U.S. patent application number 09/930245 was filed with the patent office on 2002-02-14 for soldering method and soldered joint.
This patent application is currently assigned to Fujitsu Limited. Invention is credited to Fujioka, Motoko, Kitajima, Masayuki, Shono, Tadaaki, Takesue, Masakazu.
Application Number | 20020018844 09/930245 |
Document ID | / |
Family ID | 26344891 |
Filed Date | 2002-02-14 |
United States Patent
Application |
20020018844 |
Kind Code |
A1 |
Kitajima, Masayuki ; et
al. |
February 14, 2002 |
Soldering method and soldered joint
Abstract
The present invention provides a soldering method and a soldered
joint securing a strength of joint equivalent to soldering using a
conventional Pb--Sn solder alloy without having a detrimental
effect on the environment and without causing a rise in cost. A
soldering method comprising a step of covering Cu electrodes of
electronic equipment by a rust-proofing coating consisting of an
organic compound including N and a step of forming soldered joints
on the covered Cu electrodes, by using a solder material consisting
of at least 2.0 wt % and less than 3 wt % of Ag, 0.5 to 0.8 wt % of
Cu, and a balance of Sn and unavoidable impurities. The solder
material used in the present invention further contains not more
than 3 wt % in total of at least one element selected from the
group consisting of Sb, In, Au, Zn, Bi, and Al.
Inventors: |
Kitajima, Masayuki;
(Kawasaki, JP) ; Takesue, Masakazu; (Kawasaki,
JP) ; Shono, Tadaaki; (Kawasaki, JP) ;
Fujioka, Motoko; (Kawasaki, JP) |
Correspondence
Address: |
ARMSTRONG,WESTERMAN, HATTORI,
MCLELAND & NAUGHTON, LLP
1725 K STREET, NW, SUITE 1000
WASHINGTON
DC
20006
US
|
Assignee: |
Fujitsu Limited
Kawasaki
JP
|
Family ID: |
26344891 |
Appl. No.: |
09/930245 |
Filed: |
August 16, 2001 |
Related U.S. Patent Documents
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
|
|
09930245 |
Aug 16, 2001 |
|
|
|
PCT/JP00/06389 |
Sep 19, 2000 |
|
|
|
Current U.S.
Class: |
427/58 ;
228/262.6; 228/262.61 |
Current CPC
Class: |
B23K 1/20 20130101; H05K
3/3463 20130101; B23K 1/002 20130101; H05K 3/282 20130101; B23K
35/262 20130101; Y10T 428/12903 20150115 |
Class at
Publication: |
427/58 ;
228/262.6; 228/262.61 |
International
Class: |
B05D 005/12; B23K
020/16; B23K 035/24; B23K 035/36 |
Foreign Application Data
Date |
Code |
Application Number |
Apr 17, 2000 |
JP |
PCT/JP00/02491 |
Claims
1. A soldering method characterized by comprising the following
steps of: covering Cu electrodes of electronic equipment by a
rust-proofing coating consisting of an organic compound including N
and forming soldered joints on the covered Cu electrodes, by using
a solder material consisting of at least 2.0 wt % and less than 3
wt % of Ag, 0.5 to 0.8 wt % of Cu, and a balance of Sn and
unavoidable impurities.
2. A soldering method as set forth in claim 1, characterized in
that the solder material contains 2.0 to 2.5 wt % of Ag.
3. A soldering method as set forth in claim 1, characterized in
that the solder material contains at least 2.5 wt % and less than
3.0 wt % of Ag.
4. A soldering method as set forth in any one of claims 1 to 3,
characterized in that the solder material further contains not more
than 3 wt % in total of at least one element selected from the
group consisting of Sb, In, Au, Zn, Bi, and Al.
5. A soldered joint formed by a soldering method as set forth in
any one of claims 1 to 4.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is based upon and claims priority of
International Application PCT/JP00/02491, filed on Apr. 17, 2000,
the contents being incorporated herein by reference, and a
continuation of PCT/JP00/06389.
TECHNICAL FIELD
[0002] The present invention relates to a method of soldering
electronic equipment by a lead-free solder material and a soldered
joint formed by the same.
BACKGROUND ART
[0003] Up until now, broad use has been made of lead-tin (Pb--Sn)
solder alloys for soldering various types of electrical and
electronic equipment from the viewpoint of their low melting points
and good wettability even in oxidizing atmospheres.
[0004] Pb has toxicity, so various restrictions are placed on the
handling of Pb and alloys and other materials containing Pb.
[0005] Further, the recent growing interest in protecting the
environment has been accompanied by tougher regulations on disposal
of electronic equipment and other waste using Pb-containing
alloys.
[0006] In the past, scrap electronic equipment using large amounts
of Pb-containing solder alloy was generally mainly disposed by
burial in the same way as ordinary industrial waste or general
waste.
[0007] If scrap electronic equipment using large amounts of
Pb-containing solder continues to be disposed of by burial as at
the present, the elution of the Pb is liable to have a detrimental
effect on the environment and living organisms.
[0008] In the near future, disposal of scrap electronic equipment
using large amounts of Pb-containing solder alloy only after
reclamation of the Pb will probably become mandatory.
[0009] Up until now, however, no technique has been established for
removing Pb efficiently and effectively from scrap electronic
equipment etc. Further, the cost of reclamation of Pb is liable to
cause a rise in the cost of the products.
[0010] Therefore, there is strong interest in development of a
soldering technique using a lead-free solder material.
[0011] Some lead-free solder materials have been commercialized
such as alloys of Sn with Sb (antimony), Ag (silver), Ge
(germanium), Ti (titanium), etc. added complexly, but these are
limited to special applications. This is because they do not have
the features required in general applications in which used
conventional Pb--Sn solder alloys have been used, that is, the low
melting point and good wettability, reflowability, and the freedom
from reaction with the base material to form a brittle compound
layer or embrittled layer.
[0012] Disclosure of the Invention
[0013] The present invention has as its object the provision of a
soldering method and soldered joint able to ensure a strength of
joint comparable to that of soldering using a conventional Pb--Sn
solder alloy without having a detrimental effect on the environment
and without a rise in the cost.
[0014] The object can be achieved by the a soldering method
characterized by comprising the following steps of:
[0015] covering Cu electrodes of electronic equipment by a
rust-proofing coating consisting of an organic compound including N
and
[0016] forming soldered joints on the covered Cu electrodes, by
using a solder material consisting of at least 2.0 wt % to less
than 3 wt % of Ag, 0.5 to 0.8 wt % of Cu, and a balance of Sn and
unavoidable impurities.
[0017] The solder material used in the present invention may
further contain not more than 3 wt % in total of at least one
element selected from the group consisting of Sb, In, Au, Zn, Bi,
and Al.
[0018] One of the typical applications of the present invention is
a printed circuit board of an electronic device. By covering the Cu
electrodes to be soldered by a rust-proofing coating comprised of
an organic compound including N (nitrogen), long term storability
and solder wettability are ensured.
[0019] In the past, the practice had been to nickel plate the Cu
electrodes, then gold plate them, but this had the defect of a high
cost and further a complicated plating process and therefore a long
manufacturing time. Further, there was the danger of environmental
pollution by the disposal of the waste liquor of the plating.
[0020] In the present invention, by the use of the above
rust-proofing coating, the cost is reduced and the production time
is shortened.
[0021] In the past, for rust-proofing the Cu electrodes, a resin
coating of rosin (natural pine resin), resin (synthetic resin),
etc. had been formed. Since the coating was a thick one of over 20
.mu.m, however, probing during electrical tests became difficult.
Due to this and other reasons, cleaning was necessary after the
soldering.
[0022] On the other hand, the practice has been to use a
water-soluble rust-proofing agent to reduce the thickness of the
coating so as to eliminate the cleaning after the soldering. That
is, the Cu electrodes have been cleaned by etching by a copper
sulfate solution etc., then immersed in a solution containing 1000
to 5000 ppm of a water-soluble rust-proofing agent to form a
coordinate bond coating.
[0023] In the present invention, an extremely thin rust-proofing
coating is formed by coordinate bonds (chelate bonds) by the N in
the organic compound containing N and the metal. The thickness of
the coating is believed to be less than 3000 .ANG..
[0024] As the N-containing organic compound comprising the
rust-proofing coating of the present invention, use is made of
cyclic compounds of the structural formulas shown in FIG. 1 such as
imidazole, benzoimidazole, alkylimidazole, benzotriazole,
mercaptobenzothiazole, pyrrole, thiazole, etc.
[0025] The characteristics required as a solder material are as
follows:
[0026] (1) A high wettability with the base material.
[0027] (2) The ability of soldering at a sufficiently low
temperature so as not to cause heat damage to the electronic
equipment being soldered. That is, a melting point equal to the
melting point of conventional Pb--Sn solder of 456 K (183.degree.
C.).
[0028] (3) Freedom from reaction with the base material to form a
brittle intermetallic compound or embrittled layer.
[0029] (4) The ability to be supplied in a form enabling
application to automation such as a paste, powder, or thread
solder.
[0030] (5) Freedom from poor wettability, voids, bridges, and other
defects due to oxides of the metal ingredients in the solder
material.
[0031] In particular, in soldering electrical equipment, the molten
solder has to be made to flow into narrow clearances, so the
surface tension, viscosity, fluidity, etc. of the solder material
are important.
[0032] The conventional Pb--Sn solder alloys satisfy the above
conditions well, but it has been difficult to avoid environmental
pollution due to Pb.
[0033] The solder material used in the present invention is an
Ag--Cu--Sn alloy consisting of at least 2.0 wt % and less than 3.0
wt % of Ag, 0.5 to 0.8 wt % of Cu, and the balance of substantially
Sn. Since it does not contain Pb and since the alloy ingredients
Ag, Cu, and Sn are all elements with a high safety, there is no
fear of environmental pollution. Further, the above required
characteristics are sufficiently satisfied.
[0034] The reasons for limiting the composition of the Ag--Cu--Sn
solder alloy of the present invention are explained below.
[0035] [Ag: at least 2.0 wt % and less than 3.0 wt %]
[0036] Regarding the most basic characteristic of a solder
material, its melting point, it is necessary to secure a low
melting point (not more than 220.degree. C.) equal to that of the
conventional Pb--Sn solder alloys. If the Ag content is at least
2.0 wt %, a low melting point of not more than 220.degree. C. can
be secured. If the Ag content becomes less than 2.0 wt %, the
melting point suddenly rises. On the other hand, if the Ag content
becomes 3.0 wt % or more, a large amount of needle crystals are
produced, the electronic devices short-circuit with each other, and
the reliability of the joint falls. For applications where it is
particularly necessary to prevent short-circuits due to needle
crystals and watch the reliability of the joint, it is preferable
to further limit the Ag content in the range of the present
invention to not more than 2.5 wt % since the production of needle
crystals can be substantially completely prevented. Conversely, for
applications where it is necessary to particularly keep down the
thickness of the intermetallic compound layer explained later, it
is preferable to further limit the Ag content within the range of
the present invention to not less than 2.5 wt % since the thickness
of the intermetallic compound layer can be further reduced. As the
Ag content for simultaneously satisfying both these conditions, 2.5
wt % is most preferable.
[0037] [Cu: 0.5 to 0.8 wt %]
[0038] The solder alloy and Cu electrodes are joined by the
production of an intermetallic compound at the interface between
the solder alloy and the Cu electrodes. That is, the production of
an intermetallic compound is essential. On the other hand, if the
intermetallic compound layer becomes too thick, it becomes brittle
and the strength of joint falls. Therefore, the intermetallic
compound layer is preferably formed as thin as possible at the time
of joining. It is preferable that it be resistant to growth due to
the heat history after joining. The inventors measured the
thickness of the intermetallic compound layer at the joint
interface at temperatures of up to 150.degree. C., believed to be
the heat history to which electronic equipment is subjected in the
usage environment. As a result, they discovered that when the Ag
content is in the range of the present invention and the Cu content
is in the range of 0.5 to 0.8 wt %, the thickness of the
intermetallic compound layer stabilizes and can be suppressed to
not more than about 4 .mu.m. Even if the Cu content is only a
little over the above range, the thickness of the intermetallic
compound layer increases. As a Cu content able to suppress the
thickness of the intermetallic compound layer to the thinnest, 0.7
wt % is most preferable.
[0039] Due to the above reasons, in the Ag--Cu--Sn solder alloy of
the present invention, the Ag content is limited to at least 2.0 wt
% and less than 3.0 wt % and the Cu content is limited to 0.5 to
0.8 wt %. The Ag content may in accordance with need be selected to
be in either the range of at least 2.0 wt % to not more than 2.5 wt
% or at least 2.5 wt % to less than 3.0 wt %. The most preferable
composition is 2.5 Ag-0.7 C--Sn.
[0040] In general, if the soldering temperature of electronic
equipment falls by 10 K (10.degree. C.), the lifetime of electronic
devices is said double. Reduction of the melting point of the
solder material is extremely important.
[0041] Further, the Ag--Cu--Sn solder alloy of the present
invention has properties extremely close to the main ingredient Sn,
is good in wettability with Cu, and has a high conductivity.
[0042] Further, since the amount of Ag added is small, the alloy is
provided inexpensively at the same level as a conventional Pb--Sn
alloy.
[0043] The solder alloy of the present invention can include a
total of not more than 3 wt % of one or more elements selected from
Sb (antimony), In (indium), Au (gold), Zn (zinc), Bi (bismuth), and
Al (aluminum) in addition to the above basic composition of
Ag--Cu--Sn.
[0044] These elements (in particular In and Bi) further lower the
melting point of the solder alloy and further improve the
wettability. If the total amount is over 3 wt %, however, the
appearance of the soldered joint, in particular the luster, is
degraded. Further, if the content of Bi alone is over 3 wt %, the
reliability of the joint with a Pb-containing material falls.
[0045] The solder alloy of the present invention contains as
unavoidable impurities O (oxygen), N, H (hydrogen), etc. In
particular, O is liable to make the alloy brittle, so preferably
should be kept very small in amount.
[0046] A solder alloy comprised mainly of Sn is susceptible to
oxidation of the Sn at the time of soldering. Therefore, the
soldering is preferably performed in an N.sub.2 or Ar (argon) or
other nonoxidizing atmosphere. Due to this, it is possible to
prevent poor wetting or poor electrical connection due to oxidation
of the solder alloy.
[0047] The soldering of the present invention, like the
conventional soldering, can be performed while applying an
ultrasonic wave so as to promote wetting.
BRIEF DESCRIPTION OF THE DRAWINGS
[0048] FIG. 1 is a chemical structural formula showing a specific
example of a rust-proofing coating comprised of an organic compound
containing N used in the present invention.
[0049] FIG. 2 is a graph of the change in melting point of an alloy
with respect to Ag content for a 0 to 3.5 wt % Ag-0.7 wt % Cu--Sn
solder alloy.
[0050] FIG. 3 is a graph of the change in melting point of an alloy
with respect to Cu content for a 3 wt % Ag-0 to 3 wt % Cu--Sn
solder alloy.
[0051] FIG. 4 is a graph of the relationship between the Ag content
in a 0 to 4 wt % Ag-0.7 wt % Cu--Sn solder alloy and the frequency
of occurrence of needle-shaped foreign matter.
[0052] FIG. 5 is a sectional view schematically showing a
rust-proofing coating/flux mixture expelled from the soldered joint
of the present invention.
[0053] FIG. 6 is a graph showing, by the thickness of the
.epsilon.-layer alone, the growth of the intermetallic compound
layer in the case of heating a soldered joint formed by 2 wt % Ag-0
to 1.5 wt % Cu--Sn solder alloy at 125.degree. C. and 150.degree.
C. for 100 hours.
[0054] FIG. 7 is a graph showing, by the thickness of the
.epsilon.-layer alone, the growth of the intermetallic compound
layer in the case of heating a soldered joint formed by 2 wt % Ag-0
to 1.5 wt % Cu--Sn solder alloy at 125.degree. C. and 150.degree.
C. for 100 hours.
[0055] FIG. 8 is a graph showing, by the thickness of the
.epsilon.-layer and the .eta.-layer in total, the growth of the
intermetallic compound layer in the case of heating a soldered
joint formed by 2 wt % Ag-0 to 1.5 wt % Cu--Sn solder alloy at
125.degree. C. and 150.degree. C. for 100 hours.
[0056] FIG. 9 is a graph showing, by the thickness of the
.epsilon.-layer alone, the growth of the intermetallic compound
layer in the case of heating a soldered joint formed by 2.5 wt %
Ag-0 to 1.5 wt % Cu--Sn solder alloy at 125.degree. C. and
150.degree. C. for 100 hours.
[0057] FIG. 10 is a graph showing, by the thickness of the
.eta.-layer alone, the growth of the intermetallic compound layer
in the case of heating a soldered joint formed by 2.5 wt % Ag-0 to
1.5 wt % Cu--Sn solder alloy at 125.degree. C. and 150.degree. C.
for 100 hours.
[0058] FIG. 11 is a graph showing, by the thickness of the
.epsilon.-layer and the .eta.-layer in total, the growth of the
intermetallic compound layer in the case of heating a soldered
joint formed by 2.5 wt % Ag-0 to 1.5 wt % Cu--Sn solder alloy at
125.degree. C. and 150.degree. C. for 100 hours.
[0059] FIG. 12 is a graph showing, by the thickness of the
.eta.-layer alone, the growth of the intermetallic compound layer
in the case of heating a soldered joint formed by 3 wt % Ag-0 to
1.5 wt % Cu--Sn solder alloy at 125.degree. C. and 150.degree. C.
for 100 hours.
[0060] FIG. 13 is a graph showing, by the thickness of the
.eta.-layer alone, the growth of the intermetallic compound layer
in the case of heating a soldered joint formed by 3 wt % Ag-0 to
1.5 wt % Cu--Sn solder alloy at 125.degree. C. and 150.degree. C.
for 100 hours.
[0061] FIG. 14 is a graph showing, by the thickness of the
.epsilon.-layer and the .eta.-layer in total, the growth of the
intermetallic compound layer in the case of heating a soldered
joint formed by 3 wt % Ag-0 to 1.5 wt % Cu--Sn solder alloy at
125.degree. C. and 150.degree. C. for 100 hours.
[0062] FIG. 15 is a graph showing, in comparison with a Pb--Sn
solder alloy, the strength of joint per connection terminal of an
electronic component before and after heating of a soldered joint
formed by 2.5 to 3.5 wt % Ag-0.7 wt % Cu--Sn solder alloy.
BEST MODE FOR WORKING THE INVENTION
EXAMPLE 1
[0063] The reasons for limitation of the range of Ag content in the
present invention will be explained in future detail by this
example.
[0064] The effect of the Ag content and the Cu content on the
melting point of an Ag--Cu--Sn alloy was investigated.
Specifically, the melting point of a 0 to 3.5 wt % Ag-0 to 3 wt %
Cu--Sn solder alloy was measured. FIG. 2 and Table 1 show the
change in melting point of an alloy with respect to the Ag content
for a 0 to 3.5 wt % Ag-0.7 wt % Cu--Sn solder alloy. As shown in
FIG. 2 and Table 1, a low melting point of not more than
220.degree. C. is obtained by a lower limit of the Ag content
defined in the present invention of not less than 2.0. If the Ag
content becomes less than 2.0 wt %, the melting point sharply
rises. This relation between the Ag content and melting point is
the same for the range of Cu content defined in the present
invention of 0.5 to 0.8 wt %. Note that the Pb--Sn in Table 1 is a
conventional 37 wt % Pb--Sn solder alloy.
[0065] FIG. 3 and Table 2 show the change in melting point of an
alloy with respect to the Cu content for a 3 wt % Ag-0 to 3 wt %
Cu--Sn solder alloy. It is learned that at a Cu content of a broad
range including the range of Cu of 0.5 to 0.8 wt % of the present
invention, a low melting point of not more than 220.degree. C. is
obtained. Similar results were obtained for the relation between
the Cu content and melting for an Ag content of a range of not less
than 2.0 wt % and less than 3.0 wt %.
[0066] Next, the strength of joint was investigated for a 0 to 3.5
wt % Ag-0.7 wt % Cu--Sn alloy and a 3Ag-0.5 to 1.3 wt % Cu--Sn
alloy. The joining procedure was similar to that of Example 2. As
shown in Table 3 and Table 4, the solder alloy of the range of
composition of the present invention gives a strength of joint
higher than that of a conventional Pb--Sn solder alloy.
[0067] Further, the frequency of occurrence of needle crystals
having an effect on the strength of joint was investigated. FIG. 4
shows the relation between the Ag content and the frequency of
occurrence of needle crystals (needle-shaped foreign matter) for a
0 to 4 wt % Ag-0.7 wt % Cu--Sn solder alloy. As shown in FIG. 4,
when the Ag content becomes not less than 3.0 wt %, a large amount
of needle crystals are produced. If a large amount of needle
crystals occur in this way, the electronic devices will
short-circuit between them and the reliability of the joint will
fall. For applications where it is necessary to particularly
prevent short-circuits by needle crystals and the watch the
reliability of joint, if the Ag content is limited to not more than
2.5 wt %, as shown in FIG. 4, the occurrence of needle crystals can
be substantially completely prevented, so this is further
desirable. Note that while the figure shows the results of
measurement for a 0 to 4 wt % Ag-0.7 wt % Cu--Sn solder alloy,
similar results are obtained for a range of Cu content of 0.5 to
0.8 wt % defined in the present invention.
EXAMPLE 2
[0068] The reasons for limitation of the range of Cu content in the
present invention will be explained in future detail by this
example.
[0069] Solder alloys having compositions of Sn-2.0 to 3.0 Ag-0 to
1.5Cu were prepared.
[0070] A rust-proofing coating of an alkylbenzotriazole compound
was formed as an N-containing organic compound on Cu electrodes of
printed circuit boards comprised of copper-clad laminate
boards.
[0071] Soldered joints were formed by the following procedure on
the Cu electrodes provided with the coatings by the solder
alloys.
[0072] 1) 90 wt % of a solder powder produced from each alloy
(particle size of about 20 to 42 .mu.m) and 10 wt % of a flux
(activating agent and resin component) were mixed to prepare a
solder paste. The solder paste was screen printed on the Cu
electrodes provided with the above coating to form a solder paste
layer of a uniform thickness (about 150 .mu.m).
[0073] 2) The connection terminals of the electronic device were
placed on the Cu electrodes provided with the solder paste layers.
The connection terminals were comprised of 42 Alloy (Fe-42 wt % Ni
alloy).
[0074] 3) The solder paste layers were heated to at least 498 K
(225.degree. C.) to melt the solder, then the heating was stopped
and the layers were allowed to cool to room temperature. Due to
this, soldered joints for joining the Cu electrodes and 42 Alloy
connection terminals were formed.
[0075] The rust-proofing coating comprised of the organic compound
containing N was decomposed by the heating, reacted with the acidic
flux contained in the solder paste, and was removed from the Cu
electrode/42 Alloy joint. That is, as shown in FIG. 5, the mixture
3 of the rust-proofing coating and flux is believed to have been
expelled from the interface with the Cu electrode 1 by the melted
solder alloy 2. Therefore, the rust-proofing coating covering the
Cu electrodes never remains at the soldered interface.
[0076] After the rust-proofing coating was removed, the Sn in the
molten solder alloy and the Cu in the electrodes reacted to produce
two types of intermetallic compounds (.epsilon.-phase: Cu.sub.3Sn,
.eta.-phase: Cu.sub.6Sn.sub.5) at the solder alloy/Cu electrode
interface. That is, the interface structure becomes
Cu/.epsilon.-layer/.eta.-layer/solder alloy.
[0077] Due to the production of the intermetallic compounds, the
solder alloy and the Cu electrodes are joined. That is, the
production of the intermetallic compounds is essential for the
joining. On the other hand, if too thick, the intermetallic
compound layer becomes brittle and the strength of joint falls.
Therefore, the intermetallic compound layer is preferably produced
as thinly as possible at the time of joining. Preferably, it is
resistant to growth due to the heat history after joining.
[0078] FIGS. 6 to 14 and Tables 5 to 7 show the growth of the
intermetallic compound layer in the case of heating the soldered
joints formed by the different solder alloys at 125.degree. C. and
150.degree. C. for 100 hours by the thickness of the
.epsilon.-layer alone, .eta.-layer alone, and .epsilon.-layer plus
.eta.-layer. In particular, as will be understood from FIG. 8, FIG.
11, and FIG. 14, the thickness of the intermetallic compound layer
after the above heating (total thickness) is not more than about 4
.mu.m when using a solder alloy of a composition within the range
of the present invention. In particular, by making the Cu content
0.5 to 0.8 wt % of the range of the present invention, it is
possible to stabilize and reduce the thickness of the intermetallic
compound layer. Further, the thickness of the intermetallic
compound layer tends to become smaller when the Ag content is in
the range of at least 2.5 wt % rather than the range of not more
than 2.5 wt % within the range of the present invention.
[0079] In this way, the soldered joint of the present invention is
slow in growth of intermetallic compounds and ensures a higher
reliability over the long term.
EXAMPLE 3
[0080] The strength of joint after heat treatment was investigated.
FIG. 15 shows the strength of joint in the state when bonded, after
heating at 125.degree. C. for 100 hours, and after heating at
150.degree. C. for 100 hours for a 2.5 to 3.5 wt % Ag-0.7 wt %
Cu--Sn solder alloy. From the results of the figure, it is learned
that due to the present invention, a strength of joint equal to
that of the conventional Pb--Sn solder alloy is obtained. In
particular, the strength of joint due to a conventional Pb--Sn
solder alloy falls monotonously due to heating (heat history),
while the strength of joint according to the present invention is
observed to tend to rather rise with heating.
1TABLE 1 0 to 3.5Ag-0.7Cu-Sn Ag concentration (wt %) Melting point
(.degree. C.) 0.0 226.50 0.3 226.50 1.5 222.70 2.0 218.03 2.5
217.50 3.0 217.60 3.5 218.42 Pb-Sn 183.00
[0081]
2TABLE 2 3Ag-0 to 3Cu-Sn Cu concentration (wt %) Melting point
(.degree. C.) 0.0 221.63 0.1 220.63 0.2 218.47 0.3 218.10 0.4
218.17 0.5 218.80 0.6 218.17 0.7 217.60 0.8 217.73 1.3 217.45 1.5
217.83 3.0 218.67
[0082]
3 TABLE 3 Ag concentration (wt %) Strength of joint (N) Pb-Sn 4.80
0.0 6.10 0.3 7.60 2.5 7.06 3.0 7.25 3.5 8.38
[0083]
4 TABLE 4 Cu concentration (wt %) Strength of joint (N) 0.5 8.04
0.6 8.29 0.7 7.08 0.8 8.43 1.3 7.29
[0084]
5TABLE 5 2Ag based Unit: .mu.m .epsilon.-layer .eta.-layer
.epsilon.-layer + .eta.-layer 150.degree. C./100H 125.degree.
C./100H 150.degree. C./100H 125.degree. C./100H 150.degree. C./100H
125.degree. C./100H Sn-2Ag 1.07 0.80 5.60 5.33 6.67 6.13
Sn-2Ag-0.5Cu 0.67 0.53 3.33 3.07 4.00 3.60 Sn-2Ag-0.6Cu 0.67 0.40
2.80 2.80 3.47 3.20 Sn-2Ag-0.7Cu 0.67 0.46 2.67 2.50 3.34 2.96
Sn-2Ag-0.8Cu 0.67 0.40 3.47 2.67 4.13 3.07 Sn-2Ag-1.5Cu 0.93 0.53
3.60 3.33 4.53 3.87
[0085]
6TABLE 5 2.5Ag based Unit: .mu.m .epsilon.-layer .eta.-layer
.epsilon.-layer + .eta.-layer 150.degree. C./100H 125.degree.
C./100H 150.degree. C./100H 125.degree. C./100H 150.degree. C./100H
125.degree. C./100H Sn-2.5Ag 1.00 0.80 3.50 1.90 4.50 2.70
Sn-2.5Ag-0.5Cu 0.53 0.33 2.46 2.00 2.99 2.33 Sn-2.5Ag-0.6Cu 0.70
0.30 2.00 1.90 2.70 2.20 Sn-2.5Ag-0.7cu 0.60 0.50 2.70 2.10 3.30
2.60 Sn-2.5Ag-0.8Cu 0.70 0.50 3.30 2.80 4.00 3.30 Sn-2.5Ag-1.5Cu
1.00 0.70 3.60 3.50 4.60 4.20
[0086]
7TABLE 5 3.0Ag based Unit: .mu.m .epsilon.-layer .eta.-layer
.epsilon.-layer + .eta.-layer 150.degree. C./100H 125.degree.
C./100H 150.degree. C./100H 125.degree. C./100H 150.degree. C./100H
125.degree. C./100H Sn-2Ag 0.20 0.00 4.47 4.20 4.67 4.20
Sn-3.0Ag-0.5cu 0.73 0.60 2.60 2.00 3.33 2.60 Sn-3.0Ag-0.6Cu 0.73
0.47 2.07 2.07 2.80 2.53 Sn-3.0Ag-0.7Cu 0.47 0.47 2.07 2.00 2.53
2.47 Sn-3.0Ag-0.8Cu 0.67 0.67 2.00 2.00 2.67 2.67 Sn-3.0Ag-1.3Cu
1.33 0.80 2.67 1.87 4.00 2.67
[0087] Capability of Utilization in Industry
[0088] As explained above, according to the present invention, it
is possible to secure an equivalent strength of joint to a
conventional Pb--Sn solder alloy without causing environmental
pollution by Pb and without causing a rise in cost.
* * * * *