U.S. patent application number 09/932059 was filed with the patent office on 2002-05-16 for solders.
This patent application is currently assigned to Quantum Chemical Technologies (Singapore) Pte Ltd.. Invention is credited to Chew, Kai Hwa, Pan, Wei Chih.
Application Number | 20020057986 09/932059 |
Document ID | / |
Family ID | 71993755 |
Filed Date | 2002-05-16 |
United States Patent
Application |
20020057986 |
Kind Code |
A1 |
Chew, Kai Hwa ; et
al. |
May 16, 2002 |
Solders
Abstract
A solder comprising: between 87.2% and 89.5% tin; between 4.0%
and 4.8% bismuth; between 3.5% and 4.5% indium; and between 3.0%
and 3.5% silver.
Inventors: |
Chew, Kai Hwa; (Singapore,
SG) ; Pan, Wei Chih; (Singapore, SG) |
Correspondence
Address: |
IPSOLON LLP
805 SW BROADWAY, #2740
PORTLAND
OR
97205
US
|
Assignee: |
Quantum Chemical Technologies
(Singapore) Pte Ltd.
|
Family ID: |
71993755 |
Appl. No.: |
09/932059 |
Filed: |
August 17, 2001 |
Current U.S.
Class: |
420/562 |
Current CPC
Class: |
B23K 35/262 20130101;
C22C 13/00 20130101; C22C 13/02 20130101 |
Class at
Publication: |
420/562 |
International
Class: |
C22C 013/02 |
Foreign Application Data
Date |
Code |
Application Number |
Nov 16, 2000 |
SG |
200006841-1 |
Claims
1. A solder comprising: between 87.2% and 89.5% tin; between 4.0%
and 4.8% bismuth; between 3.5% and 4.5% indium; and between 3.0%
and 3.5% silver.
2. A solder according to claim 1, wherein the solder is a
substantially lead-free solder.
3. A solder according to claim 2, wherein the solder is a lead-free
solder.
4. A solder according to claim 3, wherein the solder comprises
88.3% tin, 4.5% bismuth, 4.0% indium and 3.2% silver.
5. A method of preparing a solder, comprising the step of mixing
tin, bismuth, indium and silver such that: the proportion of tin in
the solder is between 87.2% and 89.5%; the proportion of bismuth in
the solder is between 4.0% and 4.8%; the proportion of indium in
the solder is between 3.5% and 4.5%; and the proportion of silver
in the solder is between 3.0% and 3.5%.
6. A method according to claim 5, wherein the method comprises the
addition of substantially no lead to the solder.
7. A method according to claim 6, wherein the method comprises the
addition of no lead to the solder.
8. A method according to claim 7, wherein the method comprises the
step of mixing tin, bismuth, indium and silver such that: the
proportion of tin in the solder is 88.3%; the proportion of bismuth
in the solder is 4.5%; the proportion of indium in the solder is
4.0%; and the proportion of silver in the solder is 3.2%.
9. A method of soldering, comprising the step of using a solder
comprising: between 87.2% and 89.5% tin; between 4.0% and 4.8%
bismuth; between 3.5% and 4.5% indium; and between 3.0% and 3.5%
silver.
10. A method according to claim 9, comprising the step of using a
solder comprising: 88.3% tin; 4.5% bismuth; 4.0% indium; and 3.2%
silver
11. A method according to claim 9, wherein the method comprises the
step of wave-soldering.
Description
[0001] THIS INVENTION relates to solders, and in particular to
solders which are substantially lead-free.
[0002] Many conventional solders contain lead as a major
constituent thereof. Such solders often have desirable physical
properties, and the use of lead-containing solders is widespread
among several industries, including those concerned with the
production of printed circuit boards.
[0003] However, there are increasing demands, due, for example, to
environmental considerations, for solders to be lead-free, and it
seems likely that, within the next few years, it will be a legal
requirement in several countries for solders used in the
manufacture of many items to contain little or no lead.
[0004] Previous attempts to formulate lead-free solders have met
with limited success. Conventional lead-free solders generally have
undesirable physical properties, including poor wetting properties,
low fluidity, poor compatibility with existing component coatings
and excessive drossing.
[0005] As a result, some manufacturers are finding that existing
soldering processes that have functioned effectively for many years
must be adapted to accommodate the use of lead-free solders. In
addition, existing materials that are employed in the production of
printed circuit boards may have to be replaced to be compatible
with the use of lead-free solders. This adaptation of processes and
materials is widely regarded as a poor use of resources,
particularly as the standard of articles manufactured using
conventional lead-free solders is, as described above, often below
that achievable using leaded solders.
[0006] It is an object of the present invention to seek to provide
a lead-free solder that alleviates some or all of the above
drawbacks of lead-free solders.
[0007] Accordingly, one aspect of the present invention provides a
solder comprising: between 87.2% and 89.5% tin; between 4.0% and
4.8% bismuth; between 3.5% and 4.5% indium; and between 3.0% and
3.5% silver.
[0008] Advantageously, the solder is a substantially lead-free
solder.
[0009] Preferably, the solder is a lead-free solder.
[0010] Conveniently, the solder comprises 88.3% tin, 4.5% bismuth,
4.0% indium and 3.2% silver.
[0011] Another aspect of the present invention provides a method of
preparing a solder, comprising the step of mixing tin, bismuth,
indium and silver such that: the proportion of tin in the solder is
between 87.2% and 89.5%; the proportion of bismuth in the solder is
between 4.0% and 4.8%; the proportion of indium in the solder is
between 3.5% and 4.5%; and the proportion of silver in the solder
is between 3.0% and 3.5%.
[0012] Advantageously, the method comprises the addition of
substantially no lead to the solder.
[0013] Preferably, the method comprises the addition of no lead to
the solder.
[0014] Conveniently, the method comprises the step of mixing tin,
bismuth, indium and silver such that: the proportion of tin in the
solder is 88.3%; the proportion of bismuth in the solder is 4.5%;
the proportion of indium in the solder is 4.0%; and the proportion
of silver in the solder is 3.2%.
[0015] A further aspect of the present invention provides a method
of soldering, comprising the step of using a solder comprising:
between 87.2% and 89.5% tin; between 4.0% and 4.8% bismuth; between
3.5% and 4.5% indium; and between 3.0% and 3.5% silver.
[0016] Advantageously, the method comprises the step of using a
solder comprising: 88.3% tin; 4.5% bismuth; 4.0% indium; and 3.2%
silver
[0017] Preferably, the method comprises the step of
wave-soldering.
[0018] In order that the present invention may be more readily
understood, examples thereof will now be described, by way of
example, with reference to the accompanying drawings, in which:
[0019] FIG. 1 shows a table of wetting times, in seconds, for a
plurality of different solders, at a variety of temperatures;
[0020] FIG. 2 shows a graph representing the data expressed in the
table of FIG. 1;
[0021] FIG. 3 shows a table of maximum wetting force, for a
plurality of different solders, at a variety of temperatures;
[0022] FIG. 4 shows a graph representing the data expressed in the
table of FIG. 3;
[0023] FIG. 5 shows a table of wetting times, for a solder
embodying the present invention and a prior art solder, when the
solders are applied to various coatings;
[0024] FIG. 6 presents the experimental conditions of an experiment
involving use of a solder embodying the present invention in a
wave-soldering machine; and
[0025] FIGS. 7 and 8 show the results of the experiment of FIG.
6.
[0026] As described above, conventional lead-free solders suffer
from several drawbacks, including poor wetting properties, low
fluidity, poor compatibility with existing component coatings and
excessive drossing when compared with commonly-used leaded
solders.
[0027] However, it has been found that a solder composed of a
lead-free alloy comprising between 87.2% and 89.5% tin, between 4.0
and 4.8% bismuth, between 3.5% and 4.5% indium and between 3.0% and
3.5% silver possesses significantly improved properties when
compared with conventional lead-free solders. Such a solder
embodies the present invention. Indeed, the properties of solders
embodying the present invention are comparable to conventional
leaded solders with regard to wettability, fluidity, compatibility
with existing component coatings and drossing.
[0028] In order to demonstrate the advantageous physical properties
of solders embodying the present invention, a number of tests were
carried out, as will be described below.
[0029] The first test concerned the wettability of a solder
embodying the present invention, as compared to a number of
existing lead-free solders and a conventional leaded solder
(comprising 63% tin and 37% lead). The solder of the present
invention employed in the first test comprised 88.3% tin, 4.5%
bismuth, 4.0% indium and 3.2% silver.
[0030] A first aspect of the first test comprised the measurement
of the wetting time, based on the ANSI/J Std-003 standard, for the
solders under consideration at a variety of temperatures ranging
from 235.degree. C. to 265.degree. C. In this test, a specimen of
copper was immersed in a quantity of each molten solder. A
sensitive force measuring device was connected to the copper
specimen, and arranged so that vertical forces on the specimen
could be measured and recorded.
[0031] The variation in the vertical force on the specimen during
immersion thereof in the molten solders was due to two main
factors. The first of these, the buoyancy force, arose from the
upward force exerted on the specimen due to the displacement of
solder, which was equal to the weight of solder displaced by the
specimen. Since the volume of the part of the specimen that was
immersed in the solder, and the density of the solder, are known,
this upward force can be calculated and accounted for.
[0032] The second factor is a force acting on the specimen due to
the change in contact angle between the surface of the solder and
the surface of the specimen. The wetting time in each particular
case was defined as the time taken for the wetting force acting on
the specimen to be equal to zero.
[0033] The results of the first aspect of the first test are shown
in FIG. 1. In summary, the solder embodying the present invention
exhibited a wetting time, at each of the temperatures, that was
comparable to that displayed by the conventional leaded solder. In
addition, the solder embodying the present invention exhibited a
wetting time, at all but one of the temperatures considered, that
was lower than that displayed by the any of the existing lead-free
solders. The wetting time is a measure of the rapidity with which a
solder adheres to a substrate, and clearly a low wetting time is a
desirable property for a solder. Hence, it can be seen that the
solder embodying the present invention performed better overall in
the first aspect of the first test than any of the existing
lead-free solders.
[0034] The results of the first aspect of the first test are
displayed in graph form in FIG. 2. It will be seen from this graph
that the results representing the performances of the conventional
leaded solder and the solder embodying the present invention follow
each other closely when compared to those representing the
performances of the existing lead-free solders.
[0035] A second aspect of the first test comprised the measurement
of the maximum wetting force at 2.0 seconds after immersion of the
specimen in the respective solders. The wetting force is, as
described above, the adhesive force between the solder and the
specimen. Clearly, the wetting force provides a useful indication
of the strength with which a solder binds to a substrate, and a
high wetting force is a desirable property for a solder.
[0036] The results of the second aspect of the first test are shown
in FIG. 3. To summarise these results, the solder embodying the
present invention exhibited a maximum wetting force 2.0 seconds
after immersion of the specimen therein, at each of the considered
temperatures, that was comparable to that displayed by the
conventional leaded solder. While some of the existing lead-free
solders displayed a wetting force that was close to that of the
conventional leaded solder at some of the temperatures, the solder
of the present invention displayed a wetting force that was close
to that of the conventional leaded solder at all of the considered
temperatures. It will be appreciated that such properties allow the
solder of the present invention to display similar behaviour to
conventional leaded solders under a variety of temperature
conditions, or where soldering takes place under varying
temperature conditions.
[0037] The results of the second aspect of the second test are
displayed in graph form in FIG. 4, and it will be clear from this
graph that the results representing solder embodying the present
invention follow those representing the conventional leaded solder
far more closely than those representing any of the existing
lead-free solders.
[0038] From the results of the first test, it can be seen that the
solder embodying the present invention exhibits very similar
properties, with regard to wettability, than the existing lead-free
solders studied. Clearly, this similarity in physical properties
renders the solder embodying the present invention far more
suitable for use as a replacement for the conventional leaded
solder than any of the existing lead-free solders.
[0039] A second test concerned the compatibility of the solder of
the present invention with existing component coatings. Components
on, for example, printed circuit boards, may have coatings of
various materials, and it is important that a solder to be used in
conjunction with such components adheres readily to the coatings
thereof.
[0040] In the second test, six common types of components were
plated with metallic coatings of tin, gold, silver, a tin/lead
alloy and a copper/phosphorous alloy. A comparative test was then
conducted of the wetting time displayed by a solder embodying the
present invention having a composition of 88.3% tin, 4.5% bismuth,
4.0% indium and 3.2% silver, and a conventional leaded solder
comprising 60% tin and 40% lead, when applied to each of the coated
components at a variety of temperatures, using the globule test
method, which is rather similar to that described above in relation
to the measurement of wetting force, but conducted on a smaller
scale. The flux employed in the second test was a non-clean 4%
solid content flux.
[0041] The results of the second test are presented in FIG. 5. It
will be seen from these results that the solder embodying the
present invention displays a similar wetting time to that exhibited
by the conventional leaded solder for most of the coatings, at most
of the temperatures considered (which ranged from 235.degree. C. to
265.degree. C.). In the great majority of cases, the solder
embodying the present invention and the conventional leaded solder
displayed wetting times less than half a second different from one
another.
[0042] It will be clear, from the results of the second test, that
the solder embodying the present invention is suitable for use as a
direct replacement for conventional leaded solders, with regard to
compatibility with existing component coatings.
[0043] A third test was concerned with the suitability for use of
the solder of the present invention in a wave-soldering machine. In
an example of wave-soldering, a circuit board is held just above
the surface of a quantity of molten solder. A wave is then caused
to propagate across the surface of the molten solder, of sufficient
amplitude that the crest of the wave comes into contact with the
surface of the circuit board. The wave is as wide as the circuit
board (or the portions thereof that require soldering), and as the
wave propagates across the surface of the molten solder all parts
of the downward-facing surface of the circuit board are contacted
with molten solder.
[0044] This method of application of solder to a circuit board
entails a greatly reduced risk of solder coming into contact with
the upward-facing surface of the circuit board, compared to a
method where the circuit board is dipped directly into molten
solder.
[0045] Existing lead-free solders, when used in a pot of molten
solder in a wave-soldering machine, have been known to lead to high
levels of contamination in the solder pot after a number of uses of
the wave-soldering machine. In a wave-soldering machine, it is
normal for two pot areas to be provided. When circuit boards are
soldered using such a wave-soldering machine, they are initially
manoeuvred over a first pot area, in which a preliminary "chip"
wave is passed over the downward-facing surface of the circuit
boards for the purposes of cleaning this surface. Subsequently, the
circuit boards proceed to a position over a second pot area, where
a further "laminar" wave is passed over their downward-facing
surfaces to perform the required soldering. It will be clear that,
since the chip wave is concerned with cleaning the surfaces of the
circuit boards prior to soldering, there is a risk of a build-up of
undesirable contaminants in the first pot area, and this problem
has been found to be exacerbated by the use of existing lead-free
solders.
[0046] In addition, the levels of dross present in the pot after
several uses have been found to be unacceptably high when existing
lead-free solders are employed.
[0047] In the third test, a solder embodying the present invention
having a composition of 88.3% tin, 4.5% bismuth, 4.0% indium and
3.2% silver was put to use in a conventional wave-soldering
machine. No alteration of the machine was made to accommodate the
use of the solder. The wave-soldering machine was then used to
solder circuit boards, in the same way as for a conventional solder
comprising a tin/lead alloy.
[0048] The wave-soldering machine was used at four different pot
temperatures, namely 235.degree. C., 245.degree. C., 255.degree. C.
and 265.degree. C. These temperatures are all within the range of
temperatures that would normally be used when using the
wave-soldering machine with a conventional leaded solder. In
addition, the conveyor speed (i.e. the speed at which the circuit
boards were moved over the surface of the pot) was varied such that
three different conveyor speeds were used--1 meter/minute, 1.4
meters/minute and 1.8 meters/minute. Each of these conveyor speeds
was within the normal range of conveyor speeds that would normally
be used when using the wave-soldering machine with a conventional
leaded solder. The third test was carried out in a normal air
environment.
[0049] To employ a totally lead-free process, a lead-free flux was
developed for use in the process, the lead-free flux being a RMA
13% solid content flux. At the end of each day of testing, the
level of contamination in the pot and the alloy composition of the
pot were measured and recorded. In addition, the dross in the pot
was removed and weighed to determine the amount of dross produced
by the wave-soldering process.
[0050] The experimental conditions of the of the third test are
presented in FIG. 6. The amounts of dross left in the pot at the
end of each day are presented in FIG. 7. The levels of
contamination of the solder in the pot (for both chip waves and
laminar waves, where appropriate) are presented in FIG. 8.
[0051] A person of ordinary skill in the art, from consideration of
these results, will appreciated that the levels of contamination in
the pot, as well as the amount of dross produced, are significantly
lower than those that would be encountered when using an existing
lead-free solder in an unmodified conventional wave-soldering
machine.
[0052] It will be clear that the present invention provides a
lead-free solder that is more suitable for use as a direct
replacement for conventional leaded solders than
previously-proposed lead-free solders, due to the comparable
characteristics of wettability, fluidity, compatibility with
existing component coatings and drossing exhibited by the solder of
the present invention.
[0053] An advantage of this suitability is that the need for
manufacturers to replace existing machinery, processes or component
coatings to accommodate use of a lead-free solder can be lessened
or eliminated by employing the solder of the present invention. It
will be clear that, as a result, the process of converting to use
of a lead-free solder may be rendered far simpler and more
economically viable for many manufacturers.
[0054] In the present specification "comprises" means "includes or
consists of" and "comprising" means "including or consisting
of".
[0055] The features disclosed in the foregoing description, or the
following claims, or the accompanying drawings, expressed in their
specific forms or in terms of a means for performing the disclosed
function, or a method or process for attaining the disclosed
result, as appropriate, may, separately, or in any combination of
such features, be utilised for realising the invention in diverse
forms thereof.
* * * * *