U.S. patent application number 13/800506 was filed with the patent office on 2013-12-26 for refining process for producing low alpha tin.
This patent application is currently assigned to Honeywell International Inc.. The applicant listed for this patent is HONEYWELL INTERNATIONAL INC.. Invention is credited to Brett M. Clark, Mark B. Fery, Derek E. Grove, Paul P. Silinger.
Application Number | 20130341196 13/800506 |
Document ID | / |
Family ID | 49769248 |
Filed Date | 2013-12-26 |
United States Patent
Application |
20130341196 |
Kind Code |
A1 |
Silinger; Paul P. ; et
al. |
December 26, 2013 |
REFINING PROCESS FOR PRODUCING LOW ALPHA TIN
Abstract
A method for purifying tin includes exposing an electrolytic
solution comprising tin to an ion exchange resin and depositing
electrorefined tin from the electrolytic solution. The deposited
electrorefined tin has alpha particle emissions of less than about
0.01 counts/hour/cm.sup.2 immediately after the deposition step,
and an alpha emissivity of less than about 0.01
counts/hour/cm.sup.2 at least 90 days after the deposition
step.
Inventors: |
Silinger; Paul P.; (Post
Falls, ID) ; Fery; Mark B.; (Spokane, WA) ;
Clark; Brett M.; (Spokane Valley, WA) ; Grove; Derek
E.; (Spokane, WA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
HONEYWELL INTERNATIONAL INC. |
Morristown |
NJ |
US |
|
|
Assignee: |
Honeywell International
Inc.
Morristown
NJ
|
Family ID: |
49769248 |
Appl. No.: |
13/800506 |
Filed: |
March 13, 2013 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
61661863 |
Jun 20, 2012 |
|
|
|
61670960 |
Jul 12, 2012 |
|
|
|
61714059 |
Oct 15, 2012 |
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Current U.S.
Class: |
205/81 ;
205/99 |
Current CPC
Class: |
C25C 1/14 20130101 |
Class at
Publication: |
205/81 ;
205/99 |
International
Class: |
C25C 1/14 20060101
C25C001/14 |
Claims
1. A method for purifying tin, the method comprising: exposing an
electrolytic solution comprising tin to an ion exchange resin; and
depositing electrorefined tin from the electrolytic solution.
2. The method of claim 1, wherein the electrorefined tin has alpha
particle emissions of less than about 0.01
counts/hour/cm.sup.2.
3. The method of claim 1, wherein the exposing and depositing steps
occur at least partially concurrently.
4. The method of claim 1, wherein the electrorefined tin has alpha
particle emissions of less than about 0.001
counts/hour/cm.sup.2.
5. The method of claim 1, wherein the ion exchange resin comprises
functionalized carboxylic acid from the phosphonic acids group.
6. The method of claim 1, wherein the ion exchange resin comprises
phosphomethylated functional groups.
7. The method of claim 1, wherein the ion exchange resin comprises
amino methyl phosphonic acid functional groups.
8. The method of claim 1, wherein the ion exchange resin comprises
poly(4-vinyl-pyridine) functional groups.
9. The method of claim 1, wherein a lead content of the
electrorefined tin is reduced by less than about 1% as compared to
tin prior to the exposing step.
10. The method of claim 1, wherein at least 90 days after the
deposition step, the electrorefined tin has an alpha particle
emissions of less than about 0.01 counts/hour/cm.sup.2.
11. The method of claim 1 and further comprising: detecting alpha
particle emissions from a sample of the deposited electrorefined
tin; determining a concentration of a target parent isotope in the
sample of the deposited electrorefined tin from the alpha particle
emissions detected in said detecting step and a time which has
elapsed between said detecting step and said exposing and
depositing steps; and determining an alpha emission potential of a
target decay isotope of the target parent isotope from the
determined concentration of the target parent isotope and the
half-life of the target parent isotope.
12. The method of claim 1, further comprising the additional steps,
prior to said detecting steps, of: obtaining a sample of the
deposited electrorefined tin; and heating the sample to diffuse
atoms of the target decay isotope within the sample until a uniform
concentration of atoms of the target decay isotope is obtained
throughout the sample.
13. The method of claim 1, wherein the electrolytic solution has a
tin concentration from about 60 g/L to about 180 g/L.
14. The method of claim 1, wherein the electrolytic solution
includes at least one additive.
15. The method of claim 14, wherein the additive includes at least
one member selected from the group consisting of antioxidants and
grain refiners.
16. The method of claim 14, wherein the additive is present in an
amount of at least about 0.5% by volume of the electrolytic
solution.
17. The method of claim 1 wherein exposing the electrolytic
solution to an ion exchange resin further comprises passing the
electrolytic solution from a first container through the ion
exchange resin in a second container and returning the electrolytic
solution to the first container.
18. The method of claim 1, wherein the electrolytic solution has pH
of about 1 or less.
19. The method of claim 1, wherein the electrorefined tin has a
lead concentration of at least about 1 part per million (ppm).
20. The method of claim 1, wherein at least 90 days after the step
of depositing, the electrorefined tin has an alpha particle
emissions that is at least 75% less than the alpha particle
emissions of the tin prior to the exposing and depositing steps.
Description
CROSS-REFERENCE TO RELATED APPLICATION
[0001] This application claims priority to U.S. Provisional
Application No. 61/714,059, filed Oct. 15, 2012, U.S. Provisional
Application No. 61/670,960, filed Jul. 12, 2012, and U.S.
Provisional Application No. 61/661,863, filed Jun. 20, 2012, each
of which are herein incorporated by reference in their
entirety.
FIELD OF THE INVENTION
[0002] The present invention relates to high purity tin with
reduced alpha particle emissions for the manufacture of
semiconductor equipment or the like and manufacturing methods for
producing such high purity tin.
DESCRIPTION OF RELATED ART
[0003] Solders are commonly utilized in semiconductor device
packaging and many other electronic applications. While
conventional solders have been manufactured primarily from lead,
more recent lead-free solders utilize tin and other metals as
principal components.
[0004] One challenge with respect to the use of tin solders in
electronic packaging applications is that the elemental tin
materials used to manufacture solders contain varying levels of
alpha particle emitting isotopes (also referred to as alpha
particle emitters). Alpha particle emissions (also referred to as
alpha flux) can cause damage to packaged electronic devices, and
more particularly, can cause soft error upsets and even device
failure in certain cases. This concern is compounded as device
sizes are reduced and alpha emitting solder materials are closer to
sensitive locations.
[0005] Uranium and thorium are well known as principal radioactive
elements often present in metallic containing solders, such as tin
solders, which may radioactively decay according to known decay
chains to form alpha particle emitting isotopes. Of particular
concern in tin materials is the presence of polonium-210
(.sup.210Po), which is considered to be the primary alpha particle
emitter responsible for soft error upsets. Lead-210 (.sup.210 Pb)
is a decay daughter of uranium-238 (.sup.238U), has a half-life of
22.3 years, and .beta.-decays to bismuth-210 (.sup.210 Bi).
However, due to the very short 5.01 day half-life of .sup.210Bi,
such isotope is essentially a transient intermediary which rapidly
decays to .sup.210Po. The .sup.210Po has a 138.4 day half-life and
decays to the stable lead-206 (.sup.206Pb) by emission of a 5.304
MeV alpha particle. It is the latter step of the .sup.210Pb decay
chain, namely, the decay of .sup.210Po to .sup.206Pb with release
of an alpha particle that is of most concern in metallic materials
used in electronic device applications.
[0006] Although .sup.210Po and/or .sup.210Pb may be at least in
part removed by melting and/or refining techniques, such isotopes
may remain as impurities in a tin material even after melting or
refining. Removal of .sup.210Po from a tin material results in a
temporary decrease in alpha particle emissions from the material.
However, it has been observed that alpha particle emissions, though
initially lowered, will typically increase over time to potentially
unacceptable levels as the secular equilibrium of the .sup.210Pb
decay profile is gradually restored based on any .sup.210Pb
remaining in the metallic material.
[0007] Problematically, whether an increase in alpha particle
emissions of a metallic material following a melting or refining
process will eventually reach unacceptable levels is very difficult
to assess and/or predict.
SUMMARY OF THE INVENTION
[0008] A method for purifying tin includes exposing an electrolytic
solution comprising tin to an ion exchange resin and depositing
electrorefined tin from the electrolytic solution. The
electrorefined tin can have alpha particle emissions of less than
about 0.01 counts/hour/cm.sup.2 or less than about 0.002
counts/hour/cm.sup.2. The ion exchange resin may include
sulfonated, phosphomethylated, amino methyl phosphonic acid, and
poly(4-vinyl-pyridine) functional groups and combinations of these
functional groups. The electrolytic solution may have a pH of less
than about 6 or about 1 or less.
[0009] The method for purifying tin may further include assessing
the alpha particle emission potential of the electrorefined tin,
including detecting alpha particle emissions from a sample of the
deposited electrorefined tin, determining a concentration of a
target parent isotope in the sample from the alpha particle
emissions detected in the detecting step and a time which has
elapsed between the detecting step and the exposing and detecting
steps, and determining a possible alpha emission of a target decay
isotope of the target parent isotope from the determined
concentration of the target parent isotope and the half-life of the
target parent isotope.
BRIEF DESCRIPTION OF THE DRAWINGS
[0010] FIG. 1 is a block diagram of an electrorefining system.
[0011] FIG. 2 is a plot of alpha particle emissions over time for
electrorefined tin samples.
DETAILED DESCRIPTION
I. Method of Electrorefining Tin
[0012] As described herein, tin may be electrorefined to produce
refined tin having reduced alpha particle emissions or alpha flux
when measured after the electrorefining process. The alpha particle
emissions do not necessarily remain stable after the material has
been subjected to an electrorefining process, and the alpha
particle emissions may increase or decrease over time. As described
herein, the refined tin may also have reduced alpha particle
emissions when measured a period of time following the
electrorefining process, such as 90 days after the electrorefining
process. A method for determining the alpha particle emission
potential, such as the maximum alpha particle emissions, for a
refined tin is also described herein.
[0013] Tin may be electrorefined by depositing tin ions from an
electrolytic solution onto a cathode by applying a current to the
system. An electrolytic solution containing tin or stannous ions
may be formed by dissolving or leaching tin in an acid electrolyte.
For example, tin sulfate can be formed by an electrolytic
dissolution of a 99.99% purity tin anode in an electrolyte
including 1% to 10% sulfuric acid by volume mixed with deionized
water. Suitable concentrations of soluble stannous ion in the
electrolytic solution include but are not limited to from about 10
g/L to about 200 g/L. More particularly, suitable concentrations of
soluble stannous ion in the electrolytic solution may be as low as
10, 20, 30, 40, 50, 60 g/L or as great as 80, 100, 120, 140, 160,
180 or 200 g/L or may be within any range delimited by any pair of
the foregoing values. At low tin concentrations, such as 40, 30, 20
g/L or less, the alpha particle emissions of the deposited material
may be more sensitive to the current density of the electrorefining
process than at higher tin concentrations
[0014] In certain embodiments, the electrolytic solution may be
formed by adding a commercially available tin, such as commercially
available tin having a purity level of 99.0% to 99.999% (2N to 5N),
to the acidic electrolyte. In one example, the tin may have
initial, pre-refining alpha particle emissions above about 0.001
counts/hour/cm.sup.2. In other examples, the tin may have initial,
pre-refining alpha particle emissions above about 0.002
counts/hour/cm.sup.2, above about 0.005 counts/hour/cm.sup.2, or
above about 0.01 counts/hour/cm.sup.2.
[0015] The electrolytic solution may include one or more acids.
Suitable acids for use in the acidic electrolytic solution include
but are not limited to hydrochloric acid, sulfuric acid,
fluoroboric acid, acetic acid, methane sulfonic acid, and sulfamic
acid. The acid may be mixed with water, such as deionized water.
The acid(s) of the electrolytic solution can be selected to control
the pH of the electrolytic solution.
[0016] The electrolytic solution may have a low, or acidic, pH. For
example, an electrolytic solution having an acidic pH may have a pH
of less than 7. In another example the electrolytic solution may
have a pH of less than about 6. In a further example, the
electrolytic solution may have a pH of less than about 5. In a
still further example, the electrolytic solution may have a pH of
less than about 4, less than about 3, less than about 2 or less
than about 1. The pH of the electrolytic solution may be adjusted
to optimize the effectiveness of the ion exchange resin and the
electrorefining process.
[0017] The electrolytic solution may optionally include one or more
additives. As used herein, an "additive" refers to a component of
the electrolytic solution other than the target metal to be refined
(e.g., tin), other metallic impurity components, and the acid/water
solution. The additive may be helpful for controlling one or more
properties of the electrolytic solution, the deposition process
and/or the deposited product. Each additive may be present in
amount from several parts-per-million (ppm) to several percent by
weight. For example, each additive may be present in an amount of
at least about 0.05% by volume of the electrolytic solution, at
least about 0.5% by volume of the electrolytic solution, or at
least about 1.0% by volume of the electrolytic solution.
[0018] Suitable additives include antioxidants and grain refiners.
For example, an antioxidant may be added to the electrolytic
solution to prevent spontaneous Sn.sup.2+ to Sn.sup.4+ oxidation
during electrolysis. Suitable antioxidants include, but are not
limited to, phenol sulfonic acid and hydroquinone. Suitable
commercially available antioxidants include Technistan Antioxidant,
Techni Antioxidant Number 8 available from Technic, and Solderon BP
Antioxidant available from Dow Chemical. Suitable concentrations of
an antioxidant include from about 0.05% to about 10%, from about
0.5% to about 5%, or from about 1% to about 3% by volume of the
electrolytic solution.
[0019] An organic grain refiner may optionally be added to the
electrolytic solution to limit dendritic deposition at the cathode.
Suitable organic grain refiners include, but are not limited to,
polyethylene glycol. Suitable commercially available organic grain
refiners include Technistan TP-5000 Additive, Techni Matte 89-TI
available from Technic, and Solderon BP Primary available from Dow
Chemical. Suitable concentrations of a grain refiner include from
about 0.5% to about 20%, from about 1.0% to about 15%, or from
about 3% to about 10% by volume of the electrolytic solution.
[0020] The electrolytic solution is exposed to at least one ion
exchange resin during at least a portion of the electrorefining
process. Ion exchange resins are organic compounds which include
functional groups configured to selectively capture another
material by exchanging ions with the captured material. For
example, ion exchange resins may include functional groups bonded
to a polymer matrix. In the current process, it is believed that
the ion exchange resin captures and removes alpha emitting
impurities from the electrolytic solution, such as metallic
impurities and, in particular, metallic impurities which are either
themselves capable of decay with concurrent release of an alpha
particle, such as .sup.210Po, or metallic impurities which produce
decay products with the decay products capable to decay with
concurrent release of an alpha particle, such as U and/or Th.
[0021] In one example, the ion exchange resin may be placed in a
column and the electrolytic solution may be circulated through the
column. For example, the electrolytic solution may be circulated
from a tank, through the ion exchange resin column and returned to
the tank by a pump. In this embodiment, the electrolytic solution
may be circulated through the column of ion exchange resin
concurrently with application of current to the electrolytic bath,
or alternatively, the circulation of the electrolytic solution
through the ion exchange resin may occur prior to, or after,
application of current according to a desired quantify and/or
duration. In a still further embodiment, circulation of the
electrolytic solution through the ion exchange resin and
application of current may be alternated as desired. The flow rate
through the column may be adjusted to achieve a desired contact
time between the electrolytic solution and the ion exchange resin.
In an alternative embodiment, the resin may be added directly to
the tank holding the electrolytic solution; a separate column is
not used.
[0022] Suitable ion exchange resins may include at least
functionalized carboxylic acid from the phosphonic acids group,
such as amino methyl phosphonic acid functional groups. Further
suitable ion exchange resins may include at least one functional
group selected from sulfonated, phosphomethylated, amino methyl
phosphonic acid, and poly(4-vinyl-pyridine) functional groups and
mixtures thereof. Still further suitable ion exchange resins may
include at least one functional group selected from sulfonated,
phosphomethylated, amino methyl phosphonic acid,
poly(4-vinyl-pyridine), sulfonic acid, chloromethyl, tributylamine,
di-vinyl benzene, quaternary amine, divinylbenzene, diphosphonic
acid, and iminodiacetate functional groups. Examples of
commercially available suitable ion exchange resins are presented
in Table 1, where "DVB" is divinylbenzene, "SB" is strong base,
"SA" is strong acid, "WA" is weak acid, and "Dow" is Dow Chemical
Company.
TABLE-US-00001 TABLE 1 Data for Select Ion Exchange Resins
Functional Exchange Trade name Vendor Group(s) mechanism Matrix
Monophos Resin Eichrom Sulfonated and Chelating Styrene-DVB
phosphomethylated Lewatit MonoPlus Lanxess Amino methyl Cation
Crosslinked TP 260 phosphonic acid exchange polystyrene Reillex HPQ
Vertellus Poly(4-vinyl- Anion DVB Polymer pyridine) exchange Dowex
G-26 Dow Sulfonic acid Cation Styrene-DVB, exchange gel Dowex
Optipore Dow Chloromethyl Adsorbent Styrene-DVB, L493 Macroporous
Dowex PSR-2 Dow Quatenary amine Anion Styrene-DVB, gel exchange
Dowex 21K XLT Dow Quatenary amine SB anion Styrene-DVB, exchange
gel Dowex MAC-3 Dow Carboxylic acid WA cation Polyacrylic, exchange
macroporous XZ 91419.00 Dow Quatenary amine SB anion Styrene-DVB,
resin exchange gel XUS 43568 resin Dow Di-methyl amine WB anion
Macro Styrene exchange Amberlyst A-26 Dow Quaternary amine Anion
Styrene-DVB exchange Amberlyst 15WET Dow Sulfonic acid SA cation
Styrene-DVB, exchange macroporous Amberlite IRC- Rohm
Amino-phosphate; Chelating Styrene-DVB, 747 and Haas Na+ form
macroporous Amberlite PWA 5 Rohm SB anion, NO.sub.3.sup.- Anion
Cross-linked and Haas selective exchange copolymer Diphonix resin
Eichrom Diphosphonic acid Chelating Styrene-based and sulfonic acid
polymer Lewatit TP 207 Lanxess Iminodiacetate Cation Crosslinked
exchange polystyrene
An ion exchange resin may be used alone or in combination with
other ion exchange resins. In particular, a mixed bed resin may be
used, where a mixed bed resin refers to a resin composition that
includes two or more specific resins that may have the same or
different functional groups, exchange mechanisms and/or
matrices.
[0023] Tin from the electrolytic solution is plated onto a cathode
during the electrorefining process. In some embodiments, exposing
the electrolytic solution to the ion exchange resin and
electrodeposition of the tin onto the cathode may occur at least
partially concurrently. As described further below, the
electrorefined tin may have reduced alpha particle emissions or
alpha flux.
[0024] FIG. 1 is a block diagram illustrating an exemplary
continuous tin electrorefining system 100 including tank 110,
cathode 112, first tin anode 114A and second tin anode 114B
(collectively referred to as tin anodes 114), media column 116,
pump 118, filter 120, pump 122, and rectifier 124, which is capable
of generating the required current density. One or more cathodes
112 and one or more tin anodes 114 are positioned in tank 110. As
shown in FIG. 1, tin anodes 114 may be placed on either side of
cathode 112. Tank 110 also contains an electrolytic solution
containing tin, which has been described above. The electrolytic
solution is circulated through media column 116 by pump 118 and is
returned back to tank 110. Media column 116 contains an ion
exchange resin. The flow rate through media column 116 is
calculated to achieve a specified contact time between the
electrolytic solution and the ion exchange resin. Adjusting the
flow rate through media column 116 may vary the contact time. For
example, increasing the flow rate of the electrolytic solution
through media column 116 may decrease the contact time between the
electrolytic solution and the ion exchange resin and conversely,
decreasing the flow rate of the electrolytic solution through media
column 116 may increase the contact time between the electrolytic
solution and the ion exchange resin.
[0025] System 100 may also include filter 120. The electrolytic
solution from tank 110 may be pumped through filter 120 by pump 122
and returned back to tank 110. Filter 120 may filter particulate
matter from the solution. For example, filter 120 may remove
material have a size greater than about 5 microns.
[0026] Rectifier 124 is connected to cathode 112 and anodes 114 and
provides the required current density for dissolution of tin anodes
114 and electrodepositing tin from the electrolytic solution onto
cathode 112 during the electrodepositing or electrorefining
process. A suitable current density at the cathode may be as low as
10, 15, 20, 25, 30, 35, 40 amps per square foot (ASF) or as great
as 25, 30, 35, 40, 45, 50, 55, 60, 65 or 70 ASF or may be within
any range delimited by any pair of the foregoing values. In other
embodiments, the current density at the cathode may be as low as
70, 80, 90, 100, 125 or 150 ASF or as great as 175, 200, 225, 250,
275 or 300 ASF or may be within any range delimited by any pair of
the foregoing values. In one example, the current density was
regulated at about 22 milliamps per square centimeter (mA/cm.sup.2)
(20 ASF) at cathode 112 and about 8-11 mA/cm.sup.2 (7-10 ASF) at
anodes 114.
[0027] The tin may be refined in a continuous process as described
above. For example, the steps of exposing the electrolytic solution
to an ion exchange resin and depositing the tin from the
electrolytic solution onto a cathode may occur at least partially
concurrently.
[0028] Alternatively, the tin may be refined in a step or batch
process. For example, an electrolytic solution may be formed by
electrolytic dissolution of tin anodes and a permeable membrane may
be used to prevent tin from depositing on the cathode. The
dissolution may then be stopped, and the electrolytic solution may
be exposed to an ion exchange resin for a period of time. For
example, the electrolytic solution may be passed through a column
containing the ion exchange resin or the ion exchange resin may be
added to the electrolytic solution tank. After exposure to the ion
exchange resin, the electrolytic solution may be electrodeposited
onto a cathode.
[0029] In some embodiments, the eletrorefining system may include
two or more electrodeposition processes. Each electrodeposition
process may include the same or different electrolytic solution
compositions. For example, the electrolytic solutions may include
the same or different acids and/or additive(s) and/or have the same
or different pH. One or more of the electrodeposition processes may
including an ion exchange resin as described herein, and if present
in two or more of the processes, the ion exchange resin may be the
same or different. In some embodiments, two or more
electrodeposition processes may be conducted in series or in
succession such that tin ions are electrodeposited two or more
times. For example, the electrorefining system may include
electrodepositing tin ions from an electrolytic solution containing
hydrochloric acid onto a cathode, electrolytic dissolution of the
deposited tin into a second electrolytic solution containing
sulfuric acid, and electrodepositing tin ions from the second
electrolytic solution onto a second cathode. Impurities and/or
contaminant components may be removed in each successive
electrodeposition process. Further, different impurities and/or
contaminant components may be removed based on the electrolytic
solution composition and/or the ion exchange resin of the
electrodeposition process.
[0030] In some embodiments, the electrorefined tin may not
experience a significant reduction in lead content compared to that
of the tin prior to the electrorefining process (e.g., the input or
pre-refined tin). For example, the lead content may not be reduced
by more than about 1% and particularly not by more than about 0.1%
by the electrorefining process. A suitable lead content of the tin
prior to the electrorefining process may be at least 1 ppm and more
particularly at least about 2 ppm. A suitable lead content of the
electrorefined tin may be at least about 1 ppm and more
particularly at least about 2 ppm. In some embodiments, the lead
content of the electrorefined tin may be as low as 0.01, 0.05 or
1.0 ppm or as great as 2.0, 5.0 or 10.0 ppm or may be within any
range delimited by any pair of the foregoing values.
[0031] It has been found that electrodeposited tin which is
produced by exposing the electrolytic solution to at least one ion
exchange resin during electrorefining has reduced alpha particle
emissions or alpha flux.
[0032] Although there is a relationship between a reduction in
certain impurities such as thorium and a reduction in alpha
particle emissions, a tin material having less than 1 ppm thorium
will not necessarily have a sufficient low alpha particle emissions
or alpha flux to satisfy certain industry requirements. For example
it is entirely possible to refine tin to a 6N purity level without
reducing alpha particle emissions to a suitable level. Accordingly,
in one example, electrorefined tin may be tested for alpha particle
emissions after refining using, for example, a gas flow
proportional counter such as an Alpha Sciences 1950 in the manner
described in JEDEC standard JESD221.
[0033] The overall reduction in alpha particle emissions will vary
depending on many factors including, but not limited to, the alpha
particle emissions of the input or pre-refined tin material, the
contact time of the electrolytic solution with the ion exchange
resin, and the number of passes of the electrolytic solution
through the ion exchange resin. In one example, the alpha particle
emissions of the refined tin material is reduced by at least 50%,
more particularly at least 75%, and even more particularly at least
85%, 90% or 95% compared to the alpha particle emissions of the
same material prior to deposition of the electrorefined tin. In
another example, electrorefining is carried out under conditions
suitable to reduce the alpha particle emissions of the refined tin
material to less than about 0.01 counts/hour/cm.sup.2, more
particularly less than about 0.002 counts/hour/cm.sup.2, and even
more particularly less than about 0.001 counts/hour/cm.sup.2.
[0034] It should be noted that the alpha particle emissions of tin
does not necessarily remain stable after the material has been
refined. In particular, alpha particle emissions or alpha flux of
the refined tin may increase or decrease over time due to the
residual presence and radioactive decay of various elements such as
.sup.210Pb. The increase or decrease of alpha particle emissions
over time may be referred to as alpha drift.
[0035] As described herein, it has surprisingly been found that not
only does the electrorefining process including an ion exchange
resin reduce the alpha particle emissions of the electrorefined tin
immediately after the electrorefining process but it also results
in reduced alpha drift and reduces the alpha particle emissions at
a period of time after the electrorefining process. In one
embodiment, the alpha particle emissions of the refined tin 90 days
after the electrorefining process is at least 50%, more
particularly at least 75%, and even more particularly at least 85%,
90% or 95% less than the alpha particle emissions of the same
material prior to electrorefining. In another example, the
electrorefining is carried out under conditions suitable to reduce
the alpha particle emissions of the electrorefined tin to less than
about 0.01 counts/hour/cm.sup.2, more particularly less than about
0.002 counts/hour/cm.sup.2 and even more particularly less than
about 0.001 counts/hour/cm.sup.2, when measured 90 days after the
electrorefining process.
II. Method of Determining the Alpha Particle Emission Potential of
Electrorefined Tin
[0036] A method for determining the alpha particle emission
potential of the electrorefined tin, such as the maximum alpha
particle emissions from the tin, is described herein. The described
method, for example, can be used to predict or forecast the maximum
alpha particle emissions from the tin.
[0037] As used herein, the term "target parent isotope" refers to
an isotope of interest which is present in a metallic material and
is able to decay to a daughter isotope, wherein the daughter
isotope may subsequently alpha-decay, i.e., may decay to a further
isotope with concomitant emission of an alpha particle. The term
"target decay isotope", as used herein, refers to an isotope of
interest which is a daughter isotope of the target parent isotope
and itself may subsequently alpha-decay, i.e., may decay to a
further isotope with concomitant emission of an alpha particle. The
target decay isotope may or may not be itself a direct decay
product of the target parent isotope. For example, if .sup.210Pb is
a target parent isotope, .sup.210Po may be a target decay isotope
even though .sup.210Pb decays to .sup.210Bi with subsequent decay
of .sup.210Bi to .sup.210Po.
[0038] According to the present method, the metallic material
(e.g., tin) is subjected to a secular equilibrium disruption
process. As used herein, the term "secular equilibrium disruption
process" refers to a process to which the metallic material is
subjected which at least partially disrupts the secular equilibrium
of the decay profile of at least one target parent isotope within
the metallic material. In most instances, the secular equilibrium
disruption process disrupts the secular equilibrium of the decay
profile of a target parent isotope by reducing the concentration of
the target parent isotope in the metallic material, by reducing the
concentration of a corresponding target decay isotope in the
metallic material, or by a combination of the foregoing. The
electrorefining process described herein is an exemplary secular
equilibrium disruption process. Other exemplary secular equilibrium
disruption processes include melting, casting, smelting, refining
(such as electro-chemical refining, chemical refining, zone
refining, and vacuum distillation). A secular equilibrium
disruption process may also include any combination of two or more
of the foregoing processes. Typically, in the secular equilibrium
disruption process, and particularly when the secular equilibrium
disruption process is at least in part a refining process, both the
target parent isotopes and the target decay isotopes are at least
partially removed as impurities by physical and/or chemical
separation from the bulk metallic material.
[0039] In some embodiments, the secular equilibrium disruption
process may remove substantially all of a given target decay
isotope and thereby effectively "reset" the secular equilibrium of
the corresponding target parent isotope. For example, in the case
of a metallic material including .sup.210Pb as a target parent
isotope, the secular equilibrium disruption process may
substantially completely remove all of the .sup.210Po target decay
isotope in the material, such that the secular equilibrium of
.sup.210Pb is effectively reset, wherein substantially all
.sup.210Po that is present in the material following the secular
equilibrium disruption process is generated by decay of .sup.210Pb
after the said disruption process. However, the present process may
also be practiced using secular equilibrium disruption processes
that remove only a portion of the target parent isotope and/or
target decay isotope, and the present process is not limited to
secular equilibrium disruption processes that remove substantially
all of a given target decay isotope.
[0040] In some embodiments, the secular equilibrium disruption
process may be completed in a relatively short amount of time and,
in other embodiments, the secular equilibrium disruption processes
may require a relatively greater amount of time for completion,
depending on the nature of the process and the number of processes
that together may constitute the secular equilibrium disruption
process. Therefore, the elapsed time discussed below, between the
secular equilibrium disruption process and the measurement of alpha
particle emissions of the metallic material, may be an elapsed time
between the completion of the secular equilibrium disruption
process (or processes) and the measurement of alpha particle
emissions of the metallic material.
[0041] After the metallic material (e.g., tin) is subjected to the
secular equilibrium disruption process, the alpha particle emission
of the metallic material is detected, i.e., an alpha particle
emission measurement is obtained. Although it is within the scope
of the present disclosure to obtain an alpha particle emission of
the entire metallic material in bulk form, typically a sample of
the bulk metallic material will be obtained for purposes of alpha
particle emission analysis.
[0042] A relatively thin portion of the bulk metallic material may
be obtained as a sample by a suitable method such as rolling the
bulk metallic material to provide a thin sheet of sample material,
or by any other another suitable method.
[0043] After the sample is obtained, the sample is treated by heat
in order to promote diffusion of target decay isotopes in the
sample material until such point that the concentration of atoms of
the target decay isotopes in the sample is uniform throughout the
sample volume. In many samples, there may be a larger concentration
of atoms of target decay isotopes toward the center of the sample,
for example, or otherwise in other areas of the sample such that a
concentration mismatch or gradient is present. The heat treatment
removes any such concentration mismatches or gradients by promoting
diffusion of atoms of target decay isotopes within the sample from
areas of relatively higher concentration toward areas of relatively
lower concentration such that a uniform concentration of target
decay isotopes is obtained within the sample. When such uniform
concentration is obtained, the number of atoms of target decay
isotopes within a detection limit depth of the alpha particle
detection process will be representative of and, more particularly
will correlate directly to, the uniform concentration of atoms of
target decay isotopes in the entirety of the sample. Such uniform
concentration is achieved when the chemical potential gradient of
the target decay isotopes is substantially zero and the
concentration of the target decay isotopes is substantially uniform
throughout the sample.
[0044] Stated in another way, at room temperature, the test sample
may have a chemical potential gradient, in that the concentration
of target decay isotopes is higher on one side of the sample than
another side of the sample, or at the centroid of the sample than
at the outer surfaces of the sample. Heating of the sample adjusts
the chemical potential gradient and, at a sufficient time and
temperature exposure, the chemical potential gradient is
substantially zero and the concentration of the target decay
isotopes is substantially uniform throughout the sample.
[0045] As used herein, the term "detection limit depth" refers to a
distance within a given metallic material through which an emitted
alpha particle may penetrate in order to reach a surface of the
material and thereby be released from the material for analytical
detection. Detection limit depths for .sup.210Po in selected
metallic materials are provided in Table 2 below, in microns, which
is based on the penetration of the 5.304 MeV alpha particle
released upon decay of .sup.210Po to .sup.206Pb:
TABLE-US-00002 TABLE 2 Detection limit depths of .sup.210Po in
selected metallic materials Detection limit depth of Metallic
material .sup.210Po (microns) Tin (Sn) 16.5 Aluminum (Al) 23.3
Copper (Cu) 11 Bismuth (Bi) 17.1
[0046] The detection limit depth for alpha particles of differing
energy, such as alpha particles emitted upon radioactive decay of
alpha particle-emitting isotopes other than .sup.210Po, will vary,
with the detection limit depth generally proportional to the energy
of the alpha particle. In the present method, emitted alpha
particles may be detected by use of a gas flow counter such as an
XIA 1800-UltraLo gas ionization chamber available from XIA L.L.C.
of Hayward, Calif. according the method described by JEDEC standard
JESD 221.
[0047] Target decay isotopes such as .sup.210Po are known to
diffuse or migrate within metallic materials and, in this respect,
the heat treatment of the present method is used to promote
diffusion of the target decay isotope within the material sample to
eliminate concentration gradients. In particular, target decay
isotopes, such as .sup.210Po, will have a diffusion rate J in a
given metallic material, which can be expressed according to
equation (1) below:
J = - D .differential. .phi. P o .differential. x ( 1 )
##EQU00001##
wherein: .differential..phi./.differential.x is the concentration
gradient of the target decay isotope, such as .sup.210Po; and D is
the diffusion coefficient.
[0048] The concentration gradient of the target decay isotope is
determined by measuring the alpha particle emissions at the surface
of a sample, removing a layer of material of x thickness, such as
by chemical etching, and measuring the alpha particle emissions at
the x depth. The concentration of the target decay isotope at the
original surface and at depth x is directly proportional to the
alpha particle emission at each surface, and concentration gradient
of the target decay isotope is calculated as the difference between
the concentration at one of the surfaces and the concentration at
depth x over the distance x.
[0049] To determine the polonium diffusion rate J, the polonium
alpha particle emissions from 5-5.5 MeV in a tin sample was
measured. The sample was then heated at 200.degree. C. for 6 hours,
and the alpha particle emission measurement was repeated. The
number of polonium atoms N is calculated from equation (2)
below:
N=A/.lamda..sub.Po (2)
wherein: A is the alpha particle emission measured in counts/hr;
and .lamda..sub.Po=ln 2/138.4 days, based on the half-life of
.sup.210Po.
[0050] The number of moles of polonium calculated by dividing the
number of polonium atoms N by Avogadro's number. Dividing the
difference in the number of moles of polonium by the sample area
(0.1800 m.sup.2) and the time over which the sample was heated (6
hours) yields a lower bound on the diffusion rate of
4.5.times.10.sup.-23 molm.sup.-2s.sup.-1 at 473K in tin.
TABLE-US-00003 TABLE 3 Data for diffusion rate determination
Measurement A (Counts/Hr) N (atoms) Moles Initial 24.75 1.19E+05
1.97E-19 Final 46.71 2.24E+05 3.72E-19
[0051] Based on equation (1), one may determine a suitable time and
temperature heating profile to which the sample may be exposed in
order to diffuse the target decay isotope within the sample
sufficiently to eliminate any concentration gradients, such that
detection of alpha particle emissions within the detection limit
depth of the sample is representative, and directly correlates, to
the concentration of the target decay isotope throughout the
sample. For example, for a tin sample having a thickness of 1
millimeter, a heat treatment of 200.degree. C. for 6 hours will
ensure that any concentration gradients of .sup.210Po atoms within
the sample are eliminated.
[0052] Thus, for a given metallic material and sample size, the
application of heat may be selected and controlled by time and
temperature exposure of the sample to ensure that atoms of a target
decay isotope are diffused to a sufficient extent to eliminate
concentration gradients. It has been found that, by the present
method, in providing a suitable time and temperature profile for
the heat treatment step, measurement of alpha particle emissions
from a target decay isotope present within the detection limit
depth directly corresponds to the concentration or number of target
decay isotope atoms within the entirety of the sample.
[0053] It is generally known that subjecting a metallic material to
heat promotes diffusion of elements within the material. However,
prior methods have employed heat treatment simply to increase the
number of alpha particle emissions detected over background levels
to thereby increase the signal to noise ratio of the alpha particle
emission detection.
[0054] The alpha particle emissions attributable to .sup.210Po is
expressed as polonium alpha activity, A.sub.Po, at a time (t)
following the secular equilibrium disruption process. From the
A.sub.Po and elapsed time (t), the concentration of .sup.210Pb
atoms in the sample can be calculated using equation (3):
[ 210 Pb ] 0 = .lamda. Po - .lamda. Pb .lamda. Po .lamda. Pb ( -
.lamda. Pb t - - .lamda. Po t ) ( A Po ( t ) + A Po ( t 0 ) -
.lamda. Po t ) ( 3 ) ##EQU00002##
wherein: .lamda..sub.Po=ln 2/138.4 days, based on the half-life of
.sup.210Po; .lamda..sub.Pb=ln 2/22.3 years (8,145.25 days) based on
the half-life of .sup.210Pb; and time (t) is the time which has
elapsed between the secular equilibrium disruption process and the
alpha particle emission measurement.
[0055] Due to the fact that .sup.210Pb has a 22.3 year half-life,
the .sup.210Pb concentration is substantially constant over the
time (t), particularly when the time (t) is less than three years.
Also, when substantially all of the .sup.210Po is removed in the
secular equilibrium disruption process (which may be the case when
the secular equilibrium disruption process is a strenuous refining
process, for example) the last term in equation (3) above is very
near to zero because the initial .sup.210Po concentration will be
very near to zero when the alpha particle emissions are measured
relatively soon after the secular equilibrium disruption.
[0056] The concentration of the target parent isotope may be
calculated by the above-equation (3) and, once the concentration of
the target parent isotope is calculated, the known half-life of the
target parent isotope may be used to provide an assessment or
prediction of a maximum concentration of the target decay isotope
within the material based on the re-establishment of the secular
equilibrium profile of the target parent isotope.
[0057] In other words, once the concentration of .sup.210Pb atoms
is determined using equation (3), based on the half-life of
.sup.210Pb the maximum .sup.210Po activity at re-establishment of
secular equilibrium will occur at (t)=828 days, and is calculated
from equation (4) below:
A Po ( t = 828 d ) = .lamda. Pb .lamda. Po .lamda. Po - .lamda. Pb
[ 210 Pb ] 0 ( - .lamda. Pb 828 d - - .lamda. Po 828 d ) ( 4 )
##EQU00003##
[0058] Consistent time units (i.e., days or years) should be used
across equation (3) and equation (4).
[0059] The maximum .sup.210Po activity directly correlates to a
maximum alpha particle emission of the material, and will occur at
828 days from the secular equilibrium disruption process. In this
manner, due to the fact that the present method will typically be
carried out relatively soon after the secular equilibrium
disruption process, the calculated maximum concentration of the
target decay isotope and concomitant alpha particle emission will
typically be a maximum future concentration of the target decay
isotope and concomitant alpha particle emission that the metallic
material will exhibit over a timeframe which corresponds to the
half-life of the target parent isotope.
[0060] For example, based on the half-life of .sup.210Pb, the
applicable timeframe or "window" by which a maximum possible
concentration of .sup.210Po (and thereby a peak in alpha particle
emissions) will be reached in the material will occur at 828 days
(27 months) from the secular equilibrium disruption process.
[0061] It is also possible to calculate a possible concentration of
.sup.210Po (and thereby the alpha particle emissions) at any
specified elapsed time from the secular equilibrium disruption
process. In this manner, it is possible to calculate a possible
concentration of .sup.210Po after a sufficient elapsed time from
the secular equilibrium disruption process, where the sufficient
elapsed time may be at least 200, 250, 300, 350 or 365 days from
the secular equilibrium disruption process. For example, based on
the half-life of .sup.210Pb, the applicable timeframe by which the
.sup.210Po concentration will reach 67% of the maximum possible
concentration in the material will occur at 200 days from the
secular equilibrium disruption process. Similarly, the .sup.210Po
concentration will reach 80% and 88% of the maximum possible
concentration in the material at 300 days and 365 days,
respectively, from the secular equilibrium disruption process.
[0062] Advantageously, according to the present method, after a
metallic material has been subjected to a secular equilibrium
disruption process such as by refining the metallic material, a
maximum alpha particle emission that the metallic material will
reach during the useful life of the material may be accurately
predicted. In this manner, the present method provides a valuable
prediction of the maximum alpha particle emission for metallic
materials, such as solders, that are incorporated into electronic
devices.
III. Examples
[0063] The present invention is more particularly described in the
following examples that are intended as illustration only, since
numerous modifications and variations within the scope of the
present invention will be apparent to those skilled in the art.
Unless otherwise noted all parts, percentages and ratios reported
in the following examples are on a volume basis, and all reagents
used in examples were obtained, or are available, from the
chemicals suppliers described below, or may be synthesized by
conventional techniques.
Example 1
Inclusion of Ion Exchange Resin in an Electrorefining Process
Materials Used
[0064] Monophos resin: an ion exchange resin having sulfonated and
phosphomethylated functional groups and available from Eichrom.
[0065] Lewatit MonoPlus TP 260: an ion exchange resin having amino
methyl phosphonic acid functional groups and available from
Lanxess.
[0066] Reillex HPQ Polymer: an ion exchange resin having
poly(4-vinyl-pyridine) functional groups and available from
Vertellus.
Electrorefining Process
[0067] An electrolytic solution was added to a 30 liter (L)
polypropylene tank equipped with a vertical pump for solution
agitation and filtration. A central titanium cathode and two 4N tin
anodes (one on each side of the cathode) were positioned in the
tank, and a DC power supply was connected to the cathode and anodes
for generating the required current density. During the
electrorefining process, the DC current passing between the cathode
and anodes was regulated to 22 mA/cm.sup.2 (20 ASF) at the cathode
and 8-11 mA/cm.sup.2 (7-10 ASF) at each anode.
[0068] An ion exchange resin was prewashed with at least 10 bed
volumes of deionized water and placed in a glass column. The glass
column had a diameter of approximately 1 inch and contained
approximately 77.0 cubic centimeters (4.7 cubic inches) of the ion
exchange resin. The electrolytic solution was continuously
circulated through the glass column by a magnetically coupled 1/250
HP Iwaki pump during the electrorefining process at a flow rate
between 100 and 500 mL per minute.
[0069] The tin was electrorefined for three days, and then
harvested from the cathode. The harvested tin was rinsed for five
minutes with deionized water having a purity of 5 megaohms per
centimeter. The electrorefined tin was then dried for 15 minutes at
150.degree. C., and cast at 300.degree. C.-350.degree. C. Three
crops were harvested for each example. A sample was taken from each
crop, and analyzed by an Alpha Sciences 1950 alpha counter in the
manner described in JEDEC standard JESD221 and a Varian Vista Pro
inductive coupled plasma atomic emission spectroscopy (ICP-AES) for
trace elements.
Control
[0070] The Control did not include an ion exchange resin in the
electrorefining process. A sulfuric acid electrolyte was formed by
mixing 3% sulfuric acid by volume with deionized water. Tin from
the anodes was electrolytically dissolved from high purity tin
anodes in the sulfuric acid electrolyte to form a 15 g/L solution.
Technistan Antioxidant (an antioxidant) was added at a volume
percent of 1% by volume of the total electrolytic solution and
Technistan TP-5000 additive (an organic grain refiner) was added at
a volume percent of 4% by volume of the total electrolytic
solution. The electrolytic solution had a pH of less than about 1
(calculated pH).
[0071] Electrolysis was performed at 20.degree. C. using a cathode
current density of 22 mA/cm.sup.2 (20 ASF). The cathodes were
harvested after 72 hours. The tin was cast. The casts were analyzed
by the Alpha Sciences 1950 alpha counter (in the manner described
in JEDEC standard JESD221) and the Varian Vista Pro ICP-AES. The
mean alpha particle emissions (in counts/hour/cm.sup.2) and
standard deviation ("SD") based on three samples are shown in Table
4 as measured immediately after casting ("refined alpha") and after
storage for at least 90 days ("alpha after 90 days").
TABLE-US-00004 TABLE 4 Refined tin sample data Refined alpha Alpha
after 90 days Percent Percent Starting alpha reduc- reduc- Mean SD
Mean SD tion Mean SD tion 0.0119 0.0080 0.0005 0.008 96% 0.0068
0.00046 15%
[0072] The electrorefining process of the Control, which did not
include an ion exchange resin, reduced the alpha particle emissions
by 96% immediately following the refining process. However, the
alpha particle emissions or alpha flux increased after 90 days,
resulting in an alpha reduction of only 15%.
Samples 1-3
[0073] Samples 1-3 included an ion exchange resin in the
electrorefining process. An electrolytic solution containing
sulfuric acid, deionized water, tin, Technistan Antioxidant and
Technistan TP-5000 was prepared as described above for the
Control.
[0074] Electrolysis was performed at 20.degree. C. using a cathodic
current density of 22 mA/cm.sup.2 (20 ASF). Electrolytic solution
from the main tank was pumped through the glass column which
contained the designated ion exchange resin at the designated flow
rate. The ion exchange resin and flow rates are presented in Table
5.
TABLE-US-00005 TABLE 5 Electrorefining process information Ion
Exchange Resin Flow Rate Sample 1 Monophos 450 ml/min Sample 2
Lewatit MonoPlus 320 ml/min TP 260 Sample 3 Reillex HPQ 210 ml/min
Polymer
[0075] The cathodes were harvested after 72 hours from the start of
the electrorefining process. The electrorefined tin was cast, and
the casts were analyzed by the Alpha Sciences 1950 alpha counter
(in the manner described in JEDEC standard JESD221) and the Varian
Vista Pro ICP-AES. The mean alpha particle emissions
(counts/hour/cm.sup.2) and standard deviation ("SD") for three
samples as measured immediately after casting ("refined alpha") and
at least 90 days after casting ("alpha after 90 days") are shown in
Table 6. The percent reduction ("% reduct.") of mean alpha particle
emissions based on the starting alpha particle emissions is also
shown.
TABLE-US-00006 TABLE 6 Refined tin alpha emissivity data Refined
alpha Alpha after 90 days Starting alpha % % Sample Mean SD Mean SD
Reduct. Mean SD Reduct. 1 0.0043 0.0018 0.0002 0 95 0.0005 0.0005
88 2 0.0054 0.0016 0.0001 0.0001 99 0.0007 0.0006 88 3 0.0052
0.0020 0.0001 0.0001 98 0.0005 0.0002 90
[0076] Alpha particle emissions of Samples 1-3 immediately after
refining and casting were similar to that of the control. Ninety
(90) days after casting, the alpha particle emissions of Samples
1-3 were significantly reduced compared to the control.
[0077] The lead content of the samples were analyzed before and
after electrorefining by Varian Vista Pro ICP-AES. The lead content
for Samples 1-3 are provided in Table 7.
TABLE-US-00007 TABLE 7 Refined tin lead content data Refined Pb Lot
Starting Pb (ppm) (ppm) Sample 1 1 5 5 2 5 5 3 4 5 Sample 2 1 4 5 2
4 4 3 4 4 Sample 3 1 4 4 2 4 4 3 4 4
[0078] Electrorefining did not significantly change the lead
content in Samples 1-3. Further, any measured change in lead
content is within the experimental margin of error.
Samples 4-20
[0079] Samples 4-20 included an ion exchange resin in the
electrorefining process. An electrolytic solution containing
sulfuric acid, deionized water, tin, Technistan antioxidant and
Technistan TP-5000 was prepared as described above for the
Control.
[0080] Electrolysis was performed at 20.degree. C. using a cathodic
current density of 22 mA/cm.sup.2 (20 ASF). Electrolytic solution
from the main tank was pumped through the glass column which
contained the designated ion exchange resin at the designated flow
rate. The ion exchange resin, flow rates (mL/min), alpha particle
emissions (counts/hour/cm.sup.2), including mean and standard
deviation ("SD") are presented in Table 8.
TABLE-US-00008 TABLE 8 Refined tin sample data Refined alpha Ion
Percent Sam- Exchange Flow Starting alpha reduc- ple Resin Rate
Mean SD Mean SD tion 4 Dowex G- 280 0.003 0.0005 0.0004 0.0001 87%
26 5 Dowex 227 0.003 0.0005 0.0006 0.0003 79% Optipore L493 6 Dowex
3.8 0.003 0.0005 0.0006 0.0004 80% MAC-3 7 Amberlite 3.6 0.003
0.0005 0.0003 0.0001 90% IRC-747 8 Diphonix 355 0.003 0.0005 0.0009
0.0015 69% resin 9 Amberlyst 240 0.003 0.0005 0.0017 0.0013 44%
A-26 10 Dowex 260 0.003 0.0005 0.0003 0.0003 91% PSR-2 11 Amberlyst
400 0.003 0.0005 0.0006 0.0006 79% 15WET 12 Lewatit 235 0.00413
0.00196 0.0000 0.0000 100% TP-260 13 XZ 285 0.00413 0.00196 0.0005
0.0005 88% 91419.00 resin 14 Lewatit 235 0.00413 0.00196 0.0001
0.0001 97% TP-207 15 XUS 320 0.00413 0.00196 0.0001 0.0001 98%
43568 resin 16 Amberlite 400 0.00640 0.0005 0.0005 0.0005 93% PWA 5
17 Dowex 240 0.00640 0.0005 0.0005 0.0004 92% 21K XLT 18 Dowex 210
0.00640 0.0001 0.0004 0.0001 94% G-26 19 Amberlite 430 0.00640
0.0001 0.0008 0.0001 88% IRC 747 20 Reillex 250 0.00640 0.0001
0.0009 0.0001 85% HP
[0081] The alpha particle emissions were reduced the greatest
amount in Sample 12 (100%), which included Lewatit TP-260 ion
exchange resin and was reduced the least in Sample 9 (44%).
[0082] The lead content of the samples were analyzed before (e.g.,
pre-refining) and after (e.g., post-refining) electrorefining by
the Varian Vista Pro ICP-AES. Three samples, or lots, were analyzed
for each resin tested. The lead content for Samples 4-20 are
provided in Table 9.
TABLE-US-00009 TABLE 9 Refined tin sample lead content data
Starting Refined Sample Lot Pb (ppm) Pb (ppm) 4 1 5 6 2 5 5 3 5 4 5
1 5 5 2 5 6 3 5 4 6 1 5 5 2 5 5 3 5 5 7 1 5 6 2 5 5 3 5 4 8 1 5 5 2
5 5 3 5 4 9 1 5 5 2 5 4 3 5 4 10 1 5 5 2 5 5 3 5 4 11 1 5 5 2 5 5 3
5 4 12 1 5 6 2 5 5 3 4 5 13 1 5 5 2 5 5 3 4 4 14 1 5 5 2 5 5 3 4 5
15 1 5 6 2 5 5 3 4 5 16 1 4 5 2 4 5 3 4 5 17 1 4 6 2 4 5 3 4 5 18 1
4 5 2 4 5 3 4 5 19 1 4 5 2 4 5 3 4 5 20 1 4 5 2 4 5 3 4 3
[0083] Electrorefining did not significantly change the lead
content in Samples 4-20.
Example 2
Adjustment of Tin Concentration and Current Density in an
Electrorefining Process
[0084] The effects of tin concentration and current density were
investigated in Samples 21-25. Electrolytic solutions containing
sulfuric acid, deionized water, tin, Technistan Antioxidant and
Technistan TP-5000 were prepared as described above for the
Control.
[0085] During the electrodeposition process, the electrolytic
solution from the main tank was pumped through the glass column
containing Lewatit MonoPlus TP 260 ion exchange resin. The tin was
deposited at 20.degree. C. and onto a cathode having an active area
of 72 square inches. The tin concentration of the electrolytic
solution, the cathodic current in amps and the cathodic current
density in ASF for each sample is provided in Table 10.
TABLE-US-00010 TABLE 10 Electrorefining process information Tin
Current concentration Current density Sample (g/L) (Amps) (ASF) 21
20 5 10 22 40 15 30 23 20 25 50 24 60 5 10 25 60 25 50
[0086] Before the electrorefining process, the input or pre-refined
tin had alpha particle emissions of 0.048 counts/hour/cm.sup.2. The
post-refined alpha particle emissions and elapsed time between
refining and the measurement of alpha particle emissions are shown
below in Table 11. The alpha particle emissions were measured at
multiple elapsed times for select samples.
[0087] Table 11 also includes percent reduction and the reduction
factor of the measured alpha particle emissions as compared to the
input or pre-refined alpha particle emissions. The percent
reduction was calculated by the difference between the pre-refined
and post-refined alpha particle emissions divided by the
pre-refined alpha particle emissions. The reduction factor was
calculated by the pre-refined alpha particle emissions divided by
the post-refined alpha particle emissions.
TABLE-US-00011 TABLE 11 Refined tin sample alpha emissivity data
Alpha particle Elapse emissions time Percent Reduction Sample Lot
(counts/hr/cm2) (days) reduction factor 21 1 0.02 26 58% 2.4 2
0.0138 23 71% 3.5 3 0.009 21 81% 5.3 3 0.021 47 56% 2.3 22 1 0.0137
13 71% 3.5 1 0.026 39 46% 1.8 2 0.012 9 75% 4.0 3 0.0103 6 79% 4.7
23 1 0.0136 19 72% 3.5 2 0.0094 16 80% 5.1 3 0.0095 10 80% 5.1 3
0.0301 39 37.3% 1.6 24 1 0.013 35 73% 3.7 1 0.024 54 50.0% 2.0 2
0.0093 27 81% 5.2 3 0.0058 20 88% 8.3 25 1 0.0029 14 94% 16.6 1
0.0088 34 81.7% 5.5 2 0.0035 14 93% 13.7 3 0.0016 13 97% 30.0 3
0.0093 34 81% 5.2
[0088] Sample 21, which had the lowest tin concentration and the
lowest current density, provided the least reduction in alpha
particle emissions. Sample 25, which had the highest tin
concentration and the highest current density, provided the
greatest reduction in alpha particle emissions.
[0089] A plot of the alpha particle emissions over time for each
sample is provided in FIG. 2. A linear trend line was fit to each
data set, and the equations are presented in FIG. 2. The linear
trend line for Sample 22 had a slope of 0.0005, Sample 23 had a
slope of 0.0008, Sample 24 had a slope of 0.0005 and Sample 25 had
a slope of 0.0003. A linear trend line could not be fit to the data
for Sample 21.
Example 3
Determination of Maximum Alpha Emissions in Refined Tin Samples
[0090] The present method was used to assess the maximum potential
alpha emissions in eight refined tin samples. The tin samples were
refined according to the method described herein. Test samples of
the refined tin samples were obtained by cutting an approximately 1
kilogram sample from an ingot and rolling the sample to a thickness
of 1 millimeter. The test samples were heated at 200.degree. C. for
six hours, and the alpha particle emissions of the test samples
were measured using an XIA 1800-UltraLo gas ionization chamber
available from XIA L.L.C. of Hayward, Calif. The measured alpha
particle emissions and elapsed times between refining and the
measurement of alpha particle emissions are shown below in Table
12.
TABLE-US-00012 TABLE 12 Refined tin sample data Elapsed time (t)
between Maximum refining and .sup.210Pb alpha Alpha particle
measurement of concentration particle emissions alpha particle at
time = 0 emission (alpha flux) emissions (atoms/cm.sup.2) (equation
Sample (counts/hr/cm.sup.2) (days) (equation (2)) (3)) 26 0.002 89
66 0.0056 27 0.0045 258 74 0.0063 28 0.0016 113 44 0.0037 29 0.004
272 64 0.0055 30 0.0016 211 29 0.0025 31 0.0009 32 72 0.0061 32
0.025 553 324 0.0276 33 0.0195 523 255 0.0217
[0091] From the measured alpha particle emission and the elapsed
time (t) between refining and the measurement of alpha particle
emission, the concentration of .sup.210Pb at (t)=0 can be
calculated from equation (3) above.
[0092] For example, the alpha particle emission of Sample 26 was
measured at 0.002 counts/hr/cm.sup.2 at 89 days from refining.
Based on equation (3) above, the number of .sup.210Pb atoms per
cm.sup.2 ([.sup.210 Pb].sub.0) needed to generate the measured
.sup.210Po activity, i.e., measured alpha particle emission, was
calculated to be 66. Using equation (4) above, the activity or
predicted alpha particle emission of .sup.210Po at (t)=828 days was
calculated as 0.0056 counts/hr/cm.sup.2.
[0093] In Sample 32, the alpha particle emission was measured at
0.025 counts/hr/cm.sup.2 at 523 days from refining. The value of
[.sup.210Pb].sub.0 was calculated based on equation (3) to be 255
atoms/cm.sup.2, and the maximum alpha particle emission was
calculated based on equation (4) as 0.0217 counts/hr/cm.sup.2.
[0094] As may be seen from Samples 26 and 32, the difference
between the measured alpha particle emission and the calculated
maximum alpha particle emission decreases as time (t) approaches
828 days, with the greater difference for Sample 26 attributable to
the alpha particle emission measurement being obtained early in the
secular equilibrium cycle (e.g., less time had elapsed from the
secular equilibrium disruption event) before secular equilibrium
could be re-established after refining.
Example 3
Determination of Time Required to Diffuse the Target Decay
Isotope
[0095] The time required to diffuse the target decay isotope in a
tin sample was investigated. Tin samples were refined according to
the method disclosed herein. A test sample of the refined tin
sample was obtained by cutting a sample from an ingot and rolling
the sample to a thickness of 0.45 millimeter. The test sample was
heated at 200 C for one hour, and the alpha particle emissions of
the test samples were measured using an XIA 1800-UltraLo gas
ionization chamber available from XIA L.L.C. of Hayward, Calif.
Measurement of the alpha particle emissions required about 24
hours, after which the sample was heated for one hour at
200.degree. C. and then measured for alpha particle emissions. This
process (e.g., heat for one hour followed by measurement of alpha
particle emissions) was repeated for a total of five
heat/measurement cycles. The measured alpha particle emissions and
the total hours the sample was heated at 200.degree. C. are shown
below in Table 13.
TABLE-US-00013 TABLE 13 Refined tin sample data Total Alpha
particle hour(s) emissions sample (alpha flux) heated
(counts/hr/cm.sup.2) 0 0.017 1 0.025 2 0.024 3 0.027 4 0.025 5
0.026
[0096] As can be seen from Table 13, the activity or alpha flux of
the sample increased from 0.017 counts/hr/cm.sup.2 to 0.025
counts/hr/cm.sup.2 after one hour at 200 C. That is, the activity
or alpha flux of the tin sample increased more than 50% after one
hour at 200.degree. C. As further shown in Table 13, there was no
significant change in the activity or alpha flux of the sample when
heated for more than one hour at 200.degree. C., suggesting that
one hour at 200.degree. C. was sufficient to achieve a
substantially uniform concentration of the target decay isotopes
throughout the sample.
[0097] Without further elaboration, it is believed that one skilled
in the art can, using the preceding description, utilize the
present invention to its fullest extent. The preceding preferred
specific embodiments are, therefore, to be construed as merely
illustrative, and not limitative of the remainder of the disclosure
in any way whatsoever.
[0098] In the foregoing, all temperatures are set forth uncorrected
in degrees Celsius and, all parts and percentages are by weight,
unless otherwise indicated.
[0099] From the foregoing description, one skilled in the art can
easily ascertain the essential characteristics of this invention
and, without departing from the spirit and scope thereof, can make
various changes and modifications of the invention to adapt it to
various usages and conditions.
* * * * *