U.S. patent application number 12/952962 was filed with the patent office on 2011-03-17 for soldering process.
Invention is credited to Luis A. Aguirre, Lawrence C. Kay, Erik J. Severin.
Application Number | 20110062215 12/952962 |
Document ID | / |
Family ID | 38019315 |
Filed Date | 2011-03-17 |
United States Patent
Application |
20110062215 |
Kind Code |
A1 |
Kay; Lawrence C. ; et
al. |
March 17, 2011 |
SOLDERING PROCESS
Abstract
A process by which molten solder is purified in-situ, making the
soldering process more efficient and yielding better results,
particularly for lead-free soldering. Lead-free solder becomes
practical for use since the temperature for reliable soldering is
reduced. A layer of active additive is maintained on the surface of
molten solder for scavenging metal oxide from the solder and
assimilating metal oxide into a liquid layer. The active additive
is an organic liquid having nucleophilic and/or electrophilic
groups. As an example, a layer of dimer acid maintained on a wave
soldering apparatus scavenges metal oxide from the bath, and
assimilates dross that may form on the surface. Scavenging metal
oxide cleanses the bath and lowers viscosity of the solder, and PC
boards or the like soldered on the wave have reliable solder
joints.
Inventors: |
Kay; Lawrence C.; (Sherman
Oaks, CA) ; Severin; Erik J.; (Saint Paul, MN)
; Aguirre; Luis A.; (San Antonio, TX) |
Family ID: |
38019315 |
Appl. No.: |
12/952962 |
Filed: |
November 23, 2010 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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11108400 |
Apr 18, 2005 |
7861915 |
|
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12952962 |
|
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60562964 |
Apr 16, 2004 |
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Current U.S.
Class: |
228/52 ;
75/407 |
Current CPC
Class: |
C22B 7/04 20130101; C22B
7/006 20130101; C22B 9/10 20130101; B23K 35/3618 20130101; C22B
9/103 20130101; B23K 37/047 20130101; C22B 9/106 20130101; B23K
35/3006 20130101; B23K 35/262 20130101 |
Class at
Publication: |
228/52 ;
75/407 |
International
Class: |
C22B 9/02 20060101
C22B009/02; B23K 3/06 20060101 B23K003/06 |
Claims
1. A soldering apparatus comprising: a molten solder bath
comprising a single layer of an organic liquid active additive on
at least a portion of the surface of the molten solder bath,
wherein the organic liquid active additive (i) is a liquid at the
temperature of the solder bath, (ii) comprises nucleophilic and/or
electrophilic groups, (iii) acts as an oxygen barrier to the
surface of the molten solder bath, and (iv) assimilates oxide of at
least one metal in the bath, to thereby purify the solder bath; a
soldering region where soldering occurs, and a pump effective to
circulate purified solder from the molten solder bath to the
soldering region, where the circulating purified solder is devoid
of the liquid active additive.
2. The soldering apparatus of claim 1, wherein the organic liquid
active additive comprises dimer or trimer acid.
3. The soldering apparatus of claim 2, wherein the organic liquid
active additive comprises a major portion of dimer acid.
4. The soldering apparatus of claim 3, wherein the dimer acid is
saturated.
5. The soldering apparatus of claim 3, wherein the dimer acid is
unsaturated.
6. The soldering apparatus of claim 2, wherein the dimer acid has
an average carbon number in the range of from 24 to 60.
7. The soldering apparatus of claim 1, wherein the soldering region
comprises a wave, fountain or cascade of molten solder.
8. The soldering apparatus of claim 7, wherein the pump is
effective to draw solder from near the bottom of the molten solder
bath to the soldering region such that the liquid active additive
from the surface of the solder bath is not part of the molten
solder in the soldering region for contact with a surface to be
soldered.
9. The soldering apparatus of claim 1, wherein the molten solder
bath comprises a lead-free solder.
10. The soldering apparatus of claim 1, wherein the molten solder
bath comprises a lead-tin solder.
11. The soldering apparatus of claim 1, wherein the molten solder
bath is at a temperature of no more than 260.degree. C.
12. The soldering apparatus of claim 1, wherein the amount of
liquid active additive is sufficient to maintain a layer at least a
molecule thick all across a quiescent surface of the molten solder
bath.
13. The soldering apparatus of claim 1, wherein the amount of
liquid active additive is sufficient to form a layer at least three
millimeters thick on a quiescent surface of the molten solder
bath.
14. The soldering apparatus of claim 1, wherein the liquid active
additive is liquid at room temperature.
15. A method of removing dross from molten solder, comprising: (i)
removing dross from a solder pot in a solder apparatus, (ii)
placing the dross from (i) in a receptacle, and (iii) adding a
liquid active additive comprising dimer acid to the receptacle
containing the dross to form a mixture; and (iv) heating the
mixture from (iii) above the melting point of the solder to thereby
assimilate dross in the liquid active additive and release molten
solder from the dross.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is a divisional of U.S. patent application
Ser. No. 11/108,400 filed Apr. 18, 2005, and claims the benefit of
priority of U.S. Provisional Application No, 60/562,964 filed Apr.
16, 2004, both of which are incorporated herein by reference in
their entireties.
FIELD
[0002] This invention addresses the important issue of purity or
cleanliness of a solder bath. It has been discovered that
scavenging metal oxide from molten solder is of great importance in
producing reliable and reproducible solder joints. This is of
particular importance when using lead-free solder alloys. An active
additive layer on the surface of the bath is used to scavenge and
assimilate metal oxide. This has the surprising result of reliable
lead-free solder joints produced at a temperature no more than the
260.degree. C. limit for electronic components.
BACKGROUND
[0003] Electronic components are commonly soldered to printed
circuit (PC) boards with a lead-tin solder. A maximum soldering
temperature of 260.degree. C. (500.degree. F.) has become a
standard in the industry and this limit has propagated to many
other parameters. For example, most components to be soldered to
printed circuit boards are rated for a maximum temperature of
260.degree. C. Continuous soldering apparatus is built to operate
at a maximum temperature of about 260.degree. C. Even the printed
circuit (PC) boards (sometimes called printed wiring boards, PWB)
are generally constructed for a maximum soldering temperature of
about 260.degree. C.
[0004] There is a desire to eliminate hazardous lead from solder,
and there are even moves afoot to ban the use of lead. Lead-free
solder will be required in many products which now use lead-tin
solder. Exemplary substitute lead-free solder alloys include
tin-silver and tin-silver-copper alloys having about 95-96.5% tin
and 3.5-5% silver. (There is a eutectic at 3.5% silver in the
binary Ag--Sn phase diagram.) Copper is often in the range of about
0.3 to 1%. Some tin based solders have been proposed with additions
of antimony, bismuth, indium, nickel and/or zinc. Tin is the base
for the lead-free solder alloys and is typically present as more
than 90% of the alloy.
[0005] Soldering processes have been developed which make automatic
soldering of PC boards highly reliable. Plated-through holes are
filled, ample solder fillets are almost always found, and bridging
between closely spaced connector points is rare. To achieve similar
reliability with lead-free solders such as tin-silver solder
alloys, it is generally found that soldering temperatures of 270 to
275.degree. C. (520.degree. F. or higher) are necessary. Clearly
this is higher than the conventional 260.degree. C. limit and has
the potential for damaging components. Therefore, reducing the
temperature for soldering with such lead-free substitute alloys is
highly desirable, particularly in view of the coming requirement
for use of lead-free solder.
[0006] Another issue which is of concern with respect to both the
lead-tin solders and substitute solder alloys is accumulations of
dross on the solder. Dross is an accumulation of oxides of the
metals in the solder. It can form a solid crust on the molten
solder as it accumulates during operation of soldering apparatus.
Sometimes it is appropriate to shut down continuous operating
apparatus and manually ladle dross from the solder bath. Even when
not shut down, manual removal of dross from the surface of the hot
solder is practiced. Substantial amounts of solder can be lost into
the dross, which then needs to be processed to recover and recycle
the metal. Even when dross is not visible, a small amount on the
surface of the molten solder can lead to bridging of solder between
closely spaced leads and/or failure to wet surfaces to be soldered,
so that incomplete or poor joints are obtained.
[0007] Due to study of this invention, we are now confident that
purity of the solder bath is an important factor in difficulty with
soldering. It appears that metal oxide distributed in the bath
interferes with wetting and successful soldering. The oxide may
raise solder viscosity, provide nucleation sites for
crystallization at higher temperatures than solidification in
absence of such oxides, and may cause weakness in solder joints.
Thus, in addition to visible dross on the surface, a significant
issue is purity of the molten solder bath.
[0008] It is found in practice of this invention that formation of
dross in continuous soldering apparatus can be significantly
minimized or even eliminated by durable additives. Most surprising,
the temperature at which viable soldering takes place with
lead-free solder alloys has been reduced by as much as 30.degree.
F. (16 to 17.degree. C.). Soldering temperature for tin-silver
alloys can be brought below the 260.degree. C. limit.
[0009] Furthermore, there is a surprising reduction in viscosity of
the molten metal in a wave solder apparatus, for example. This may
contribute to the excellent solder joints obtained at
plated-through holes in PC boards. Such improvements in solder
joints are also due to better wetting as shown by wetting balance
tests. Cleanliness of the solder bath is believed responsible.
[0010] A variety of wave soldering, fountain soldering and cascade
soldering systems which may be used in practice of this invention
are described and illustrated in ASM Handbook, Volume 6, Welding,
Brazing, and Soldering. Exemplary apparatus, as illustrated in FIG.
7 which is largely copied from Metals Handbook, page 1088,
comprises a large vat or "solder pot" in which molten solder 10 may
be held at the desired soldering temperature. A pump (not shown)
draws solder from near the bottom of this molten mass and forces it
upwardly through one or more slot nozzles 11 from which the solder
flows laterally like a waterfall, either in one direction or both
directions from the slot, and back into the vat. The upper surface
of the flowing solder is commonly referred to as a "wave".
[0011] When such a wave soldering apparatus is used for soldering,
a printed circuit board 12 is moved across the apparatus so that
the lower face of the PC board contacts the upper surface of the
wave 13 of molten solder. Molten solder wets the surfaces to be
soldered, and wicks into the plated-through holes and around leads,
and makes good solder joints therebetween. In such automatic
apparatus PC boards are fed into the wave in close succession for
high capacity production. There are also so-called fountain
soldering machines and cascade soldering systems with which this
invention is useful.
[0012] Sometimes a portion of the solder in wave soldering
apparatus overflows into a secondary reservoir and molten solder
returns from the reservoir to the larger solder pot. Dross forming
on the solder due to oxidation upon exposure to air also overflows
and accumulates in the secondary reservoir, from which it may be
removed. Some dross also may flow along the surface of the wave.
There is appreciable turbulence where such "waterfalls" of molten
solder meet the surface of the solder bath, providing surfaces
where metal oxides or dross may form. The deleterious effects of
such phenomena are ameliorated by this invention.
[0013] In practice of this invention, a sufficiently extensive
liquid active additive layer is maintained on the molten solder
bath during the soldering process for maintaining purity or
cleanliness of the bath. The layer provides the surprising result
of significantly lowering the temperature at which reliable solder
joints are obtained. The liquid layer preferably comprises a
material that is stable at the temperature of the bath, effectively
bars oxygen in air from reaching a quiescent surface of the bath,
and has the ability to assimilate oxide of at least one metal in
the bath and remain liquid for a commercially acceptable time.
Typically, the material comprises an organic molecule with
nucleophilic and/or electrophilic end groups. Carboxylic --COOH end
groups are particularly preferred.
[0014] An exemplary substance comprises a dimer acid such as
described in greater detail hereinafter. A dimer acid has
previously been used as a cover or oxygen barrier material on the
surface of a bath of molten metal as lead and tin are melted
together to formulate a lead-tin solder alloy. Minor amounts of
dimer acid have been formulated into soldering flux
compositions.
SUMMARY
[0015] In an embodiment of practice of this invention, a liquid
layer of active additive for scavenging and assimilating metal
oxide is introduced onto a solder bath, and a surface to be
soldered is contacted with the molten solder. The invention
comprises scavenging metal oxide from a bath of molten metal.
Furthermore, the invention comprises assimilating oxidized metal in
an active additive.
BRIEF DESCRIPTION OF THE FIGURES
[0016] FIG. 1 illustrates the steps of lowering and raising a test
coupon in a wetting balance test and a plot of force versus time
during such a wetting balance test.
[0017] FIG. 2 illustrates schematically the types of wetting curves
one may see in such wetting balance tests.
[0018] FIG. 3 is a graph of force versus time from wetting balance
tests.
[0019] FIG. 4 is a graph of force versus time for wetting balance
tests at a series of temperatures.
[0020] FIG. 5 is a graph of force versus time for wetting balance
tests illustrating practice of this invention.
[0021] FIG. 6 illustrates a "bag" for additive for a solder
bath.
[0022] FIG. 7 illustrates semi-schematically and in partial
transverse cross section, an exemplary soldering apparatus which
may be used in practice of this invention.
[0023] FIGS. 8 and 9 are metallographic cross sections of solder
joints.
DETAILED DESCRIPTION
[0024] This invention comprises a process by which molten solder is
purified in-situ, making the soldering process more efficient and
yielding better results, particularly for lead-free soldering.
Lead-free solder becomes practical for use since the temperature
for reliable soldering is reduced.
[0025] In a preferred embodiment of practice of this invention, a
liquid active additive layer is maintained on a molten solder bath
during a soldering process for maintaining purity or cleanliness of
the bath. The active additive comprises a material that scavenges
metal oxide from the molten metal, that is stable at the
temperature of the bath, that effectively bars oxygen in air from
reaching a quiescent surface of the bath, and has the ability to
assimilate oxide of at least one metal in the bath. The active
additive should remain liquid for a commercially acceptable time.
Typically, the material comprises an organic molecule with
nucleophilic and/or electrophilic end groups. Carboxylic end
groups, such as in a dimer acid, are particularly preferred.
[0026] Although the active additive is stated to be stable at the
temperature of the bath, this does not mean that it is stable for
an infinitely long time. As described hereinafter, even a "stable"
additive gradually degrades, oxidizes and/or becomes loaded with
assimilated metal to the extent that it is viscous or gummy after a
period of exposure to the harsh conditions of a molten solder bath.
On the other hand, a material that vaporized rapidly, smoked badly
or quickly degraded and became solid, would not be considered
stable.
[0027] The description commences with an outline of an easily
understood example of a soldering process with details and
variations, as appropriate, added later. Wave soldering is
convenient as a way of describing the subject matter.
[0028] Thus, in its simplest form, an active additive is added to
the molten solder in a wave soldering apparatus. The active
additive is an organic liquid of lower density than solder and
quickly spreads across at least the exposed quiescent surface of
the molten solder bath. Metal oxide dross formation decreases and
already formed dross on the surface is collected in a darkening
liquid that appears to include active additive and assimilated
metal oxide. Metal oxides in the molten solder are promptly
scavenged when the active additive is added to the bath. The
resulting cleansed or purified solder bath has lowered viscosity,
and surprisingly, allows reliable solder joints to be formed at
lower temperatures than previously believed feasible with
conventional lead-free solder. Although not measured, it is
believed that the cleansed molten metal wets solid surfaces to be
soldered more effectively than does metal which still contains
metal oxides.
[0029] Sufficient active additive is put onto the bath of molten
solder to form a layer across the exposed quiescent surface of
solder in the pot in apparatus such as illustrated in FIG. 7.
Preferably a sufficient amount of the active additive is added to
provide a layer on the quiescent surface of the bath that will last
at least a full shift of a work day, or at least four hours so that
maintenance is not required more often than that. A printed circuit
board (or other object to be soldered) is brought into contact with
at least a surface of the molten solder so that solder wets metal
surfaces on the board and components, and flows to fill
plated-through holes, secure electrical leads, cover contact pads,
etc. In a wave soldering apparatus, the PC board contacts the top
of the wave of molten solder pumped from near the bottom of the
bath. Additive may not be present on the dynamic surface of the
wave or in a turbulent area where the wave falls into the bath, but
the active additive enhances soldering by scavenging metal oxide
from the main volume of the bath.
[0030] Preferably, the amount of additive is sufficient to promptly
assimilate metal oxide from the surface of the bath. Preferably one
forms a layer with a thickness of as much as three millimeters, or
even more, on at least a portion of the surface of the molten
solder. Such a thick layer is desirable since it can remain
effective at least four hours and ordinarily at least a full day
before the bath should be cleaned. A thinner layer may be suitable
when the apparatus is operated for shorter periods. One might, for
example, form a relatively thinner layer initially and later add
additional material to form a thicker layer. Or a thinner layer
might be used and skimmed and replenished more often. Preferably,
enough additive is used to maintain a liquid layer which has
assimilated metal oxide rather than allowing solid-appearing dross
to accumulate on the surface.
[0031] Other conventional aspects of the soldering process need not
be described, such as, for example: application of flux to the PC
board before soldering, use of a hot air knife or the like for
removing excess solder, or any desired prior or subsequent cleaning
considered desirable for such a PC board. Examples of soldering
other objects besides PC boards need not be described.
[0032] Residues of additive do not appear to remain on PC boards to
which solder has been applied from a bath on which dimer acid, for
example, has been added. Benign solvents for cleaning any such
residues exist, such as iso-propyl alcohol and aqueous solutions
containing surfactants, for example. Toluene is effective for
dissolving and removing dimer acid, which is a presently preferred
active additive.
[0033] A dimer acid is a high molecular weight di-carboxylic acid
which is liquid (typically viscous at room temperature), stable and
resistant to high temperatures. It is produced by dimerization of
unsaturated or saturated fatty acids at mid-molecule and often
contains 36 carbons. (For example, a trimer acid which contains
three carboxyl groups and 54 carbons is analogous. A trimer of
shorter fatty acid chains with about 36 total carbons would be
equivalent.) Fatty acids are composed of a chain of aliphatic
groups containing from 4 to as many as 30 carbon atoms (although
commercially useful fatty acids have up to 22 carbon atoms) and
characterized by a terminal carboxyl group, --COOH. The generic
formula for all carboxylic acids above acetic acid is
CH.sub.3(CH.sub.2).sub.xCOOH. The carbon atom count includes the
--COOH group.
[0034] Fatty acids may be saturated or unsaturated. In some cases
there may be dimers of mixed saturated and unsaturated fatty acids.
Exemplary saturated fatty acids include palmitic acid (C16) and
stearic acid (C18). Unsaturated fatty acids are usually
vegetable-derived and comprise aliphatic chains usually containing
16, 18 or 20 carbon atoms with the characteristic end group --COOH.
Among the most common unsaturated acids are oleic acid, linoleic
acid and linolenic acid, all C18. Saturated fatty acids are
preferred in practice of this invention. They are more stable at
elevated temperature than unsaturated fatty acids with appreciable
double bonds. Aromatic fatty acids are also known, for example
phenyl-stearic, abietic acid and other fatty acids derived from
rosin. Rosin acids comprise C20 monomers and may contain a
phenanthrene ring (e.g. abietic and pimaric acids). Dimers
containing phenyl rings are quite acceptable when the rings are
linked (if more than one is in a molecule) solely at one corner so
that the molecule has "flexibility". Phenyl rings are effectively
flat and may stack to form a monomolecular film on molten solder.
The aromatic dimer acids may also be more thermally stable than
similar carbon number aliphatic dimer acids.
[0035] The dimers (and higher oligomers) of fatty acids may be
dimers of like fatty acids or copolymers of different fatty acids.
This can be seen from the mass spectrometer analysis of composition
of one commercial grade of "dimer acid" found useful in practice of
this invention. As set forth in Tables I to III, the "dimer acid"
was found to be about 89% dimer, about 6% monomer (fatty acids) and
5% timer acid.
[0036] The commercially available monomeric fatty acids used to
make dimers can vary appreciably depending on the source of raw
materials. The proportions of different acids present differs as
between coconut oil, peanut oil, palm oil, olive oil, corn oil,
safflower oil, tung oil, rapeseed oil, tall oil, distilled tall
oil, oils from marine sources, etc. Such oils may be blended for
still further variations.
[0037] The dimerized molecules may have considerable variation due
to source of fatty acid and/or polymerizing parameters. For
example, one might consider a dimer as an X-shaped structure of
four aliphatic chains with primary hetero atoms or reactive end
groups on one or more of the chains. There may be various lengths
of all four chains depending on where the source materials linked.
The typical two --COOH end groups on a dimer acid may be on the
ends of adjacent chains or on the ends of opposite chains. The
hetero atoms at the ends of chains may be the same or different,
and although two is typical, there may be one or more active end
groups on individual molecules.
[0038] Instead of a neat X such as might be found in an
8,9-substituted C18 alkane, the side chains on a C18 chain might
not be directly opposite, but may be found at essentially any
location along such a chain. (For example, side chains might be at
positions 3 and 12, or 3 and 9, or almost any other combination.)
The hetero atoms may be essentially along the length of such a
chain instead of at the end of a carbon chain. Also, not all
molecules in a mixture need to be the same and probably never
are.
[0039] Thus, a broad variety of dimers, trimers and higher polymers
can be made depending on the raw material monomers and the
polymerization conditions and/or catalyst. For example, just one
manufacturer of commercial "dimer acids" offers about two dozen
different grades, and there are numerous manufacturers annually
producing about 235 million pounds of such products. Many of these
dimer acids include varying proportions of monomer, dimer and
trimer. Most are made from tall oil feedstocks, but other fatty
acid sources are also prevalent.
[0040] Commercially available dimer acids may have mixed dimers,
i.e., dimers where the two fatty acids are different from each
other, and there may be mixes of saturated and unsaturated fatty
acids which are dimerized. Since dimerization occurs at a site of
unsaturation, starting with unsaturated fatty acids may result in
the preferred saturated dimers.
[0041] Exemplary commercially available dimer acids and trimer
acids include AVER13, AVER17, AVER18 and AVER19 available from Aver
Chemical, Yuanda Group of Yichun City, JiangXi Province, China;
Century 1156, Unidyme 11 Unidyme 14, Unidyme 14R, Unidyme 18,
Unidyme 22, Unidyme 27, Unidyme 35, Unidyme 40, Unidyme 60, Unidyme
M-9, Unidyme M-15, Unidyme M-35, Unidyme T-17, Unidyme T-18, and
Unidyme T-22 available from Arizona Chemical Company of Dover, Ohio
and Picayune, Miss.; Empol 1008, Empol 1018, Empol 1022, Empol 1040
and Empol 1062 available from Cognis Group of Cincinnati, Ohio and
Kankakee, Ill.; MeadWestvaco DTC 155, DTC 175, DTC 180, DTC 195,
DTC 275, DTC 295, DTC 595, and SCTO available from MeadWestvaco of
Stamford, Conn.; a dimer acid identified as PM200 which is 80 to
90% dimer acid, 10 to 20% trimer acid and a maximum of 5% monomer
acid available from Samwoo Oil Chemical Co of Yangjugun, KYE,
Korea; products from Resolution Performance Products, Lakeland,
Fla.; Pripol 1006, Pripol 1009, Pripol 1013, Pripol 1017 and Pripol
2033 available from Uniqema of London, England and Wilmington,
Del.; Empol 1010, Empol 1014, Empol 1016, Empol 1018, Empol 1022,
Empol 1024, Empol 1040, and Empol 1041 available from Brown
Chemical Co. (distributor) of Paterson, N.J.; Pacific Dimer Acid
from Pacific Epoxy Polymers, Inc., of Richmond, Mo.; and various
dimer acid products from Lianyou Products of Hianjin, China; Kodia
Company Limited of Changsha. China; and Zhejiang Yongzai Chemical
Industry Co. of Zhejiang, China. This list is not believed to be
comprehensive and other dimer acids and the like may be
commercially available from these or other vendors.
[0042] In addition to dicarboxylic dimer acids, nucleophilic or
electrophilic substitutions for the --COOH group, per se, may also
be equivalent. Some acceptable end groups might not be considered
to be electrophilic or nucleophilic in strictest chemical terms but
are still capable of complexing or forming non-covalent (e.g.
dative) bonds with metal oxides. For purposes of this application
such end groups are considered within the scope of "nucleophilic
and/or electrophilic". For example, other additives comprise
amines, alcohols, thiols, phosphenes, and amides, as dimers and/or
trimers. Other additives may be suitable if they do not
disassociate at the temperature of the molten solder bath comprise
esters, anhydrides, imides, lactones and lactams. (For example,
ERISYS GS-120, a glycidyl ester of linoleic acid dimer, available
from Specialty Chemicals Inc. of Moorestown, N.J.)
[0043] Thus, the additive may comprise the hydrocarbon moiety of a
dimer and/or trimer of fatty acid and at least one nucleophilic or
electrophilic group on the hydrocarbon moiety. It is preferable
that there are at least two nucleophilic or electrophilic groups
and more specifically that the groups are carboxylic.
[0044] For practice of this invention, it is considered that dimers
and/or trimers of fatty acids having at least eight carbon atoms
(C8) can be used. Instead of a dimer of fatty acid with about 18
carbon atoms, a trimer of a lower molecular weight fatty acid may
have properties sufficiently similar to a dimer acid to be used as
an additive on a solder bath.
[0045] The active additive need not always have a hydrocarbon
moiety corresponding to a dimer of fatty acid. In other words, an
appropriate additive is an organic molecule with a hydrocarbon
moiety, and functional group(s) which are nucleophilic or
electrophilic to capture tin oxide and/or other oxide of metal in
the bath. For example, a long chain hydrocarbon (preferably
saturated) split near one end with a side chain and nucleophilic or
electrophilic groups on one or both ends of the split is
acceptable.
[0046] There are properties of the active additive to the solder
bath that are important for commercial applications. For example,
the additive is liquid at the temperature of molten solder in the
bath, and has sufficient stability against oxidation and
sufficiently low vapor pressure to remain as an active liquid layer
on the bath of molten solder, preferably for at least four hours
and even better, a full day. The active additive includes an
organic material having one or more nucleophilic and/or
electrophilic end groups and has the ability to scavenge and
assimilate oxide of at least one metal in the bath and preferably
remain effective for at least a full work shift and more preferably
about one day. Preferably the layer of active additive effectively
bars oxygen in air from reaching the quiescent surface of the
solder. It is also desirable that the additive be non-corrosive,
non-conductive and non-hydrophilic so that there is no detriment in
the event of residue of additive on a PC board or other object
soldered, and there is no need for supplemental cleaning.
[0047] Since the number of commercially available dimer acids
and/or trimer acids and other suitable nucleophilic- and/or
electrophilic-group containing molecules is quite large and the
number of possibilities within the scope of "active additives" is
even larger, there is some probability that there are substances
with some of these properties which will not be fully effective as
described, and therefore not be suitable for practice of this
invention.
[0048] For example, a dictionary definition of fatty acid goes down
to 4 carbon atoms in the monomer. A dimer of this material would
probably be inappropriate for any of a number of reasons. For
example, it may have a vapor pressure that is too high (or boiling
point that is too low), so that it could not be used on a molten
solder bath; it may have a flash point that is too low for use on a
solder bath at 260'C; etc. A higher oligomer of such short chain
fatty acids, might, however, be suitable. Failure to have some of
the properties mentioned above may readily eliminate some candidate
materials.
[0049] Fortunately, there is a quick, easy and inexpensive test for
screening a candidate active additive material to avoid those that
are unsuitable. Clearly, one skilled in the art can eliminate some
substances by simply knowing some of the physical properties, such
as viscosity, vapor pressure, boiling point, flash point, oxidation
stability, etc., (some materials may be unacceptably smoky or give
off other fumes, for example). Some candidate substances may
remain, where it is uncertain whether they will work well. Those
can be found by a screening test. Furthermore, there may be
substances that pass the screening test and do in fact work, but
are not commercially practical because of the need to operate for
longer periods of time at high temperature. Some materials degrade
more rapidly than others and may not be deemed commercially usable,
although operable.
[0050] The screening test is simple. Solder flow is started in an
apparatus such as a wave soldering apparatus and the flow of solder
observed. A small amount of the candidate substance is added onto
the solder bath. When a candidate substance is operable, there is a
prompt visually discernable change in the flow characteristics of
the solder. The solder in a "waterfall" over a weir or through a
slot appears more fluid, as if there is a reduction in viscosity.
Irregularities in the surface of the wave diminish. Dross on the
surface of the solder seems to collect in one or a few regions of
sludge, with other areas of the surface of the solder previously
containing floating dross becoming shiny and clean. Solid dross may
disappear as it is assimilated by the liquid additive. The changes
might be quantified, but that is not necessary for screening. Only
a small amount of material needs to be added, i.e., 50 to 100
milliliters or less in a typical small wave soldering apparatus to
produce a visually discernable change and to obtain good soldering
characteristics from the bath. Larger amounts may be added for
evaluating longer term stability of the additive on the molten
solder bath. An exemplary use further characterizing the screening
test is described hereinafter.
[0051] A surprising result of adding an active additive to the
surface of a solder bath in wave soldering apparatus is an almost
immediate reduction in viscosity of the molten metal. When the
active additive is poured onto a bath without active additive, and
maybe with some visible dross, the height of the wave promptly
increases. In wave soldering apparatus, the metal that flows into
the wave is drawn from near the bottom of the solder bath, so the
floating active additive liquid is not part of the solder passing
through the pump. Without change in pump pressure, there is a quite
noticeable change in wave height. A wave previously grazing the
bottom of PC boards passed over the wave in automatic apparatus,
may rise enough to now overflow the top of a board, for example.
Pump pressure may, therefore, be reduced or the boards passed at a
slightly higher elevation.
[0052] There appears to be solubility or at least dispersion of
metal oxide in molten metal, such as dispersion of tin oxide in
tin. (The solubility of oxygen in tin, for example, is very low.)
It only takes a small amount of metal oxide to change the rheology
of molten metal. Even a small concentration of high melting point
materials in the molten metal may raise the viscosity of the metal.
(This has been suggested to occur in lead-tin solder alloys.) An
active additive layer added to a molten solder bath appears to
scavenge and assimilate at least some of the metal oxide dispersed
in the molten solder, thereby purifying or cleansing the solder,
and lowering the viscosity of the molten metal. This could explain
the visually discernable change in the flow characteristics in a
wave soldering apparatus upon addition of an active additive, as
well as the improved wetting by solder on components being
soldered.
[0053] In an exemplary situation, a layer of oxide dross was
allowed to accumulate on the surface of solder in a small
commercial wave soldering apparatus operated for three eight hour
shifts. The solder pot had a surface area of about 10 by 14 inches
(25.times.35 cm) including the area of the wave. About 1/3 or more
of the surface was "quiescent" in that it was not in the flowing
wave. About 150 to 200 ml of a dimer acid active additive was added
to the apparatus and formed a layer that appeared to be about 3-4
mm thick. Floating dross was largely assimilated into the liquid
layer within about a half minute.
[0054] Surprisingly, after two or three minutes, viscosity of the
liquid metal pumped into the wave appeared to be decreased since
the wave height was noticeably increased as compared with wave
height before the active additive layer was formed. This is
regarded as evidence that metal oxide is being scavenged from the
molten metal. The apparatus was operated with PC boards passed
across the wave and soldered for another 24 hours. It is estimated
that about 500 PC boards were soldered while the active layer was
maintained on the bath. The layer was then dark (rather like
chocolate) and gummy, but still effective for assimilating metal
oxide. The volume of the layer had increased about 50 to 100% from
its original thickness. It is believed that a significant part of
the change in the layer is due to thermal degradation of the active
additive material.
[0055] Thus, an aspect of this process is reducing viscosity and
improving purity of a solder bath by adding a stable liquid active
additive with nucleophilic and/or electrophilic end group(s) that
scavenge oxides from the molten solder. A preferred nucleophilic
end group is --COOH. By reducing viscosity by cleansing or
purifying the bath of metal oxides, lower soldering temperatures
can be used. Further, metal oxide is assimilated in the liquid
active additive layer. It is of particular significance that
scavenging metal oxide from the bath of molten metal enhances
wetting of solid (e.g. copper) surfaces to be soldered.
[0056] One surprising aspect of this invention is that the
temperature at which reliable soldering takes place with lead-free
solder alloys such as tin-silver and tin-silver base alloys has
been reduced to no more than 260.degree. C. Thus, the soldering
process comprises contacting a PC board or the like to be soldered
with molten solder at a temperature of up to 260.degree. C. This
occurs when an active additive has been applied to the surface of
the molten solder. Comparable joint soldering reliability from a
bath without the active additive requires a temperature higher than
260.degree. C.
[0057] Wetting balance tests show the effectiveness of an active
additive which scavenges oxides from the metal on wetting of
lead-free solder on copper. In a wetting balance test, a test
coupon is lowered into molten solder and allowed to wet the metal
surface before withdrawing the coupon from the bath. FIG. 1
illustrates the steps of lowering and raising the coupon and a plot
of force versus time during such a test. Point a corresponds to the
moment the sample reaches the surface of the solder bath. Point b
is at the end of immersion of the sample in the solder and
indicates the Archimedean push due to density differences between
the sample and the solder. Point c is when the buoyancy force is at
equilibrium, i.e., when there is no force applied by the wetting
balance apparatus. Point d illustrates the maximum force as a
sample is wetted by solder. Point e is a spike of force as the
sample is lifted out of the solder bath. Line f shows the force or
weight of the sample out of the solder bath at the end of the
test.
[0058] FIG. 2 illustrates schematically some types of wetting
curves one may see in such tests. The nature of the wetting
performance is indicated for each schematic graph.
[0059] In the tests described herein, the wetting balance apparatus
was a "Must II" model from Concoat. About ten pounds (about 4.5
kg.) of SAC 305 alloy was in a pot with a surface area of about 48
square inches (310 sq cm). This alloy has 3% silver, 0.5% copper
and balance tin. Test coupons were like pieces of PC board with
copper on one face. A test coupon is 1/2 inch (1.27 cm) wide and
was immersed in the solder one inch (2.54 cm). All test coupons
were "fresh" with a conventional OSP (oxygen solder protection)
sealer on the surface. The OSP sealer inhibits oxidation of the
copper before soldering. Shortly before immersion, Type R flux was
applied on the copper surface. (Type R flux is a conventional flux,
about 25% by weight water-white gum rosin and balance isopropyl
alcohol. It evaporates or "burns off" rapidly at soldering
temperatures.) The solder in the pot was quiescent (i.e., there was
no flow). Before a sample coupon was immersed, a flat blade was
used to push visible dross and/or additive away from the area where
the coupon was to be immersed.
[0060] In a pair of tests, coupons were immersed in SAC 305 ahoy
solder at 235.degree. C., and in neither case was there any wetting
after eight seconds in the solder pot. FIG. 3 is a graph of force
versus time from these tests, One coupon had slight wetting after
about eight seconds. In effect, this was non-wetting. (235.degree.
C. is a typical temperature for solder reflow with conventional
lead-tin solder alloy.)
[0061] Coupons were also immersed at 245, 255 and 265.degree. C.,
respectively, and those tests are illustrated the graph of FIG. 4.
The coupon immersed at 245.degree. showed retarded poor wetting
(after about four seconds). The coupon at 255.degree. showed slow
poor wetting (after about 1.5 seconds). The coupon at 265.degree.
showed good wetting (at less than 3/4 second). There was no
additive on the bath during these tests.
[0062] About two fluid ounces (about 60 ml) of dimer acid was added
to the solder pot and allowed to spread to the edges. When pushed
away with a blade, about 1/3 of the surface of the molten solder
had a layer of dimer acid with a thickness estimated as about 1/4
inch (about 6 mm). No visible dimer acid was in the region where
the coupons were immersed. There was no visible dross on the
surface. Three test coupons were immersed and in each test there
was good wetting at 235.degree. C. FIG. 5 is a graph illustrating
these results. Each sample reached the zero force axis at about 0.3
seconds and was fully wetted in no more than 3/4 second. (It may be
noted that in a typical wave soldering process, a PC board is in
contact with molten solder in the wave for about two seconds or
even as much as four seconds.)
[0063] After dimer acid was apparently cleaned from the pot and
dross was allowed to form, coupons showed significantly retarded
wetting at 235.degree. C. There was no wetting before about two
seconds on any of three coupons. Reasonable wetting was found after
about four seconds.
[0064] Remarkably, the appearance of a solder joint surface is
changed by floating a layer of active additive on the surface of
the solder bath in wave soldering apparatus or the like. A good
quality conventional solder joint of lead-tin alloy has a smooth
shiny surface, and operators doing soldering rely on that
appearance to assess whether there are good joints. The surface of
a lead-free solder such as a tin-silver-copper alloy is typically
rather rough looking or grainy, even when an acceptable joint has
been produced. There may also be what seem to be flow lines or
patches of ordered irregularities on the surface. These are
subjective observations of the joint appearance which are not
quantified, but are apparent to an experienced operator either with
the naked eye or with small magnification.
[0065] It has been found that the surface of a lead-free solder
joint formed from a melt where active additive is present on the
surface of a solder pot generally has the smooth (non-textured)
shiny appearance of a conventional lead-tin solder joint. Such
smooth surface can be seen on the top and bottom of a joint. When a
PC board is soldered in a wave soldering apparatus, the "bottom" of
the board is brought into contact with the top of the wave of
solder. Molten solder flows through a plated-through hole in the
board and along a lead in the hole to form a joint that extends
through to the "top" of the board. When such a solder joint is made
without use of active additive on the wave solder apparatus, there
may be a subtle difference in the appearance of the joint on the
top and bottom surfaces. The surface on the bottom appears smoother
and the surface on the top of the joint appears rougher. However,
when active additive is used on the solder bath, the top and bottom
surfaces are quite similar in appearance and generally smooth and
shiny.
[0066] Furthermore, the metallographic appearance of such a
lead-free solder differs depending on whether active additive is
used or not used.
[0067] A tin-silver alloy solder includes a eutectic so that upon
solidification from a melt there is a two phase structure; a
basically tin phase and a silver rich phase (probably an
intermetallic compound). Copper and other additional alloying
elements may be present in low enough amounts to remain soluble in
one of these phases or may be present as a third phase in such
small quantities and grain size that they are not noticeable in a
magnified cross section at 100.times., for example. A cross section
(etched with KOH solution, for example) shows large areas of tin
grains and smaller areas of silver-rich grains.
[0068] When the solder comes from a bath without an active additive
layer, the tin-rich grains tend to be somewhat elongated or
non-symmetrical. When the solder comes from a bath with an active
additive layer, the tin-rich grains are more rounded or
symmetrical. The differences have not been quantified, but are
readily observed by an experienced operator. FIG. 8 illustrates in
magnified cross section a representative solder joint formed by
wave soldering with lead free solder from a bath without the use of
a layer of active additive floating on the molten solder bath. FIG.
9 is a similar cross section of a representative solder joint
formed at by the same technique with a layer of active additive
floating on the molten solder bath.
[0069] These visual observations of the surface and grain structure
of solder with and without use of active additive in the process
are "averages". In other words, an observation of one joint or
cross section may not clearly indicate whether a joint was made
with or without active additive. An individual joint may be
ambiguous, although other times even a single joint is enough to
distinguish processes with and without active additive. When a
group of joints made by one process are examined, use or non-use
can be distinguished.
[0070] An aspect of this invention comprises minimizing formation
of dross on molten solder. When molten solder is exposed to air,
there is oxidation of the metal. These oxides (usually called
dross) form on the surface and accumulate during operation of a
continuous soldering apparatus, such as wave soldering machine.
There are several problems associated with dross formation.
[0071] Dross can interfere with sound soldering of printed circuit
boards. For example, in severe situations it may inhibit wetting of
the surfaces to be soldered and result in poor or incomplete
joints. The presence of dross is also implicated in bridging of
solder between closely spaced electrical leads or connection pads.
When dross accumulates in a continuous soldering apparatus, it is
sometimes necessary to shut down the operation of the machine and
manually ladle floating dross from the surface. Even if ladled from
the solder bath during continuous operation, dross removal involves
clear hazards around molten solder. Furthermore, the dross is a
waste of the solder, and the metal removed as dross must be
replaced. With lead-tin solders, dross is a hazardous waste.
[0072] It is found that when an active additive layer is added to a
surface of the molten solder in wave soldering apparatus, for
example, the formation of dross is diminished. The presence of a
film of active additive on surfaces exposed to air apparently
serves to block air from reaching the metal surface and thereby
inhibits oxidation. Additionally, the active additive scavenges
metal oxides from the bulk of the solder bath.
[0073] Dross that may form on exposed areas of the molten solder
surface is assimilated into the active additive layer. Dross formed
on a solder bath typically includes metal oxide and entrained
solder metal when the dross forms in absence of an active additive.
As much as 3/4 or more of the dross may be in the form of entrained
solder. In practice of this invention, it appears that the metal
oxide portion of dross is retained in the additive layer and
metallic portions entrained in dross (if any) are restored to the
bath, so that the total amount of solder lost into dross is greatly
diminished. It does not appear that any appreciable amount of
unoxidized metal is entrained in the active additive. Thus, less
solder is consumed during soldering and costs are thereby reduced
since less waste is produced.
[0074] It is found that metal-containing dross can be heated in
contact with active additive and entrained metal in the dross is
released as metal oxide is assimilated in the additive. Thus, dross
removed from a solder pot in wave solder apparatus, for example,
when no active additive is used, may be skimmed off and processed
to recover solder. The dross is heated above the melting point of
the solder under a layer of active additive. The layers may be
stirred for enhanced contact to speed processing. A pool of molten
solder forms and/or grows under the additive layer and the
remainder of the dross is assimilated by the liquid additive.
[0075] The active additive with assimilated oxidized metal may be
roasted for recovering tin and other metals (e.g. silver). Some tin
ores are commonly roasted in coal-fired or oil-fired
firebrick-lined rotary kilns (or reverberatory furnaces) at up to
650.degree. C. preparatory to eliminating impurities. The metal
laden additive may be used as some of the input fuel or simply
added to the ores and burned in the kiln. An oxidizing roast is
employed since a reducing roast can yield undesirable smoke and tin
oxide is the most common form of the metal in tin ores. A
chloridizing roast (with NaCl) in oxidizing conditions may be used
to separate tin from silver, which is recovered as fume.
[0076] Although it is believed that at least a mono-molecular film
forms over quiescent parts of the exposed surface of the molten
solder, it is likely that areas of solder surface in a dynamic or
turbulent situation are not completely covered with such a film.
Thus, where there is considerable turbulence (such as where a wave
falls to the surface of the bath of solder in the bath) or rapid
flow (such as on part of a wave), a continuous film may not exist.
Even if the film is not continuous throughout the surface, it is
beneficial in minimizing dross formation as well as continually
scavenging metal oxides from the bulk of the solder bath.
[0077] Oxidation to form dross may require nucleation sites to form
dross that would interfere with soldering. By removing most of the
oxide and isolating it from locations where dross interferes,
nucleation sites are diminished and dross formation is likewise
reduced. In other words, dross continues to be formed, but a lower
rate. What dross does form is captured and assimilated by the
active additive and removed from harm's way. Furthermore, the situs
of dross formation may be shifted from where it would be
detrimental to locations where it is more readily captured.
[0078] At a minimum, the addition to the solder bath should be
sufficient to maintain a substantially continuous film on a
quiescent surface of the molten solder. No detriment has been
recognized from having excesses of the additive beyond what is
required to maintain a continuous film. Apparently, the active
additive does not form a continuous layer on turbulent or
significantly dynamic areas of the surface such as on the wave in a
wave soldering apparatus or where the wave falls onto the quieter
areas. Dross can be seen forming in such non-quiescent areas, but
the dross is assimilated into the active additive upon contact.
[0079] It has been found desirable to add enough active additive to
the surface of a solder pot in wave soldering apparatus to form a
floating layer of appreciable thickness, e.g. about 1/4 to 1 cm on
at least a portion of the surface of the bath. This amount permits
the apparatus to be operated for a day or more before bath
maintenance (except for adding solder to replace that used on the
PC boards). The layer forms a barrier which prevents oxidation of
the solder in the bath. Small amounts of oxidation occur on the
surface of the wave and these bits of floating oxide "waterfall"
back toward the bath. Such new metal oxide is promptly assimilated
by the floating layer and essentially disappears.
[0080] How thick a layer of active additive to place on a bath is
somewhat dependent on the volume of the solder in the bath. An
important function of the active additive is to scavenge metal
oxide from the molten solder. Thus, instead of being determined
mainly by surface area, the amount of solder is a better measure of
the amount additive to be used on a bath. As an order of magnitude,
about 100 ml. of active additive appears appropriate per 100 kg. of
solder. That is more than enough for initial scavenging and permits
continued operation of the bath for an extended time. After a bath
has been cleaned of oxides in the molten metal, volume is less
significant and the amount of additive maintained on the bath is
related more to surface area and to turbulent activity that exposes
metal to air so that oxides form.
[0081] A dimer acid when added onto a bath is nearly water-white
clear. The layer gradually darkens as metal oxide is assimilated by
the organic additive. The layer gradually takes the appearance of
tea, milky tea, cocoa, coffee with cream and black coffee. It is
believed that the darkening is partially due to degradation of the
organic material and partially due to assimilating metal oxide.
Degradation may be due to polymerization, decomposition or
oxidation, and possibly involves all of these processes. A darkened
"gummy" layer forms and when skimmed off, at least a film of active
additive typically remains on a quiescent surface of the molten
metal, and continues to be effective in assimilating metal oxide,
barring contact of air and the metal surface and maintaining low
amounts of oxide in the metal.
[0082] When the active additive layer is on a dynamic bath, such as
in a wave soldering apparatus, such darkening occurs, but
apparently at a lower rate than on a quiescent bath. The layer of
organic liquid on the bath remains on quieter areas of the bath,
but may be pushed away from the turbulent region where the wave
falls into the bath. As the active layer darkens its viscosity
seems to increase so that it gradually advances toward the foot of
the wave, and may eventually encounter the metal flowing off the
top of the wave.
[0083] Upon "resting" on a quiescent bath for a period after a wave
is turned off, for example, a gummy dark layer may separate and
when this is removed a layer of active additive remains beneath it.
It is believed that the degraded material is merely stirred into
the active material in such a way that it is not visually observed
as a distinct layer. It can be desirable to intermittently remove
degraded or spent material.
[0084] Although the active additive remains as a liquid on the bath
even after degraded, it may include dispersed solids. The gummy
material is not readily separated to see if there are solids
present, but it does appear that metal oxide particles are
dispersed in the additive. After about five hours of operation of a
small laboratory scale wave solder apparatus (which generates dross
much more rapidly than in commercial scale apparatus) it was found
that a gummy liquid removed from the bath was about 70% by weight
metal oxide solids. Although viscous and a solid crust may form in
areas, the additive continues to behave as a liquid, albeit quite
viscous, at the temperature of the bath. It is also found that
effectiveness of the additive can be maintained by adding fresh
active additive even after it becomes quite viscous. When the
additive is left on the bath for an extended period (for example,
overnight) with the wave turned off, a thin crust of degraded
material forms on the surface.
[0085] When quite dark and gummy, effectiveness of the additive may
be diminished and the entire visible layer of organic material may
be removed from the bath. One easy removal technique is to add a
powdered absorbing agent to the liquid and vacuum it off the
surface of the bath when visible liquid is no longer seen.
Absorbing agents of the sort commonly used for soaking up fuel or
motor oil spills, or even absorbent kitty litter are effective.
Such an agent is mixed with finely divided diatomaceous earth or
silica gel; about a 50-50 mixture for good effectiveness. The
proportions are not critical and 25-75 to 75-25 mixes have been
found acceptable.
[0086] The degraded active additive layer may also be removed by a
high temperature resistant "sponge". For example, after the
production scale operation described above, a piece of aramid fiber
(Kevlarm) woven fabric about three inches by eight inches was
placed on the surface. The tight weave fabric was up to 1/4 inch
thick. Degraded material wetted the aramid and was soaked into the
fabric. The floating patch of fabric was pushed around the surface
to pick up additive along the edges of the pot, and when lifted
off, it was found that almost all of the visible additive layer was
removed with the patch. Downtime for removing the degraded additive
and replacing it with fresh was about three to four minutes,
certainly less than five minutes.
[0087] It has been found that a costly aramid fiber "sponge" is not
essential. Degraded active additive has been successfully removed
from a bath of molten solder by swabbing with an ordinary cotton
terry cloth towel. The rag is not in contact with the solder enough
to sustain appreciable damage or leave any residue on the bath.
Thus, an inexpensive cotton rag or other fiber wetted by the active
additive can be used for removing spent additive.
[0088] It can be noted that in some commercial scale apparatus
dross is removed from the solder pot about once per shift. When a
suitable active additive is applied it appears that removal of
spent material is desirable only about once per day. In other
words, operating time between surface cleanings is roughly
tripled.
[0089] After a period of use some of the viscous, tar-like additive
on a solder pot may adhere to walls of the pot or other surfaces
and be somewhat hard to remove. Thus, it may be desirable for long
term usage to add and remove active additive or the like without
forming free floating visible layers of the liquid on the surface
of the molten solder. This may be accomplished by containing a
principal portion of the active additive layer in a "tea bag" which
is placed on the bath and can be lifted off when effectiveness is
diminished. The visible layer of additive is contained within the
bag and additive sufficient to form at least a monolayer of
additive over a quiescent surface of the bath can pass through
pores in the bag.
[0090] An exemplary "tea bag" or permeable envelope for containing
additive is illustrated in FIG. 6. A simple hollow envelope of fine
mesh material contains active additive. The mesh may be metal that
is not readily wetted by solder in the bath or may be a high
temperature plastic such as aramid resin (melting point about
500.degree. C., e.g. Kevlarn9. The pore size in the mesh is only
small enough to avoid large amounts of the liquid additive from
leaving the bag and is large enough to allow contact of the
additive with solder for scavenging oxides and assimilating any
dross that forms. The bag need not be rectangular as shown and may
be made simply by sealing or stitching edges of a couple flat
sheets with additive therebetween. A complex shape may be used to
provide convolutions along the edges floating at the surface of the
bath for increasing the surface area through which additive and/or
metal oxide can pass.
[0091] Since the effectiveness of active additive layer on the
molten solder in a continuous soldering apparatus may be degraded
or depleted during use, it may be desirable to replace the
substance at about the same rate it is depleted. This may be
accomplished by intermittently manually swabbing off or aspirating
some of the active additive and adding a small amount of the
substance to the solder bath to maintain an effective layer.
Alternatively, this may be automated to intermittently or
periodically add remove and add small amounts of the substance
during operation of the apparatus.
[0092] When the active additive is a viscous liquid (as is often
the case), it can be readily dispensed (drop wise, for example) by
any of a variety of available liquid dispensers. The viscosity may
be reduced by use of suitable solvents such as toluene, hexane,
octane, isopropyl alcohol, butyl alcohol, hexanol or the like. The
desired rate of renewal of the layer is readily found empirically.
Spent or degraded additive may be removed by automated "swabbing"
with an aramid sponge as described above, or liquid may be
aspirated off the surface.
[0093] Particularly useful materials for changing the rheology of a
dimer acid or similar active additive are fatty acid monomers or
short chain esters (e.g. a methyl butylate or dibutylate ester).
For example, a C8 carboxylic acid may serve a dual function of
reducing viscosity of a dimer acid and capturing metal oxide that
would otherwise appear in troublesome dross. Such shorter
carboxylic acids may be found in the low end distillates from tall
oil or other organic fatty acid mixtures. Palm oil or the like may
reduce viscosity enough to be practical in some applications.
Esters with carbon numbers of C8, C10, C12 and the like may be used
when they have reasonably low vapor pressure at room temperature
and suitably modify the rheology of the active additive.
Perfluorinated fatty acids and the like may also be used for
modifying rheology of the active additive. Low molecular weight
adjuvants may vaporize or oxidize rapidly upon addition onto a
molten solder bath. It is desirable to avoid much use of adjuvants
that are smoky or emit unpleasant or noxious fumes.
[0094] The active additive may be diluted with essentially
ineffective ingredients without destroying effectiveness.
Substantial dilution may reduce the time the active ingredient
remains effective or accelerate the need to remove degraded
material. For example, a small amount of carnauba wax (up to about
1%) has been added to a dimer acid to produce a rather pleasant
odor when heated. A ten percent dilution with carnauba wax did not
significantly reduce effectiveness or lifetime. A dilution to about
70% dimer acid and 30% wax noticeably reduced useful life, but did
not seem to reduce effectiveness. Useful life was reduced since the
mixture got dark and gummy quicker than undiluted dimer acid. Thus,
the liquid layer on the molten solder preferably has a major
portion of the active additive, i.e., more than about 50%. One may
also add coloring agents to the active additive without
detriment.
[0095] A characteristic of the active additive is that it
"assimilates metal oxide" from the molten metal or dross, or
"assimilates oxide of at least one metal in the bath". This is
intended to encompass assimilating metal in its oxidized state. It
is not known exactly how the "metal oxide" is retained in the
active additive layer. It is not known whether metal oxide is
substituted in a molecule or entrapped in the additive, and it may
be both. There may be chelating, sequestering, reaction, or simply
surrounding. For example, if a reactive group on the active
additive is an amine, the metal ion may attach to the additive
molecule and release water. The active additive scavenges and
assimilates metal oxide since it has a greater affinity for metal
oxide than does the molten metal.
[0096] Thus, active additive on molten solder and used in
continuous solder apparatus may gradually degrade by saponification
in the course of eliminating metal oxides. There can be covalent or
dative (coordinate) bonding between the organic additive end
group(s) and a metal oxide. Most likely, micelles of the active
additive effectively entrap oxides. In effect, a number of
molecules of the organic liquid encompass a molecule or group of
molecules of metal oxide. Such assimilation of the metal oxide
leaves the additive as a liquid, although the viscosity may be
increased (viscosity of the liquid has not been quantitatively
measured and that is nearly meaningless since viscosity seems to
gradually increase). Metal oxide may not assimilated as distinct
stoichiometric molecules, and that is not important. There may be
"oligomers" of metal oxide with loose bonding of a few apparently
stoichiometric molecules.
[0097] It is possible that active additives with nucleophilic or
electrophilic end groups are forming "heavy metal soaps" in the
heat of the molten solder alloy. These soaps are structures where
the carboxyl group is complexed to a metal ion, for example, tin,
at an end of an aliphatic chain, for example. When carboxylic end
groups are present, tin may substitute for hydrogen in the --COOH
group (two such groups for divalent tin). Evidence for this is the
presence of tin detected in the dimer acid used on a solder
bath.
[0098] An exemplary reaction is
(R--COOH).sub.2+Sn0=(R--COO).sub.2Sn
where (R--COOH).sub.2 represents the dimer acid. When tin has a
valence of four as in Sn0.sub.2 the product is (R--COO).sub.4Sn by
combination of two dieters with the tin oxide. Like most salts,
these heavy metal soaps have a high heat tolerance, which may help
explain why the additive does not rapidly degrade in the harsh
environment of the molten alloys.
[0099] If desired, one may improve heat tolerance by minimizing
unsaturation in the active additive molecules. Decreasing
unsaturation increases heat tolerance by encouraging tight
molecular packing. Thus, for example, Sigma-Adrich product 432369,
a hydrogenated dimer acid, may provide enhanced heat tolerance as
compared with unsaturated counterparts. Furthermore, aromatic dimer
acids or the like have enhanced thermal stability. Di-carboxy
phenyl acids that are analogs of phthalic acid may be particularly
useful.
[0100] Halogenated materials may be added to active additives for
enhanced heat stability, such as nonadecafluorodecanoic acid or
poly(dimethylsiloxane-co-dimer acid, bis(perfluorododecyl)
terminated; Sigma-Aldrich products 177741 and 434906, respectively
(Sigma-Aldrich Inc. of Madison, Wis.). Other dimer acid products
from Sigma-Aldrich include their products 430307, 191043, 191035,
191019, 434647 and 434655.
[0101] The active additive is not behaving as a flux in the
soldering process. The function of a flux in soldering is to remove
the oxide film from the base metal by reacting with or otherwise
loosening that film from the base metal surface. The molten flux
then forms a protective blanket in the vicinity of the joint which
prevents re-formation of the oxide film until molten solder
displaces the flux and reacts with the base metal to form an
intermetallic bond. The relationship among solder, base metal and
flux is such that a flux that is optimum for one solder composition
is not necessarily the best flux for a different composition. The
active additive is scavenging metal oxide from the molten solder
and may never contact the solid surfaces to be soldered. Flux may
also be used on the solid surfaces to facilitate soldering during
practice of this process. Fluxing action is a separate, independent
function.
[0102] When using a layer of additive on a solder bath in a wave
soldering apparatus, for example, the PC boards being soldered
typically have a conventional flux added to the surface before
contacting the wave of molten solder. Some fluxes contain rosin
acid and/or fatty acids such as listed in Metals Handbook, (9th
Edition, Vol. 6, Welding, Brazing and Soldering, page 1082). Such
fatty acid fluxes are not suitable for forming a layer of additive
on a molten solder bath for a number of reasons, the most cogent
being stability at the temperature of the bath. Dimerized rosin is
used in fluxes and is found insufficiently stable for cleansing a
solder bath. Many such materials emit noxious fumes or smoke when
maintained at bath temperatures.
[0103] Active additive is not believed to be present on the top of
the wave of solder in a wave soldering apparatus, for example,
since the molten solder in the wave is pumped from the bottom of
the bath, far below the floating layer of additive. The additive is
neither soluble in nor easily dispersed in the metal. Although
active additive readily and quickly spreads to form at least a
monomolecular film on the surface of the molten metal, it is not
believed capable of rapidly "climbing" the wave to contact the PC
boards or the like being soldered. Such a layer forms in quiescent
areas of the bath and a continuous film is not believed present in
turbulent areas. No residue of additive has been found on boards
wave soldered when the bath has a layer of dimer acid on the
surface.
[0104] The substance added to the solder in a continuous process
may be added continually, such as intermittently or periodically,
and continuous addition is not believed to be needed. It also
appears adequate to intermittently remove spent liquid residues
(including dross) from the surface of the solder and where this is
done repeatedly, there is, in effect, continual removal. If
desired, removal of spent residues may be automated so that it
becomes more nearly continuous.
[0105] As noted above, dirtier acid and/or trimer acid suitable for
use in practice of this invention is not necessarily pure dimer of
one fatty acid. An example has been given of a dimer acid which
includes small amounts of monomer and trimer. What could be termed
a "trimer acid" having a substantial proportion of trimer of fatty
acids, may be suitable. Thus, for example, a trimer acid having
about two-thirds trimer and one-third dimer may be quite
satisfactory, particularly if the fatty acid(s) used to make the
trimer have small carbon numbers. A predominantly trimer acid
composition with suitable carbon number may be preferable to a
predominantly dimer acid composition, since it is suggested that a
trimer acid degrades more slowly than a dimer acid.
[0106] Dimer acids and trimer acids effective in a soldering
process can be made from fatty acids having about 18 carbon atoms,
including the carbon in the carboxyl group. Readily available fatty
acids from vegetable sources generally have an even number of
carbon atoms. A number of C18 fatty acid monomers are mentioned
above. An example of a C16 fatty acid monomer is palmitic acid.
Since they are easily available and inexpensive, dimer acids made
from fatty acids with carbon numbers ranging from about C14 to C22
are preferred. Dimer and/or trimer acids with higher carbon numbers
are probably suitable for some soldering applications but are not
readily commercially available. They may also be useful on zinc
baths used for dip galvanizing.
[0107] When the carbon number is lower than about twelve, it is
believed desirable to employ trimers or higher polymers or
dendrimers to achieve adequate carbon moiety lengths for good film
forming properties and assimilation of metal oxides. Thus, it is
preferred that the dimer acid or equivalent have a carbon number in
the range of from about 24 to 60. Best results seem to be available
with dimer acid with a carbon number in the range of from about 28
to 44. When speaking of carbon number it will be recognized that
this is commonly an "average" for the dimer acid or the like since
such materials are commonly a mixture of dimers of different fatty
acids and may include monomers, trimers and dendrimers with higher
and lower carbon numbers. Dendrimers may be particularly useful
since there can be several reactive sites without diminishing other
desirable properties of the additive.
[0108] The process of scavenging metal oxide from a solder bath is
particularly effective with lead-free solders. It is suitable for
conventional lead-tin solders, but subjectively seems to offer
fewer advantages. It has been found that an active additive is more
effective on a bath of lead-free solder than on a lead-tin solder
alloy bath.
[0109] A "skin" of dross can sometimes be seen on the surface of a
wave in wave soldering apparatus, for example. The skin travels
across the surface of the solder pot until it reaches the active
additive, whereupon it is assimilated into the additive. It is not
known if this dross includes entrained metal or is largely oxidized
metal. If there is metallic solder in the dross, it is released and
returns to the solder bath as oxidized metal is assimilated in the
additive.
[0110] A much more visible layer is formed on the dynamic wave in a
lead-tin solder bath than on a lead-free solder bath. This is
believed to be a dynamic effect as lead oxidizes more readily or
rapidly than tin in the conditions of soldering apparatus. (An
analogy may occur at room temperature. A surface of lead becomes
dull grey as a layer of visible oxide forms. A surface of tin, on
the other hand, remains shiny and metallic appearing. This is
believed due to formation of a thin, transparent layer of tin oxide
that passivates the tin surface and inhibits further oxidation.)
The high density of lead and its compounds may also play a role. A
skin of lead-containing oxide may push further across a quiescent
surface toward the active additive than a similar skin of lead-free
oxide. A layer of active additive on a lead-free solder bath
appears homogeneous even after assimilating appreciable oxidized
metal. A dispersion of what appear to be fine particles may be seen
in lower parts of an active additive layer on a lead-tin solder
bath. Only slight stirring is sufficient to disperse the particles
throughout the layer of additive so that it appears homogeneous. It
may also occur that the active additive "wets" a high tin,
lead-free molten solder surface than it does a surface of lead-tin
solder. This could result in less area on the lead-free solder
which is not covered by a thin layer of the additive, and
therefore, is less exposed to air.
[0111] Use of an active additive is particularly appropriate for
tin-silver solders and tin-based ternary solder alloys, including,
for example, tin-silver alloys with additions of copper, nickel,
bismuth, antimony, zinc and/or indium. It is also effective for
"pure" tin baths. So far as is known, the soldering process is also
independent of the solder apparatus in which it is used.
[0112] Although described in context of wave soldering of PC boards
with components in place, the invention is also useful for
pre-tinning PC boards or component leads and other soldering
processes. For example, freshly manufactured PC boards have
conductive areas coated with solder by contact of the board with
molten solder, somewhat the same way as in a wave solder apparatus.
A blast of hot air is then used to blow away excess solder on
contact pads and even from plated-through holes. The technique for
preparing PC boards is called Hot Air Solder Leveling (HASL). This
pre-tinning is used to protect the copper leads from oxidation
during the interval between making of the board and mounting
components on the board, as well as to facilitate soldering of
components in place. Pre-tinning of component leads with solder is
for similar purposes.
[0113] In addition to soldering PC boards and the like, a soldering
process as described herein may be employed for other products. For
example, automotive radiator cores are often soldered by dipping
the cores in a bath of molten solder. A layer of active additive on
the bath facilitates such soldering. Costume jewelry and other
products are often soldered and the process is suitable for such
uses, as well.
[0114] Dross is a troublesome issue when tin plating steel,
manufacturing float glass, making bullets or lead shot, making toy
figurines and other processes involving molten metals, and solving
such problems by use of this invention is also feasible. When the
active additive is suitably resistant to elevated temperatures, the
process may be used for hot dip galvanizing. Such an active
additive may be a trimer or aromatic compound, for example, and may
be solid at room temperature without departing from principles of
this invention. Other uses for such a process will be apparent to
those skilled in the art.
[0115] Following are the Tables referred to above.
TABLE-US-00001 TABLE I Monomeric fatty acids, relative and absolute
amounts Monomers % of monomers Amount in sample Stearic 48% 2.9%
Oleic 43% 2.6% Linoleic 9% 0.5% Total 100% 6%
TABLE-US-00002 TABLE II Dimeric fatty acids, relative and absolute
amounts Dimers % of dimers Amount in sample oleic-stearic 3% 2.7%
oleic-oleic 18% 16.0% linoleic-oleic 46% 40.9% linoleic-linoleic;
linolenic-oleic 14% 12.5 linolenic-linoleic 9% 8.0
linolenic-linolenic 8% 7.1 mass 276-linolenic 3% 2.7% Total 101%
90%
TABLE-US-00003 TABLE III Trimeric fatty acids, relative and
absolute amounts Trimers % of trimers Amount in sample
oleic-oleic-oleic 14% 0.7% oleic-oleic-linoleic 46% 2.3%
oleic-linoleic-linoleic 26% 1.3% linoleic-linoleic-linolenic 13%
0.7 Total 99% 5%
* * * * *