Tin lead plating solution

An acid solution for displacement plating of a reflowable tin lead alloy having a melting point not exceeding 500.degree. F. over a cupreous surface. The displacement plating solution contains stannous ions in an amount of from 0.05 to 0.50 moles per liter, plumbous ions in an amount of from 0.01 to 0.15 moles per liter, a total content of stannous ions and plumbous ions in an amount of from 0.05 to 0.60 moles per liter and a ratio of stannous ions to plumbous ions varying from about 1.0:3.0 to 50.0:1.0. The solution is characterized by an ability to plate the tin lead alloy at a plating rate of at least 100 microinches per 15 minutes. The deposit obtained is capable of reflow and is useful in the manufacture of printed circuit boards.

BACKGROUND OF THE INVENTION 
1. Introduction 
This invention relates to a metal plating solution for the deposition of 
reflowable, thick deposits of tin lead alloys such as solder over metal 
surfaces such as cupreous based substrates including copper, brass, bronze 
and other alloys of copper. The invention also includes methods for 
accomplishing deposition of such alloys by immersion plating. The 
invention is especially useful for the manufacture of printed circuit 
boards 
2. Description of the Prior Art 
Tin lead alloy having a composition of between 60 and 65 percent tin and 35 
to 40 percent lead (solder) is coated onto copper traces in the 
manufacture of printed circuit boards. The alloy may function as an etch 
resist and permits attachment of components to a circuit board. For this 
latter use, the alloy must melt and reflow at a reasonably low temperature 
to avoid damage to the circuit during the heating of the board and must 
have an adequate thickness for surface mounting of components. 
Conceptually, there are four methods available for coating tin lead alloys 
onto copper traces in printed circuit manufacture. These methods include 
(1) hot air leveling, (2) electroless plating, (3) immersion plating and 
(4) electroplating. The coating method selected for a specific fabrication 
process is dependent upon the required properties of the alloy deposited 
on the substrate and the specific circuit fabrication method used. Each 
coating process has distinct advantages and disadvantages and the metal 
deposit obtained from each differs from the deposits obtained by other 
plating methods. 
It is known in the art that to be suitable for printed circuit board 
manufacture, following reflow, a tin lead alloy deposit over copper 
circuit lines must be pore free, reasonably thick, should possess a 
uniform cross section and should be bondable to subsequently attached 
components. It is also known that as metal is deposited onto a substrate 
from solution in a plating process, the deposit obtained is often not 
coherent and may contain numerous pores due to the nature of the plating 
reaction. Pores cannot be tolerated as they permit migration of corrosive 
chemicals to the surface of the board during subsequent processing steps 
in the fabrication process. Pores are typically eliminated in a tin lead 
alloy deposit by heating the board with the alloy deposit to a temperature 
in excess of the alloy's melting point whereby the deposit melts and pores 
are eliminated by reassembly of the deposit, a process known in the art as 
"reflow" of the deposit In addition, a tin lead alloy must be bondable 
This means the alloy must melt to permit attachment of components to a 
circuit board To reflow a tin lead alloy and to bond components thereto, 
the tin lead alloy should melt at a relatively low temperature to avoid 
damage to the circuit by heating to an excessively high temperature and 
the deposit must be adequately thick A thin deposit will not reflow and 
may not contain the minimum mass of metal required to bond a component to 
a circuit board, particularly if the component is surface mounted on the 
board. To obtain a low melting tin lead alloy, the deposit desirably has a 
concentration of tin and lead at or close to the low melting eutectic of 
tin and lead (approximately 63% tin and 37% lead by weight--i.e., solder). 
Electrolytic plating of tin lead alloys is disclosed by Schlabac and Ryder, 
Printed and Integrated Circuitry, McGRaw Hill Book Company, Inc., New 
York, 1963 at page 146. Electrolytic plating of tin lead alloys is one 
method of choice in the industry but suffers certain disadvantages. For 
example, for electrolytic deposition, a circuit board must be racked as 
part of an electrolytic cell which is a labor intensive step. In addition, 
due to uneven current densities as a consequence of a circuit pattern over 
which the tin lead alloy is to be deposited, the throwing power of the 
electrolytic bath does not provide desired uniformity and deposits of 
uneven thickness are obtained. This problem is aggravated further if the 
circuit board is one having high aspect ratio through holes. Further, 
deposits of a 60/40 percent tin lead alloy, the alloy of choice, requires 
close control of the solution composition and operating conditions of the 
electrolytic cell. Finally, once the tin lead alloy is coated onto the 
circuit, and the copper not coated with the alloy is etched, the resultant 
copper circuit traces are discontinuous and damaged circuits are not 
readily reworked because electrolytic deposition cannot take place over a 
discontinuous conductive substrate. 
Hot air leveling is also a method conventionally used in the art for 
providing a tin lead deposit over copper circuit lines in printed circuit 
manufacture. In this process, the circuit board is floated or immersed in 
a molten tin lead bath. The molten coating obtained is of non uniform 
thickness and is then made more uniform by passing the board with the tin 
lead coating beneath a stream of hot air to level the coating or make it 
uniform. Notwithstanding the use of the hot air stream, the coating 
obtained is thin at the heel of the hole and non uniform and with high 
aspect ratio holes, thin in the center of the hole, the step of hot air 
leveling is time consuming and labor intensive and with high aspect holes, 
the coating is thin at the edge of the pads. 
Electroless plating baths for each of tin and lead, separately, are known 
in the art. For example, an electroless tin plating bath is disclosed by 
Warwick and Shirley, The Autocatalytic Deposition of Tin, Trans. Inst. 
Met. Fin. 58 9 (1980) pp. 9-14 and by Juergen, Chemical Deposition of Tin 
and Tin Lead, European Institute of Printed Circuits, Deposition for PCB; 
Conference: Economics and Cost Savings, Conference location: Munich, West 
Germany, Conference Date: 1983, Monthly number: EIM8506-033415, 1983. A 
publication showing the electroless codeposition of tin and lead is not 
known. It is believed that electroless solder plating baths are difficult 
to formulate because an autocatalytic solution must contain a mixture of 
tin and lead as well as a complexing agent and reducing agent for each in 
a single plating solution. 
Immersion plating is an electroless plating process, but is given a 
separate classification by the art. In immersion plating, deposition is by 
displacement of an elemental metal from a substrate by metal ions in a 
plating solution while in electroless plating, in accordance with the art 
accepted definition, plating takes place primarily by autocatalytic 
reduction of metal ions from solution. 
Since immersion plating is by displacement, immersion plating, like 
electroless plating, does not employ an external electric current but 
rather is an electrochemical displacement reaction which depends upon the 
position the substrate metal occupies in the electromotive series relative 
to the metal to be deposited from solution. Plating occurs when the metal 
from a dissolved metal salt is displaced by a more active (less noble) 
metal that is immersed in the solution. Since copper is more noble than 
either tin or lead, it would appear that copper should not be plated by an 
immersion process to give a tin lead deposit. However, when complexed, 
under acidic conditions, the electropotentials of the tin and lead 
complexes relative to copper reverse making the utilization of immersion 
plating for the manufacture of printed circuit boards possible However, 
major limitations in the use of immersion plating for circuit fabrication 
exist. These limitations include a relatively slow plating rate, 
difficulty in obtaining a desired alloy and limited deposit thickness. 
Limited deposit thickness is due to the fact that the immersion plating 
reaction is self limiting because as the coating builds to full thickness, 
the metal deposited from solution masks the underlying base metal which 
functions as the reducing agent and is required for displacement thereby 
preventing further displacement. Additionally, as the displaced base metal 
is dissolved in solution, it becomes a contaminant in solution in 
increasingly high concentration thereby progressively slowing the rate of 
displacement. Typical deposit thickness for an immersion tin deposit in 
the prior art is 50 to 100 microinches, mainly because of the foregoing 
problems in building the deposit to greater thicknesses. 
If the above disadvantages with immersion plating could be overcome, there 
would be advantages to immersion plating over other methods for depositing 
tin lead alloy in circuit manufacture. Compared to electroless and 
electroplating, there is no hydrogen generation during the plating process 
and no concomitant pitting or similar plating discontinuities in the 
deposit. Also, the immersion plating process is not subject to surface 
roughness as found in electroplating processes due to "drag-over" from 
precleaners, anode corrosion and the like. Further, since electroless 
baths contain both a metal to be plated and a reducing agent, the bath is 
potentially unstable subject to spontaneous plating whereas, in immersion 
plating, there is no problem with spontaneous plate-out due to inherent 
instability of the bath. Moreover, with immersion plating, neither an 
electrically continuous circuit nor attachments of electrical contacts are 
required nor is there a need to maintain a precise current. Finally, an 
immersion deposit is generally uniform in thickness. 
In spite of the potential advantages of immersion plating described above, 
the prior art generally dismissed immersion plating processes for use in 
printed circuit fabrication because the prior art believed that thick, 
bondable (solderable) deposits could not be obtained by immersion plating. 
As stated in Printed and Integrated Circuitry, supra, at page 138, 
(immersion deposits are) "limited in thickness, porous, and often poorly 
adherent and, therefore, of limited interest". The self limiting feature 
and thickness of immersion tin and lead plating procedures were believed 
to make soldering impossible and consequently, plating baths for immersion 
deposits of tin and lead were of minimal interest to the art. 
In U.S. Pat. No. 4,194,913, incorporated herein by reference, an immersion 
plating composition is disclosed for deposition of tin lead alloys. In 
accordance with the teachings of this patent, an immersion plating 
solution is disclosed comprising stannous chloride in an amount of from 10 
to 150 grams per liter of solution, lead chloride in an amount of from 1 
to 12 grams per liter, sodium hypophosphite in an amount of from 10 to 100 
grams per liter, thiourea in an amount of 40 to 100 grams per liter, 
hydrochloric acid in an amount of from 40 to 100 milliliters per liter and 
gelatin in an amount of from 0.1 to 10 grams per liter. In this patent, 
patentee states that the plating composition of the patent provides a 
faster plating rate and a deposit of increased thickness compared to that 
possible using prior art tin lead alloy immersion plating compositions. 
Though it is believed that improved immersion deposits are obtained using 
the compositions of said patent compared to those known to the art prior 
to said patent, the immersion deposits of said patent are nonetheless of 
inadequate thickness, are not readily reflowed and consequently, are 
believed to be unsuitable for the commercial manufacture of printed 
circuit boards 
In European published application No. 0 167 949, also incorporated herein 
by reference, (hereafter the "European Application"), an immersion tin 
plating solution that may also contain lead is disclosed as consisting of 
a tin salt of a fluorine containing mineral acid in an amount of from 1 to 
50 grams per liter, a lead salt of a fluorine containing mineral acid in 
an amount of from 1 to 10 grams per liter, a fluorine containing mineral 
acid in an amount sufficient to provide a pH varying between 0 and 1, and 
a sulfur containing complexing agent in an amount of from 10 to 400 grams 
per liter. Though the patent application concentrates primarily on 
immersion tin plating solutions, an example of a tin lead plating solution 
is given. However, thick, reflowable deposits of tin and lead are not 
obtained from the compositions disclosed in the patent and the 
compositions are believed to be unsuitable for the commercial fabrication 
of printed circuit boards. 
SUMMARY OF THE INVENTION 
The subject invention provides an immersion plating solution capable of 
plating a thick, porous adherent tin lead alloy deposit, is capable of 
reflow, typically at a temperature of below about 500.degree. F. and has a 
melting point at or near the melting point of a tin lead solder eutectic 
alloy. The invention also provides methods for reflow of said alloys and a 
process for the manufacture of printed circuit boards using the alloys. 
The invention disclosed herein is based upon a combination of discoveries. 
One such discovery is that to reflow a tin lead alloy immersion deposit, 
it is necessary that the deposit be thick and porous deposit--i.e., a 
deposit having a thickness of at least 100 microinches and preferably at 
least 150 microinches or greater. Another discovery of this invention is 
the realization that to obtain a thick deposit capable of reflow, it is 
necessary to utilize a plating solution favoring a porous structure rather 
than a dense deposit as desired in the prior art. 
In accordance with the invention, it has been found that to obtain a thick, 
porous deposit of a tin lead alloy as required for reflow, it is necessary 
to use an immersion plating solution containing a relatively high metal 
content--i.e. preferably in excess of 0.10 moles per liter of total metal 
with a tin to lead ratio of at least 1 to 1. A further discovery of the 
invention is that both the tin salt and the lead salt, and preferably the 
acid used to provide a desired pH, all have a flourine containing anion. 
In addition, it has been discovered that the displacement reaction favors 
tin and it is desirable to add a lead promoter to maintain a desired 
concentration of lead relative to tin and to obtain a thick deposit. 
Furthermore, it has been found that dissolved copper is not a contaminant, 
but is highly desirable to promote plating rate. Finally, it has been 
found that to obtain as thick a deposit as possible, it is desirable to 
add certain exaltants to the formulation to promote plating rate. 
Following deposition of a tin lead alloy from the immersion plating 
solution described above, the deposit formed has a crystalline porous 
structure with alternating layers of lead and tin. The first layer is 
typically a relatively thin layer rich in lead followed by a relatively 
thick layer of richer in tin. If deposition is permitted to continue to 
full thickness where plating terminates due to the unavailability of 
copper necessary for the displacment reaction to occur, the top layer will 
typically be richer in lead. Though this structure was unplanned and an 
artifact of the plating solution, it has been found to be highly desirable 
if the part with the solder coating is to be stored for any extended 
period of time because the initial layer of rich in lead prevents 
migration of copper with time and formation of a copper tin intermetallic 
compound undesired in circuit manufacture. Moreover, the top layer 
enriched in lead protects the coating from oxidation during storage. 
Following immersion deposition of the alloy of this invention, the alloy 
obtained is crystalline and may be reflowed by heating the alloy to a 
temperature above its melting point for a time sufficient to form a dense 
coherent coating. Reflow procedures and conditions are known in the art. 
Prior to reflow of the tin lead deposit of the invention, the deposit is 
porous with a matte gray coloration and an observable sparkle in the 
appropriate light indicating it is crystalline and suitable for reflow. 
After reflow, the alloy is dense and shiny and believed to be a true 
homogeneous alloy of tin and lead. It is an advantage of this invention 
that the porous deposit can be stored prior to reflow without loss of 
reflow capability and as aforesaid, without contamination

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
A tin lead alloy plating solution in accordance with this invention will 
have the following composition: 
TABLE I 
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Component Broad Range Preferred Range 
______________________________________ 
Sn.sup.++ 0.05 to 0.50 0.10 to 0.30 
moles/liter moles/liter 
Pb.sup.++ 0.01 to 0.15 0.02 to 0.10 
moles/liter moles/liter 
Sn.sup.++ + Pb.sup.++ 
0.05 to 0.60 0.10 to 0.35 
moles/liter moles/liter 
Ratio Sn.sup.++ :Pb.sup.++ 
1.0:3.0 to 50.0:1.0 
1.0:1.0 to 10.0:1.0 
Complexing agent 
0.05 to 2.50 0.50 to 2.00 
moles/liter moles/liter 
Lead Promoter 0.00 to 0.60 0.05 to 0.25 
moles/liter moles/liter 
pH controller to pH 0 to 1.5 
to pH 0 to 1.2 
Thickness promoter 
0.00 to 1.00 0.001 to 0.20 
moles/liter moles/liter 
Buffer to maintain pH 
to maintain pH 
Water to 1 liter to 1 liter 
______________________________________ 
The tin (stannous) and lead (plumbous) ions are provided to the bath in the 
form of a solution soluble salt, preferably of a fluorine containing acid. 
Suitable sources of the salt include tin and lead fluoride, tin and lead 
fluoroborate or tin and lead fluorosilicate. Preferred sources for both 
the tin and the lead ions are their respective fluoroborate salts. 
The concentration of the tin and lead ions in solution are set forth in 
Table I. It should be noted that their concentrations are relatively high 
compared to that which is conventional in the art and these higher 
concentrations are desirable to obtain a deposit capable of reflow as 
required for purposes herein. It is believed that the high concentration 
of metal ions is partly responsible for a rapid rate of deposition and the 
formation of a deposit that is porous and of relatively low density. 
Preferably, the thickness of the deposit prior to reflow is at least 100 
microinches, preferably, is at least 150 microinches and more preferably, 
the thickness of the deposit prior to reflow exceeds 200 microinches. 
Though not wishing to be bound by theory, it is believed that a thick 
deposit is lower in concentration of copper and copper interferes with the 
reflow capability of a tin lead alloy deposit, especially after storage 
following deposition and prior to reflow. Moreover, a thick deposit is 
desirable for surface mounting of components to the board as the mass of 
material is needed to hold the component to the board. 
The ratio of the tin to lead ions is as stated in Table I. One object of 
this invention is formation of a tin lead alloy having a concentration of 
tin and lead close to the low melting tin lead eutectic comprising 63 
percent tin and 37 percent lead. The ratio of tin to lead in the plating 
solutions of the invention, as recorded in Table I, and subject to the use 
of a lead promoter in solution as discussed below, permits deposition of 
alloys with tin ranging between 50 and 82 percent and lead ranging between 
50 and 18 percent. The preferred solutions deposit alloys with tin ranging 
between about 56 and 74 percent and lead ranging between 44 and 26 
percent. 
The plating solutions of the invention contain a complexing agent in an 
amount at least sufficient to complex all of the metal in solution and 
consequently, the complexing agent is present in an amount at least 
equimolar with the tin and lead content in solution. Preferably, the 
complexing agent is present in an amount in excess of the stoichiometric 
amount required for complete complexation of all metal initially present 
in solution. Where the minimum content of the tin and lead ions in 
solution is at least 0.05 moles per liter, the minimum content of the 
complexing agent is also at least 0.05 moles per liter of solution. The 
broad and preferred ranges for the concentration of the complexing agent 
are set forth in Table I. 
The complexing agent used in the immersion plating solution of the 
invention is one preferably capable of complexing both tin and lead ions. 
Sulfur containing complexing agents are preferred. Preferably, the sulfur 
containing complexing agent is an aliphatic compound containing sulfur and 
nitrogen, especially thiourea or a thiourea substituted with a lower alkyl 
group having from 1 to 4 carbon atoms such as tetramethyl thiourea. 
In metal plating solutions containing a high concentration of metal as 
desired herein, for reasons not fully understood, it is believed that tin 
deposits at a more rapid rate than lead. It is believed that this is not 
observed with tin lead immersion plating compositions having a lower total 
metals content. To avoid obtaining an alloy with an undesirably high tin 
content or an undesirably low lead content, it is desirable to add a 
component to the plating solution that promotes the rate of lead 
deposition relative to tin by changing the displacement potential of lead 
relative to tin whereby the rate of deposition of lead is increased 
relative to the rate of deposition of tin. Suitable lead promoters include 
hypophosphorus acid, sodium hypophosphite, glycerin, urea, amino acetic 
acid, etc. Hypophosphorus acid and an alkali metal salt of hypophosphite 
are preferred. 
The concentration of the lead promoter in solution suitable for purposes of 
this invention is set forth in Table I. Though general and preferred 
ranges are given, the relative rate of codeposition of tin and lead is 
subject to several variables such as solution pH, solution temperature and 
the total combined metals content of the solution. Therefore, some routine 
experimentation may be required to find a suitable concentration for a 
given formulation and a given desired tin lead alloy. 
The immersion plating solutions of this invention are acidic solutions 
having a pH ranging between about 0 to 2. Any mineral acid may be used to 
acidify the solution, but it is preferred that the acid have a cation 
common to the acid salts of tin and lead and therefore, fluorine 
containing acids such as hydrofluoric acid, fluoroboric acid and 
fluorosilic acid are suitable with fluoroboric acid being most preferred. 
A buffering agent such as boric acid should be used to maintain solution 
pH. 
In a preferred embodiment of the invention, a thickness promoter is added 
to solution. The thickness promoter is a solution soluble salt of a metal 
of the Group IVb, Vb or VIb of the periodic table of the elements. Such 
metals include molybdenum, zirconium, titanium, chromiun, vanadium, etc. 
Titanium salts are most preferred promoters for purposes of this invention 
though the method by which they promote deposit thickness is not fully 
understood. Titanium added in the form of titanium trichloride dissolved 
in hydrochloric acid constitutes a preferred thickness promoter for 
Thickness promoter purposes of this invention. The titanium ion 
concentration varies between 0 and 1.0 moles per liter and preferably 
between 0.001 and 0.20 moles per liter. 
Chloride ion is another desirable addition to the bath as it appears to 
promote deposit thickness. The chloride ion concentration preferably 
varies between 0 and 0.5 moles per liter and more preferably, between 
0.0001 and 0.20 moles per liter. The most preferred concentration for both 
the titanium ions and/or the chloride ions is that concentration 
sufficient to provide a deposit of a thickness of at least 100 microinches 
during the first 10 minutes of plating and more preferably, of a thickness 
exceeding 150 microinches during the first 10 minutes of plating. 
An additional desirable component in the solution is a flourine containing 
acid, and preferably flouroboric acid in minor amount sufficient to 
provide the required pH as set forth in Table I. 
In addition to the above solution components, it has been observed that 
limited amounts of dissolved copper is a desirable component of the bath. 
As is known in the art, when displacement or immersion plating over 
copper, copper metal dissolves and is replaced by the plating metal 
through a displacement reaction. For this reason, copper goes into 
solution as plating continues. In accordance with this invention, it has 
been discovered that the dissolved copper is a desirable component in the 
bath. For this reason, with a fresh plating solution, it is desirable to 
initially introduce copper ions into solution prior to use of a freshly 
made plating bath. This can be accomplished by deposition of the alloy 
over scrap copper to introduce copper into the solution before depositing 
the immersion alloy on a circuit. In this respect, it is desirable that 
the dissolved copper content be at least 0.01 moles per liter of solution 
and preferably, variability within a range of from at least 0.04 to 0.20 
moles per liter. As the concentration of dissolved copper increases beyond 
0.20 moles per liter, the plating bath may yield deposits that vary in 
alloy composition and thickness relative to a bath having an acceptable 
concentration of copper. 
The immersion plating solutions of this invention are used in conventional 
manner. A substrate having a cupreous surface is prepared for deposition 
by etching the cupreous metal with an etchant such as an ammonium 
bifluoride--hydrogen peroxide etchant. The substrate is then immersed in 
the immersion plating solution of the invention for a time sufficient to 
form a tin lead deposit having a thickness of at least 100 microinches and 
preferably ranging between 150 and 350 microinches. Typically, the plating 
time required to obtain a deposit of this thickness is between about 10 
and 30 minutes. The temperature of the plating solution affects the rate 
of deposition and deposit thickness. Plating temperatures may range from 
about room temperature to close to the boiling point of the plating 
solution but preferably range from about 100.degree. F. to about 
175.degree. F. with a plating temperature of about 150.degree. F. being 
more preferred. 
The deposits formed in accordance with the invention are porous and matte 
gray in color with an observable sparkle dispersed throughout the deposit. 
It has been observed that if a visible sparkle is not observable, 
difficulties might be encountered during efforts to reflow the alloy. The 
deposits are non homogeneous and have an initial thin layer with the lead 
comprising at least 50 percent of the alloy and typically between 55 and 
75 percent by weight followed by a relatively thick layer rich in tin with 
the tin typically comprising in excess of 50 percent of the alloy and when 
plating is permitted to continue until it stops, overcoated with a 
relatively thick layer rich in lead and having a composition approximating 
that of the first alloy layer. For reasons given above, this is a 
desirable layered deposit if the alloy is to be stored prior to reflow. 
The immersion deposits of the invention are reflowed in conventional 
manner. Reflow involves heating the alloy above its melting point and 
holding the alloy at this temperature for a short period of time, 
typically for less than one minute. A preferred method involves floating a 
part with the alloy deposit on hot oil. Prior to reflow, the part is 
preferably warmed and following reflow, the part is preferably quenched at 
elevated temperature, all in accordance with conventional practice in the 
art. It is a characteristic of this invention that deposits having a 
thickness of less than about 100 microinches will not reflow. In addition, 
it is believed that prior to the invention disclosed herein, immersion 
deposits of lead and tin and capable of reflow were not in commercial use 
and may not have been known in the art. 
Immersion deposition of tin lead alloys in accordance with this invention 
is an improvement over electrolytic deposition of solder in circuit 
manufacture because for electrodeposition of solder, the cupreous metal 
over which the solder is plated must have electrical continuity for 
plating to take place. This means that in the manufacture of a circuit 
board, photomask must be applied to a copper clad circuit board substrate 
material in an image pattern, solder plated onto the copper 
electrolytically, the photomask removed and the underlying copper etched. 
If a defect in the solder deposit is found, the circuit board cannot 
readily be repaired because once the circuit is formed by etching, there 
is no continuity over the surface of the copper and solder cannot be 
plated onto the copper conductors. In contrast, the immersion plating 
solutions of this invention do not require continuity for deposition and 
if a defect is found, the solder can be stripped and immersion plating 
repeated. The invention will be better understood by reference to the 
examples that follow: 
EXAMPLES 1 TO 3 
This example represents the relationship between thickness of a deposit and 
its ability to reflow. The following formulation was prepared wherein 
solution components are set forth in grams per liter of solution unless 
otherwise indicated. 
______________________________________ 
Example 1 
Example 2 Example 3 
______________________________________ 
Stannous fluoroborate 
23.0 46.0 70.0 
Plumbous fluoroborate 
7.4 15.0 22.0 
Fluoroboric acid 
2.0 5.0 7.5 
Thiourea 42.0 85.0 127.0 
Hypophosphorus acid 
14.0 9.2 14.0 
Boric acid 3.0 6.0 3.0 
Titanium trichloride 
0.0 1.0 2.0 
Hydrochloric acid 
0.0 0.05 0.1 
Dissolved copper 
0.01 0.01 0.01 
Water to 1 liter 
to 1 liter 
to 1 liter 
Solution pH 0.8 0.8 0.8 
Plating temperature 
71.0.degree. C. 
71.0.degree. C. 
71.0.degree. C. 
Plating time 10 minutes 
10 minutes 
10 minutes 
______________________________________ 
In the above formulations, the tin and lead contents were as follows: 
______________________________________ 
Example Sn.sup.++ 
Pb.sup.++ Sn.sup.++ + Pb.sup.++ 
Number moles moles Sn.sup.++ /Pb.sup.++ 
moles 
______________________________________ 
1 0.079 0.019 4.2:1 0.098 
2 0.160 0.039 4.1:1 0.200 
3 0.240 0.058 4.1:1 0.300 
______________________________________ 
Copper clad epoxy glass laminate was prepared for plating by abrasive 
scrubbing or by immersion in an acid soak cleaner followed by immersion in 
a mild copper etching solution and a fluroboric acid soak with rinsing 
after each treatment except for the fluroboric acid soak. A part was then 
immersed into one of the aforesaid plating solutions for a period of ten 
minutes. The deposit thickness of each part was determined and the 
deposits reflowed by fluxing, warming at 250.degree. F. in oil for about 5 
to 20 seconds, reflowed in hot oil at 400.degree. F. for from 5 to 20 
seconds and quenched in oil at 250.degree. F. The parts were then rinsed, 
cleaned and dried with the following results. 
______________________________________ 
Deposit 
Example 
Deposit Thickness Alloy % 
Number Appearance microinches 
Reflow Tin Lead 
______________________________________ 
1 matte grey, 100 yes 56 44 
slight sparkle 
2 matte grey 207 yes 67 33 
sparkling 
3 light matte grey 
252 yes 78 22 
slight sparkle 
______________________________________ 
Example 2 constitutes the most preferred embodiment of the invention. 
Example 1 reflowed, but the deposit was of a quality inferior to the 
deposit obtained from Examples 2 and 3. Example 2 is preferred because the 
alloy is close to the tin lead solder eutectic.