U.S. patent application number 09/774011 was filed with the patent office on 2002-05-02 for lead-free zinc-containing solder paste.
Invention is credited to Hirata, Masahiko, Nagashima, Takashi, Taguchi, Toshihiko, Takaura, Kunihito, Yoshida, Hisahiko.
Application Number | 20020050305 09/774011 |
Document ID | / |
Family ID | 18553513 |
Filed Date | 2002-05-02 |
United States Patent
Application |
20020050305 |
Kind Code |
A1 |
Taguchi, Toshihiko ; et
al. |
May 2, 2002 |
Lead-free Zinc-Containing Solder Paste
Abstract
A solder paste having a powder of a Zn-containing solder alloy
such as an Sn--Zn based alloy in admixture with a soldering flux
such as a rosin flux is improved by adding from 0.1% to 5.0% by
weight of a glycidyl ether compound such as alkyl, alkenyl, or aryl
glycidyl ether. The improved solder paste has increased resistance
to aging and to concomitant deterioration in solderability caused
by reaction of Zn in the solder alloy with ingredients in the flux
and has a substantially extended shelf life.
Inventors: |
Taguchi, Toshihiko;
(Kitakatsushika-gun, JP) ; Takaura, Kunihito;
(Mouka-shi, JP) ; Hirata, Masahiko; (Kashiba-shi,
JP) ; Yoshida, Hisahiko; (Osaka, JP) ;
Nagashima, Takashi; (Kyoto-shi, JP) |
Correspondence
Address: |
Michael Tobias
Suite 304
1730 K Street, N.W.
Washington
DC
20006
US
|
Family ID: |
18553513 |
Appl. No.: |
09/774011 |
Filed: |
January 31, 2001 |
Current U.S.
Class: |
148/24 ;
148/23 |
Current CPC
Class: |
B23K 35/3613 20130101;
B23K 35/025 20130101; B23K 35/3612 20130101; B23K 35/262
20130101 |
Class at
Publication: |
148/24 ;
148/23 |
International
Class: |
B23K 035/34 |
Foreign Application Data
Date |
Code |
Application Number |
Feb 4, 2000 |
JP |
2000-28034 |
Claims
What is claimed is:
1. A lead-free solder paste comprising a powder of a
zinc-containing, lead-free solder alloy in admixture with a
soldering flux with a glycidyl ether compound being added to the
soldering flux in an amount of from 0.1% to 5.0% by weight based on
the flux.
2. The lead-free solder paste according to claim 1 wherein the
glycidyl ether compound has the following formula: 3where R is a
saturated or unsaturated, aliphatic or aromatic hydrocarbon group
which may optionally contain at least one hydroxyl group, and n is
an integer from 1 to 4.
3. The lead-free solder paste according to claim 1 wherein the
zinc-containing solder alloy is an Sn--Zn based solder alloy.
4. The lead-free solder paste according to claim 3 wherein the
zinc-containing solder alloy is an Sn--Zn--Bi based solder
alloy.
5. The lead-free solder paste according to claim 1 wherein the
soldering flux is a rosin flux.
6. The lead-free solder paste according to claim 5 wherein the
rosin flux is an activated rosin flux containing an activator.
7. The lead-free solder paste according to claim 6 wherein the
activated rosin flux contains a thixotropic agent.
Description
BACKGROUND OF THE INVENTION
[0001] The present invention relates to a solder paste and more
particularly to a lead-free, zinc-containing solder paste which
comprises a powder of a zinc-containing, lead-free solder alloy in
admixture with a soldering flux.
[0002] Sn--Pb alloys have been used in soldering since ancient
times, and they are still the most popular solders for soldering
electronic components to printed circuit boards or other
substrates.
[0003] Sn--Pb alloys have a eutectic composition of approximately
63% Zn with a balance of Pb. This composition has a low melting
temperature of 183.degree. C., thereby making it possible to
perform soldering in a temperature range of from 220.degree. C. to
230.degree. C., in which range there is no substantial thermal
damage to heat-sensitive electronic components. The eutectic Sn-Pb
alloy, called eutectic solder, has extremely good wettability and
solderability, and since it does not have a difference between its
liquidus and solidus temperatures (i.e., no solidification
temperature range exists), solidification occurs instantaneously
during soldering when the melting point is reached. As a result,
even if vibrations or mechanical shocks are applied to parts to be
soldered during soldering, cracking or detachment of the resulting
soldered joints does not occur.
[0004] Discarded electronic appliances including televisions,
radios, audio or video recorders, computers, copying or printing
machines, etc. are generally disposed of in landfills, since such
appliances are composed of various materials such as synthetic
resins used for housings and printed circuit boards, and metals
used for wires and other electric connections and frames, which are
not suitable for disposal by incineration.
[0005] In recent years, the phenomenon of acid rain has become
serious due to discharge of sulfur oxide into the atmosphere by
extensive use of fossil fuels such as gasoline and fuel (heavy)
oils. Acid rain penetrates into the ground and causes dissolution
of the solders present in discarded electronic appliances buried in
the landfills, thereby producing contamination of the groundwater
with lead. If such contaminated groundwater is ingested by people
for many years, the accumulation of lead in their bodies may result
in lead poisoning (plumbism).
[0006] From this viewpoint, it has been desired in the electronics
industry to use a lead-free solder alloy for soldering electronic
components. Conventional lead-free solder alloys are Sn-based
alloys such as Sn--Ag, Sn--Sb, Sn--Bi, and Sn--Zn alloys.
[0007] Sn--Ag alloys form a eutectic composition of Sn--3.5Ag, but
the melting temperature, i.e., eutectic temperature of this
composition is relatively high (221.degree. C.). Even if this
eutectic composition having 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 270.degree. C., which may
cause thermal damage to heat-sensitive electronic components during
soldering, thereby deteriorating or even destroying their
functions.
[0008] Of Sn--Sb alloys, an Sn--5Sb alloy has the lowest melting
temperature, but its melting temperature is as high as 235.degree.
C. on the solidus line and 240.degree. C. on the liquidus line.
Therefore, its soldering temperature is in the range of from
280.degree. C. to 300.degree. C., which is still higher than that
of an Sn-3.5Ag alloy, and thermal damage to heat-sensitive
electronic devices cannot be avoided.
[0009] Sn--Bi alloys have a eutectic composition of Sn-42% Bi with
a melting temperature of 139.degree. C., which is considerably
lower than that of the above-described conventional Sn--Pb eutectic
solder (183.degree. C.). Therefore, Sn--Bi alloys may be considered
to be potentially usable as lead-free solders from the viewpoint of
melting temperatures. However, Sn--Bi alloys are too brittle and
hard to meet the mechanical properties such as tensile strength and
elongation that are required for solder alloys.
[0010] Sn--Zn alloys have a eutectic composition of Sn-9% Zn with a
melting temperature of 199.degree. C. This eutectic composition is
advantageous in that its melting temperature is close to that of
conventional Sn--Pb eutectic solder (183.degree. C.). Another
advantage of Sn--Zn alloys is that their mechanical properties are
superior to those of Sn--Pb alloys. However, Sn--Zn alloys have
poor solderability.
[0011] In order to improve the solderability of Sn--Zn alloys and
further enhance their mechanical properties, a number of solder
alloys based on an Sn--Zn alloy and containing one or more
additional elements such as Ag, Cu, Bi, In, Ni, and P have been
proposed.
[0012] With these improved Sn--Zn based solder alloys containing
one or more additional elements, a considerably satisfactory
solderability can be achieved as long as these alloys are used in
the form of wire solder for soldering with a soldering iron along
with an appropriate soldering flux. However, when these Sn--Zn
based solder alloys are used in the form of a solder paste, which
is a mixture of a powder of such a solder alloy and a soldering
flux in a viscous fluid, they do not work successfully or not
exhibit satisfactory solderability. Thus, a solder paste formed
from an Sn--Zn based solder alloy may cause non-wetting or
dewetting during soldering whereby the areas of s a substrate to be
soldered are not wetted by the solder completely and have solder
balls thereon. Even though the solder after soldering appears by
visual observation to be sound or wet the areas completely, it may
contain internal pit-like voids at the interface between the solder
and the substrate, as can be seen when the solder is peeled
off.
[0013] The solderability of a solder paste formed from an Sn--Zn
based solder alloy can be improved by using an activated flux
containing a strong activator which can effectively enhance the
spreading of the molten solder alloy. However, the strong activator
can react with zinc (Zn) present in the solder alloy to oxidize or
corrode it in a short period of time and cause the solder to lose
its metallic nature, resulting in a significant deterioration in
solderability. Thus, a solder paste formed from an Sn--Zn based
solder alloy or other Zn-containing solder alloy (such a solder
paste being hereinafter referred to as "Zn-containing solder paste)
generally suffers the problem of detrimental alterations, i.e.
aging, after storage for a relatively short period.
[0014] The detrimental alterations with time (hereinafter referred
to as aging) of a Zn-containing solder paste appear as a change in
viscosity. Thus, a Zn-containing solder paste immediately after it
is prepared has an appropriate viscosity which makes it easy to
stir with a spatula or stirring rod and which is suitable for
application by screen printing or feeding with a dispenser.
However, after it is stored for a certain period of time on the
order of one or two weeks, it has an increased viscosity due to
aging and is difficult to stir.
[0015] When such an aged solder paste having an increased viscosity
is applied to a printed circuit board by screen printing or with a
dispenser and then heated in a reflow furnace, the solder may not
entirely melt or a large quantity of oxides formed in the solder
alloy may cause the formation of solder balls as the solder melts.
Even with a freshly prepared Zn-containing solder paste, if reflow
soldering is conducted in an oxygen-containing atmosphere such as
air, the molten solder may not spread adequately and good
solderability may not be obtained. Therefore, reflow soldering must
be performed in an inert gas atmosphere, thereby adding to
operating costs.
SUMMARY OF THE INVENTION
[0016] It is an object of the present invention to provide a
Zn-containing solder paste which is less susceptible to aging and
has an extended shelf life.
[0017] It is another object of the invention to provide a
Zn-containing solder paste which exhibits satisfactory
solderability when reflow soldering is performed in air.
[0018] It has been found that the addition of a glycidyl ether
compound to a flux used to prepare a Zn-containing solder paste has
an effect of stabilizing the resulting solder paste against aging
and improving the solderability of the solder paste, although the
mechanism for this effect has not been clearly elucidated.
[0019] Such a favorable effect of the glycidyl ether compound on
stability and solderability of a solder paste is prominent
particularly with Zn-containing solder pastes, but it can be
achieved to some extent with other solder pastes. Thus, the
addition of a glycidyl ether compound to a flux is generally
effective in all kinds of solder pastes in order to retard aging
and improve solderability.
[0020] According to one aspect of the present invention, a solder
paste comprises a powder of a solder alloy, preferably of a
lead-free solder alloy, in admixture with a soldering flux with a
glycidyl ether compound being added to the soldering flux.
[0021] More particularly, the present invention provides a
lead-free solder paste comprising a powder of a zinc-containing,
lead-free solder alloy in admixture with a soldering flux (namely,
a Zn-containing solder paste) wherein a glycidyl ether compound is
added to the soldering flux, preferably in an amount of from 0.1%
to 5.0% by weight of the flux.
[0022] The glycidyl ether compound is a compound having at least
one glycidyl ether moiety 1
[0023] in the molecule.
[0024] In a preferred embodiment, the Zn-containing solder alloy is
an Sn--Zn based alloy including an Sn--Zn alloy, and more
preferably an Sn--Zn--Bi alloy.
[0025] In accordance with the present invention, a Zn-containing
solder paste, which has been considered to have a shelf life of
approximately one week when stored at 25.degree. C. or below, can
be stored for 4 weeks or longer without a significant aging,
thereby facilitating the practical use of Zn-containing solder
pastes.
BRIEF DESCRIPTION OF THE DRAWING
[0026] The attached drawing is a graph showing the change as a
function of time in the viscosity of Zn-containing solder pastes
prepared in the Examples during storage at 25.degree. C.
DESCRIPTION OF PREFERRED EMBODIMENTS
[0027] The aging of a conventional Zn-containing solder paste
during storage occurs due to the high reactivity of Zn with an acid
or alkali which is normally present in the flux as an activator or
other additive, whereby Zn is selectively corroded (oxidized) in
the solder paste. The Zn-containing solder alloy in a solder paste
is in the form of a fine powder having an increased surface area,
which accelerates the corrosion of Zn. The addition of a glycidyl
ether compound to the soldering flux used to form a Zn-containing
solder paste according to the present invention can stabilize the
solder alloy against corrosion and retard aging of the solder
paste.
[0028] The aging of a solder paste appears as deteriorations in
various properties of the solder paste with time. Particularly in
the case of a Zn-containing solder paste, such deteriorations can
be evaluated by an increase in viscosity of the solder paste.
[0029] In the context of the present invention, the solderability
of a solder paste is evaluated by the degree of spreading and the
formation of solder balls when the paste is applied to a substrate
and heated in a reflow furnace in air or other oxygen-containing
atmosphere.
[0030] As described previously, these properties of conventional
Sn-Zn solder alloys have been improved by addition of one or more
elements such as Ag, Bi, In, Ni, and P to the alloys. This approach
is successful when these alloys are used in the form of wire solder
for soldering with a soldering iron, but it is not effective when
the alloys are used in the form of solder pastes. It was difficult
in the prior art to use a solder paste comprising an Sn--Zn or
other Zn-containing solder alloy due to rapid aging and concomitant
increase in viscosity and deterioration in solderability. The
present invention can effectively retard the aging of such a solder
paste and facilitates practical use thereof.
[0031] The Zn-containing solder alloy used in the solder paste
according to the present invention may be any Zn-containing solder
alloy, but it is preferably an Sn--Zn based solder alloy. An Sn--Zn
based solder alloy is normally comprised predominantly of Sn (tin)
and contains Zn (zinc), and it may optionally contain one or more
additional alloying elements. Non-limiting examples of Sn--Zn based
solder alloys include Sn--Zn alloys such as an Sn--9%Zn alloy,
Sn--Zn--Bi alloys such as an Sn--8%Zn--3%Bi alloy, Sn--Zn--Ag
alloys such as an Sn--9%Zn--0.2%Ag alloy, and Sn--Zn--Bi--Ag alloys
such as an Sn--8%Zn--11%Bi--0.1%Ag alloy.
[0032] As described above, the concept of the present invention can
be applied to other solder alloys such as conventional Sn--Pb
solder alloys, Sn--Ag solder alloys, and Sn--Bi alloys. Thus,
solder pastes of these solder alloys can be improved with respect
to aging by adding a glycidyl ether compound to the flux used to
form the solder paste. However, the improvement is particularly
significant in Zn-containing solder pastes since they are highly
susceptible to aging as discussed above.
[0033] Generally, a powder of a solder alloy used to form a solder
paste may be prepared by the gas atomizing or centrifugal atomizing
technique. The average particle diameter of the powder is usually
in the range of from 200 to 400 mesh or even smaller.
[0034] The soldering flux which is admixed with a powder of a
Zn-containing solder alloy to form a solder paste and to which a
glycidyl ether is added is not limited to a specific class, and it
may be the same as those which have been used in conventional
solder pastes, e.g., those pastes containing a powder of an Sn--Pb
eutectic solder alloy.
[0035] A typical flux used in a solder paste is a rosin flux. Also
in the solder paste according to the present invention, it is
preferable to use a rosin flux as a soldering flux, although other
soldering fluxes, particularly non-water soluble fluxes such as
those based on a synthetic resin may be used.
[0036] The rosin flux used in the present invention is preferably
an activated rosin flux which contains an activator. An activated
rosin flux comprises a rosin as a main ingredient and minor amounts
of an activator and optionally one or more other additives such as
a thixotropic agent, these ingredients being dissolved in a
solvent. The rosin may be a natural rosin, also called colophony,
or a modified rosin, or a mixture of these. Useful activators are
amine hydrohalides, particularly amine hydrobromides such as
diphenylguanidine hydrobromide, cyclohexylamine hydrobromide,
triethanolamine hydrobromide, and the like, although other
activators may be used. Non-limiting examples of the thixotropic
agent are hardened castor oil and fatty acid amides such as
stearamide. Non-limiting examples of the solvent are
.alpha.-terpineol and alkylene glycol ethers such as diethylene
glycol monohexyl ether.
[0037] A typical composition of an activated rosin flux on a weight
basis is as follows:
[0038] 40%-60% of a rosin and/or a modified rosin,
[0039] 3%-8% of a thixotropic agent,
[0040] 0.5%-3% of an activator, and
[0041] 30%-50% of a solvent.
[0042] Each of these ingredients may be comprised of one or more
compounds. Other additives including a co-activator such as an
organic halide compound may be present in the rosin flux.
[0043] In accordance with the present invention, a glycidyl ether
compound is added to the soldering flux during or after the
preparation of the flux. The flux is then uniformly admixed with a
powder of a Zn-containing solder alloy to prepare a solder paste.
The resulting solder paste has improved resistance to aging and
improved solderability. Thus, the solder paste can be stored for a
prolonged period, e.g., 4 weeks or more, without substantial
detrimental alterations such as an increase in viscosity, and can
be used in reflow soldering in air to form satisfactory soldered
joints.
[0044] Preferably, the glycidyl ether compound has the following
general formula (I): 2
[0045] where R is a saturated or unsaturated, aliphatic or aromatic
hydrocarbon group which may optionally contain at least one
hydroxyl group, and n is an integer from 1 to 4.
[0046] The aliphatic hydrocarbon group includes a cyclic aliphatic
hydrocarbon groups. When R is a monovalent group, it may be an
alkyl group, preferably having 3 to 20 carbon atoms, such as
propyl, n-butyl, sec-butyl, amyl, hexyl, 2-ethylhexyl,
2-methyloctyl, decyl, dodecyl, tridecyl, or stearyl; an alkenyl
group such as allyl; or an aryl group such as phenyl, naphthyl,
biphenyl, or tolyl.
[0047] Non-limiting examples of the compound of Formula (I) having
a monovalent R group (n=1) include allyl glycidyl ether, propyl
glycidyl ether, n-butyl glycidyl ether, phenyl glycidyl ether,
biphenyl glycidyl ether, tolyl glycidyl ether, 2-ethylhexyl
glycidyl ether, sec-butyl phenyl glycidyl ether, 2-methyloctyl
glycidyl ether, dodecyl glycidyl ether, stearyl glycidyl ether, and
the like. Non-limiting examples of the compound of Formula (I)
having a polyvalent R group (n.gtoreq.2) include ethylene glycol
diglycidyl ether, sorbitol polyglycidyl ether, glycerol triglycidyl
ether, trimethylolpropane polyglycidyl ether, and the like.
[0048] The glycidyl ether compound is preferably added to the
soldering flux in an amount of 0.1% to 5.0% by weight based on the
flux. Of course, two or more glycidyl ether compounds may be added
together in a total amount of 0.1% to 5.0% by weight. If the amount
is less than 0.1% by weight, a substantial improvement in
resistance to aging and in solderability cannot be obtained.
Addition of more than 5% by weight of the glycidyl ether compound
has an adverse effect on solderability. Preferably the glycidyl
ether compound is added to the flux in an amount of 1.0% to 3.0%
and more preferably 1.5% to 3.0% by weight.
[0049] In the solder paste according to the present invention, the
proportions of the soldering flux and the Zn-containing solder
alloy powder in a solder paste are not critical but they are
usually in the range of from 5% to 50% and preferably from 5% to
30% of the soldering flux and from 95% to 50% and preferably from
95% to 70% of the solder alloy powder on a weight basis.
[0050] The Zn-containing solder paste according to the present
invention has improved resistance to aging. Therefore, it can be
stored for a prolonged period while keeping its viscosity within
such a range that the solder paste can be smoothly applied by
screen printing or with a dispenser and without a substantial
deterioration in solderability. As a result, the stored solder
paste can be subjected to reflow soldering with little or no
formation of solder balls or oxides of the solder alloy, thereby
achieving reflow soldering with a Zn-containing solder paste in a
reliable manner.
[0051] Since a Zn-containing solder alloy is capable of having a
melting point close to that of the most common Sn--Pb eutectic
solder alloy, Zn-containing solder pastes can be used to perform
reflow soldering in a reflow furnace designed for reflow soldering
with conventional Sn--Pb solder pastes. Thus, Zn-containing solder
pastes are advantageous in that they make it possible to perform
lead-free reflow soldering in existing reflow soldering facilities.
However, due to their short storage period of about one week or so,
the application of Zn-containing solder pastes has been limited in
the prior art. The present invention can eliminate or alleviate
this limitation of Zn-containing solder pastes.
[0052] 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, all
percentages are by weight unless otherwise indicated.
EXAMPLES
[0053] In each of the following examples, solder pastes consisting
of 10% of a soldering flux and 90% of a solder alloy powder were
prepared by thoroughly mixing these two constituents. The fluxes
used in the examples had the compositions shown in Table 1. The
solder alloy powder was a powder of a Zn-containing alloy having
the composition: Sn--8%Zn--3%Bi. The resulting solder pastes were
evaluated with respect to aging (increase in viscosity) and
solderability (reflowing properties) in the manner described
below.
[0054] [Testing Method for Aging]
[0055] A freshly prepared solder paste to be tested was stored in a
thermostatic chamber at 25.degree. C. for maximum 5 weeks while the
viscosity of the paste was determined at regular intervals. The
resistance to aging of the solder paste was evaluated by the length
of time before the viscosity of the solder paste increased to 350
pascal-seconds [Pa.s] or higher, which is no longer suitable for
use by screen printing or with a dispenser, and ranked as
follows:
1 .circleincircle. (Excellent): Four weeks or more, .largecircle.
(Good): Two weeks or more but less than four weeks, X (Poor): Less
than two weeks.
[0056] Thus, the above-mentioned length of time is indicative of
the shelf life of the solder paste.
[0057] [Testing Method for Solderability]
[0058] Solderability was tested by applying a solder paste to be
tested, which was either freshly prepared or stored as above, to a
printed circuit board and heating at 230.degree. C. in air to
simulate heating in a reflow furnace in air. The solderability was
evaluated by the reflowing properties during heating by observing
the conditions of spreading of the solder on the boards and the
formation of solder balls due to oxidation of the solder alloy to
form infusible oxides and ranked as follows:
2 .circleincircle. (Excellent): No solder balls observed,
.largecircle. (Good): Some solder balls observed, X (Poor): Little
or no melting.
[0059] Table 1 shows, in addition to the composition of the
soldering flux used in weight percent, the resistance to aging as
evaluated above and the solderability after storage for 7 days.
[0060] The change in viscosity with time of the solder pastes of
Examples 1 and 2 and Comparative Example 1 during storage for 35
days is shown in the attached drawing. The change in the
solderability (reflowing properties) with time until storage for 25
days is given in Table 2.
3TABLE 1 Ingredient of flux with figures in wt % or Comparative
items tested with results Example Example on the 7th day 1 2 3 4 1
2 Polymerized rosin 44 41 45.5 43 46 39 Diphenylguanidine HBr 2 2 2
2 2 2 Hardened castor oil 5 5 5 5 5 5 Phenyl glycidyl ether 2 5 --
-- -- 7 2-ethylhexyl glycidyl ether -- -- 0.5 3 -- --
2,3-dibromo-1-propanol 2 2 2 2 2 2 .alpha.-terpincol 45 45 45 45 45
45 Change in viscosity .circleincircle. .circleincircle.
.largecircle. .circleincircle. X .circleincircle. Solderability
(Reflowing) .circleincircle. .largecircle. .circleincircle.
.circleincircle. .largecircle. X
[0061]
4TABLE 2 Example Solderability after stored for the indicated days
Number Initial (day 0) 3 days 7 days 14 days 25 days Comparative
.circleincircle. .largecircle. .largecircle. X X Example 1 Example
1 .circleincircle. .circleincircle. .circleincircle.
.circleincircle. .circleincircle. Example 2 .largecircle.
.largecircle. .largecircle. .largecircle. .largecircle.
[0062] As can be seen from the drawing and Table 1, the
Zn-containing solder pastes of examples 1 and 2 formed from an
activated rosin flux to which a glycidyl ether compound was added
showed a very slow increase in viscosity with time, and their
viscosities remained at a level suitable for use by screen printing
or with a dispenser after storage for 35 days.
[0063] In contrast, the Zn-containing solder paste of Comparative
Example 1, which illustrates a conventional solder paste formed
from an activated rosin flux, showed a rapid increase in viscosity
with time. Its viscosity exceeded 300 pascal-seconds on the seventh
day and soon after increased to 3 50 pascal-seconds or higher which
is no longer suitable for use. As can be seen from Tables 1 and 2,
such a solder paste having an increased viscosity showed
deteriorated solderability with the formation of fine solder balls
or of an oxidized alloy. Such deteriorated solderability was
already observed approximately on the third day.
[0064] Thus, it is apparent that the addition of a glycidyl ether
compound to a flux used to form an Zn-containing solder paste is
effective for retarding aging of the solder paste and concomitant
deterioration of the solderability thereof, thereby providing the
solder paste with a significantly extended shelf life.
[0065] However, the glycidyl ether compound, if added in an
excessively large amount, adversely affects the solderability of
the solder paste, as shown in Comparative Example 2.
[0066] 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.
* * * * *