations are almost universally 
based on inorganic ammine complexes of palladium, such as palladosamine 
chloride, Pd(NH.sub.3).sub.4 Cl.sub.2, or palladium diaminodinitrite, 
Pd(NH.sub.3).sub.2 (NO.sub.2).sub.2, which is more commonly known as 
palladium P-salt. Plating formulations containing inorganic ammine 
palladium complexes are ordinarily operated at pH values between about 8 
and 10, and ordinarily contain a slight excess of ammonium hydroxide in 
the electrolyte in order to stabilize the palladium ions in solution. 
The operation of palladium plating formulations containing ammonium ions at 
alkaline pH incurs several disadvantages. These disadvantages include: 
(a) fumes of ammonia are evolved from the plating bath during operation, 
necessitating adequate ventilation for operator safety, 
(b) frequent replenishment of ammonium hydroxide is necessary for stability 
and pH control, and 
(c) it is well-known in the art that metals such as nickel and copper and 
the alloys thereof are rapidly tarnished by alkaline ammoniacal solutions 
and, therefore, in order to plate such materials with palladium from a 
plating formulation as described above, it is ordinarily necessary to 
employ a strike coating of gold or silver to protect the surface of the 
work prior to introduction into the palladium plating bath. 
Accordingly, it is an object of this invention to provide a formulation for 
the electrodeposition of palladium deposits having the usually desired 
characteristics of brightness, lustre, ductility, evenness of 
distribution, freedom from stress, high current efficiency, etc., which 
formulation is free of added ammonium ion so as to be applicable for 
plating palladium deposits onto substrates including nickel, copper, and 
alloys thereof, without requiring the application of a strike coating 
prior to palladium plating. This and other objects of the invention will 
become apparent to those skilled in this art from the following detailed 
description. 
SUMMARY OF THE INVENTION 
This invention relates to palladium coating baths and the use thereof, and 
more particularly to baths employing palladium complexed with an organic 
polyamine. The use of imide and organic polyamine brightener are also a 
subject of the invention. 
DESCRIPTION OF THE INVENTION 
It has been found that certain organic ligands can be reacted with simple 
inorganic salts of palladium such as the sulfate, nitrate, halides 
(chloride, bromide, iodide), etc., to yield soluble organopalladium 
complexes which are stable in aqueous solution, and which can be 
discharged by means of an electric current to produce deposits of metallic 
palladium with high current efficiency at solution pH values from 
approximately 3 to 7, i.e., in the range from neutral to moderately acid. 
In general, it is not advisable to electrodeposit palladium under very 
strongly acid conditions because it is difficult under such conditions to 
avoid cogeneration of large amounts of hydrogen which can permeate the 
crystal lattice of palladium and lead to very high deposit stresses. 
Ligands which are useful for the purposes of this invention are aliphatic 
and cycloaliphatic polyamines containing 2 to about 8 carbon atoms and 2 
to about 5 amino groups. The aliphatic polyamines are preferably of the 
formula 
EQU H.sub.2 N--(CH.sub.2 CH.sub.2 NH).sub.x --(CH.sub.2).sub.y --CH.sub.2 
CH.sub.2 NH.sub.2 
where x is 0-3 and y is 0-4. The cycloaliphatic polyamines are preferably 
of the formula 
##STR1## 
where m is 3-4. Thus, among diamines, effective ligands include 
ethylenediamine; 1,2-propylenediamine; 1,3-propanediamine; 
1,4-butanediamine; pentamethylenediamine; hexamethylenediamine; 
cyclopentanediamine; and cyclohexanediamine. Among polyamines having more 
than two amino groups, effective ligands include diethylenetriamine, 
triethylenetetramine, and tetraethylenepentamine. 
The complexes are prepared by simply mixing the inorganic palladium salt 
and the organic polyamine. If desired, this can be accomplished by adding 
the organic polyamine to a palladium containing plating bath which is 
preferably free of ammonia and inorganic ammines. The amount of organic 
polyamine is that sufficient to provide at least one amino functional 
group per palladium atom. Since divalent palladium ordinarily exhibits a 
coordination number of four, the soluble organopalladium complexes of this 
invention are preferably prepared using a molar ratio of two moles of the 
polyamine complexing agent per gram atomic weight of palladium. The amount 
of the complex can be about 1-180 grams per liter and is preferably about 
30-90 grams per liter. The other constituents of the plating solution can 
be the conventional constituents. 
It has been further found that in an aqueous electroplating solution of pH 
from about 3 to 7 containing palladium in the form of a soluble 
organopalladium complex described above, together with a supporting 
electrolyte such as phosphate, citrate, malate, etc., as commonly employed 
in the art for the purpose of providing electrical conductivity and/or pH 
control for the said solution, a substantial brightening effect is 
produced by the addition to the solution of an organic imide of the 
formula 
##STR2## 
wherein each substituent R, independently, can be hydrogen, alkyl or 
alkoxy; the alkyl and alkoxy groups not exceeding 5 carbon atoms in size. 
Typical of these imides are succinimide; 2-methyl succinimide, 2,2,3,4 
tetramethyl succinimide; maleimide; and the like. 
The concentration of organic imide required to achieve a brightening effect 
is not critical, and may be varied from about 1 to about 50 grams per 
liter of electroplating solution. 
It has further been found that in an aqueous electroplating solution of pH 
from about 3 to 7 such as has been described above, containing palladium 
in the form of a soluble organopalladium complex together with a suitable 
supporting electrolyte and an added organic imide as hereinabove 
described, a further improvement in the brightness and appearance of the 
electrodeposited palladium can be obtained by including in the 
electroplating solution a quantity of the organic polyamine ligand beyond 
that forming a part of the soluble organopalladium complex. In order to 
simplify the chemical makeup of the electroplating solution it is 
convenient, but not absolutely necessary, to employ as the organic 
polyamine the same chemical species used as a ligand to form the 
particular organopalladium complex in solution. For example, in an 
electroplating solution as hereinabove described in which palladium is 
present as an ethylenediamine complex, it is convenient to add a quantity 
of free ethylenediamine to the solution for the purpose of improving the 
brightness of the electrodeposit, although 1,2-propylenediamine, for 
example, will produce a similar effect. The quantity of organic polyamine 
which is effective for brightening purposes of this invention may vary 
considerably depending on the chemical nature of the additive and the 
composition of the electroplating solution, but in general will be in the 
range from about 1-50 grams of polyamine additive per liter of 
electroplating solution.

In order to illustrate the present invention, some examples are given 
below: 
EXAMPLE 1 
Sufficient water was employed to form one liter of a palladium 
electroplating solution containing the following: 
8 grams palladium in the form of palladium bis (ethylenediamine) sulfate 
120 grams monopotassium phosphate 
15 grams succinimide 
The solution pH was adjusted to 6.0 by adding potassium hydroxide. A test 
panel was plated in this solution in a Hull cell for two minutes at 1 
ampere at 50.degree. C. A deposit of palladium was obtained which was 
mirror-bright and haze free at current densities from near zero to about 
20 mA/cm.sup.2. 
EXAMPLE 2 
To one liter of the palladium electroplating solution of Example 1 was 
added 3.6 grams of ethylenediamine. The pH of the solution was readjusted 
to 6.0 with phosphoric acid. A test panel was plated in this solution in a 
Hull cell for two minutes at 1 ampere at 50.degree. C. A mirror-bright and 
haze free deposit of palladium was obtained at current densities from near 
zero to about 40 mA/cm.sup.2. 
EXAMPLE 3 
An electroplating solution was formed as in Example 1, except that in place 
of succinimide, maleimide was employed at a concentration of 7.5 grams per 
liter. The solution pH was adjusted to 4.0 with phosphoric acid. A test 
panel was plated in this solution in a Hull cell for two minutes at 1 
ampere at 25.degree. C. A bright deposit of palladium was obtained at 
current densities from near zero to about 40 mA/cm.sup.2. 
EXAMPLE 4 
An electroplating solution was formed as in Example 1, except that the 
palladium employed was in the form of palladium bis (1,2-propylenediamine) 
sulfate. A test panel was plated in this solution in a Hull cell for 2 
minutes at 1 ampere at 50.degree. C. A deposit of palladium was obtained 
which was mirror-bright and haze free at current densities from near zero 
to about 15 mA/cm.sup.2. 
EXAMPLE 5 
To one liter of the electroplating solution of Example 4 was added 3.5 
grams of 1,2-propylenediamine, and the solution pH was readjusted to 6.0 
with phosphoric acid. A test panel was plated in this solution in a Hull 
cell for two minutes at 1 ampere at 50.degree. C. A deposit of palladium 
was obtained which was mirror-bright and haze free at current densities 
from near zero to about 40 mA/cm.sup.2. 
EXAMPLE 6 
An electroplating solution was formed as in Example 1, except that the 
palladium employed was in the form of palladium bis (1,3-propanediamine) 
sulfate. The pH of the solution was adjusted to 4.0 with added phosphoric 
acid. A test panel was plated in this solution in a Hull cell for two 
minutes at 1 ampere at 50.degree. C. A bright deposit of palladium was 
obtained at current densities from near zero to about 35 mA/cm.sup.2. 
EXAMPLE 7 
An electroplating solution was formed as in Example 1, except that the 
palladium employed was in the form of palladium diethylenetriamine 
sulfate. The solution pH was adjusted to 4.0 with phosphoric acid. A test 
panel was plated in this solution in a Hull cell for two minutes at 1 
ampere at 60.degree. C. A bright deposit of palladium was obtained at 
current densities from near zero to about 10 mA/cm.sup.2. 
It will be appreciated by those skilled in the art that various changes and 
modifications can be made in the present invention without departing from 
the spirit and scope thereof. The embodiments disclosed herein were for 
the purpose of illustrating the invention only and were not intended to be 
limited thereto.