U.S. patent number RE30,434 [Application Number 05/972,088] was granted by the patent office on 1980-11-11 for electroless tin and tin-lead alloy plating baths.
This patent grant is currently assigned to AMP Incorporated. Invention is credited to Thomas F. Davis.
United States Patent |
RE30,434 |
Davis |
* November 11, 1980 |
Electroless tin and tin-lead alloy plating baths
Abstract
Salt compositions and baths thereof useful in, and methods for,
immersion plating of tin and tin-lead alloys which give greatly
increased deposition rates and thicker coatings of better quality
accomplished by incorporating into immersion plating tin bath
compositions soluble plumbous salts in the amount of from 0.5 grams
per liter calculated on the basis of elemental lead to the maximum
amount soluble in the bath and a sulphur-containing complexing
agent for the tin and lead such as thiourea or a thiourea-type
derivative. Preferably the salt elements are stannous chloride,
lead chloride, sodium hypophosphite (as a solubility enhancer) and
with hydrochloric acid used as a agent for adjusting the pH in the
resulting bath from 0.5 to 1.0.
Inventors: |
Davis; Thomas F. (Harrisburg,
PA) |
Assignee: |
AMP Incorporated (Harrisburg,
PA)
|
[*] Notice: |
The portion of the term of this patent
subsequent to October 11, 1994 has been disclaimed. |
Family
ID: |
25519147 |
Appl.
No.: |
05/972,088 |
Filed: |
December 21, 1978 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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Reissue of: |
574979 |
May 6, 1975 |
04093466 |
Jun 6, 1978 |
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Current U.S.
Class: |
106/1.22;
427/437; 427/443.1 |
Current CPC
Class: |
C23C
18/31 (20130101); C23C 18/48 (20130101) |
Current International
Class: |
C23C
18/16 (20060101); C23C 18/48 (20060101); C23C
18/31 (20060101); C23C 003/02 () |
Field of
Search: |
;106/1.22,1.25
;204/43S,53,54R ;427/437,43E |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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106864 |
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Jun 1974 |
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JP |
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50-57927 |
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May 1975 |
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JP |
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319057 |
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Dec 1969 |
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SE |
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659976 |
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Oct 1951 |
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GB |
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455171 |
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Apr 1975 |
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SU |
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Other References
IB.M. Tech. Disclosure Bulletin, vol. 9, No. 3, Aug. 1966, p. 226.
.
Russian Inventor Certificate No. 1882632/22-1, Dec. 1974. .
Chem. Abstracts, vol. 83, p.2083u (1975)..
|
Primary Examiner: Hayes; Lorenzo B.
Claims
I claim:
1. A salt composition, when diluted with 1 liter of water and
sufficient hydrochloric acid to provide a solution having a pH of
0.5 to 1, is suitable for immersion plating .[.tin-lead alloys.].
.Iadd.essentially tin .Iaddend.at increased rates, the salt
composition .[.comprising.]. .Iadd.consisting essentially of
.Iaddend.the proportions of:
2. A salt composition for immersion plating a tin-lead alloy, when
diluted with 1 liter of water and sufficient hydrochloric acid to
provide a solution having a pH of 0.5 to 1, said salt composition
.[.comprising.]. .Iadd.consisting essentially of .Iaddend.the
proportions of:
3. A composition according to claim 1 further comprising gelatin
from about 0.1 g.
4. A composition according to claim 1 further comprising an
alkylphenoxy polyethanol surfactant from about 0.1 g.
5. A composition according to claim 2 further comprising gelatin
from about 0.1 g.
6. A composition according to claim 2 further comprising an
alkylphenoxy polyethanol surfactant from about 0.1 g. .Iadd. 7. A
salt composition as defined in claim 2 for the immersion plating of
tin-lead alloys containing about 90-95% lead, wherein the
composition contains about 15 to 25 grams of lead chloride, 6 to 10
grams of tin chloride, 60 to 100 grams of sodium hypophosphite, and
100 to 150 grams of thiourea..Iaddend..Iadd. 8. A salt composition
as defined in claim 2 for the immersion plating of tin-lead alloys
containing about 60% tin and 40% lead which contain 15 to 25 grams
of tin chloride dihydrate, 8 to 12 grams of lead chloride, 60 to
100 grams of sodium hypophosphite, and 100 to 150 grams of
thiourea. .Iaddend..Iadd. 9. A salt composition as defined in claim
2 for the immersion plating of tin-lead alloys containing about 10%
lead and 90% tin, said composition containing about 6 grams of lead
chloride, about 25 grams of tin chloride dihydrate, about 60 to 100
grams of sodium hypophosphite and about 100 to 150 grams of
thiourea. .Iaddend.
Description
This invention relates to an immersion bath suitable for the
deposition of essentially tin and of tin-lead alloys on copper,
brass, other copper alloys and similar metal substrates (as
discussed below). This invention also includes salt compositions
useful in, and methods for, accomplishing the deposition of tin and
tin-lead alloys by immersion plating. This invention, though not
specifically so limited, finds particular usefulness in forming a
protective and bondable (i.e., solder bondable) layer of tin over
circuitry elements and in other electronic devices.
BACKGROUND OF THE INVENTION
There are three widely used methods for plating metal onto a
substrate, and in some recent developments of plating circuitry on
a non-conductive flexible plastic substrate all three may in fact
be employed. These methods are (1) electroless plating, (2)
immersion plating, and (3) electroplating. The choice of which
method to employ for a given plating step is often controlled as
much by the choice of the substrate or disadvantages of the
respective methods as vis-a-vis the other methods. Although both
methods (1) and (2) may be broadly characterized as electroless
plating, this latter term as used hereinafter will be restricted to
its narrower definition (involving reduction of the plating metal
from the plating bath, as discussed below).
Some of the electroplating disadvantages are: a conductive path is
required for the desired circuit pattern; due to uneven current
densities, the "throwing power" of electrolytic baths can never
reach the desired uniformity of deposit; e.g. with irregularly
shaped substrates. The alkaline stannate electrolytic baths have
greater throwing power than the acid stannous baths and require
less critical control, but because these are restricted to the four
valent stannate form these have a lower cathode efficiency.
Electrolytic deposits of tin are corrosion resistant and non-toxic,
possess good solderability with good softness and ductility.
However, to provide corrosion protection of a substantial nature
the tin deposits should be thick enough to be virtually non-porous.
Recommended thicknesses by the Tin Research Institute for plating
of tin on copper is 0.5 mils both for soldering and for resisting
atmospheric corrosion.
Tin, or even tin-lead alloys, are electrolytically deposited in
thicknesses typically from 0.2 to 2.0 mils when used in printed and
other circuitry to provide a solderable finish, a contact material,
or an etchant resist. Tin can be readily deposited from acid
solutions at room temperature. Where a lower melting point material
is required, tin-lead alloys, such as the typical 60%-40% solder
can also be deposited, but require much closer control of the
solution composition and operating conditions, making it more
costly (see Printed and Integrated Circuitry, Schlabach and Rider,
McGraw-Hill Book Company, Inc., New York, 1963, at page 146).
Electroless plating involves the use of a plating bath without the
imposition of any electric current where the substrate is plated by
reduction of the plating metal from a solution of a salt of a
plating metal. The plating solution contains controlled reducing
agents which are generally either catalyzed by the surface of the
substrate, or by some catalytic metal emplaced onto the surface
both to initiate the reduction and to give good adherence. Since
the plated-on-surface is autocatalytic, an electroless process can
be used to build up good thicknesses. Furthermore since an
electroless process is not dependent upon current densities, the
resulting coating is of excellent uniformity.
However, reducing agents in electroless baths must be controlled in
order to avoid spontaneous reduction of the metal in the bath e.g.
to a fine powder. The reduction is not localized at the surface of
the substrate, hence considerable losses may occur. Moreover, with
copper-based substrates the electroless tin baths are affected
adversely by contaminants such as cyanides, lead, zinc, manganese,
and cadmium (see Metals Handbook, Vol. 2., p. 642; copyright 1964;
American Society for Metals).
Immersion plating, like electroless plating does not employ an
electric current. Immersion plating, sometines called galvanic
plating, is an electrochemical displacement reaction which depends
on the position that the substrate metal occupies in the
electromotive series with respect 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. This requirement indicates one limitation to the
process, since in the standard electromotive series test
conditions, copper is more noble than tin and under those
conditions could not be placed by an immersion process to give a
tin plate coating. However under acidic conditions, the relative
electrode potentials reverse making the utilization of this process
possible.
Major limitations of immersion plating in the past have been in the
slow plating speeds and limited thicknesses. Immersion plating is
generally self limiting, because as the plating coating builds up
it tends to mask the underlying base metal thereby preventing
further replacement; additionally as the displaced base metal is
dissolved into the solution, it becomes an increasing contaminent
in the bath progressively slowing the rate of displacement. Normal
deposit thickness from immersion tin processes of the prior art is
50 to 100 microinches (i.e., 0.05-0.1 mils), mainly because of the
foregoing problems in building the deposit to greater
thicknesses.
An advantage of the immersion processes over many of the other
aforementioned processes is the absence of hydrogen generation (or
other gases) on the plating surface, thereby avoiding pitting or
similar plating discontinuties. Also the immersion plating process
is not subject to the surface roughness found in electroplating due
to "drag-over" from precleaners, anode corrosion, and the like.
In comparison to electroless baths, there is no problem with the
bath decomposing. With immersion plating, the circuit need not be
electrically continuous nor need it have electrical contacts
attached. There is no need for maintaining a precise current and
the deposit is uniform in thickness. However in spite of these
latter advantages, the prior art generally dismissed such immersion
plating process for use with printed circuitry because it was noted
that deposition of the more noble metal continued only in the
presence of exposed base metal so that such deposits were "limited
in thickness, porous, and often poorly adherent" and therefore of
"limited interest" (see Printed and Integrated Circuitry, supra, at
page 138).
Tin and some solders are subject under certain conditions to growth
from their surfaces of metallic filaments known as "whiskers."
These dendritic formations can in time and under proper conditions
project from the surface to a length (as much as one-fourth of an
inch) sufficient to short out adjacent circuitry when used for fine
resolution electronic application printed circuits. The growth rate
is encouraged slightly by elevated temperature and humidity, but is
greatly promoted by high stresses (causing growth in a matter of
hours). Such high stresses occur in thin tin or solder coatings
(less than about 100 microinches) and will be higher in
electrodeposited coatings because of the stresses which are
impressed by the flow of current, not present in the electroless
and immersion coating methods. As the prior art immersion coating
method for depositing tin on copper are self limiting generally at
a thickness of below 100 microinches, to a maximum limited to
approximately 200 microinches; tin plating from the prior art
immersion methods can be particularly susceptible to whisker growth
not just in the original formation but in subsequent use.
Applicant has been unable to find any immersion process for plating
a tin-lead alloy in the literature, and is aware of only one
system, recently available on the market (a fluoroborate and
immersion process) which codeposits any lead, and that only less
than 1% (at 20-30 microinches of the tin maximum in 20 minutes at
an operating range of 125.degree.-150.degree. F.). The lead present
in the latter system appears to have no effect on the rate of
deposit and is intended solely to give a cosmetic effect.
The self limiting feature of prior art immersion tin plating
procedures also made it more difficult to solder the resulting tin
plating because of its limited thickness.
From the foregoing it can be appreciated that due to various
disadvantages inherent in electroplating and electroless plating,
immersion plating would for a great many applications be a more
desirable method if a greater plating rate could be achieved and if
a thicker final plating coating were possible, all with reduced
whisker growth. Therefore objects of the present invention include
methods and baths for immersion plating which overcome or minimize
by a great order of magnitude each of these aforementioned
limitations. More specifically, an object of the present invention
is a novel method giving a faster rate of immersion plating so as
to be more competitive with the other types of plating methods. A
further object is with an immersion plating method giving
essentially tin or tin-lead alloys of commercially useful thickness
of the order of magnitude including 0.5 mils.
DESCRIPTION OF THE INVENTION
In accordance with the present invention, it has now been
discovered that by including lead salts in an immersion tin plating
bath, and an appropriate combination of cooperating elements the
rate of deposition of tin can be increased by up to 500% in an
immersion plating process. Furthermore, unlike electroplating
processes, this increased rate of deposition has been achieved
without promoting whisker growth; and, in fact, the small amount of
lead present in the resulting tin plate appears to inhibit such
whisker growth. Additionally, the tin immersion plating baths
prepared according to the present invention result in a thicker
coating of tin being plated than was readily achieved by the prior
art immersion-type processes. (Even faster plating rates giving
thicker plating can be achieved for tin-lead alloy, equally
preferred in this invention).
Thus the immersion process of the present invention results in a
more protective and more readily bondable layer of tin being plated
to metallic copper, and in a significantly shorter time.
In the following Table is set forth a bath illustrating a preferred
embodiment of the present invention for plating essentially tin at
greatly enhanced plating rates.
TABLE I ______________________________________ TIN IMMERSION RATING
BATH Primary Range Grams/ Extended Range Solution Make Up Liter
Grams/Liter ______________________________________ a. Stannous
Chloride (SnCl.sub.2), hydrous 20-40 10-150 b. Lead Chloride
(PbCl.sub.2) 2-15 1-12 c. Sodium Hypophosphite (NaH.sub.2 PO.sub.2
. H.sub.2 O) 10-100 10-100 d. Thiourea (NH.sub.2 . CS . NH.sub.2)
40-100 .Iadd.40.Iaddend..[.15.].-100 e. Hydrochloric Acid (HCl)
40-100 40-100 (ml/l) f. Gelatin 1.0-4.0 0.1-10
______________________________________ Extended operating range:
100.degree.-200.degree. F.
Extended operating range: 100.degree.-200.degree. F.
Items c, d and e may be individually substantially in excess of 100
g/l, if the other two are lower; otherwise lead tends to plate out
at percentages in excess of 2%.
The above disclosed baths have been used to deposit 175 microinches
of tin on metallic copper in 5 minutes in an operating temperature
range of 140.degree. to 180.degree. F. The maximum rate of deposit
so far attained is 100 microinches in one minute. The amount of
lead deposited with the tin in these first baths typically ranges
from above 0 to about 2% and in the specific preferred embodiments
was less than one percent. This is a deposition rate of at least
500% faster than is known in prior art baths such as disclosed in
the Shipley U.S. Pat. No. 3,303,029, issued Feb. 7, 1967; e.g. a
bath of Example I thereof containing SnCl.sub.2 (20 g/l), thiourea
(75 g/l), NaH.sub.2 PO.sub.2 H.sub.2 O (16 g/l), wetting agent (1
g/l), and HCl (conc.) (50 ml/l), with an operating temperature of
150.degree.-180.degree. F. giving a rate of deposit of 53
microinches in 5 minutes; 95 microinches in 10 minutes; 132
microinches in 30 minutes; and 189 microinches in 60 minutes.
Immersion plating baths of the present invention having lead salts
added have already achieved a plating rate of up to 300 microinches
in 5 minutes.
The bath has been adjusted by HCl to a strongly acidic condition in
order to permit the immersion plating of tin on copper. In the
illustrated embodiment the pH is about 1. The presence of lead in
the bath eliminates the use of sulfuric acid because of the
formation of insoluble lead sulfate. Since the bath is a
hydrochloric acid based bath, the lead and tin should preferably be
introduced as chlorides to prevent the buildup of other ions in the
bath. Other soluble salts could be used; and as yet, no long term
adverse effects on this system are known. The lead and tin oxides
could also be used, since they would be converted to the chlorides
by the hydrochloric acid. Other non-precipitating strong acids in
this system are indicated; such as hydrobromic acid, etc. The
sodium hypophosphite makes the tin chloride more soluble in the
water bath, probably assists the HCl in keeping the tin ion in the
stannous state (since immersion plating will not proceed from the
stannic state), and permits the immersion plating to proceed at a
lower temperature. Thiourea functions as a complexing agent for the
stannous and plumbous chlorides. Other complexing agents
(preferably non-toxic) that form chelates with both metals
(especially when bound through a sulfur atom) could be substituted
(including thiourea derivatives such as tetramethyl thiourea).
Gelatin is added as a surfactant giving a smoother deposition
(leveling effect). Other suitable surfactants, soluble and stable
in this system could be substituted. For example recent tests have
shown GAF's IGEPAL (trademark), i.e., alkylphenoxypoly
(oxyethylene) ethanols, are even better than gelatin in these
systems as grain refiners.
The tin and lead should both be added in the plumbous and stannous
state. The percentages of each is determined by the total
concentration of the system and the operating temperature. An
increase in temperature increases the rate of deposit, and an
increase in the concentration of lead with respect to the
concentration of tin will increase the rate of deposit. Thus it
will be appreciated that the percentage of lead in the deposit will
depend on the particular bath used.
Traces of lead in the foregoing bath increases not only the
immersion plating rate of tin on copper, but also the ultimate
thickness achieved. In a comparison of the typical prior art bath
set forth above with a bath prepared according to the present
invention as set forth in Table I, the maximum practical plating
thickness which could be achieved by the former was about 100
microinches and took nearly 30 minutes to form. In contrast the
bath in Table I plated 175 microinches in only 5 minutes.
As indicated earlier an equally important aspect of the present
invention is its use in plating solder and other tin-lead alloys of
thicknesses up to 1/2 mil by immersion techniques, and consequently
with more uniform plating and at less expense than the heretofore
accepted standard prior art electroplating techniques.
Table II below is an example of a bath according to another
preferred embodiment of the present invention, specifically adapted
for depositing a tin-lead solder (60-40). It will be noted that as
the percentage of lead significantly increases the percentage molar
ratio of tin to lead in the bath becomes close to the ratio of the
metal content of the alloy plated. Baths of this type have
successfully plated alloys of up to 95% lead content.
TABLE II ______________________________________ PbCl.sub.2 8.05 g/l
SnCl.sub.2 . 2H.sub.2 O 16.35 g/l NaH.sub.2 PO.sub.2 . 2H.sub.2 O
80 g/l Thiourea 150 g/l HCL Cl 80 ml/l
______________________________________ Operating temperature
170.degree. F. Rate of deposition 200 microinches in 5 minutes Rate
of deposition 300 microinches in 10 minutes
Immersion baths capable of plating tin-lead alloys containing
percentages of tin in excess of approximately 2% apparently are
heretofore unknown in the literature. As illustrated in Table II it
has been discovered that these baths can not only incorporate
plumbous salts as rate of deposit enhancers in immersion tin
plating, but in fact can be adjusted to give tin-lead alloys of any
desired ratio, (a surprising development).
It has been found that within the 20 to 80% range the percentages
of the lead and tin in the baths closely follow the percentages
actually plated out in the resulting alloys. Outside this range the
linear relationship no longer holds true and proportionally more of
the metal which is in greater concentration plates out than would
be indicated by the percentage composition of the bath.
It should also be noted that for all ratios of tin to lead the
preferable total lead plus tin content of the bath is 30 and 40
grams per liter. Because of the greater solubility of stannous
chloride, wider variation from this is possible in the lower lead
content alloys than otherwise; nevertheless, even with the latter
alloys the preferred baths are close to the 30 to 40 gram per liter
range.
Baths for plating alloys having 90 to 95% lead will have bath
concentrations of 15 to 25 grams per liter of lead chloride and 6
to 10 grams per liter of tin chloride dihydrate. In order to plate
60-40 tin--lead solder the preferred ratios are 15 to 25 grams per
liter of tin chloride dihydrate and 8 to 12 grams per liter of lead
chloride. In order to plate an alloy having 10% lead - 90% tin, the
bath should preferably have 6 grams per liter of lead chloride and
25 grams per liter of tin chloride dihydrate. Note that lead
chloride is not soluble at concentrations in these baths in excess
of approximately 25 grams per liter.
For these latter baths, the other ingredients should have
concentrations of 60-100 grams per liter of sodium
.[.hyposulfite;.]. .Iadd.hypophosphite; .Iaddend.100-150 grams per
liter of thiourea and 50-100 milliliters per liter of hydrochloric
acid. Concentrations above the latter ranges are permissible but
serve no useful purpose and therefore are uneconomical.
The preferred pH range for all of these baths are from about 0.5 to
about 1.0.
As obtained from the above baths, the plated-on deposit was tested
by a Scotch tape adhesion test and very good adhesion was
confirmed. Similarly the solder wettability of the tin plating was
found to be excellent.
The substrates on which this bath can be used to plate tin or
tin-lead alloys include metallic copper, copper alloys (in all
their various forms, including printed circuits which hve been
built up upon organic dielectrics such as a flexible support of a
polyimide, Mylar, polyolefins, and such as rigid boards of
impregnated fiberglass, epoxy and the like), lead, and any such
other metals displaceable with tin or tin and lead from a
thiourea-type complex. The improved printed circuit for example
could involve a flexible polyimide tape which has its surface
selectively activated by palladium and a thin film of copper
deposited thereon in the printed circuit pattern by an electroless
process which electroless copper plating has been further built up
by the electrolytic deposition from copper strike; which in turn
has been still further built up, if necessary, by conventional
electrolytic depositions. Finally the copper printed circuitry is
coated with a protective bondable layer of tin or tin-lead alloy
according to the present invention.
The copper parts to be plated should be cleaned first, by any
normal cleaning method. Sulfuric acid and persulfate should be
avoided, however, to prevent the drag-in of any sulfates.
Note that the invention in its broader aspects includes baths
substantially the same as the specifically disclosed preferred
embodiments, but which omit sodium hypophosphite altogether. Such
baths are not preferred because the solubility of the bath will be
decreased requiring higher temperatures and/or lower concentrations
(with possibly some slight modification of the ratio of the
remaining components--as will be understood by those skilled in the
art having benefit of the disclosure herein).
There is no specific preferred concentration or makeup of the bath.
That depends on the operating temperature, rate of deposit, and
percentage of lead desired. However, the following baths are
further illustrative of the present invention:
TABLE III ______________________________________ SnCl.sub.2 .
2H.sub.2 O 21.8 gm/l PbCl.sub.2 110 gm/l Thiourea 80 gm/l HCl
(conc.) 50 ml/l NaH.sub.2 PO.sub.2 16 gm/l With 1 g/l gelatin
______________________________________ Operating temperature
160.degree.-180.degree. F. Thickness 2 min. at 170.degree. F.-120
microinches Thickness 5 min. at 170.degree. F.-175 microinches
.[.Deposits tin in excess of 98% .]. .Iadd.Deposits 60% tin 40%
lead (.+-.5-10%)
TABLE IV ______________________________________ SnCl.sub.2 .
2H.sub.2 O 20 g/l PbCl.sub.2 4 g/l Thiourea 100 g/l HCl 80 ml/l
NaH.sub.2 PO.sub.2 90 g/l With 1 g/l gelatin
______________________________________ Operating temperature
--140-180.degree. F. Thickness--5 min. at 170.degree. F.-85
microinches Thickness--5 min. at 140.degree. F.--55 microinches
Deposits tin in excess of 98%?
TABLE V ______________________________________ SnCl.sub.2 .
2H.sub.2 O 20 g/l PbCl.sub.2 6 g/l Thiourea 120 g/l HCl 90 ml/l
NaH.sub.2 PO.sub.2 H.sub.2 O 100 g/l Gelatin 1 g/l
______________________________________ Operating temperature
.sup.2/3.degree.-180.degree. F. Thickness 5 min. at 140.degree. F.
28 microinches Thickness 5 min. at 170.degree. F. 135 microinches
Deposits tin in excess of 98%
TABLE VI ______________________________________ SnCl.sub.2 .
2H.sub.2 O 10 g/l PbCl.sub.2 15 g/l Thiourea 100 g/l HCl 80 ml/l
NaH.sub.2 PO.sub.2 60 g/l ______________________________________
Operating temperature 170.degree.-180.degree. F. Thickness 5 min.
300 microinches Deposits: 90% lead 10% tin
* * * * *