U.S. patent application number 10/046781 was filed with the patent office on 2003-01-02 for tin-silver soldering alloy.
This patent application is currently assigned to MITSUI MINING & SMELTING COMPANY, LTD.. Invention is credited to Nakahara, Yuunosuke, Ninomiya, Ryuuji.
Application Number | 20030003012 10/046781 |
Document ID | / |
Family ID | 18934471 |
Filed Date | 2003-01-02 |
United States Patent
Application |
20030003012 |
Kind Code |
A1 |
Nakahara, Yuunosuke ; et
al. |
January 2, 2003 |
Tin-silver soldering alloy
Abstract
A tin--silver soldering alloy comprising 3 to 4% by weight of
silver, 5 to 10% by weight of bismuth, 0 to 5% by weight of indium,
and 0.1 to 1.5% by weight of zinc, with the balance being tin.
Inventors: |
Nakahara, Yuunosuke;
(Saitama, JP) ; Ninomiya, Ryuuji; (Saitama,
JP) |
Correspondence
Address: |
YOUNG & THOMPSON
745 SOUTH 23RD STREET 2ND FLOOR
ARLINGTON
VA
22202
|
Assignee: |
MITSUI MINING & SMELTING
COMPANY, LTD.
TOKYO
JP
|
Family ID: |
18934471 |
Appl. No.: |
10/046781 |
Filed: |
January 17, 2002 |
Current U.S.
Class: |
420/562 |
Current CPC
Class: |
B23K 35/262 20130101;
C22C 13/02 20130101 |
Class at
Publication: |
420/562 |
International
Class: |
C22C 013/02 |
Foreign Application Data
Date |
Code |
Application Number |
Mar 19, 2001 |
JP |
2001-77769 |
Claims
What is claimed is:
1. A tin--silver soldering alloy comprising 3 to 4% by weight of
silver, 5 to 10% by weight of bismuth, and 0.1 to 1.5% by weight of
zinc, with the balance being tin.
2. A tin--silver soldering alloy comprising 3 to 4% by weight of
silver, 5 to 10% by weight of bismuth, 5% by weight or less of
indium, and 0.1 to 1.5% by weight of zinc, with the balance being
tin.
Description
BACKGROUND OF THE INVENTION
[0001] The present invention relates to a tin--silver soldering
alloy and, more particularly, to a tin--silver soldering alloy
having satisfactory elongation characteristics to secure high joint
reliability.
[0002] Eutectic or nearly eutectic Pb--Sn alloys are known as
typical soft solders. Zn--Cd alloys having higher strength than
eutectic Pb--Sn alloys are also known. These known soldering alloys
are problematical due to the harm of lead or the adverse influence
of cadmium vapor on workers and unacceptable from the standpoint of
recent environmental issues.
[0003] Various Sn--Ag soldering alloys have been proposed as
leadless solder free of harmful Pb, Cd, etc. However, because the
Sn--Ag alloys have a higher melting point than the currently
prevailing Pb--Sn alloys (melting point: 183.degree. C. with an Sn
content of 37 wt %), they need an increased soldering temperature,
which may involve thermal influences on parts. Therefore, to lower
the melting point of Sn--Ag solder has been of great concern.
[0004] The melting point of Sn--Ag solder can be lowered by
addition of a third element or third and fourth elements. Indium,
bismuth and copper are usually added for their abundance or
producibility. For example, an Sn--Ag--Cu solder can have its
melting point reduced to about 217.degree. C. at the lowest and
requires addition of In and Bi for further reduction. However, use
of In is limited from economical considerations. Addition of Bi is
effective in lowering the melting point and improving tensile
strength but, in turn, reduces elongation at break. Seeing that the
elongation characteristics of solder make a great contribution to
reliability of solder joints, addition of Bi is considered to
reduce the joint reliability. It has also been pointed out that
solder having a Bi content of 5% by weight or more undergoes
appreciable reduction in strength on some kinds of plating
materials of electronic board lands and electrode materials of
electronic components. Reduction in joint reliability caused by Bi
addition has now come to be a problem to be solved and, on the
other hand, there still has been a demand for solder having a
melting point around 205.degree. C. for application to electronic
components.
[0005] Accordingly, it has been desired to develop Bi-containing
solder having a Bi content of 5% by weight or more and yet
promising high joint reliability. It seems that high joint
reliability of solder will be obtained through stability of the
solder structure, slow growth of the reactive layer on the joint
interface, satisfactory elongation characteristics of the soldering
alloy, and the like.
SUMMARY OF THE INVENTION
[0006] An object of the present invention is to provide a
tin--silver soldering alloy having satisfactory elongation
characteristics to secure high joint reliability.
[0007] As a result of investigations, the present inventors have
found that the interface of a tin--silver soldering alloy having a
given bismuth content or a given bismuth/indium content can be
modified so as to suppress growth of the reactive layer by adding a
specific amount of zinc that diffuses in copper easily. The object
of the invention is accomplished based on this finding.
[0008] The present invention provides a tin--silver soldering alloy
comprising 3 to 4% by weight of silver, 5 to 10% by weight of
bismuth, and 0.1 to 1.5% by weight of zinc, with the balance being
tin.
[0009] The present invention also provides a tin--silver soldering
alloy comprising 3 to 4% by weight of silver, 5 to 10% by weight of
bismuth, 5% by weight or less of indium, and 0.1 to 1.5% by weight
of zinc, with the balance being tin.
[0010] The tin--silver soldering alloy according to the present
invention exhibits satisfactory elongation characteristics and
thereby secures high joint reliability.
BRIEF DESCRIPTION OF THE DRAWINGS
[0011] The present invention will be more particularly described
with reference to the accompanying drawings, in which:
[0012] FIG. 1 schematically illustrates the method of measuring
joint strength.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0013] The tin--silver soldering alloy of the present invention has
an Ag content of 3 to 4% by weight. While the optimum Ag content is
3.5% by weight, the above range is acceptable from considerations
to production yield in solder preparation.
[0014] The Bi content ranges from 5 to 10% by weight. A Bi content
lower than 5% results in an increased melting point and reduced
tensile strength. A Bi content higher than 10% results in reduced
joint reliability.
[0015] Where In is also incorporated, its content is 5% by weight
or lower. An In content exceeding 5% is not favorable from the
viewpoint of cost performance.
[0016] The tin--silver soldering alloy of the present invention is
characterized by having a zinc content of 0.1 to 1.5% by weight.
Addition of zinc brings about improved joint reliability. A zinc
content less than 0.1% or more than 1.5% fails to produce the
effect of improving joint reliability. The tin--silver soldering
alloy of the present invention thus exhibits improved joint
reliability owing to the specific zinc content.
[0017] The present invention will now be illustrated in greater
detail with reference to Examples. In tables 1 and 2, the figures
of the alloy compositions indicate the content of the respective
following elements in percent by weight.
EXAMPLES 1 TO 5 AND COMPARATIVE EXAMPLES 1 AND 2
[0018] Raw materials were weighed out to give the compositions
shown in Table 1 (the balance was Sn) totally weighing 10 kg. The
mixture was put in a graphite crucible and melted at 300.degree. C.
in the air in an electric oven. After complete melting, the melt
was stirred thoroughly to avoid gravity segregation.
[0019] For testing the tinsilver soldering alloy thus prepared, two
copper plates 10 mm wide, 30 mm long and 1 mm thick were prepared.
As shown in FIG. 1, the solder was applied to a 5 mm wide and 10 mm
long tip area of each copper plate, and the tips were
overlap-joined to make a test specimen. The test specimen was
pulled in the direction shown in FIG. 1 on an Instron tensile
tester to measure a joint strength. The measurement was made
immediately after joining (0 hr) and after 1000 hours each at
100.degree. C., and a deterioration rate (%) was calculated. The
results obtained are shown in Table 1.
1 TABLE 1 Joint Strength (MPa) Deterioration Alloy Composition 0 hr
1000 hrs Rate (%) Example 1 3Ag--5Bi--0.1Zn 34.37 31.67 8 Example 2
3Ag--5Bi--0.5Zn 32.52 30.43 6 Example 3 3Ag--5Bi--1Zn 32.2 32.02 1
Example 4 3Ag--5Bi--1.5Zn 32.24 31.22 3 Example 5
3.5Ag--3In--6Bi--1Zn 32.23 33.21 -3 Comparative 3Ag--5Bi 35.82 29.6
17 Example 1 Comparative 3.5Ag--3In--6Bi 37.04 29.6 20 Example
2
[0020] As is apparent from the results in Table 1, incorporation of
a given amount of zinc reduces the deterioration rate of joint
strength (Examples 1 to 5), whereas the solder containing no zinc
(Comparative Examples 1 and 2) undergoes considerable deterioration
in strength.
[0021] After the tensile testing, the cut area of the specimen was
polished, and the joint interface of the solder was observed under
an energy dispersive scanning electron microscope (SEM) to measure
the thickness of the reactive layer on the interface at 10 points
of the SEM micrograph. The average reactive layer thickness is
shown in Table 2.
2 TABLE 2 Reactive Layer Thickness (.mu.m) Alloy Composition 0 hr
1000 hrs Example 1 3Ag--5Bi--0.1Zn 1.6 2.44 Example 2
3Ag--5Bi--0.5Zn 2.03 2.13 Example 3 3Ag--5Bi--1Zn 1.16 1.66 Example
4 3Ag--5Bi--1.5Zn 1.03 1.92 Example 5 3.5Ag--3In--6Bi--1Zn 1.11 1.3
Comparative 3Ag--5Bi 2.31 2.93 Example 1 Comparative
3.5Ag--3In--6Bi 2.57 3.51 Example 2
[0022] As can be seen from Table 2, the solder formulae of Examples
1 to 5 are generally slow in growth of the interfacial reactive
layer as compared with the solder of Comparative Examples 1 and
2.
[0023] Where solder contains no zinc (Comparative Examples 1 and
2), a Cu--Sn reactive layer is usually formed to make a Cu/Cu--Sn
reactive layer/solder structure in the joint interface. It was
confirmed that existence of zinc in the solder provided a Cu/Cu--Sn
reactive layer/Cu--Zn reactive layer/solder structure in the
interface. It is considered that the Cu--Zn reactive layer
suppresses the growth of the Cu--Sn reactive layer. It is noted
that the solder having a zinc content of 1% by weight (Examples 3
and 5), while showing a slight growth of the reactive layer after
reflow, stably maintained the initial interfacial structure.
* * * * *