Process for plating solder

An apparatus and process are described for plating tin-lead solder on metallic surfaces. Particularly significant is the high plating rate and the limited amount of plating in designated areas of the metallic surface. Solder is introduced into the plating solution from anode baskets holding tin-lead solder. This apparatus and process are particularly adaptable for continuous strip plating on metallic electrical connectors.

TECHNICAL FIELD 
The invention involves apparatus and process for electroplating lead-tin 
solder. 
BACKGROUND OF THE INVENTION 
Solder electroplating is used extensively in modern technology both as a 
protective coating and to render metallic surfaces solderable for various 
applications. Solder plating is highly advantageous principally because of 
low cost (as compared to gold electroplating, for example) and the need in 
many applications for a protective coating over various metals. Solder 
plating can also be done quite rapidly and can be used to cover large 
areas. 
In modern technology, it is highly desirable to have high solder plating 
rates as well as high throughput of material. This is particularly true 
for electrical connectors which are used in large amounts in various 
electrical equipment. Particularly significant in recent years has been 
the development of continuous strip plating systems which have high 
material throughputs and relatively low economic investment. Such systems 
also have the advantage of reduced equipment necessary for venting and 
rinsing. Solder plating is discussed in some detail by B. F. Rothschild, 
in U.S. Pat. No. 3,554,878 issued Jan. 12, 1971. 
SUMMARY OF THE INVENTION 
The invention is apparatus and process for rapid electroplating of solder. 
The invention applies to tin-lead solder with 60.+-.2 weight percent tin, 
remainder lead. This composition is nominally referred to as 60/40 
tin-lead solder. Although impurities up to 5 weight percent are tolerable, 
higher purity levels are usually preferred (for example, up to one weight 
percent) for greater uniformity of properties and better protection of 
underlying metal. The apparatus is particularly adaptable to rapid 
plating, often in the range of 200-1,000 amperes per square foot. Best 
results in terms of plating quality and rapid production of plated 
material are obtained in the range of 300-400 amperes per square foot. The 
apparatus is quite useful in conjunction with other apparatus in a strip 
line plating system. Particularly unique is the fact that this apparatus 
design permits solder plating without excessive solder buildup on the 
upper and lower end of the piece being plated.

DETAILED DESCRIPTION 
In general terms, the invention is a solder plating cell and process 
uniquely suited for rapid solder plating in a strip line plating system. 
The plating cell is also shaped so as to prevent excessive solder buildup 
on the tips (top and bottom) of the piece being plated. The plating 
procedure and cell is particularly adaptable to plating electrical 
connector pieces and electrical contact pieces because of the quantity 
used and the advantage in using 60/40 tin-lead solder in electrical 
applications. 
The plating cell is best described as a closed or semi-closed system, with 
input and output ports to circulate plating solution. The strip to be 
plated moves down the center of the cell guided by slots or grooves on top 
and bottom which also prevent excessive buildup of solder on top and 
bottom of the piece being plated. Electrical contact is made to the strip 
being plated generally using brushes or rollers. This arrangement 
electrically connects the strip to a power supply so as to make the strip 
to be plated the cathode in the plating cell. 
The anode is made up of a metal basket located on either one or both sides 
of the metal strip being plated and extending along the length of the 
cell. The basket contains pieces of 60/40 tin-lead solder which dissolve 
(are electrolytically oxidized) during the electroplating process. Such 
dissolution of solder replenishes the tin and lead in the solder plating 
bath. Although pure tin metal or pure lead metal may be put into the anode 
basket, this procedure would change the composition (tin/lead ratio) of 
the bath. The metal basket may be made up of any inert conducting 
material. Certain copper-nickel alloys such as monel are ideal, 
particularly when a fluoroboric acid plating solution is used. The metal 
basket is electrically connected to one end of the power supply so as to 
make it the anode of the plating cell. 
Various plating solutions may be used in the plating procedure. Solder 
plating solutions are described by B. F. Rothschild, in U.S. Pat. No. 
3,554,878 issued Jan. 12, 1971. A typical plating solution is set forth in 
the table below: 
______________________________________ 
Concentration in grams/liter 
Range Optimum 
______________________________________ 
Stannous tin 12-20 15 .+-. 2 
Lead 8-14 10 .+-. 1 
Free fluoboric acid 
350-500 400 .+-. 20 
Temperature 60.degree.-100.degree. F. 
75.degree. F. .+-. 5.degree. F. 
______________________________________ 
The range of concentration in the "optimum" column reflects either the 
accuracy with which the optimum concentration and temperature can be 
determined or the accuracy with which the concentration and temperature 
can be controlled. This solder plating solution is particularly 
advantageous for strip line plating because of high plating rates 
permitted. Particularly advantageous is a plating rate between 200 and 
1,000 amperes per square foot. Higher plating rates may be used but the 
quality of the plating may occasionally be affected. Lower plating rates 
yield excellent results but are not as advantageous economically because 
of slower throughput. 
This plating solution is also advantageous because it plates out 60/40 
tin-lead solder. Nominally, 60/40 tin-lead solder has approximately 
60.+-.2 weight percent tin, remainder lead. This composition is highly 
desirable for several reasons. It is the eutectic point for tin-lead. This 
makes it ideal for soldering. The 60/40 tin-lead solder is also quite hard 
and, therefore, has excellent wear qualities. Excellent wear qualities are 
particularly advantageous for electrical connectors, electrical contact, 
etc. because it leads to long lifetimes. 
An understanding of the invention is facilitated by a description of the 
drawing. FIG. 1 shows a view in perspective of the solder plating cell 10. 
The solder plating cell is made up of an electroplating container 11. The 
electroplating container is made of an inert material usually a plastic 
preferably a clear plastic such as plexiglass. The strip to be plated 12 
moves down the center of the plating cell along the long dimension in a 
channel. The strip is held in position in the channel by small notches or 
grooves on top 13 and bottom 14 of the channel path of the metal strip. 
Solder plating solution is circulated through the plating cell by means of 
an input tube 15 and output tube 16. The role of input and output tubes 
may be reversed. The anode 17 is located on both sides of the metal strip 
and along the length of the solder plating cell. Some screening is 
required in the structure of the anode to permit circulation of plating 
solution into the anode structure where the tin-lead solder is located. 
The anode basket may be made of all screening but the particular model 
shown here only has screening on the side of the anode facing the strip 
being plated. The metal strip 12 enters the cell through a long narrow 
slot 18 and exits the cell through a similar slot not shown. 
FIG. 2 shows a sectioned view of the plating cell. The section is along the 
length of the cell and shows metal strip 12, the anode 17 and container 
walls 11. 
FIG. 3 shows a sectioned view perpendicular to the length of the plating 
cell. The metal strip 12 is shown together with the upper slot 13 and the 
lower slot 14. Also shown is the inert container walls 11, and anode 17. 
The anode is made up of solid inert metal 19, and a metal screening 20 
which permits plating solution to circulate inside the anode basket. 
A particular advantage of the solder plating cell is its adaptability for 
use in a multicell strip line plating apparatus. Such an apparatus 
generally includes cleaning cells, rinsing cells, acid dipping cells and 
drying apparatus. A typical strip line for solder plating has a cleaning 
cell, a rinsing cell, an acid dipping cell, another rinse cell, a solder 
plating cell, a third rinse cell, and a drying cell. Generally, this strip 
line or metal strip to be plated is pulled through all of these cells 
which are aligned one after the other. 
The nature of the cleaning cell generally depends on the contaminate to be 
removed from the surface of the strip being plated. If this is an oil or 
grease from a cutting machine, as is usually the case, an alkaline 
detergent is usually used. A typical alkaline detergent is alconox. 
Electrolytic cleaning in which the strip is either cathode or anode may be 
used. 
The cleaning cell is followed by a rinse cell generally made up of a water 
spraying apparatus that washes off solution from the previous 
electrochemical cell. 
This cell is followed by an acid dip cell generally used to remove oxide 
from the metal surface. The nature of the acid depends on the composition 
of the metal strip. For copper containing metal strips, a mixture of 
nitric acid and sulfuric acid is usually used. For nickel containing metal 
strips, fluoboric acid or mixture of fluoboric acid and sulfuric acid is 
often used. The dipping cell is followed by a rinse cell and subsequently 
followed by a solder plating cell. A rinse cell is then used to remove 
solder plating solution. The metal strip is dried generally by blowing 
nitrogen or dry onto the metal strip. 
Highly advantageous is a process for making electrical connectors from pins 
plated in the solder plating cell described above. Solder plating on such 
pins yields excellent protection. The pin surfaces are easily solderable 
and exhibit good electrical contact to other surfaces. An additional 
important advantage is the low cost of the process and the speed with 
which such pins can be solder plated. In addition, continuous strips of 
the pins may be processed either through the solder plating cell alone, or 
the entire array of electrochemical cells referred to above as the strip 
line plating apparatus. Particularly advantageous is immediate sequential 
processing since surfaces are not contaminated between processing steps, 
and large amounts of inventory need not be built up between processing 
steps. Further, pins are rapidly produced with high yield and low cost. 
Initial capital costs are generally low and it is particularly convenient 
for real time control, including pH measurement, temperature control, etc.