 |
Description  |
|
|
TECHNICAL FIELD
The invention relates to electronic circuit board manufacturing, and to
those steps in the procedure which follow soldering of components. In
particular, the invention concerns methods to clean soldering flux from
assembled or partially assembled circuit boards.
BACKGROUND ART
The manufacturing and assembly of electronic circuit boards is a complex
and multistep process resulting in products of considerable and well-known
utility. In general, a circuit is superimposed on a metal surface by
patterning areas of conductivity and resistance. This is done using a
suitable photoresist coating as protection and etching unprotected areas.
The resultant is a set of metal layers on the board. As part of this
process, in addition, certain features of this process involve procedures
which utilize protection of areas, which are not to be affected, using
adhesive tape. After completion of these processes, the tape is removed,
and the adhesive cleansed from the board. The adhesives subject to
cleaning are generally silicone or vinyl polymers.
After the unassembled boards have been fabricated, the components which
will serve as gates, resistors, capacitors, etc. are soldered to the
fabricated board. The soldering process, which results in a board
assembled with its components, uses fluxes to insure good bonding between
the metal of the fabricated board and the metal contact of the component
through the interdigitation of the solder into each metal surface. To
achieve this, the solder must be capable of wetting the surface and there
can be no impurities impeding the integration. The flux, therefore, is
used to degrease and decontaminate the surface to allow free access of the
solder. Finally, the remainder of the flux must be cleansed from the
assembled board.
The invention provides a superior composition for removing the flux
residues in an efficient and thorough manner. The composition contains, in
admixture, a terpene or terpenol, a surfactant, and an aprotic polar
organic solvent. The admixture may be dispersed in water for use,
depending on the nature of the flux to be removed.
Compositions of these substances have been sold for removal of adhesive
residues at the above-mentioned, different, earlier stage of the process.
However, the chemical and physical nature of the tape residues are very
different from the flux compositions, and as far as Applicants are aware,
these compositions have not heretofore been used for flux removal.
The fluxes which are subject to removal by the compositions of the
invention are those customarily used in circuit board manufacture. These
are of two major types: synthetic water soluble and rosin-containing
fluxes. Rosin is a by-product of the distillation of pine tree sap and
contains about 90% rosin acids and 10% neutral material, both of which are
basically tricyclic isoprenoid condensates. In the operation of this flux,
the rosin acids react with the surface metal oxide layer and do not attack
the freshly exposed metal. The rosin-containing fluxes may also be
activated using organic amines, acids, amides or halogen-containing
materials and are thus used to produce rosin mildly activated (RMA) or
rosin-activated (RA) fluxes. Most of the activating substances,
apparently, decompose to liberate hydrochloric acid so that metal chloride
salts formed from reaction with the clean metal surface become suspended
in the flux.
The synthetic or water-soluble fluxes are also used in soldering
electronics assemblies; the active components are usually amine
hydrochlorides or hydrobromides along with water-soluble organic acids.
These activators are dissolved in a solvent such as a glycol ether,
alcohol, or their blends with water. Occasionally these fluxes can be
washed away simply by using water; however, because they are so reactive,
their use in electronics components may be disadvantageous. These fluxes
can also be cleaned away usefully using the compositions of the invention,
as their water solubility often is not maintained during the soldering
process; while some of the residues remain water-soluble, others are not.
For example, decomposition products caused by heating of the organic
materials are often insoluble, and the chlorides of certain metals, for
example lead chloride, may be tightly bonded to the substrate. Therefore,
simple rinsing with water is often inadequate to remove the putatively
"water-soluble" residues generated from these fluxes.
There is no dearth of compositions which have been tried for solder flux
removal in the electronic board manufacturing process. For example, U.S.
Pat. No. 4,640,719 describes compositions with a minimum of 60% terpene
with the balance of the composition being a surfactant. A typical
composition illustrated as the most preferred embodiment contains 90%
limonene, and the 10% detergent component composed of 6.5%
poly(7)ethoxynonyl phenol; 2.1% poly(10)ethoxynonyl phenol, and 1.4%
sodium dioctyl sulfosuccinate. This composition is asserted to be capable
of removing adhesives as well as flux; however, no data are given to
support this asserted property.
U.S. Pat. No. 4,276,186 to Bakos et al describes a composition of at least
50% by weight of N-methyl-2-pyrrolidone and at least 5% by weight of a
water-miscible alkanolamine, with the balance of 0-35% of a nonpolar
hydrocarbon or halogenated hydrocarbon solvent. This composition is said
to be useful in cleaning solder flux or flux residues from substrates. The
cleaning process evidently is advanced by relatively high temperatures.
The Printed Circuits Handbook Koombs, C. F. ed., on pages 16-2 to 16-13
describes methods and compositions used for cleaning soldered circuit
boards and freeing them of flux residues. This description shows the
effect of various single component solvents such as perchloroethylene and
various fluorocarbons to dissolve fluxes. Other alternatives include
saponification of the acidic components in rosin-based solvents, and the
use of water, although in large quantities with incomplete results, to
remove the residues of putatively water-soluble fluxes.
DISCLOSURE OF THE INVENTION
The invention provides a composition which is capable of removing residues
of a variety of soldering fluxes with surprising efficiency and
convenience. Furthermore, the materials used in the composition are
relatively environmentally safe as compared to the chloroflurocarbons. The
proportions of materials in the composition are adjustable according to
the substrate flux; however, the composition itself, which can, when in
use, be mixed with water or not, is of the following components: 25-90%
terpene and/or terpenol; 1-10% surfactant; and the remainder, or 5-65%, of
a polar aprotic solvent. As the terpene or terpenol is the least polar
component, its proportion will be varied according to the polarity of the
residue; the major portion of the offset of the proportion of terpene
component lies in the amount of the polar aprotic solvent.
Therefore, the invention is directed to a method to remove soldering flux
residues from assembled or partially assembled circuit boards by
contacting the circuit board to be cleaned with the composition of the
invention for a time period effective to solubilize or mobilize the flux
residue, followed by removal of the cleaning composition.
MODES OF CARRYING OUT THE INVENTION
The compositions of the invention are useful for cleaning the residues of
fluxes from assembled or partially assembled electronic circuit boards.
The fluxes generally utilized leave residues which include terpene-derived
acids and other tricyclics, and degradation products thereof, from
rosin-based fluxes, and end products of the various organic monomers used
in synthetic fluxes, as well as insoluble or complexed salts of inorganic
ions from the synthetic flux type. The residues therefore differ in
mechanical and chemical character from the tape residues encountered at
earlier stages of manufacture. The tape residues are generally polymeric,
nonaromatic adhesives such as polyvinyl or polyacrylic structures of high
molecular weight, and silicone. The adhesive residues are problems for the
fabricators of the bare board without components; the flux residues which
are the targets of the method of the invention are on assembled or
partially assembled boards wherein components have been soldered to the
basic board, and thus are problems for assemblers.
The compositions have three major components. A major portion of the
composition--25-90%, and preferably, in most cases, 50-80%--is terpene
and/or terpenol or mixture of either or both which contributes a nonpolar
character to the composition. This element of the composition is essential
because of nonpolar components occurring in the flux residues. This is
particularly true of residues of rosin-based fluxes, since these are
themselves terpenoid by-products. As used herein, "terpene" refers to a
polyunsaturated oligomer of isoprene units and includes such
representative examples as limonene and pinene. "Terpenols" refers to the
hydration products of the terpenes, which contain one or more alcohol
functional groups resulting from the addition of water to one or more of
the double bonds contained in the terpenes. Of course, mixtures of the
terpenes and terpenols are satisfactory and, in fact, may be more
economically convenient than purified single components. These materials
can be obtained from the distillate of pine oil or of the oils extracted
from citrus fruit. The optimal percentage of the terpene or terpenol
components depends on the composition of the residue--the more polar the
residue, the lower the percentage of terpene or terpenol desirable in the
cleaning composition.
There is a variable range of desirable compositions depending on the nature
of the flux residue, although the generally described composition is
satisfactory for a wide range of flux residue targets. A high percentage
of abietic acid, pimeric acid, or dehydroabietic acid as would be
contributed by rosin in the flux is better removed by compositions
containing higher amounts of terpenes and/or terpenols in the cleaning
composition. On the other hand, organic water-soluble fluxes which
contain, at least initially, water-soluble organic compounds, especially
hydrohalide salts such as hydrobromides and hydrochlorides, require less
terpene or terpenol in the cleaning composition. (Entirely inorganic
fluxes such as zinc chloride are generally not used in electronics
assembly; therefore, the requirements for cleaning of these fluxes are not
considered here.)
The second component of the composition is a polar aprotic solvent, such as
dimethylsulfoxide (DMSO), dimethylformamide (DMF), N-methylpyrrolidone
(NMP), butyl cellusolve or ethylene glycol monobutyl ether (BC), and
tripropylene glycol monomethyl ether (TPGME) . In general, low molecular
weight forms of polyethylene glycol ethers are suitable in the
composition. The amount of polar aprotic solvent is in the range of 5-65%
and, like the percentage of terpene/terpenol, is a function of the nature
of the flux residue--the more polar synthetic organic fluxes requiring a
higher percentage of the solvent. In general, the terpene/terpenol
component and polar aprotic solvent component are balanced to meet the
particular requirements of the flux residue encountered.
The third component of the cleaning compositions of the invention is 1-10%
surfactant. The percentage depends on ease of rinse desired. The nature of
the surfactant depends on the ratio of nonpolar (terpene/terpenol) to
polar (polar aprotic solvent) component. Mixtures of surfactants are
generally used. Highly desirable surfactants include non-ionic surfactants
such as the polyethoxylated alcohols, alkylphenols, and dialkylphenols, as
well as anionic surfactants such as the aryl and alkyl sulfonates,
carboxylates and phosphates. Cationic detergents are generally less
preferred, because they typically are not as free rinsing as other
classes. Amphoterics may be used but most commercially available
amphoterics are too polar to provide satisfactory emulsification. These
surfactants are designed to be used in shampoos or aqueous high salt
compositions. Generally, the percentage of surfactant is less if an ionic
surfactant is employed as these forms are more readily effective; higher
percentages of the nonionic forms are forms are used. Particularly
preferred as surfactants are ethoxylated nonyl phenols and high molecular
weight sulfonates.
In carrying out the method of the invention, the cleaning composition
described herein is used neat or is emulsified or otherwise mixed with
water for application to the assembled or partially assembled circuit
board. The composition can be applied in a dip tank, in a vat wherein
agitation is provided, or the composition can be sprayed or brushed onto
the board. Contact with the cleaning composition is maintained for a time
sufficient to remove the flux residue, generally from 20 sec to 10 min,
more typically 2-3 min, shorter times being required if agitation is
provided, for example by stirring the composition or by ultrasonic
agitation. The temperature of treatment is generally ambient temperature.
Slightly elevated temperatures can also be used, but are not necessary.
When sufficient contact has been made with the circuit board to mobilize
and detach the flux residue, the cleaning composition containing the
residue is removed from contact with the board, which may optionally be
further washed with water, with increasing dilutions of cleaning
composition and water, and finally with water.
If desired, the cleaning composition can be reclaimed and reused in further
operations, using generally accepted techniques.
The following examples are intended to illustrate but not to limit the
invention.
EXAMPLE 1
Comparison of Various Polar Aprotic Solvents in Invention Compositions
15 1.times.2 inch coupons were cut from a board obtained from Advanced
Circuit Services, and Kester 1429 flux (composition) was applied on half
of one side of each coupon. 5 coupons were heavily coated and placed in a
300.degree. F. oven for 5 min; 5 were lightly coated and placed in a
240.degree. F. oven for 5 min, and the remaining 5 coated with a medium
coating were allowed to dry at room temperature and dipped in molten
solder.
After cooling to room temperature, each sample was cleaned with a
composition containing 80% limonene, 6.5% poly(7)ethoxynonyl phenol, 2.1%
poly(10)ethoxynonyl phenol, and 1.4% sodium dioctyl sulfosuccinate, plus
10% aprotic polar organic solvent. The control was the EC-1 described in
U.S. Pat. No. 4,640,719, which is as described above except that the 10%
polar aprotic organic solvent is replaced with an additional 10% of
limonene. The cleaning compositions which contain solvent are dramatically
more efficient in removing flux residue from these samples, as is shown in
Table 1.
TABLE 1
______________________________________
Time for Complete Removal
Organic
Solvent Baked (300.degree. C.)
Baked (240.degree. C.)
Dipped
______________________________________
None (EC-1)
* 20 sec 20 min
NMP 10 sec 5 sec 2 min
TPGME 20 sec 7 sec 2 min,
20 sec
BC 15 sec 6 sec 1 min
DMSO 20 sec 12 sec 2 min,
20 sec
______________________________________
*Within 2 min, most of the flux was removed. However, a heavy scum was
left on the part. The parts removed from the other solutions had only a
light film remaining.
An additional 5 coupons were heavily treated with Kester 1429 flux, allowed
to dry at room temperature, and then dipped in molten solder so that the
results shown in Table 1, column 3 were repeated but with a heavier
coating of the flux. The results of this variation are shown in Table 2.
TABLE 2
______________________________________
Solvent Time for Complete Removal
______________________________________
None (EC-1) Most removed in 2 min, however still
incomplete after 15 min
NMP 3 min, 30 sec (90% removed in 2 min)
TPGME 5 min, 10 sec
BC 3 min, 25 sec
DMSO 4 min, 30 sec
______________________________________
Following the same procedure for the dipped coupons, various fluxes were
tested with the invention compositions and the time required for complete
flux removal is shown in Table 3. The values are given in minutes (') and
seconds (").
TABLE 3
______________________________________
Kester #
Type % Solids EC-1 NMP BC DMF
______________________________________
1429 SA ? 20' 2' 1' 2'
197 RMA 37 1',20"
1',20"
1',45"
40"
185 RMA 36 18" 16" 20" 25"
1544 RA 68 2' 3' 2',45"
2',25"
1585 RA 36 1-1.5'
1-1.5'
1-1.5'
1-1.5'
1588 RA 20 40",1'
30",55"
30" 45"
1515S RA 16 50",25"
30" 25" 25"
932-M1 RMA 13 >13'* 1',45"
8'* 2',20"
______________________________________
*white residue remains
In these evaluations, substitution of the polar aprotic solvent for a
portion of the limonene resulted in a more efficient cleaning of the board
for some fluxes. The extent of improvement depends on the nature of the
flux used and on the pretreatment of the coated coupon.
* * * * *
|
|
 |
|
 |
Description |
|
