U.S. patent application number 09/970036 was filed with the patent office on 2002-04-11 for solder paste.
Invention is credited to Kurata, Ryoichi, Nakamura, Sinzo, Takahashi, Hiroshi.
Application Number | 20020040624 09/970036 |
Document ID | / |
Family ID | 18786766 |
Filed Date | 2002-04-11 |
United States Patent
Application |
20020040624 |
Kind Code |
A1 |
Nakamura, Sinzo ; et
al. |
April 11, 2002 |
SOLDER PASTE
Abstract
A solder paste comprises a lead-free solder alloy powder and a
flux and is suitable for use in reflow soldering of electronic
parts at a soldering temperature of 230.degree. C. or below with
minimized damage to the electronic parts to form soldered joints of
good bond strength. The solder alloy powder is a mixture of from 10
to 30 vol % of a first powder of an Sn--Bi alloy consisting
essentially of 10-45 wt % of Bi and a balance of Sn and from 70 to
90 vol % of a second powder of an Sn--Zn alloy consisting
essentially of 9-15 wt % of Zn and a balance of Sn. The mixture
gives an alloy having a composition upon melting which consists
essentially of 7-11 wt % of Zn, 1-5 wt % of Bi, and a balance of
Sn.
Inventors: |
Nakamura, Sinzo; (Mouka-shi,
JP) ; Kurata, Ryoichi; (Kawachi-gun, JP) ;
Takahashi, Hiroshi; (Utsunomiya-shi, JP) |
Correspondence
Address: |
Michael Tobias
Suite 304
1730 K Street, N.W.
Washington
DC
20006
US
|
Family ID: |
18786766 |
Appl. No.: |
09/970036 |
Filed: |
October 4, 2001 |
Current U.S.
Class: |
75/252 ; 148/23;
420/562 |
Current CPC
Class: |
B23K 35/025 20130101;
C22C 13/02 20130101; C22C 13/00 20130101; H05K 3/3463 20130101;
B22F 2998/00 20130101; B23K 35/3613 20130101; H05K 3/3485 20200801;
B23K 35/262 20130101; H05K 2201/0272 20130101; B23K 35/0244
20130101; B22F 2998/00 20130101; C22C 1/04 20130101 |
Class at
Publication: |
75/252 ; 148/23;
420/562 |
International
Class: |
C22C 013/02 |
Foreign Application Data
Date |
Code |
Application Number |
Oct 5, 2000 |
JP |
2000-306006 |
Claims
What is claimed is:
1. A solder paste comprising a solder alloy powder and a flux,
wherein the solder alloy powder is a mixture of from 10 to 30 vol %
of a first powder of an Sn--Bi alloy consisting essentially of
10-45 wt % of Bi and a balance of Sn and from 70 to 90 vol % of a
second powder of an Sn--Zn alloy consisting essentially of 9-15 wt
% of Zn and a balance of Sn, the mixture having a composition which
consists essentially of 7-11 wt % of Zn. 1-5 wt % of Bi, and a
balance of Sn.
2. A solder paste according to claim 1 wherein the Sn--Bi solder
alloy for the first powder has a composition consisting essentially
of 10-35 wt % of Bi and a balance of Sn.
3. A solder paste according to claim 1 wherein the Sn--Zn solder
alloy for the second powder has a composition consisting
essentially of 9-13 wt % of Zn and a balance of Sn.
4. A solder paste according to claim 1 wherein the first powder is
present in the powder mixture in a proportion of from about 12% to
about 25% by volume and the remainder is constituted by the second
powder.
5. A solder paste according to claim 1 wherein the mixture of the
first and second powders has a composition consisting essentially
of 7-10 wt % of Zn, 2-4 wt % of Bi, and a balance of Sn.
6. A solder paste according to claim 1 wherein the flux is a rosin
flux.
7. A solder paste according to claim 1 wherein the flux is present
in the solder paste in a proportion of 5%-20% by weight.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] This invention relates to a solder paste suitable for use in
reflow soldering of electronic parts mounted on printed circuit
boards. More particularly, it is concerned with a solder paste made
of a lead-free solder alloy of an Sn--Zn--Bi ternary system.
[0003] 2. Description of the Related Art
[0004] Electronic parts for use in electronic appliances can be
mounted on printed circuit boards either by insertion mounting
technology (IMT) (also called through hole mounting technology or
TMT) or surface mounting technology (SMT).
[0005] Electronic parts to be mounted by IMT comprise electronic
elements having elongated rod-like or pin-like leads projecting
therefrom. They are normally soldered by the flow soldering
technique. In a typical process of IMT, electronic parts are
mounted on one surface of a printed circuit board by inserting the
leads of each part into through holes formed in the board until the
leads pass through the holes, and the leads are secured to lands
formed on the opposite surface of the board around the through
holes by flow soldering, which includes the steps of application of
a flux, preheating, contact with a molten solder, and cooling.
[0006] IMT is disadvantageous in that there is a limit to the
extent to which the overall size of the electronic parts, including
leads, can be reduced, particularly in the case of discrete single
function parts such as resistors and capacitors. Another problem of
IMT particularly encountered with transistors and pin grid arrays
(PGA's) having a great number of leads is that it is difficult to
insert all the leads completely into the through holes of the
board, and some leads may be bent by failure to be in alignment
with the through holes.
[0007] In view of these problems of IMT, SMT has recently been
employed widely. Electronic parts to be mounted by SMT are called
surface mount devices (SMD's) and they include chip-type, discrete,
single-function parts such as resistors and capacitors as well as
IC packages such as quad flat packages (QFP's) and small outline
packages (SOP's).
[0008] SMD's are soldered on the same surface of a printed circuit
board on which they are mounted without their leads being inserted
through holes. Therefore, SMD's are not suitable for soldering by
flow soldering, in which the areas to be soldered must be contacted
with molten solder. If SMD's are soldered by flow soldering, they
may come into direct contact with the molten solder, which may
result in thermal damage thereto, or the molten solder may be
prevented from attaching to all the areas to be soldered due to the
uneven surface of the printed circuit board.
[0009] For this reason, most SMD's are soldered by reflow
soldering, In a typical process of SMT employing reflow soldering,
a solder paste which is comprised of a solder alloy powder
uniformly mixed with a flux is applied to the surface of a printed
circuit board on which SMD's are to be mounted by printing or
dispensing. Thereafter, SMD's are disposed on that surface of the
printed circuit board, and the board with the SMD's disposed
thereon is heated in a reflow furnace to a soldering temperature
sufficient to allow the solder alloy powder to melt in order to
perform soldering. The above-described problems of flow soldering
when applied to soldering of SMD's can be eliminated by reflow
soldering.
[0010] Reflow soldering causes the entire printed circuit board to
be heated along with the electronic parts (SMD's) disposed thereon.
In order to minimize adverse thermal effects of this indicating on
the printed circuit board and SMD's and prevent bumping (sudden
splashing) of the solder paste during heating, the reflow furnace
may be designed such that preheating is performed at a temperature
in the range from 100 to 170.degree. C. before main heating to the
soldering temperature, thereby minimizing the duration of main
heating.
[0011] The soldering temperature, which is the peak temperature for
main heating, is normally from 20 to 40.degree. C. above the liquid
us temperature of the solder alloy powder used in the solder paste
in order to completely melt the powder, and it varies to a certain
extent depending on the size and thickness of the printed circuit
board and the packing density of the electronic parts mounted
thereon. The soldering temperature is recommended to be as low as
possible in order to minimize adverse thermal effects particularly
on the electronic parts disposed on the board. Therefore, it is
preferable to use a soldering alloy powder having a low liquid us
temperature in order to lower the soldering temperature.
[0012] The solder alloy powder used in a conventional solder paste
is a powder of an Sn--Pb solder alloy, particularly an Sn--Pb
eutectic solder alloy, in view of its low melting temperature of
183.degree. C. and good solderability, Soldering with a
conventional solder paste prepared from an Sn--Pb eutectic solder
alloy powder is generally performed at a temperature of 230.degree.
C. or below after preheating to minimize the duration of main
heating. Such soldering conditions are generally sufficient to
avoid thermal damage to SMD's and printed circuit boards during
reflow soldering.
[0013] When electronic appliances are to be discarded, they are
typically disposed of in landfills, where they may be brought into
contact with rain, which has recently become acidic. Such acid rain
causes the Sn--Pb alloy 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, it has recently
been recommended to use a lead-free solder alloy in the electronics
industry. This recommendation also applies for a solder paste for
use in reflow soldering.
[0014] Typical lead-free solder alloys are Sn-based alloys
comprising a major proportion of Sn and a small amount of at least
one additional element such as Ag, Cu, Bi, Sb, and Zn.
[0015] Sn--Ag alloys have a eutectic composition of Sn-3.5Ag with a
melting temperature of about 220.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 250.degree. C. or above, which may cause thermal
damage to electronic parts during reflow soldering. Addition of a
small amount of Bi and/or In to an Sn--Ag alloys can decrease the
solidus temperature of the alloy, but its liquidus temperature is
not decreased significantly, so the soldering temperature still
remains high. Sn--Ag alloys have another problem of poor surface
gloss of soldered joints formed therefrom, leading to a decrease in
attractiveness of the soldered products.
[0016] Sn--Cu alloys have a melting temperature of 227.degree. C.
for a eutectic composition of Sn-0.7Cu, which makes the soldering
temperature too high to avoid thermal damage to electronic parts
during reflow soldering. These alloys have another problem of poor
solderability. The addition of Bi and/or In is not effective to
significantly decrease the liquidus temperature of these alloys, as
is the case with Sn--Ag alloys.
[0017] Sn--Bi alloys have a very low melting temperature of
139.degree. C. for a eutectic composition of Sn-57Bi. Therefore,
they make it possible to perform reflow soldering even at a lower
temperature than the conventional Sn--Pb eutectic solder alloy with
no concern about thermal damage to electronic parts. However, such
alloys are very brittle due to the presence of a large proportion
of Bi, and the resulting soldered joints are readily detached when
subjected to even a mild mechanical impact.
[0018] Sn--Zn alloys have a eutectic composition of Sn-9Zn with a
melting temperature of 199.degree. C., which is low enough to make
it possible to perform reflow soldering at a soldering temperature
of 230.degree. C. or below to minimize thermal damage. The alloying
element Zn or zinc is harmless to human bodies, and it is an
abundant and inexpensive element. Therefore, Sn--Zn lead-free
solder alloys are advantageous from the viewpoints of safety and
economy.
[0019] Although reflow soldering with a solder paste prepared from
a powder of an Sn--Zn solder alloy, particularly Sn-9Zn alloy, can
be performed at a temperature 230.degree. C. or below, it takes a
considerable length of time for main heating to completely melt the
solder alloy powder present in various locations on the surface of
a printed circuit board due to a local temperature difference in
the course of heating caused by variations in thickness and
packaging density. In other words, an extended time for main
heating is necessary to melt the solder alloy powder completely in
all the applied spots on the board. As a result, even when the
soldering temperature is as low as 230.degree. C. or slightly
below, the extended duration of main heating in a reflow furnace
may cause thermal damage to electronic parts.
[0020] Another problem of an Sn--Zn solder alloy is that it has
poor solderability and tends to form soldered joints having
numerous voids and traces of dewetting. Voids are vacancies like
air bubbles formed inside soldered joints, particularly at the
interface between soldered joints and their parent material, i.e.,
a printed circuit board. A soldered joint having numerous voids
therein has a reduced bonding area to the parent material and hence
a reduced bond strength, and it tends to be readily detached when a
weak force is applied to the electronic part secured by the
soldered joint.
[0021] Dewetting is a phenomenon in which molten solder which has
initially wetted the surface of a parent material and spread
thereon is partially or totally repelled by the parent material and
constricted into a narrow area. The dewetting phenomenon also leads
to a reduction in bonding area between the soldered joint and the
parent material and weakens the bond strength of the resulting
soldered joint,
SUMMARY OF THE INVENTION
[0022] Thus, there is a need to provide a solder paste prepared
from a lead-free, low-melting solder alloy powder which can be used
in reflow soldering at a soldering temperature of 230.degree. C. or
below without extending the time for main heating and which can
form soldered joints having good bond strength without forming
numerous voids and causing dewetting during reflow soldering.
[0023] According to the present invention, the above-described need
can be fulfilled by a solder paste prepared from a mixture of a
major proportion of an Sn--Zn alloy powder and a minor proportion
of a non-eutectic Sn--Bi alloy powder.
[0024] Thus, the present invention provides a solder paste
comprising a solder alloy powder and a flux, wherein the solder
alloy powder is a mixture of from 10 to 30 vol % of a first powder
of an Sn--Bi alloy consisting essentially of 10-45 wt % of Bi and a
balance of Sn and from 70 to 90 vol % of a second powder of an
Sn--Zn alloy consisting essentially of 9-15 wt % of Zn and a
balance of Sn, the mixture having a composition which consists
essentially of 7-11 wt % of Zn, 1-5 wt % of Bi, and a balance of
Sn.
[0025] Other features and advantages of the present invention will
be apparent by reading the following description.
DESCRIPTION OF PREFERRED EMBODIMENTS
[0026] When two solder alloys in powder form having the same
liquidus temperature are simultaneously heated to the same
temperature to melt, a solder alloy having a lower solids
temperature molts completely in a shorter period since it begins to
melt earlier at the lower solidus temperature than the other solder
alloy having a higher solidus temperature.
[0027] It has been found that addition of a certain third element
to an Sn-9Zn eutectic alloy to lower the solidus temperature is
effective for reducing the time for main heating required to
completely melt a solder alloy powder during reflow soldering.
Candidates for such a third metal include In and Bi, which are
known to have an effect of lowering the melting temperature of an
alloy to which they are added.
[0028] Among these, In (indium) is expensive, and the addition of
In increases the reactivity of the resulting alloy. As a result, a
solder paste formed from an In-containing solder alloy powder and a
flux tends to have a viscosity which is rapidly increased in a
short period due to a reaction of the solder alloy powder with the
flux. The solder paste having an increased viscosity is no longer
suitable for use in application by screen printing with a mask or
through needle-like fine orifices of a dispenser.
[0029] Bi (bismuth) is less expensive than In, and it is also less
reactive with a flux. The addition of a small proportion of Bi to
an Sn--Zn alloy having an approximately eutectic composition can
lower the solidus temperature of the alloy without causing a rapid
increase in the viscosity of a solder paste.
[0030] For this reason, according to the present invention, Bi is
added to the Sn--Zn alloy to lower the solidus temperature of the
alloy, thereby reducing the time for main heating required to
completely melt a powder of the alloy during reflow soldering. Bi
is present in the resulting Sn--Zn--Bi alloy in a proportion of
from 1% to 5% by weight. Addition of less than 1% by weight of Bi
is not effective for lowering the solidus temperature of an Sn--Zn
alloy. If added in a proportion of more than 5%, Bi makes the alloy
brittle and causes the formation of soldered joints which have a
decreased mechanical strength and may readily be detached upon
application of a mechanical impact or vibration thereto.
[0031] In order to keep the soldering temperature below 230.degree.
C. which is required to avoid thermal damage to electronic parts
during reflow soldering, the solder alloy used in the present
invention should have a liquidus temperature of at most 205.degree.
C., which is only slightly higher than that of the Sn-9Zn eutectic
alloy (199.degree. C.). For this purpose, the proportion of Zn
present in the alloy is limited to a range of from 7% to 11% by
weight.
[0032] Thus, a lead-free solder alloy having a composition
consisting essentially of 7-11 wt % of Zn, 1-5 wt % of Bi, and a
balance of Sn has an increased difference between its solidus and
liquidus temperatures. For example, an Sn-8Zn-3Bi alloy has a
solidus temperature of 190.degree. C. and a liquidus temperature of
196.degree. C., and the difference between the solidus and liquidus
temperatures is 6.degree. C. When a solder paste prepared from a
powder of this solder alloy is applied to a printed circuit board
and heated in a reflow furnace, partial melting of the solder alloy
powder begins as soon as the surface temperature of the printed
circuit board reaches 190.degree. C. (liquidus temperature)
particularly in those areas of the board a having a lower heat
capacity. As the surface temperature exceeds 196.degree. C.,
melting of the is entire solder alloy powder begins, and when it
reaches the soldering temperature, e.g., 230.degree. C., complete
melting of the solder alloy powder has occurred already or occurs
shortly.
[0033] The above-described Sn--Zn--Bi ternary solder alloy has the
advantage of being capable of melting completely when heated for a
short period at a temperature of 230.degree. C. or below to avoid
thermal damage to electronic parts during reflow soldering.
However, it still has the problems of forming soldered joints
having numerous voids and many dewetting spots in reflow
soldering.
[0034] The present inventors found that these problems of the
Sn--Zn--Bi alloy can be eliminated by using a mixture of two solder
alloy powders having different solidus temperatures from each
other. Specifically, the solder alloy powder used in a solder paste
according to the present invention is a mixture of a first Sn--Bi
alloy powder and a second Sn--Zn alloy powder. The two alloy
powders are mixed in a proportion that gives an alloy upon melting
which consists essentially of 7-11 wt % of Zn, 1-5 wt % of Bi, and
a balance of Sn.
[0035] The present inventors found that the formation of voids and
dewetting encountered in reflow soldering occur by the following
mechanism. When a solder paste consisting of a powder of an
Sn-8Zn-3Bi solder alloy, for example, and a flux is heated for
reflow soldering, the flux, which is an organic composition having
a softening temperature much lower than the solidus temperature of
the solder alloy, liquefies first at the preheating stage in a
temperature range of from 100 to 170.degree. C. The liquefied flux
is initially retained in the interstices between particles of the
solder alloy powder, but as the temperature increases, it vaporizes
to form gas. As soon as the temperature of the solder alloy powder
exceeds 190.degree. C., which is the solidus temperature of the
solder alloy, it begins to melt all at once while entrapping
therein some of the gas formed from the flux, resulting in the
formation of voids. Dewetting is caused by the fact that the
Sn-8Zn-3Bi solder alloy itself does not have good wetting
properties sufficient to maintain the molten solder alloy in a well
spread condition and the molten solder alloy is repelled by the
parent material (printed circuit board) to cause dewetting.
[0036] According to the present invention, a mixture of two solder
alloy powders is used in which a first powder is of an Sn--Bi
solder alloy having good wetting properties and a lower solidus
temperature and a second powder is of an Sn--Zn solder alloy having
relatively poor wetting properties and a higher solidus
temperature.
[0037] The first powder of an Sn--Bi solder alloy is not of a
eutectic composition of Sn-57Bi, but the Sn--Bi alloy has a
composition consisting essentially of 10-45 wt % of Bi and a
balance of Sn. The composition of this alloy is selected so that
the difference between the solidus and liquidus temperatures of the
alloy is at least 25.degree. C. and preferably at least 30.degree.
C. The Sn--Bi alloy for the first powder has a solidus temperature
of 139.degree. C. and a liquidus temperature of 169.degree. C. or
above,
[0038] The Sn--Zn solder alloy for the second powder has a
composition consisting essentially of 9-15 wt % of Zn and a balance
of Sn. The composition makes it possible to perform reflow
soldering by main heating at a temperature of 230.degree. C. or
below. The Sn--Zn alloy has a solidus temperature of 199.degree.
C., which is 60.degree. C. higher than that of the Sn--Bi alloy for
the first powder (139.degree. C.).
[0039] In the course of heating a solder flux according to the
present invention to a soldering temperature, the first powder of
an Sn--Bi alloy having a lower solidus temperature begins to melt
at a temperature of 139.degree. C. (solidus temperature of the
alloy) in a preheating stage, and melting of the first powder
continues at least until the powder is heated to the liquidus
temperature thereof (169.degree. C. or higher). Thus, the first
powder takes an extended time to completely melt due to a big
difference between the solidus and liquidus temperatures of the
alloy, and the powder is kept in a highly viscous, partially molten
state for an extended time, during which time the liquefied flux,
which is much less viscous than the partially molten solder alloy,
can be separated and expelled from the solder alloy and released by
vaporization. Therefore, it is much less likely that the liquefied
flux will be entrapped within the molten solder to form voids.
[0040] The second Sn--Zn alloy powder begins to melt after the
first Sn--Bi alloy powder has melted completely or almost
completely since its solidus temperature is much higher than that
of the first alloy powder and even higher than the liquidus
temperature thereof. Thus, before melting of the second powder, the
surface to be soldered is covered by the molten alloy from the
first powder which has good wetting properties, so the second
solder alloy can spread sufficiently upon melting despite its
relatively poor wetting properties, thereby minimizing
dewetting.
[0041] The first and second solder alloy powders are mixed in such
a proportion that the resulting mixture contains from 10% to 30% of
the first solder alloy powder ad from 70% to 90% of the second
solder alloy powder by volume. The proportion of at least 10% by
volume of the first alloy powder is sufficient to fill the
interstices formed between particles of the second alloy powder
with molten first alloy after the first alloy melts earlier so that
any flux remaining in the interstices can be expelled by the molten
first alloy to avoid the formation of voids. The presence of more
than 30% by volume of the first alloy powder in the mixture may
cause slumping (over-spreading of the solder paste) or formation of
fine solder balls during reflow soldering.
[0042] The proportions of the first and second solder alloy powders
in the mixture are also selected so as to give an Sn--Zn--Bi alloy
upon melting which has a composition consisting essentially of 7-11
wt % of Zn, 1-5 wt % of Bi, and a balance of Sn, for the reason
mentioned previously. Thus, although the Sn--Bi alloy of the first
powder is rather brittle, the mixture of the first and second
powders can form soldered joints having good bond strength.
[0043] In a preferred embodiment,
[0044] the Sn--Bi solder alloy for the first powder has a
composition consisting essentially of 10-35 wt % and more
preferably 12-30 wt % of Bi and a balance of Sn, and/or
[0045] the Sn--Zn solder alloy for the second powder has a
composition consisting essentially of 9-13 wt % and more preferably
greater than 9 wt % and not greater than 11 wt % of Zn and a
balance of Sn, and/or
[0046] the first powder is present in the powder mixture in a
proportion of from about 12% to about 25% by volume and the
remainder is constituted by the second powder, and/or
[0047] the mixture of the first and second powders has a
composition consisting essentially of 7-10 wt % of Zn, 2-4 wt % of
Bi, and a balance of Sn. The Zn content of the mixture is more
preferably at least 7 wt % and lower than 9 wt %.
[0048] The solder alloy powders may be produced in a conventional
manner, e.g., by gas atomization or centrifugal atomization. The
particle size of the solder alloy powders is typically in the range
of from 200 to 400 mesh, but finer powders may be used. All the
solder powders need not have the same particle size. One or both of
the first and second powders in the solder alloy powder mixture may
be comprised of two or more different alloys which have
compositions falling within the prescribed ranges.
[0049] The flux used in the solder paste according to the present
invention may be any flux suitable for use in soldering of
electronic parts and in preparing a solder paste. The flux is
preferably a rosin flux, which is a non-water soluble, rosin-based
flux. The rosin in the flux may be either a natural rosin or a
modified rosin including a polymerized rosin. The flux usually
further contains an activator and a solvent and may have a pasty
consistency. The flux is usually present in a solder paste in a
proportion of 5-20 wt %.
[0050] The solder paste according to the present invention may be
used for reflow soldering in a conventional manner in a reflow
furnace. The heating may be performed in two stages while the
temperature is kept for a while at a constant preheating
temperature in the range of about 100 to 170.degree. C. before
heating to the temperature for main heating, which is preferably
230.degree. C. or below, or the temperature may be elevated
gradually to the temperature for main heating through the
preheating temperature range. Since the first solder alloy powder
begins to melt in the preheating temperature range, the duration of
melt heating can be reduced to melt the entire solder powders
completely and form soldered joints.
[0051] The following examples are presented to further illustrate
the present invention. These examples are to be considered in all
respects as illustrative and not restrictive, In the examples, the
figures preceding the element Zn or Bi indicates its proportion in
weight percent.
EXAMPLES
Example 1
[0052] A solder paste was prepared by uniformly mixing 900 grams of
a solder alloy powder and 100 grams of a pasty rosin flux. The
solder alloy powder was a mixture of the following two alloy
powders:
[0053] 20 vol % of a first powder of an Sn-15Bi alloy, and
[0054] 80 vol % of a second powder of an Sn-10Zn alloy.
[0055] The solder alloy powder mixture gives an Sn-8Zn-3Bi alloy
upon melting.
Example 2
[0056] A solder paste was prepared by uniformly mixing 900 grams of
a solder alloy powder and 100 grams of the same rosin flux as was
used in Example 1. The solder alloy powder was a mixture of the
following two solder alloy powders:
[0057] 15 vol % of a first powder of an Sn-20Bi alloy, and
[0058] 85 vol % of a second powder of an Sn-10Zn alloy.
[0059] The solder alloy powder mixture gives an Sn-8Zn-3Bi alloy
upon melting.
Comparative Example 1
[0060] A solder paste was prepared by uniformly mixing 900 grams of
a powder of an Sn-8Zn-3Bi alloy and 100 grams of the same rosin
flux as was used in Example 1
Comparative Example 2
[0061] A solder paste was prepared by uniformly mixing 900 grams of
a solder alloy powder and 100 grams of the same rosin flux as was
used in Example 1. The solder alloy powder was a mixture of the
following two solder alloy powders:
[0062] 30 wt % of a first powder of an Sn-57Bi alloy, and
[0063] 70 wt % of a second powder of an Sn-9Zn alloy.
[0064] Thus, both the alloys used had eutectic compositions. The
solder alloy powder mixture gives an Sn6.2Zn-17.1Bi alloy upon
melting.
[0065] Using each of the solder pastes prepared in the above
Examples and Comparative Examples which were applied to a printed
circuit board by screen printing, SMD's disposed on the printed
circuit board were soldered by reflow soldering with such a heating
profile in a reflow furnace that the duration in the temperature
range from 150 to 170.degree. C. in a preheating stage was about 30
seconds and the duration at the peak temperature of 230.degree. C.
in a main heating stage was from 20 to 60 seconds.
[0066] The solder pastes of Examples 1 and 2 could form soldered
joints with little voids and dewetting spots, and they uniformly
deposited on the areas to which they had been applied by screen
printing.
[0067] In contrast, numerous voids and dewetting spots were found
in the soldered joints formed with the solder paste of Comparative
Example 1. The soldered joints formed with the solder paste of
Comparative Example 2 showed a decreased number of dewetting spots
but still included numerous voids.
[0068] Thus, a solder paste according to the present invention can
be used to perform reflow soldering of electronic parts by exposure
to the main heating temperature of 230.degree. C. or below for a
short period in a reflow furnace to completely melt the solder
alloys present therein, thereby minimizing thermal damage to the
electronic parts and occurrence of bonding failures. In additions
the resulting soldered joints formed from the solder paste are
reliable, since the formation of voids and dewetting spots which
weaken the bond strength of the soldered joints is prevented. As a
whole, the performance of the solder paste is superior to that of
any conventional solder paste prepared from a lead-free solder
alloy powder.
[0069] It will be appreciated by those skilled in the art that
numerous variations and modifications may be made to the specific
embodiments of the present invention described above without
departing from the spirit or scope of the invention as broadly
described.
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