Method of cleaning printed circuit boards using water

Solder pastes having vehicles including blends of low boiling point alcohols and relatively high boiling point alcohols are described which leave residues which may be cleaned using only water are described. The low boiling point alcohols have a boiling point range of between about 65.degree. and about 150.degree. C. whereas the high boiling point alcohols have a boiling point in rhe range of about 150.degree. to about 270.degree. C. The solder pastes also use water-soluble organic acids as fluxing agents such as compounds of the formula: ##STR1## where R is an electron withdrawing group such as fluorine, chlorine, bromine, iodine, sulfur, hydroxyl, nitrile, and benzyl. Other suitable formic-acid fluxing agents include, but are not limited to, adipic acid, polyacrylic acid, methacrylic acid and polymethacrylic acid. The compounds clean oxides from the printed circuit boards (PCBs) under assembly and then volatilze leaving a residue to be cleaned away. The cleaning step involves rinsing with water. No undesired residue remains indicating that the organic acids of the invention are effective in cleaning boards. Malic acid is a preferred organic acid fluxing agent.

CROSS-REFERENCES TO RELATED APPLICATIONS 
This application is related to U.S. patent application Ser. No. 07/524,540, 
filed on May 17, 1990 which relates to solder flux formulations containing 
malic acid; U.S. patent application Ser. No. 583,129, filed on Sept. 17, 
1990 which relates to solder pastes incorporating acrylic acid therein to 
improve fluxing action; and U.S. patent application Ser. No. 07/521,871 
filed May 11, 1990 relating to solder paste vehicles using blends of 
monofunctional and polyfunctional alcohols. 
FIELD OF THE INVENTION 
The invention relates to solder paste formulations, and in one aspect, more 
particularly relates to solder paste formulations that produce a residue 
that is cleanable with water. 
BACKGROUND OF THE INVENTION 
Solder formulations, also known as solder creams or solder pastes, are 
homogeneous blends of a soft solder alloy typically in a powder form 
dispersed in a liquid medium conventionally containing a fluxing 
composition or flux, an organic solvent, and a thickening agent which will 
give the desired viscous or paste-like consistency to the solder 
formulation. Such solder formulations can be applied to the surfaces of 
locations in a number of various ways, such as by screen printing, or by 
means of a dispenser such as a syringe, or simply by dipping the site to 
be soldered into the solder paste formulation so that the viscous paste 
adheres to the site, such as an electronic component lead. 
Recently, solder paste formulations have been used increasingly by the 
electronics industry, particularly in the automated manufacture of printed 
circuits in which leadless miniature electronic components are surface 
mounted on a printed circuit board (PCB) to which a solder paste 
formulation has previously been applied, such as by screen printing. The 
PCB is then subjected to a sufficiently high temperature, for example by 
means of a heated conveyor belt, to cause the flux and solder alloy in the 
formulation to liquefy and contact the electronic component leads so that 
on subsequent cooling of the PCB, the components will remain soldered on 
the PCB. 
The rheological properties of the solder paste dictate the screen 
printability of the material. Hence, in order to obtain good line 
definition on the PCB, the rheology of the paste must fall within a 
relatively narrow process window. Solder paste rheology is largely 
determined by those components of the solder paste which are collectively 
known as the vehicle. Generally, solder paste vehicles consist of solvents 
to dissolve the rosin fluxes or fluxing agents and the activators, and 
other additives to achieve the proper rheology. Currently, cyclohexanol 
(sorbitol) derivatives and various Cellosolve.TM. mono- and dialkyl ethers 
of ethylene glycol and derivatives thereof are used in commercial solder 
pastes. Problems encountered with conventional solvents include poor 
solubility of the organic acid fluxing agents in the solvents--which is 
sometimes addressed by using more solvent than is considered desirable, 
and an inability to obtain high viscosities (poor rheological control). 
Many of these components do not volatilize under reflow conditions and 
therefore result in a residue which needs to be removed by 
chlorofluorocarbon (CFC), terpene, aqueous, exotic gas or other cleaning 
techniques. Some of these materials are hazardous or toxic and CFCs have 
been linked to the underside degradation of atmospheric ozone. Thus, it 
would be an improvement in the art to eliminate residues and the materials 
used to remove them. 
For some uses in the electronics industry, it is desirable to use as the 
flux composition of the solder formulation a material which is 
non-corrosive and which will provide, after the heating and cooling steps, 
flux residues which are themselves non-corrosive and non-conducting. For 
this reason, rosin-based flux compositions are widely used in the 
commercially available solder paste formulations specifically made for use 
in the manufacture of surface mounted electronic components. 
Alternatively, more reactive fluxing compositions may be used, which leave 
residues which are corrosive and/or conductive. Often a somewhat corrosive 
fluxing composition is desired so that the oxides which form on the metal 
surfaces to be soldered may be removed to permit the subsequently formed 
solder bond to be stronger both physically and electrically. However, it 
is necessary to remove these residues formed by means of either aqueous or 
organic solvent systems to ensure that the resulting solder circuit is 
non-corrosive. 
The use of solder paste formulations containing such rosin-based or more 
reactive fluxes has a number of disadvantages. First, because the 
non-corrosive residues (such as rosins) tend to be sticky, they prevent 
repetitive automatic testing of the circuit. Rosin-based fluxes tend to 
leave copious amounts of residue on the circuit. Additionally, such 
residues are unsightly and therefor, as with the corrosive flux residues 
which are also unattractive, will need to be removed. The removal step 
involves extra production equipment, time and material. 
Secondly, flux residues tend to be hygroscopic and may thereby cause 
spattering. Thirdly, some fluxes permit solder particles in the paste to 
move away from the solder site and give rise to the formation of numbers 
of discrete small balls of soft solder around the soldered joints, which 
can create electrical short circuits. 
Because of these and other disadvantages, it is desirable and often 
essential to meet specifications, to remove the flux residues and any 
solder balls as much as possible. Often, however, their removal is 
difficult or impossible, particularly from areas of the PCB underneath the 
electronic components. 
As noted, a common procedure is to use an aqueous or organic solvent in the 
removal of flux residues. Water is preferred because it will not leave an 
objectionable residue itself. Organic solvents are more effective, but 
less desirable because they are more expensive and particularly because 
they are more troublesome to dispose of. A particular class of organic 
solvents that had attained widespread use was the halocarbons, such as the 
chlorofluorocarbons (CFCs), because they would volatilize after cleaning. 
However, these materials are particularly inert and their eventual 
decomposition is involved in the undesirable depletion of atmospheric 
ozone. Thus, the solder paste formulations of the present invention do not 
contain and do not require CFCs for residue removal. 
Thus, for these and other reasons the prior solder fluxing compositions are 
less preferred, and it would therefore be advantageous to discover a new 
fluxing composition that would avoid one or more of these disadvantages. 
For example, it would be advantageous to provide a solder pastes whose 
residues could be easily cleanable by water. 
SUMMARY OF THE INVENTION 
Accordingly, it is an object of the present invention to provide a solder 
paste fluxing composition that would provide an oxide removing agent 
during the soldering process. 
It is another object of the present invention to provide a novel solder 
paste which is an oxide removing agent that would either readily 
volatilize or be readily removed with water. 
It is yet another object of the invention to provide a solder paste 
composition that would accomplish the above objectives, but also be easy 
to formulate. 
In carrying out these and other objects of the invention, there is 
provided, in one form, a method of mounting electronic components on a 
printed circuit board providing a residue cleanable with water which 
involves first formulating a solder paste by mixing together a solder 
paste vehicle having a solvent blend selected from the group consisting of 
a blend of low boiling point alcohols having a boiling point in the range 
of about 65.degree. and about 150.degree. C. and high boiling point 
alcohols having a boiling point in the range of about 150.degree. and 
about 270.degree. C.; a fluxing agent; and a solder powder comprising a 
metal selected from the group consisting of lead, tin, antimony, silver 
and mixtures thereof. Next, the solder paste is applied to a metallic 
surface to be soldered. At least one electronic component is placed on the 
metallic surface to be soldered. Then the solder paste is caused to reflow 
and form a solder bond producing a residue on the metallic surface. 
Finally, the residue is removed by washing the surface with water.

DETAILED DESCRIPTION OF THE INVENTION 
It has been discovered that water is an inexpensive, readily available and 
readily disposable solvent for washing away the residues left from a 
solder reflow operation, such as the assembling of electronic components 
to a printed circuit board (PCB) using particular solder paste 
formulations. Typically, an organic acid is used in the solder paste to 
serve as an effective fluxing agent by removing the oxides from the 
metallic surfaces to which the component is to be bonded. The organic acid 
should be soluble in water so that water may wash away the residue after 
the bonding step. 
Suitable organic acid materials that are soluble in water include compounds 
of the formula: 
##STR2## 
where R is an electron withdrawing group, which added as the sole or joint 
fluxing agent to solder pastes is an effective flux for the soldering of 
solders such as tin/lead, tin/lead/silver and tin/lead/silver/antimony to 
metals such as copper, aluminum, etc. In one aspect, the R group may be 
fluorine, chlorine, bromine, iodine, sulfur, nitrile, hydroxyl, benzyl or 
some other electron withdrawing group. Phrased another way, R must be 
electronegative, although there is no specific degree of electronegativity 
required. 
Other suitable organic fluxing agents include, but are not limited to, 
adipic acid, acrylic acid, polyacrylic acid, methacrylic acid and 
polymethacrylic acid. These excellent fluxing compositions produce 
substantially less residue than conventional fluxing agents based on rosin 
chemistry, and in some cases produce no residue at all. Where residue is 
present, however, it may be easily and quickly removed with formic acid. 
In one aspect of the invention where R is hydroxyl, the formula above 
represents malic acid, HOOCCH.sub.2 CH(OH)COOH. As will be explained, 
malic acid was unexpectedly found to perform surprisingly better than some 
of the other organic acids screened. It has also been discovered that 
there are a number of ways to implement this concept. 
More specifically, these formic acid-soluble organic acids have been found 
to serve as good fluxing agents for soldering Sn/Pb and Sn/Pb/Ag 
compositions to copper over and above some of the other organic acids. It 
will be appreciated, however, that other solders are expected to be useful 
in conjunction with malic acid, and that other metals besides copper are 
expected to be effectively cleaned and bonded to by formulations 
containing malic acid. The flux can be prepared in a variety of ways, 
including, but not limited to: 
(1) The addition of organic acid powder to a solder paste vehicle. 
(2) A solution of the water-soluble acid in a number of solvents, 
including, but not limited to alcohols such as isopropanol and 
Cellosolve.TM. derivatives. 
With these solder formulations, tests have shown that these organic acids 
are effective fluxing agents, resulting in solder reflow bonds having 
minimum residue, which can be easily removed by water. The addition of the 
organic acids acid to the paste vehicle is straightforward since the 
material is a solid powder at room temperature. 
The fluxing action of these organic acids appears to be derived from the 
ability of the acid to reduce surface oxides. Interestingly, malic acid 
can also be used directly as a flux for solder balls or spheres. 
The solder used in this process should be untreated solder powder. The 
metals of the solder may include, but are not limited to lead, tin, 
antimony, silver and mixtures thereof. These types of solder metals were 
found to exhibit excellent reflow characteristics. 
In the instances where malic acid is used in conjunction with a solvent to 
provide a vehicle for a solder formulation, in one aspect the proportion 
of malic acid in the vehicle ranges from about 0.1 to about 60 wt. % of 
the vehicle, and in a preferred aspect from about 0.3 to about 40 wt. % of 
the vehicle. 
It will be appreciated that the proportion of organic acid as a proportion 
of the solder paste vehicle will vary depending on the particular 
formulation. For example, high temperature solder pastes or pastes for 
highly oxidized metal surfaces may require a different organic acid 
proportion to those outlined above. The balance of the fluxing composition 
may be any of the customary materials. It will be appreciated that 
although the other common materials, such as the rosins, may be used in 
conjunction with the compounds of the invention, that some of these 
typical materials contribute to flux residues and should not be employed 
to take full advantage of the water-soluble characteristics provided by 
the flux compositions of the present invention. 
Current solder pastes are based on rosin fluxing agents which degrade 
during reflow to give dark brown, crusty residues that are commonly 
removed with chlorofluorocarbon solvents. By way of contrast, all residues 
from the solder pastes of this invention are completely water soluble, 
eliminating the use of CFCs in printed circuit board cleaning operations. 
Suitable alcoholic solvents for dissolving the fluxing agents in these 
pastes are a blend of at least two types of alcohols: low boiling point 
alcohols and relatively high boiling point alcohols. The low boiling point 
alcohols boil within a range of about 65.degree. to about 150.degree. C., 
preferably from about 70.degree. to about 130.degree. C. In turn, the high 
boiling point alcohols boil within a range of about 150.degree. to about 
270.degree. C., preferably from about 150.degree. to about 220.degree. C. 
Generally, the low boiling point alcohols have a molecular weight range 
from about 46 to about 130, preferably from about 88 to about 130. The 
relatively higher boiling point alcohols have a molecular weight range 
from about 60 to about 180, preferably from about 100 to about 180. 
Representative low boiling point alcohols include, but are not limited to 
ethanol; 2-methyl-1-propanol; neopentyl alcohol; 2,2-dimethyl-3-pentanol; 
isopropanol; 1-pentanol; 2-pentanol; and mixtures thereof and the like. 
Representative high boiling point alcohols include, but are not limited to 
2-butoxy-ethanol; 2-(2-ethoxyethoxy)ethanol; octanols; pentanediols; 
dodecanol; 3-phenyl-1-propanols; benzyl alcohol; 
2,3-dimethyl-2,3-butanediols; dimethycyclohexanols; ethylene glycol; 
naphthaleneethanol; and mixtures thereof and the like. 
The solder pastes of this invention may optionally contain paraformaldehyde 
as a blowing agent and formic acid generator. In one embodiment of the 
present invention, the solder paste has the following formulation: 
about 4 to about 10 wt. % solvent blend; which may include: 2-5% low 
boiling point alcohol; and 2-5% high boiling point alcohol; 
about 1 to about 5wt. % organic acid fluxing agent (e.g. malic acid and/or 
adipic acid and/or polyacrylic acid); 
about 85 to about 94 wt. % solder powder; and 
0 to about 5 wt. % paraformaldehyde. 
Other specific formulations would also be expected to be useful. Further, 
in a finalized formulation, various thixotropes, in a proportion of about 
1-2%, will likely be added in order to achieve proper rheology for optimum 
screen printing. In one embodiment, if both malic acid and polyacrylic 
acid are used, a preferred proportion of malic acid is about 1%, and that 
of polyacrylic acid from about 0.5 to 1%. 
The proposed solder pastes containing a water-soluble organic acid fluxing 
agent can replace current solder fluxing systems based on rosin additives. 
For example, malic acid can replace the acid in conventional abietic 
acid-based fluxes. The addition of malic acid to the solder pastes will 
achieve excellent solder reflow properties and eliminate the residue 
problems that plague conventional solder pastes. This lack of residue 
reduces the need for any board cleaning with ozone-depleting CFCs after 
solder reflow. 
Solder pastes with the compositions given above were formulated and tested. 
The pastes exhibited good screen printability and excellent reflow 
characteristics for pastes containing 2 to 5% organic acids. Lower 
concentrations of acids did result in reflow; however, insufficient 
wetting of copper surfaces due to inadequate oxide removal occurred. The 
introduction of a low molecular weight alcohol is required to speed the 
dissolution of the acid and the paraformaldehyde. The higher boiling point 
alcohol is required in order to give the paste proper rheological 
properties both at room temperature and at soldering temperatures. 
Reflowing the solder in an open atmosphere resulted in the formation of a 
small amount of clear to white residue on the top of the solder joint. The 
residue could be easily removed by rinsing with water. The length of the 
water rinse varied with the paste formulation. The shortest rinsing times 
(10 seconds) were observed for the malic and polyacrylic acid based 
pastes, while pastes employing adipic acid were found to require several 
minutes of rinsing. The formulation of a water cleanable solder paste is 
significant because it will eliminate the use of CFCs in printed circuit 
board cleaning. Boards can be cleaned either by rinsing briefly in water 
or soldering in a nitrogen/water atmosphere. 
With any of these organic acid fluxes and methods of this invention, no 
retooling would be required in the existing assembly line. If some residue 
does remain with certain of these systems, it will further be appreciated 
that it may be washed away with water. Depending on the exact organic 
fragments from the non-metallic compounds, the water to rinse them will 
probably need to be treated as well. Nevertheless, these concerns are 
appreciably less than those presented by the CFC cleaning agents or other 
organic cleaning agents. 
The invention will be described in more detail with respect to the 
following illustrative examples. 
EXAMPLES 1-15 
Various organic acids were added, in quantities ranging from about 10 to 
about 100 mg., to an aluminum pan containing 10 to 15 30 mil diameter 
solder spheres. A couple of drops of isopropyl alcohol (IPA) were also 
added to the pans. The pans were heated on a hot plate to temperatures 
above the melting point of the tin/lead solder spheres. Whether or not the 
solder balls coalesced was observed. Coalescence is a measure of whether 
reflowing of the solder and wetting of the pan occurred. The following 
organic acids were evaluated. 
TABLE I 
______________________________________ 
Organic Acid Screening 
Solder ball 
coalescence 
Example Acid occurred? 
______________________________________ 
1 Abietic acid Yes 
2 Adipic acid Yes 
3 Ascorbic acid Yes 
4 Acrylic acid Yes 
5 Citric acid Yes 
6 2-Furoic acid Yes 
7 Malic acid Yes 
8 Polyacrylic acid Yes 
9 Acetic acid No 
10 Cyclohexane carboxylic acid 
No 
11 Formic acid No 
12 Hexanoic acid No 
13 4-Hydroxybutyric acid/Na salt 
No 
14 Maleic acid No 
15 Oxalic acid No 
______________________________________ 
EXAMPLE 16 
Water Soluble Residue Solder Paste Formulation 
A solder paste was formulated with the following composition: 
0.85544 g--Solder powder: 63% Sn/36.65% Pb/0.35% Sb 
0.06190 g--Paraformaldehyde (Aldrich Chemical Co., 95%) 
0.04659 g--Malic Acid (Aldrich, 99%) 
0.15591 g--Isopropanol (IPA, Fischer Chemical Co. &gt;99.9%) 
0.00965 g--2-Butoxy ethanol (Aldrich, &gt;99%) 
The solder paste was placed in copper plated pans and reflowed. 
Temperatures were measured by a surface thermometer and thus may have read 
lower than the actual temperature of the pan. 
The solder paste reflowed and the pan was removed from the heat source when 
the temperature was 220.degree.-250.degree. C. A white-bubbly residue was 
visible under a microscope at 30.times.. The residue was removed by 
physically scraping it from the surface or by dissolving the residue by 
placing the pan in an ultrasonic water bath for 1 to 3 minutes. 
EXAMPLES 17-20 
Additional Solder Paste Formulations 
The following solder pastes were formulated as noted. 
EXAMPLE 17 
______________________________________ 
Amount, g. 
Wt. % Component 
______________________________________ 
0.87712 80.0 Solder powder 
0.01632 1.5 Paraformaldehyde 
0.06401 5.8 Malic acid 
0.05875 5.4 IPA(isopropyl alcohol) 
0.08157 7.4 2-(2-Ethoxyethoxy)ethanol (Aldrich, &gt;99%) 
______________________________________ 
This formulation produced good reflow. 
EXAMPLE 18 
______________________________________ 
Amount. g. 
Wt. % Component 
______________________________________ 
2.7908 90.7 Solder powder (Sn/Pb/Sb) 
0.0307 1.0 Paraformaldehyde 
0.0026 0.08 Malic acid 
0.1197 3.9 2-(2-Ethoxyethoxy)ethanol 
0.1315 4.3 IPA 
______________________________________ 
This formulation did not produce good reflow results. 
EXAMPLE 19 
To 0.9385 g. of the formula of Example 18, the following were added: 
______________________________________ 
Amount. g. Component 
______________________________________ 
0.0479 IPA 
0.0822 Paraformaldehyde 
0.0797 2-(Ethoxyethoxy)ethanol 
______________________________________ 
This formulation did not produce good reflow results. 
EXAMPLE 20 
To 1.4771 g. of the formulation of Example 18, the following were added: 
______________________________________ 
Amount. g. Component 
______________________________________ 
0.0949 2-(Ethoxyethoxy)ethanol 
0.0558 IPA 
0.0079 Malic acid 
______________________________________ 
The formulation of Example 20 cause some reflow, but copper/solder wetting 
was not adequate. A white residue remained after reflow that was removed 
by 10 seconds in an ultrasonic water bath, leaving behind a shiny solder 
surface. 
EXAMPLE 21 
Water Soluble Residue Solder Paste Formulation 
A solder paste was formulated with the following composition: 
89.8% 2.0192 g--Solder powder: 63% Sn/36.65% Pb/0.35% Sb 
0.4% 0.0091 g--Malic Acid (Aldrich Chemical Co., 99%) 
0.3% 0.0074 g--Adipic Acid (Aldrich, &gt;99%) 
1.4% 0.0319 g--Paraformaldehyde (Aldrich, 95%) 
4.0% 0.0902 g--Isopropanol (IPA, Fischer Chemical Co. &gt;99.9%) 
4.0% 0.0900 g--2-(2-Ethoxyethoxy) ethanol 
The solder paste was placed in copper plated pan and heated until reflow 
occurred (190.degree.-250.degree. C.). A light brown residue formed after 
reflow which was not as soluble in water as the case of malic acid alone. 
Addition of one drop of concentrated formic acid (88%, Fisher Scientific) 
to the paste mixture in the pan prior to reflow did not result in the 
degree of wetting that was observed for the initial paste. However, a 
second drop of formic acid was added to the paste at 250.degree. C. This 
resulted in removal of all visible residue. 
EXAMPLE 22 
Polyacrylic Acid Solder Paste Formulation 
A solution of polyacrylic acid (average molecular weight of about 2000% and 
isopropanol was added to a copper plated can containing about ten 
commercially available solder balls. Solder ball reflow resulted in a 
small amount of residue that disappeared after 10 seconds in a hydrosonic 
water bath. During the first run, the pan was accidentally dropped while 
the residue was still a liquid. The residue ran off the pan leaving no 
visible residue. 
EXAMPLE 23 
Malic Acid Solder Paste 
A very low residue, water cleanable solder paste was formulated with the 
following composition. 
90.0% Solder powder 
1.0% Malic acid (Aldrich) 
7.0% Solution of 17% Polyacrylic acid in 2-(2-ethoxyethoxy)ethanol 
0.3% Ethylenediamine tetraacetic acid 
2.0% 2,5-Hexanediol (Aldrich 99%) 
This paste was tested in a manner similar to that of Example 16. The reflow 
characteristics were good and resulted in very minimal residues. All 
residues that were present were removed by 1 minute rinses with water. 
Screen printability was also excellent. 
EXAMPLE 24 
Screen Printability of Malic Acid Solder Paste 
The following formulation was prepared: 
40.4671 g--Solder Powder 
0.8582 g--Malic Acid 
1.6176 g--Polyacrylic acid (PAA)/2-(2-ethoxyethoxy)ethanol solution, 
(18% PPA) 
1.1520 g--Saturated malic acid/2-(2-butoxyethoxy)ethanol solution 
(primarily solvent) 
1.0176 g--2,5-Hexanediol 
Additional solder powder (about 5 g) was added to achieve proper viscosity 
for screen printing. This result may be achieved by adding less 
hexanediol, on the order of about 1-1.5%. Although the print definition 
after screening of this material was poor, appearance after reflow was 
good. A small amount of white residue was observed after reflow. Rinsing 
for 30 seconds in water was found to remove the residue. 
EXAMPLE 25 
Screen Printability of Malic Acid Solder Paste 
A solder paste formulation of the following materials was made: 
48.5623 g--Sn/Pb Solder Powder 
0.9195 g--Malic acid 
1.7152 g--18% PAA in 2-(2-ethoxyethoxy) ethanol 
1.1905 g--Saturated malic acid in 2-(2-butoxyethoxy) ethanol 
0.4214 g--2,5-Hexanediol 
The screen printability of the paste was excellent. The screen printing was 
performed both by hand and using the automatic screen printer. Reflow 
characteristics were excellent. A small amount of white-to-clear residue 
remained after reflow, which was readily removed by water rinses. 
It is apparent that the use of the compounds of this invention provide 
useful fluxing agents in solder paste vehicles. Malic acid is an 
especially useful organic acid fluxing agents that performs surprisingly 
better than other acids, particularly in leaving low portions of residue. 
Additional embodiments of the invention include, but are not limited to 
soldering and assembling in a water/nitrogen atmosphere. The discovery of 
a solder paste with water-soluble residues is significant because it will 
eliminate the use of CFCs in printed circuit board cleaning. 
It will be appreciated that many modifications may be made in the exact 
implementation of the invention illustrated in the above examples which 
would still fall within the spirit and scope of the invention as claimed 
herein. For example, it is anticipated that the processing conditions, 
modes or sequences of addition of the vehicles and fluxing compositions, 
and exact combinations of flux components may be altered to optimize the 
invention by one skilled in the art. It is also expected that the method 
of this invention could be used to facilitate assembly of PCBs by having 
solder paste containing the water-soluble organic acid fluxing agents 
screened thereon.