U.S. patent application number 10/244477 was filed with the patent office on 2003-01-30 for lead-free solder alloys.
This patent application is currently assigned to Senju Metal Industry Co., Ltd.. Invention is credited to Katoh, Rikiya, Taguchi, Toshihiko, Toyoda, Yoshitaka.
Application Number | 20030021719 10/244477 |
Document ID | / |
Family ID | 24232133 |
Filed Date | 2003-01-30 |
United States Patent
Application |
20030021719 |
Kind Code |
A1 |
Taguchi, Toshihiko ; et
al. |
January 30, 2003 |
Lead-free solder alloys
Abstract
A lead-free solder alloy which has a relatively low melting
temperature and which is suitable for soldering electronic devices
consists essentially of from 5 to 9 mass % of Zn, from 2 to 15 mass
% of Bi, optionally from 0.001 to 1 mass % of P or from 0.001 to
0.1 mass % of Ge, and a balance of Sn. The solder alloy has a
liquidus temperature of at most 220.degree. C.
Inventors: |
Taguchi, Toshihiko;
(Kitakatsushika-gun, JP) ; Katoh, Rikiya;
(Sohka-shi, JP) ; Toyoda, Yoshitaka; (Satte-shi,
JP) |
Correspondence
Address: |
Michael Tobias
#40
1717 K Street, N.W., Suite 613
Washington
DC
20036
US
|
Assignee: |
Senju Metal Industry Co.,
Ltd.
|
Family ID: |
24232133 |
Appl. No.: |
10/244477 |
Filed: |
September 17, 2002 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
10244477 |
Sep 17, 2002 |
|
|
|
09559062 |
Apr 28, 2000 |
|
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Current U.S.
Class: |
420/562 |
Current CPC
Class: |
B23K 35/262
20130101 |
Class at
Publication: |
420/562 |
International
Class: |
C22C 013/02 |
Claims
What is claimed is:
1. A lead-free solder alloy for use in soldering electronic devices
to a printed circuit board, which consists essentially of: from 5
to 9 mass % of Zn, from 2 to 15 mass % of Bi, and a balance of
Sn.
2. A lead-free solder alloy as set forth in claim 1 wherein the
total content of Zn and Bi is at most 18%.
3. A lead-free solder alloy as set forth in claim 1 wherein 5-8% of
Zn is contained.
4. A lead-free solder alloy as set forth in claim 1 containing
5-10% of Bi.
5. A lead-free solder alloy as set forth in claim 1 having a
liquidus temperature of at most 210.degree. C.
6. A lead-free solder alloy as set forth in claim 1 having a
solidus temperature of at least 160.degree. C.
7. A lead-free solder alloy for use in soldering electronic devices
to a printed circuit board, which consists essentially of: from 5
to 9 mass % of Zn, from 2 to 15 mass % of Bi, from 0.001 to 1 mass
% of P and/or from 0.001 to 1 mass % of Ge, and a balance of
Sn.
8. A lead-free solder alloy as set forth in claim 7 wherein the
total content of Zn and Bi is at most 18%.
9. A lead-free solder alloy as set forth in claim 7 wherein 5-8% of
Zn is contained.
10. A lead-free solder alloy as set forth in claim 7 wherein 5-10%
of Bi is contained.
11. A lead-free solder alloy as set forth in claim 7 having a
liquidus temperature of at most 210.degree. C.
12. A lead-free solder alloy as set forth in claim 7 having a
solidus temperature of at least 160.degree. C.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates to solder alloys which are
completely free from lead and are suitable for use in soldering of
electronic devices without producing thermal damage.
[0003] 2. Description of the Related Art
[0004] Sn--Pb alloys have long been used for soldering in the
electronics industry, and they are still the most popular alloys
for soldering electronic devices to printed circuit boards or other
substrates.
[0005] When electronic appliances such as televisions, radios,
audio or video recorders, computers, pocket telephones, and copying
or printing machines are to be discarded, they are typically
disposed of in landfills, since they commonly include large amounts
of synthetic resins (used for housings and printed circuit boards)
and metals (used for frames and connecting wires) which are not
suitable for incineration.
[0006] In recent years, acid rain (the phenomenon in which rain
becomes highly acidic due to discharge of sulfur oxide into the
atmosphere by extensive use of fossil fuels such as coals,
gasolines, and fuel (heavy) oils) has become increasingly serious.
Acid rain causes the solders used in discarded electronic
appliances present in landfills to dissolve and contaminate
groundwater. If groundwater contaminated with lead is ingested by a
person for many years, the accumulation of lead in the person's
body may result in lead poisoning. For this reason, there is a need
for a lead-free solder alloy in the electronics industry.
[0007] Conventional lead-free solder alloys are Sn-based alloys
such as Sn--Ag and Sn--Sb alloys. Of Sn--Ag alloys, an Sn-3.5Ag
alloy has a eutectic composition with a melting temperature of
221.degree. C. Even if this composition, which has the lowest
melting temperature among Sn--Ag alloys, is used as a solder alloy,
the soldering temperature will be as high as from 260.degree. C. to
280.degree. C., which may cause thermal damage to heat-sensitive
electronic devices during soldering, thereby deteriorating their
functions or rupturing the devices. Of Sn--Sb alloys, an Sn-5Sb
alloy has the lowest melting temperature, but its melting
temperature is still as high as 235.degree. C. at the solidus line
and 240.degree. C. at the liquidus line. Therefore, the soldering
temperature is in the range of from 280.degree. C. to 300.degree.
C., which is higher than that of an Sn-3.5Ag alloy, and thermal
damage to heat-sensitive electronic devices cannot be avoided.
[0008] In view of the relatively high melting temperatures of
Sn--Ag and Sn--Sb alloys, many attempts to lower their melting
temperatures have been proposed. See, for example, Japanese Patent
Applications Laid-Open (JP A1) Nos. 6-15476(1994), 6-344180(1994),
7-1178(1995), 7-40079(1995), and 7-51883(1995).
[0009] The solder alloys disclosed in these Japanese patent
applications contain a large proportion of Bi and/or In (indium) in
order to lower their melting temperatures. Although Bi and In are
both effective for decreasing the melting temperatures of Sn--Ag
and Sn--Sb solder alloys, the addition of Bi and/or In in a large
amount is accompanied by a number of problems. Addition of Bi in a
large proportion makes the solder alloys very hard and brittle. As
a result, it is impossible or difficult to subject the solder
alloys to plastic working to form wire, and when the solder alloys
are used to solder electronic devices, the soldered joints may be
readily detached when subjected to only a slight impact. Addition
of indium in a large proportion to solder alloys is undesirable due
to its very high cost.
[0010] In order to avoid thermal damage to electronic devices
during soldering, the soldering temperature should generally be at
most 250.degree. C. In order to perform soldering at a temperature
of at most 250.degree. C., it is desirable that the liquidus
temperature of the solder alloy be at most 220.degree. C. and
preferably at most 200.degree. C.
[0011] However, when attempting to lower the melting temperatures
of Sn--Ag and Sn--Sb solder alloys by addition of Bi and/or In, it
is difficult to decrease the liquidus temperature of the alloys to
200.degree. C. or below unless Bi and/or In is added in a large
amount. Furthermore, even though it is possible to provide a solder
alloy having a liquidus temperature lowered to 200.degree. C. or
less by addition of Bi and/or In, the solidus temperature thereof,
at which solidification of the alloy is completed, may be
excessively lowered, so that it takes a prolonged period of time to
completely solidify the solder alloy in soldered joints formed by
soldering. As a result, if the soldered joints are subjected to any
vibration or impact before they are completely solidified, they may
crack.
[0012] Another problem of conventional lead-free solder alloys is
that those lead-free alloys having liquidus temperatures which are
low enough to be close to their solidus temperatures do not have
satisfactory mechanical properties such as tensile strength and
elongation, thereby forming soldered joints which have poor bonding
strength or which are liable to be detached upon impact.
SUMMARY OF THE INVENTION
[0013] It is an object of the present invention to provide
lead-free solder alloys having a liquidus temperature which is less
than 220.degree. C. and preferably less than 200.degree. C. and a
solidus temperature, at which solidification of the alloy is
completed or substantially completed, which is 160.degree. C. or
higher, and preferably 170.degree. C. or higher.
[0014] It is another object of the present invention to provide
lead-free solder alloys which have good bonding strength when used
for soldering.
[0015] A more specific object of the present invention is to
provide lead-free solder alloys having the following
properties.
[0016] 1) The alloys can be used at a soldering temperature below
250.degree. C. and preferably from 230.degree. C. to 240.degree. C.
so as to prevent thermal damage to heat-sensitive electronic
devices during soldering.
[0017] 2) The alloys have excellent solderability.
[0018] 3) The alloys have a narrow solidification temperature range
between the liquidus and solidus temperatures such that the alloys
are rapidly solidified after soldering in order to prevent the
resulting soldered joints from cracking when subjected to vibration
or an impact immediately after soldering, the temperature range
being close to the eutectic temperature of an Sn--Pb alloy
(183.degree. C.).
[0019] 4) The alloys produce soldered joints having a bonding
strength which is high enough to prevent the joints from being
detached when subjected to an impact.
[0020] 5) The alloys can be easily subjected to plastic working to
form wire so that the alloys can be used for soldering with a
soldering iron.
[0021] The present inventors found that alloys consisting
essentially of Zn, Bi, optionally one of P and Ge, and a balance of
Sn in specific proportions can provide solder alloys having a low
liquidus temperature which enables dip soldering to be performed in
a temperature range at which electronic components being soldered
will not undergo thermal damage. Furthermore, it was found that by
appropriately adjusting the proportions of Zn and Bi, the solidus
temperatures of the alloys can be close to their liquidus
temperatures, thereby enabling molten solder to rapidly solidify
following soldering to avoid problems such as cracking or
detachment of soldered joints. In addition, the alloys have a
tensile strength and ductility which enables them to be plastically
formed into wire suitable for use with a soldering iron. Thus,
these solder alloys can be satisfactorily used in place of
conventional Sn--Pb alloys, and because these solder alloys are
lead free, they prevent contamination of groundwater by lead which
occurs with conventional Sn--Pb alloys.
[0022] According to one aspect of the present invention, a
lead-free solder alloy consists essentially of from 5 to 9 mass %
of Zn, from 2 to 15 mass % of Bi, and a balance of Sn.
[0023] According to another aspect of the present invention, a
lead-free solder alloy consists essentially of from 5 to 9 mass %
of Zn, from 2 to 15 mass % of Bi, P and/or Ge, and a balance of
Sn.
DESCRIPTION OF PREFERRED EMBODIMENTS
[0024] The present invention will now be described in detail. In
the following description, all percents concerning alloy
compositions are by mass.
[0025] The lead-free solder alloys according to the present
invention are intended as a substitute for conventional Sn--Pb
alloys. Therefore, it is desired that the melting temperatures,
i.e., the liquidus and solidus temperatures of these alloys be
close to the eutectic temperature of Sn--Pb alloys (183.degree.
C.). The liquidus temperatures are preferably at most 220.degree.
C., more preferably at most 210.degree. C. and still more
preferably at most 200.degree. C. As long as a solder alloy has a
liquidus temperature below 220.degree. C., soldering can be
performed at a temperature below 250.degree. C., thereby
eliminating or minimizing thermal damage to heat-sensitive
electronic devices. The solidus temperatures of the solder alloys
are preferably above 160.degree. C., more preferably above
170.degree. C., and still more preferably above 180.degree. C. If
the solidus temperatures are lower than 160.degree. C., it takes a
long time for the alloys to solidify after soldering, and if the
resulting soldered joints are subjected to any vibration or impact
before they are completely solidified, they will crack.
[0026] The bonding strength of a soldered joint is correlated to
the tensile strength of the solder alloy used, and the level of
tensile strength required for solder alloys varies depending on the
purposes of soldering. The tensile strength (at break) required for
solder alloys used to solder electronic devices is at least 49 Mpa
(5 kgf/mm.sup.2). A solder alloy having a tensile strength less
than 49 Mpa (5 kgf/mm.sup.2) is not reliable, since soldered joints
formed therefrom may be detached when subjected to impact.
[0027] It is generally desirable for solder alloys to have a
percent elongation which is high enough for the alloys to be
deformed into wire by plastic working in order for the alloys to be
used in the form of wire when soldering is performed with a
soldering iron. In this case, at least 10% elongation is normally
desirable for solder alloys in order to perform plastic working
smoothly.
[0028] The solder alloys according to the present invention meet
these requirements for mechanical properties, i.e., a tensile
strength at break of at least 49 Mpa (5 kgf/mm.sup.2) and an
elongation of at least 10%. Preferably, they have a tensile
strength of at least 10 kgf/mm.sup.2 and/or an elongation of at
least 20%.
[0029] According to one aspect of the present invention, a
lead-free solder alloy consists essentially of from 5 to 9 mass %
of Zn, from 2 to 15 mass % of Bi, optionally one or more of P in an
amount of from 0.001 to 1.0 mass % and Ge in an amount of from
0.001 to 0.1 mass %, and a balance of Sn. The proportions of the
above components were selected for the following reasons.
Zn: 5%-9%
[0030] In an Sn--Zn alloy containing a relatively large amount of
Bi, if the content of Zn (zinc) is less than 5% or more than 9%,
the alloy will not have a liquidus temperature below 200.degree. C.
The Zn content is therefore 5%-9%, more preferably from 5%-8%, and
still more preferably 6.5%-7.5%.
Bi: 2%-15%
[0031] Addition of Bi (bismuth) to an Sn--Zn alloy is effective for
decreasing the melting temperature of the alloy. However, the
melting point of the alloy is not appreciably lowered if Bi is
added in an amount of less than 2%. On the other hand, addition of
more than 15% Bi to an Sn--Zn alloy makes the alloy so hard and
brittle that it is difficult to apply plastic working to deform the
solder alloy into wire. Furthermore, after soldering is completed,
the resulting soldered joints may be readily detached when
subjected to impact. Therefore, the content of Bi in the alloy is
2%-15%, preferably 5%-10%, more preferably 7%-9%, and still more
preferably 7.5%-8.5%.
[0032] In a preferred embodiment, the total content of Zn and Bi is
18% or less. More preferably, the total content of Zn and Bi is 15%
or less.
P: 0.001%-1.0%
[0033] The addition of P (phosphorus) to an Sn--Zn--Bi alloy is
effective for preventing oxidation during heating.
Ge: 0.001%-0.1%
[0034] The addition of Ge (germanium) to an Sn--Zn--Bi alloy is
also effective for preventing oxidation during heating.
[0035] The present invention will now be further illustrated by the
following examples, which are to be considered in all respects as
illustrative and not restrictive.
[0036] Molten solder alloys having the compositions shown in the
following table were cast into tensile test rods each having a
central neck portion measuring 50 mm in length and 10 mm in
diameter according to JIS specifications. The tensile test rods
were used to determine the tensile strength and elongation at break
also shown in the table of each solder alloy. The table further
includes the solidus and liquidus temperatures of each solder alloy
determined by differential thermal analysis.
[0037] In the table, alloys 2, 3, 6, 7, 16, and 17 are alloys
according to the present invention, while the remaining alloys are
comparative examples.
[0038] A molten solder alloy of Alloy No. 2 was poured into a
solder bath of an automatic dip soldering apparatus and used to
solder electronic devices to printed circuit boards while the
temperature of the molten solder alloy was maintained at
240.degree. C. Visual inspection of the soldered electronic devices
on the printed circuit boards showed no signs of thermal damage or
deterioration. The other solder alloys according to the present
invention in the table can be used for soldering in the same
manner. Since all the solder alloys according to the present
invention shown in the table have liquidus temperatures of at most
220.degree. C., dip soldering can be performed at a temperature of
molten solder alloy (soldering temperature) of 250.degree. C. or
below. Therefore, thermal damage to electronic devices can be
eliminated or minimized. Also it is noted that the solidus
temperatures of these alloys are all at least 160.degree. C.,
giving narrow solidification temperature ranges, thereby ensuring
that the molten solder alloy in soldered joints is rapidly
solidified in a short period after soldering and the formation of
cracks is minimized.
1 Melting Alloy Temperature (.degree. C.) Tensile Elonga- Alloy
Composition (mass %) Solidus Liquidus Strength tion No. Sn. Zn Bi
Other Temp. Temp. (MPa) (%) 1 bal. 9 -- -- 199 199 65.7 64 2 bal. 8
3 -- 188 199 84.3 40 3 bal. 6 8 -- 175 196 106.9 10 4 bal. 2 11 --
166 215 104.9 20 5 bal. 4 11 -- 166 202 103.0 16 6 bal. 6 11 -- 165
197 106.9 10 7 bal. 8 11 -- 168 218 113.8 10 8 bal. 2 16 -- 135 206
104.9 14 9 bal. 4 16 -- 135 196 103.0 13 10 bal. 6 16 -- 135 192
106.9 14 11 bal. 8 16 -- 133 233 111.8 10 12 bal. 2 22 -- 136 200
99.0 20 13 bal. 4 22 -- 135 189 98.1 18 14 bal. 6 22 -- 134 211
96.1 5 15 bal. 8 22 -- 135 272 114.7 9 16 bal. 8 3 P0.01 188 199
85.3 38 17 bal. 8 3 Ge0.05 189 199 81.0 43
[0039] All the solder alloys according to the present invention in
the table had a tensile strength of at least 5 kgf/mm.sup.2 and at
least 10% elongation. Thus, these solder alloys can form soldered
joints with sufficient bonding strength which will not be dislodged
upon impact, and the alloys can be satisfactorily subjected to
plastic working to form wire.
[0040] In the comparative examples having a Bi content of greater
than 15%, the solidus temperatures of the alloys were far below the
liquidus temperatures, making it difficult to solidify these alloys
after soldering without problems such as the formation of cracks or
the dislodging of soldered joints.
[0041] It will be appreciated by those skilled in the art that
numerous variations and modifications may be made to the invention
as described above with respect to specific embodiments without
departing from the spirit or scope of the invention as broadly
described.
* * * * *