U.S. patent application number 10/376526 was filed with the patent office on 2004-07-22 for solder composition substantially free of lead.
Invention is credited to Nagase, Takashi, Takemoto, Tadashi, Uetani, Takashi, Yamazaki, Morio.
Application Number | 20040141873 10/376526 |
Document ID | / |
Family ID | 32709246 |
Filed Date | 2004-07-22 |
United States Patent
Application |
20040141873 |
Kind Code |
A1 |
Takemoto, Tadashi ; et
al. |
July 22, 2004 |
Solder composition substantially free of lead
Abstract
This invention provides solder that is substantially free of
lead that minimizes corrosion from occurring in the soldering
equipment such as the tip of the soldering iron and soldering
dipping tank. Without substantial, if any, amount of lead in the
solder, the solder does not expose lead into the environment. The
solder composition also includes element(s) that extend the life of
the soldering tip or other equipments associated with soldering.
The solder composition may include Sn (tin) along with Co (cobalt)
and Fe (iron). Tin may make up the most of the content in the
solder composition. The two elements cobalt and iron substantially
inhibit tin from corroding the iron, thereby substantially
preventing the soldering iron tip and dipping solder tank from
corroding. The composition of the solder may also include Ag
(silver) to improve the mechanical strength and wettability of the
solder. Alternatively, the composition may include Ni (nickel) to
improve the wettability of the solder as well, along with other
elements.
Inventors: |
Takemoto, Tadashi; (Osaka,
JP) ; Nagase, Takashi; (Nara, JP) ; Uetani,
Takashi; (Osaka, JP) ; Yamazaki, Morio;
(Osaka, JP) |
Correspondence
Address: |
Squire, Sanders & Dempsey L.L.P
14th Floor
801 S. Figueroa Street
Los Angeles
CA
90017-5554
US
|
Family ID: |
32709246 |
Appl. No.: |
10/376526 |
Filed: |
February 27, 2003 |
Current U.S.
Class: |
420/557 |
Current CPC
Class: |
B23K 35/262 20130101;
C22C 13/02 20130101; B23K 35/362 20130101; B23K 35/0227 20130101;
B23K 2101/38 20180801 |
Class at
Publication: |
420/557 |
International
Class: |
C22C 013/00 |
Foreign Application Data
Date |
Code |
Application Number |
Jan 22, 2003 |
JP |
2003-013989 |
Claims
What is claimed is:
1. A solder composition having substantial content of tin by mass,
the solder composition further comprising: between about 0.2% and
about 5% of silver by mass of the solder composition; and between
about 0.01% and about 1.0% of iron by mass of the solder
composition.
2. The solder composition according to claim 1, further including:
between about 0.1% and about 5.0% of bismuth by mass of the solder
composition.
3. The solder composition according to claim 1, further including:
between about 0.1% and about 2.50% of cobalt by mass of the solder
composition.
4. The solder composition according to claim 1, further including:
flux to formulate the solder composition into a wire.
5. A solder composition having substantial content of tin by mass,
the solder composition further comprising: between about 0.2% and
about 5% of silver by mass of the solder composition; and between
about 0.01% and about 1.0% of cobalt by mass of the solder
composition.
6. A solder composition having substantial content of tin by mass,
the solder composition further comprising: between about 0.2% and
about 5% of silver by mass of the solder composition; and between
about 0.01% and about 1.0% of nickel by mass of the solder
composition.
7. A solder composition having substantial content of tin by mass,
the solder composition further comprising: between about 0.2% and
about 5% of silver by mass of the solder composition; between about
0.01% and about 1.0% of iron by mass of the solder composition;
between about 0.01% and about 1.0% of cobalt by mass of the solder
composition; and between about 0.01% and about 1.0% of nickel by
mass of the solder composition, where the combined mass of the
iron, cobalt, and nickel is less than about 1.0% of the solder
composition.
8. A solder composition having substantial content of tin by mass,
the solder composition further comprising: between about 0.2% and
about 5% of silver by mass of the solder composition; between about
0.01% and about 1.0% of nickel by mass of the solder composition;
and between about 0.01% and about 1.0% of cobalt by mass of the
solder composition, where the combined mass of the nickel and
cobalt is less than about 1% of the solder composition.
9. A process for soldering electrical components comprising:
providing a lead-free solder including tin (Sn) as its main
component and cobalt (Co), iron (Fe), and nickel (Ni) each in the
amount of between about 0.01 to 1 percent by mass, and the combined
amount of cobalt, iron and nickel being equal to or smaller than
about 1 percent by mass; heating said lead free solder with a
soldering iron to melt said lead free solder; and applying said
melted lead free solder to an electrical component.
10. The process of claim 9 wherein said lead free solder further
comprises silver (Ag) in the amount of between about 0.2 to 5
percent by mass.
11. The process of claim 9 wherein said lead free solder contains
copper (Cu) in the amount of between about 0.1 to 2.5 percent by
mass.
12. A lead free solder composition for use in soldering electrical
components wherein said lead free solder composition has a
corrosive level approximately equal to lead based solder
compositions, said lead free solder comprising:
13. Tin (Sn) as its main component and cobalt (Co), iron (Fe), and
nickel (Ni) each in the amount of between about 0.01 to 1 percent
by mass, and the combined amount of cobalt, iron and nickel being
equal to or smaller than about 1 percent by mass.
14. The lead free solder of claim 12 further comprising silver (Ag)
in the amount of between about 0.2 to 5 percent by mass.
15. The lead free solder of claim 12 further comprising copper (Cu)
in the amount of between about 0.1 to 2.5 percent by mass.
Description
1. RELATED APPLICATION
[0001] This application claims priority to a Japanese Patent
Application no. 2003-013989 filed Jan. 22, 2003, entitled
"Lead-Free Solder and Electronic Parts Using It," which is hereby
incorporated by reference.
BACKGROUND OF THE INVENTION
[0002] 2. Field of the Invention
[0003] This invention relates to solder compositions for bonding
electronic parts. In particular, this application relates to lead
(Pb)-free solder composition that has tin (Sn) as the main
element.
[0004] 3. General Background
[0005] In the electronic industry solders are used for connecting
electrical leads and parts. A common solder contains tin (Sn) and
lead (Pb) as its main elements with the lead content generally
determining the eutectic temperature. For example, a lead base
solder may contain 37% by mass. In recent years, a lead-free solder
(or solder with small percentage by mass of lead equivalent to the
contents of inevitable impurities) have been developed and used.
The lead-free solders have been used because the lead in the solder
contributed to acid rain, which in turn raises environmental
concerns. Many lead-free solders have been proposed as discussed in
the unexamined Japanese patent application No. H8-132277. When
lead-free solder is used with a manual soldering iron, however, the
useful life of the tip of the soldering iron is substantially
shortened when compared to using solders with significant lead
content.
[0006] One of the reasons for the shortened life of the tip is that
the corrosion rate of the tip is increased with prior lead-free
solder compositions. The soldering tip is usually made of copper or
copper alloy, and its surface is plated with iron to prevent the
solder from corroding the surface. The corrosion rate increases as
the temperature of the soldering iron tip increases. The lead-free
solder used for manual soldering generally has a higher melting
point than the lead based solder. Accordingly, the operating
temperature of the tip must be increased to solder with the
lead-free solder as compared to the solder with lead content. With
the increase in temperature of the tip, the iron plate on the tip
corrodes faster thereby shortening the life of the tip.
[0007] Another reason for the shortened life of the tip is that the
lead-free solder has more tin content than the lead solder. Tin is
more reactive with the iron of the soldering iron tip further
increasing the corrosion rate of the tip. In addition, the
corrosion rate may be accelerated due to effects such as that of
flux that is generally contained in solder used for manual
soldering.
[0008] The corrosion problems have occurred in other soldering
applications as well. For instance, in soldering applications where
printed circuit boards with electronic components are immersed into
melted lead-free solders, corrosion can occur to the walls of the
solder tank, as well as the feed propeller and heating section. One
of the reasons for the corrosion is that the solder tank is usually
made of stainless steel, which has a large content of iron. The
lead-free solders have larger amounts of particulates such as
oxides, sludge, as well as tin, compared to the conventional
lead-containing solder. The tin in the solder and iron in the
stainless steel react to corrode the irons. In addition, the
melting point of the lead-free solder is higher, which further
accelerates the corrosion rate. These factors accelerate the
corrosion rate of the dipping solder tank and shorten the useful
life of the tank.
INVENTION SUMMARY
[0009] This invention provides a solder composition that is
substantially free of lead to minimize exposing lead to the
environment, and inhibit corrosion from occurring in the soldering
equipment such as the tip of the soldering iron and solder dipping
tank. The composition of the solder includes tin (Sn) along with
cobalt (Co) and iron (Fe). Tin may make up the most of the content
in the solder composition. The two elements cobalt and iron
substantially inhibit tin from corroding the iron to substantially
prevent the soldering iron tip and dipping solder tank from
corroding. The composition of the solder may also include silver
(Ag) to improve the mechanical strength and wettability of the
solder. Alternatively, the composition may include nickel (Ni) to
improve the wettability of the solder.
[0010] Other systems, methods, features and advantages of the
invention will be or will become apparent to one with skill in the
art upon examination of the following figures and detailed
description. It is intended that all such additional systems,
methods, features and advantages be included within this
description, be within the scope of the invention, and be protected
by the accompanying claims.
BRIEF DESCRIPTION OF THE DRAWINGS
[0011] The invention can be better understood with reference to the
following figures. The components in the figures are not
necessarily to scale, emphasis instead being placed upon
illustrating the principles of the invention. Moreover, in the
figures, like reference numerals designate corresponding parts
throughout the different views.
[0012] FIG. 1(a) illustrates a front view of a testing apparatus
for conducting the corrosion test.
[0013] FIG. 1(b) is a cross-sectional view of the leading end of a
test specimen used for the corrosion test.
[0014] FIG. 2 is a graph illustrating the results of the corrosion
test.
DETAILED DESCRIPTION
[0015] While various embodiments of the invention have been
described, it will be apparent to those of ordinary skill in the
art that many more embodiments and implementations are possible
within the scope of this invention. Accordingly, the invention is
not to be restricted except in light of the attached claims and
their equivalents.
[0016] This invention provides a solder composition (also referred
to as lead-free solder) that is substantially free of lead. The
solder composition includes tin and silver, where tin makes up the
largest percentage by mass of the solder composition. In addition,
the solder composition may include either alone or in some
combination such elements as iron, nickel, and cobalt. These
elements may be incorporated into the solder composition to inhibit
corrosion and improve the wettability. Besides these elements,
other elements may be incorporated to the solder composition to
inhibit corrosion from occurring in the soldering equipment, and to
improve the strength and wettability of the solder.
[0017] The solder composition may include between about 0.01% and
about 1.0% of iron by mass. With less than about 0.01% by mass of
iron in the solder composition, there may not be enough iron in the
composition to inhibit corrosion. On the other hand, with the
content of iron in the solder composition in excess of about 1% by
mass, the solder alloy may change to an oxidized state. In
addition, the melting point of the solder composition may increase
causing the soldering temperature to rise. Reducing the soldering
temperature reduces the corrosion rate. Accordingly, the iron
content in the solder composition may be between about 0.01% and
about 1% by mass. In particular, the iron content in the solder
composition may be between about 0.02% and about 1.0% by mass.
[0018] The content of cobalt in the soldering composition may be
between about 0.01% and about 1.0% by mass. With less than about
0.01% by mass of cobalt in the solder composition, there may not be
enough of cobalt in the composition to inhibit corrosion. On the
other hand, with the content of cobalt in the solder composition in
excess of 1% by mass, cobalt segregation may occur in the solder
alloy. In addition, the melting point of the solder composition may
increase causing the soldering temperature to rise. Accordingly,
the cobalt content in the solder composition may be between about
0.01% and about 1% by mass. In particular, the cobalt content in
the solder composition may be between about 0.02% and about 0.5% by
mass.
[0019] The solder composition may also include nickel in the amount
between about 0.01% and 1.0% by mass to minimize corrosion and
improve the wettability. With less than 0.01% by mass of nickel in
the solder composition, there may not be enough of nickel in the
composition to inhibit corrosion. On the other hand, with the
content of nickel in the solder composition in excess of 1% by
mass, the solder alloy may change to an oxidized state. In
addition, the melting point of the solder composition may increase
causing the soldering temperature to rise. Accordingly, the nickel
content in the solder composition may be between about 0.01% and
about 1% by mass. In particular, the nickel content in the solder
composition may be between about 0.02% and about 0.5% by mass.
[0020] Besides the individual content of iron, cobalt, and nickel,
the total content of iron, cobalt, and nickel may also affect the
melting point of the solder. That is, if the total content of iron,
cobalt, and nickel exceed 1.0% by mass of the solder composition,
then the melting point of the solder may rise and the corrosion
rate may increase. Accordingly, the total content of iron, cobalt,
and nickel may be less than about 1.0% by mass of the solder
composition, and in particular less than about 0.7% by mass. The
total content of iron, cobalt, and nickel, however, is not limited
to being less than 1% by mass, and above percentage may be adjusted
to minimize the corrosion effect on the solder composition.
[0021] The lead composition may include silver in the amount
between about 0.2% and about 5.0%. With less than 0.2% by mass of
silver in the soldering composition, there may not be silver
content to improve the mechanical strength and wettability of the
solder composition. On the other hand, incorporating more than
about 5% by mass of silver may incur additional cost to formulating
the solder. Accordingly, silver in the amount between about 0.2%
and about 5.0% by mass of the solder composition may be used. In
particular, silver content of between about 2.0% and 4.0% by mass
may be used in the solder composition.
[0022] The solder composition may also include copper in the amount
between about 0.1% and about 2.5% by mass. Incorporating copper
into the solder composition may improve the wettability and
decrease the melting point of the solder. Incorporating less than
0.1% of copper by mass may not be enough to improve the wettability
and decrease the melting point. On the other hand, incorporating
more than 2.5% by mass of copper into the solder composition may
increase the viscosity of the solder in such a way that soldering
defects may occur. Accordingly, the solder composition may include
between about 0.1% and 2.5% by mass of copper. In particular,
copper content in the solder composition may be about 0.2% and 1.0%
by mass.
[0023] In addition to the above elements, the solder composition
may include bismuth in the amount between about 0.1% and 5.0% by
mass of the solder composition to improve the mechanical strength
and decrease the melting point. Incorporating less than 0.1% of
bismuth by mass into the solder composition may not improve the
mechanical strength or decrease the melting point. On the other
hand, incorporating more than about 5% by mass of bismuth into the
solder composition may result in coarse solder alloy. Accordingly,
the solder composition may include between about 0.1% and 5.0% by
mass of bismuth, and in particular, between about 0.2% and 3.0% by
mass.
[0024] The solder composition may include other elements such as
zinc (Zn), stibium (Sb), indium (In), manganese (Mn), chromium
(Cr), and palladium (Pd), to the extent that none of these elements
adversely affect the iron, nickel, cobalt, silver, copper, and
bismuth.
[0025] For manual soldering, the solder composition that is
substantially lead-free may be shaped like a hollow wire containing
flux as a core component. Electrical components such as a printed
circuit board that has been soldered with lead free solder is
beneficial to the environment because lead is not exposed to the
environment. Also, with the solder composition described above,
there is no trade off in terms of cost because the life of the iron
tip and other equipment associated with soldering is approximately
the same or longer as the life of the iron tip when soldering with
lead based solder.
[0026] The lead composition may be formulated to shape like a solid
wire or hollow wire having flux as its core component for use in
manual soldering. In such application, the solder composition
inhibits the iron-plated layer of the soldering iron tip from being
corroded, thereby extending the life of the tip. In addition, other
elements may be added to the solder composition to improve the
wettability and the soldering efficiency.
[0027] The lead composition may be also formulated to be stored in
the dipping solder tank for flow soldering application or used as
solder paste for reflow soldering application. In this application,
the solder composition inhibits the tank wall, feed propeller, and
heating section of the dipping solder tank from being corroded,
thereby extending the life of soldering equipment and increasing
the wettability. That is, the solder composition may be used for
soldering electrical components to a printed circuit board. In this
application, even if the printed circuit board is discarded and
later exposed to acid rain, minimal lead, if any, is released to
the environment. In addition, the lead-free solder extends the life
of the soldering iron tip and dipping solder tank and the
replacement intervals, thereby increasing the productivity and
reducing the cost of manufacturing the electronic parts.
[0028] The follow test results describe the corrosion test:
[0029] First Test:
[0030] FIGS. 1(a) and 1(b) illustrate a front view of the testing
apparatus 1 for the corrosion test. The soldering iron 2 has a tip
that is coupled to a test specimen 3. The Test specimen 3 is
equivalent to a soldering iron tip. To measure the corrosion on the
tip, test specimen 3 is shaped like a semi-cylindrical rod. The
testing apparatus 1 allows the temperature of test specimen 3 to be
maintained at a given value by controlling the heater that is
provided in soldering iron 2. The testing apparatus 1 also includes
a solder feeder 4 that is coupled to a solder specimen 5. The
solder feed 4 is capable of moving axially as indicated by the
arrow in FIG. 1(a). The test specimen 3 and the solder specimen 5
may be positioned so that they are axially aligned. As the solder
feeder 4 moves the solder specimen 5 towards the test specimen 3,
the leading end of solder specimen 5 makes contact with the test
specimen 3. When the temperature of test specimen 3 is high enough,
the leading end of solder specimen 5 melts and corrosion occurs in
the test specimen 3. In this corrosion test, the solder specimen 5
was repeatedly melted and measured for the corrosion amount on the
test specimen 3.
[0031] FIG. 1(b) is an enlarged view of the leading end of the test
specimen 3. The test specimen 3 includes an iron-plated layer 12
(about 200 .mu.m to about 300 .mu.m) formed on the outer surface of
copper-made test specimen base 10 shaped like a round rod. A thin
Cr-plated layer 14 (about 2 .mu.m to about 10 .mu.m) is also formed
on the iron-plated layer 12. The outer diameter of test specimen 3
is 5.4 mm. At the tip of the test specimen 3, an exposed area 13 is
formed on the Cr-plated layer 14 with a diameter of 3 mm. This in
turn exposes the iron-plated layer 12 in the exposed area 13. The
solder specimen 5 is positioned so that it makes contact with the
exposed iron-plated layer 12 around the exposed area 13. For the
corrosion test, the corrosion amount on the iron-plated layer 12 in
the iron-exposed area 13 is measured as indicated by the dotted
line 18.
[0032] The solder specimen 5 used for the test had a diameter of
1.0 mm and contained flux. Approximately 3 percent of flux was
impregnated at the center of the solder. Rosin-based or
halogen-based type flux may be used. For this test, halogen-based
flux was used.
[0033] Table 1 shows the test results of nine solder specimens that
were evaluated. The test results excluded the flux components and
the inevitable impurities. Column 1 lists the solder specimens
indicated by numbers S1 through S6, and CS21 through CS23. The
remaining columns indicate the test results, which are explained
below in conjunction with FIG. 2. The numerical values in the table
1 represent percent by mass in relation to the composition. Test
specimens S1 through S6 are solders where the large percentage by
mass is tin, 3.5% of silver by mass, and remaining balance of
iron-family of elements such as iron, nickel, and cobalt. For
instance, specimen S1 is a solder containing 3.5% of silver and
0.02% of iron, with the rest being tin (hereafter referred to as
Sn-3.5 Ag 0.02 Fe). Similarly, test specimen S2 hereinafter
referred to as Sn-3.5 Ag-0.05 Fe; test specimen S3 hereinafter
referred to as Sn-3.5 Ag-0.1 Ni; test specimen S4 hereafter
referred to as Sn-3.5 Ag-0.1 Ni-0.05 Fe; test specimen 5 hereafter
referred to as Sn-3.5 Ag-0.5 Co; and test specimen S6 hereafter
referred to as Sn-3.5 Ag-1.0 Co.
[0034] Test specimens CS21 through CS23 are reference specimens
used for the comparison purposes. Reference test piece CS21 is
eutectic tin-lead solder that contains tin and 37% of lead by mass.
Reference test pieces CS22 and CS23 are lead-free solder that have
been used. CS22 is Sn-3.5 Ag, and CS23 is Sn-3.5 Ag-0.75 Cu.
1 Specific Char- Sample acter No. No. Sn Pb Ag Cu Fe Ni Co S1 101
remainder 3.5 0.02 S2 102 remainder 3.5 0.05 S3 103 remainder 3.5
0.1 S4 104 remainder 3.5 0.05 0.1 S5 105 remainder 3.5 0.5 S6 106
remainder 3.5 1 CS21 (for 201 remainder 37 comparison) CS22 (for
202 remainder 3.5 comparison) CS23 (for 203 remainder 3.5 0.75
comparison)
[0035] Referring back to FIGS 1(a) and 1(b), for testing
conditions, the temperature of the test specimen 3 was set at
300.degree. C., 350-400.degree. C. (at intervals of 10.degree. C.),
425.degree. C. and 450.degree. C. The solder specimen 5 was fed 5
mm every 3 seconds towards the test specimen 3. This was done up to
5,000 times. When solder specimen 5 was fed 6 or 7 times to the
test specimen 3, the melted solder dropped spontaneously.
[0036] After the solder specimen 5 was fed 5,000 times, the test
specimen 3 was cut along the central line in a coaxial direction.
The corrosion amounts of iron-plated layer 12 at the upper,
central, and lower parts of iron-exposed area 13 were measured. The
maximum was assumed as the measured value. FIG. 2 shows the
results, where the horizontal axis shows the temperature (.degree.
C.) of the test specimen 3, and the vertical axis represents the
corrosion amount (.mu.m). The results of reference specimens CS21,
CS22 and CS23 are shown as broken lines 201, 202 and 203,
respectively.
[0037] The broken lines 202 and 203 corresponding to the specimens
CS22 (Sn-3.5 Ag) and CS 23 (Sn-3.5 Ag-0.75 Cu), respectively, are
conventional lead-free solder, which showed 3 to 5 times more
corrosion occurring than the broken line 201 (Sn-37Pb) for the
reference specimen CS21. In contrast, lines 105 and 106
corresponding to the test specimens S5 (Sn-3.5 Ag-0.5 Co) and S6
(Sn-3.5 Ag-1.0 Co), respectively, where cobalt was added, showed
almost similar corrosion amounts as compared to the line 201.
Accordingly, this test result demonstrates that the lead-free
solder added with cobalt improves the resistance to corrosion.
[0038] Silver was added to the reference specimens CS21 through
CS23 for mechanical strength of the solder composition, and
therefore, excluded from the evaluation of the corrosion
resistance. It was, however, noted that silver had an effect of
improving the mechanical strength and wettability.
[0039] Lines 101 and 102 corresponding to the specimens S1 (Sn-3.5
Ag-0.02 Fe) and S2 (Sn-3.5 Ag-0.05 Fe), respectively, added with
iron showed larger corrosion amounts than the line 201. However,
these corrosion amounts were smaller than characteristics 202 and
203. Accordingly, adding iron to the lead-free solder improves the
corrosion resistance.
[0040] Line 103 corresponding to the specimen S3 (Sn-3.5 Ag-0.1 Ni)
added with nickel showed a larger corrosion amount than the lines
202 and 203. However, it was noted that adding nickel improved
wettability. Line 104 corresponding to the test specimen S4 (Sn-3.5
Ag-0.1 Ni-0.05 Fe) added combination of nickel and iron. This
combination improved corrosion resistance as compared to the line
103. In particular, at 400.degree. C. or lower, the line 104
resulted in better corrosion resistance than the lines 202 and 203.
In general, the operating temperature of the soldering iron tip,
which is used for bonding electronic parts, is 350.degree. C. to
400.degree. C. When the solder is used for such an application, a
practical effect of improving the corrosion resistance can be
obtained. Cobalt may be used instead of iron to be combined with
nickel. In this case, as illustrated by lines 105 and 106, a level
equivalent to or better corrosion resistance as shown by line 104
may be expected. Accordingly, the addition of nickel plus iron or
nickel plus cobalt to the solder composition improves the
wettability, as well as the corrosion resistance within the range
of operation temperatures such as below 400.degree. C. The total
content of iron, nickel, and cobalt in the solder composition may
be between about 0.01% and 1.0% by mass, and in particular between
about 0.01% and 0.7% by mass.
[0041] Second Test:
[0042] In the second test, a corrosion test was conducted as
follows: the solder composition was melted and stored in the solder
tank, a round iron bar (simulating the dipping solder tank) was
dipped in the tank and the average corrosion amount was
measured.
[0043] Table 2 shows the test results of four solder specimens that
were evaluated in this test. The first column in table 2 lists the
four specimens S11, S12, CS31, and CS32. S11 contains tin, 3.5% of
silver, and 0.023% of iron by mass of the solder composition. S12
contains tin, 3.5% of silver, and 0.016% of iron by mass. CS31 and
CS32 were also tested as references for comparison purposes.
Reference piece CS31 is the eutectic tin-lead solder that contains
tin and 37% of lead by mass. Reference piece CS32 is typical
lead-free solder, which contains tin and 3.5% of silver by
mass.
2 Sample No. Sn Pb Ag Fe S11 remainder 3.5 0.023 S12 remainder 3.5
0.016 CS31 (for remainder 37 comparison) CS32 (for remainder 3.5
comparison)
[0044] For testing conditions, the temperature of the four solder
specimens S11 and S12, and reference pieces CS31 and CS32 were set
at 350.degree. C., 400.degree. C., and 450.degree. C.; and the
dipping duration of the round iron bar at 2, 4, 6, and 8 hours.
Table 3 shows the test results. The first column indicates the data
numbers (D1 through D7), the second column indicates the specimens,
the third column indicates the dipping temperature (.degree. C.),
and the fourth column indicates the duration (h) in hours for test
conditions, and the fifth column list the corrosion amount (.mu.m)
of the round iron bar after dipping. For example, Data 1 of table 3
shows that when the round iron bar of test piece S11 was dipped for
4 hours at 400.degree. C., its corrosion amount was 2 .mu.m. Data
D1 through D7 similarly show the respective results.
3 Temperature for Time for Erosion Saturation Saturation Amount
Data No. Sample (submerge) .degree. C. (submerge) h .mu.m D1 S11
400 4 2 D2 S12 350 6 3 D3 CS31 (for 450 2 8 D4 comparison) 450 8 60
D5 CS32 (for 450 2 30 D6 comparison) 400 4 25 D7 350 6 15
[0045] The comparison between the data D3 and D4 reveals that the
corrosion amount increased as the reference piece CS31 was dipped
for a longer time, e.g. from 8 hours to 60 hours. The comparison
between the data D3 and D5 demonstrates that the corrosion amount
of the conventional lead-free solder (tin and 3.5 % of silver by
mass) was 3 times higher than that of the eutectic tin-lead solder.
The comparison between the data D5 through D7 demonstrate that the
dipping duration was increased from 2 to 4 and 6 hours, but the
temperature was lower from 450.degree. C. to 400.degree. C. and
350.degree. C., respectively. The result shows that corrosion
amount decreased from 30 to 25 and 15 .mu.m, respectively,
indicating that the dipping temperature was influential with regard
to corrosion on the specimen within the above dipping temperature
and duration ranges.
[0046] The comparison between the data D1 and D6 demonstrates that
for same testing conditions in terms of temperature and dipping
duration (400.degree. C. for 4 hours), the corrosion amount reduced
from (2 .mu.m) to (25 .mu.m), respectively. Similarly, the
comparison between the data D2 and D7 demonstrates that the
corrosion amount reduced from (3 .mu.m) to (15 .mu.m),
respectively, for the same testing conditions (350.degree. C. for 6
hours). These comparisons indicate that the addition of iron to the
solder composition that is substantially free lead decreased the
corrosion amount.
[0047] The comparison between the data D1 and D2 indicates that
between temperature range of 350.degree. C. and 400.degree. C., the
corrosion amount of the lead-free solder added with iron was less
affected.
[0048] The above test results indicate that adding iron to the
solder composition inhibits melted solder from corroding the walls
of the dipping solder tank and related equipment. As noted from the
first test, cobalt may be used instead of iron, or the combination
of iron and cobalt may be used to inhibit corrosion from occurring.
Alternatively, the solder composition may be modified to minimize
corrosion from occurring such as: (1) about 0.2% and 5% of copper
by mass; or (2) about 0.1% and 5.0% of bismuth by mass.
[0049] The solder composition according to this invention is not
limited to the elements or the percentage of mass for the elements
used above for the purpose of the two tests. For example, the
solder composition may be modified to incorporate copper between
about 0.2% and about 5.0% by mass. The solder composition may also
incorporate bismuth between about 0.1% and about 5% by mass.
[0050] In addition, other combination of elements not tested in
table 1 may be incorporated into the soldering composition. For
instance, the solder composition may include tin as its main
element along with nickel, cobalt, and iron, where nickel and
cobalt may each make up between 0.01% and 1.0% by mass with the
total mass of iron, nickel, and cobalt being equal to or less than
1.0% by mass of the solder composition. Accordingly, alternative
elements and different combination of elements may be incorporated
into the solder composition to improve the mechanical strength and
wettability of solder while inhibiting the tip of the soldering
iron and the dipping solder tank from corroding.
* * * * *