Aqueous zinc solution resistant to precipitation

There is disclosed an aqueous chemical solution that consists essentially of zinc ions, a source of hydroxides, and silicon ions. The ratio of silicon to zinc (Si:Zn) is, by weight from about 1:5 to about 1:250. Within the ratio range, the silicon ions inhibit precipitation of zinc from the chemical solution. This is particularly useful when the chemical solution is an electrolyte for the deposition of zinc or a zinc containing compound onto a metallic structure where a critical zinc content must be maintained to impart adequate adhesion of a coated substrate to a polymer adhesive.

CROSS REFERENCE TO RELATED APPLICATION 
This patent application relates to U.S. Pat. No. 5,098,796 entitled 
"Chromium-Zinc Anti-Tarnish Coating on Copper Foil" by Lifun Lin et al. 
that issued on Mar. 24, 1992 and which is incorporated by reference in its 
entirety herein. 
BACKGROUND OF THE INVENTION 
This invention relates to a zinc containing aqueous solution that resists 
zinc precipitation. More particularly, silicon is added to an alkaline, 
aqueous electrolyte that contains zinc ions in a specified zinc to silicon 
ratio. 
Alkaline aqueous chemical solutions containing zinc are widely used such as 
for galvanizing steel and providing tarnish resistance to copper and 
copper alloys. One limitation with these alkaline solutions is that zinc 
continuously precipitates from the solution, both during use and while the 
solution is not in use. 
To maintain the chemical solution in optimum operating condition, the 
solution must be filtered to remove precipitated zinc metal, frequently 
analyzed to determine the present zinc content and replenished to replace 
the precipitated zinc. These steps are time consuming and the chemical 
solution is out of service during the filtering, analyzing, replenishing 
sequence. U.S. Pat. No. 5,098,796 to Lin et al. discloses an electrolyte 
for codepositing a mixture of chromium and zinc on a metallic substrate 
that is an alkaline, aqueous zinc containing solution. The codeposited 
layer imparts the substrate with tarnish resistance at room temperature 
and at elevated temperatures, up to about 220.degree. C. The codeposited 
layer also improves the adhesion of the substrate to a polymer adhesive or 
to a polymer molding resin. 
The present inventor has now discovered that when the zinc content of this 
electrolyte drops below 0.8 grams per liter (g/l), adhesion of the 
anti-tarnish coating to a copper substrate is impaired. 
Therefore, to eliminate lost time while the chemical solution is being 
filtered, analyzed and replenished and to maintain the enhanced adhesion 
of a codeposited layer of chromium and zinc, there is a need for an 
alkaline zinc containing electrolyte that resists zinc precipitation. 
SUMMARY OF THE INVENTION 
Accordingly, it is an object of the invention to provide an alkaline, 
aqueous chemical solution that contains zinc ions and inhibits zinc 
precipitation from the solution. It is a feature of the invention that the 
addition of silicon to the solution inhibits the precipitation of zinc. 
Yet another feature of the invention is that the solution may further 
contain water soluble hexavalent chromium ions for the electrodeposition 
of a codeposited layer of chromium and zinc. 
Among the advantages of the invention is that by inhibiting zinc 
precipitation, the chemical solution is effective for longer periods of 
time without requiring zinc salt additions. Another advantage is that when 
the chemical solution is an electrolyte, more of the zinc is deposited 
onto a substrate. Yet another advantage of the invention is that by 
inhibiting precipitation of the zinc, a more accurate calculation of the 
bath chemistry is possible and it is easier for an operator to ensure that 
the zinc content remains above a critical minimum value. 
In accordance with the invention, there is provided an aqueous chemical 
solution that consists essentially of from about 0.07 g/l to about 30 g/l 
of zinc ions, an amount of silicon ions effective to inhibit precipitation 
of the zinc and from about 3 g/l to about 150 g/l of a hydroxide source. 
The electrolyte is prepared by supplying an aqueous solution having a pH in 
excess of 7. Dissolving silicon in that aqueous medium, from a water 
soluble silicon source, in an amount that provides a silicon ion content 
effective to inhibit precipitation of zinc from the electrolyte and then, 
subsequent to the addition of the silicon ions, dissolving from about 0.07 
to about 30 grams per liter of zinc ions into the aqueous medium. 
The above stated objects, features and advantages will become more apparent 
from the specification that follows. 
DETAILED DESCRIPTION 
The precipitation of zinc ions from an alkaline, aqueous solution is 
inhibited by the presence of a concentration of silicon ions effective to 
inhibit precipitation of the zinc ions. The most effective concentration 
of silicon ions is dependent on the zinc concentration. The ratio, by 
weight, of silicon to zinc (Si:Zn) is from about 1:250 to about 1:5 and 
more preferably, the ratio, by weight, is from about 1:10 to about 1:60. 
When the silicon content is less than 1:250, the beneficial effect of the 
silicon is lost. When the silicon content exceeds 1:5, the integrity of 
the deposit obtained from the chemical solution is impacted. For example, 
when in an electrolytically codeposited layer of chromium and zinc, when 
the silicon content exceeds 1:5, the adhesion promoting properties of the 
codeposited layer are impaired. 
Most preferably, the ratio, by weight of silicon to zinc in the chemical 
solution is from about 1:10 to about 1:20. 
The silicon ions are provided from any suitable, water soluble silicon 
containing compound. A preferred group of compounds are the silicates such 
as sodium silicate or potassium silicate, as well as mixtures thereof. 
Other water soluble silicon containing groups of compounds such as silanes 
are also satisfactory. 
The chemical solution is made alkaline by adding a suitable caustic or 
salt. Preferably, alkalinity is provided by a hydroxide source such as 
sodium hydroxide, potassium hydroxide or ammonium hydroxide. Most 
preferred is sodium hydroxide. 
The zinc ions are supplied in the form of a soluble zinc compound. The zinc 
compound may be soluble in the caustic, such as zinc oxide (ZnO), or in 
the aqueous component of the solution. The concentration of zinc ions is 
from about 0.07 g/l to about 30 g/l and, preferably, from about 0.3 g/l to 
about 10 g/l. 
The aqueous solution may be used for any desired purpose such as 
electrolytic or non-electrolytic coating or for anodization. In one 
preferred embodiment, the aqueous medium is utilized as an electrolyte for 
the deposition of zinc or a zinc containing compound onto a metallic 
substrate. Among the metals that may be combined with zinc in the 
electrolyte for codeposition are chromium, copper, nickel, aluminum, tin 
and iron, as well as mixtures thereof. 
A zinc compound containing chromium is useful as an anti-tarnish coating on 
copper or copper alloy substrates, such as foils and leadframes. 
Chromium ions, in the form of hexavalent chromium ions, are added to the 
electrolyte as a hexavalent chromium salt such as sodium dichromate 
(Na.sub.2 Cr.sub.2 O.sub.7.2H.sub.2 O). Preferably, the chromium (VI) ions 
are present in an amount of from about 0.05 g/l to about 5 g/l and more 
preferably in an amount of from about 0.3 g/l to about 1 g/l. 
When preparing an aqueous medium according to the invention., it is 
desirable to add the silicon to the bath prior to adding the zinc. This is 
because without the silicon addition, the zinc immediately begins to 
precipitate from the solution and it is difficult to accurately maintain a 
desired zinc content. 
One method to produce the aqueous solution of the invention is to first 
supply an aqueous medium that has a pH in excess of 7 and preferably, in 
excess of 11. This aqueous medium may be prepared by adding to water, 
preferably deionized water, from about 3 to about 150 grams per liter of a 
hydroxide source such as sodium hydroxide or potassium hydroxide. The 
water is either at room temperature or heated above room temperature 
during addition of the hydroxide source. 
The silicon ions are then added to the aqueous medium by adding a suitable 
amount of a silicate such as sodium silicate to achieve the desired 
silicon to zinc (Si:Zn) ratio, by weight, of between about 1:250 and about 
1:5. If the aqueous medium is to contain metal ions other than zinc, they 
are also added at this time. 
Finally, the necessary amount of a soluble zinc salt is added to the bath 
to a zinc ion content of from about 0.07 to about 30 grams per liter. 
The advantages of the alkaline, aqueous medium of the invention will become 
more apparent from the examples that follow.

EXAMPLES 
An aqueous electrolyte containing 14.4 g/l sodium hydroxide, 1 g/l zinc 
ions and 0.5 g/l chromium (VI) ions was prepared and had a pH of 13.4. 
This electrolyte was heated to a temperature of 60.degree. C. and became 
part of an electrolytic cell having stainless steel anodes and a copper 
alloy C194 (composition by weight: 2.1-2.6% iron, 0.05-0.20% zinc, 
0.015-0.15% phosphorous and the balance copper) substrate as the cathode. 
A current density of 10 milliamps/centimeter.sup.2 (mA/cm.sup.2) was 
impressed across the electrolytic cell for a time of 10 seconds. 
A codeposited layer of chromium and zinc was deposited on the copper alloy 
C194 substrate. This codeposited layer had strong adhesion to the 
substrate as determined by a tape test. A piece of SCOTCH TYPE 600 tape 
(3M Corporation, Minneapolis, Minn.) having a length of at least 5.1 
centimeters (2 inches) was pressed firmly against the coated substrate. 
The tape was then removed at an angle of 180.degree., ie. pulled back on 
itself, in one rapid motion at a speed of about 2.5 centimeters (1 inch) 
per second. 
The substrate was then evaluated, both visually and under a microscope at 
magnifications of up to 20X. The presence of adhesive on the substrate 
indicates a coating layer having good adhesion to the substrate and 
imparting good adhesion to a polymer. If at least 20% of the surface area 
of the substrate that was covered with the tape retains adhesive, the 
coating is considered to have passed the tape test. The coated copper C194 
substrate passed the tape test. 
The coated substrate was then evaluated for oxidation resistance by heating 
to 175.degree. C., in air for 1.5 hours. No discoloration of the coated 
substrate was detected, indicating that the coating layer provided 
satisfactory oxidation resistance. 
The electrolyte was then divided into three portions. One portion was kept 
silicon free. Sodium silicate was added to the other two portions at a 
level of 20 ppm silicon (Si:Zn ratio=1:50) in a first portion and 100 ppm 
silicon (Si:Zn ratio=1:10) in a second portion. After storage for 64 hours 
at room temperature with no current flowing through the electrolyte, the 
zinc content of each bath was determined. It should be recognized that the 
rate of zinc precipitation would be much more rapid at elevated 
temperatures such as when the electrolyte is operated at 60.degree. C. The 
results are summarized in Table 1. 
TABLE 1 
______________________________________ 
Zinc Remaining 
Silicon Content 
after 64 Hours 
Comments 
______________________________________ 
0- 0.48 g/l visible precipitate 
20 ppm 0.88 g/l visible precipitate 
100 ppm 1.0 g/l no visible precipitate 
______________________________________ 
After completion of the 64 hour storage period, copper alloy C194 
substrates were electroplated in the silicon-free electrolyte and the 100 
ppm silicon bath. Electroplating was again at 10 mA/cm.sup.2 for 10 
seconds with an electrolyte temperature of 60.degree. C. The silicon-free 
bath had poor tape test results, indicative of a coating layer deposited 
from an electrolyte containing less than about 0.8 gm/l zinc. 
Copper alloy C194 substrates plated in the 100 ppm silicon bath after 
completion of the 64 hour storage period showed no degradation in adhesion 
properties believed to be because the zinc content remained above the 
critical 0.8 gm/l level. These samples also showed no degradation in 
oxidation resistance when heated to 175.degree. C. for 11/2 hours. 
It is apparent that there has been provided in accordance with this 
invention a zinc-containing aqueous solution that is resistant to 
precipitation that fully satisfies the objects, means and advantages set 
forth hereinbefore. While this invention has been described in combination 
with specific embodiments thereof, it is evident that many alternatives, 
modifications and variations will be apparent to those skilled in the art 
in light of the foregoing description. Accordingly, it is intended to 
embrace all such alternatives, modifications and variations as fall within 
the spirit and broad scope of the appended claims.