INDUSTRIAL AND ENGINEERING CHEMISTRY
1114
VOL. 29, NO. 10
tion curves of dyes and vitamins from the oil are not appreciably sensitive to moderate divergence from uniformity so that this variation is not yet significant in practice. The low-boiling fractions of the constant-yield oil are pale yellow in color, odorless, but slightly bitter in taste. Those boiling above 180” C. are brownish yellow, odorless, and bland in taste. TABLE 1x1. DATAON CONSTANT-YIELD MIXTURET-175 -CompositionFraction of blend Weight T-170 used (Table 11) fra%on Crams
6 7 8
9 10 12 “ l3 16 l4
3.1 7.7 10.6 5.6 4.3 2.5 1.6 1.7 3.5 4.1
7 -
Fraotion
1
Residual gas Temp. Dressure
c:
Mm.
x
I10 190 210 T E M P E R A T U R E , OC
150
230
250
Weight
108 Cram*
7 8 9 10
1.4 1.3 1.2 1.2 1.2 1.3 1.5 1.7 1.8 2.0
2.2 1.9 2.5 2.9 3.3 3.8 4.0 3.5 3.0 2.9
16
250
1.2
7.3
6
130
FIGURB 2. YIELDCURVE
100 110 120 130 140 150 160 170 180 190
2 3 4 5
110
Distillation
Acknowledgment The authors wish to thank K. C. D. Hickman for suggestions and advice.
Literature Cited Burch, C. R., Proc. Roy. SOC.(London), A123, 271-84 (1929). Hickman, K. C. D., IND. ENG.CHEM.,29, 968 (1937). Hilditch, T. P., and Rigg, J. G . , S. Chem. SOC.,1935, 1774-8. Long, J. S., Kittelberger, W. W., Scott, L. K., and Egge, W. S., IND. ENG.CHEM.,21,950-5 (1929). ( 6 ) Marcus, J. K., J. Biol. Chem., 80,9-14 (1928). (6) Washburn, E. W., IND.ENQ.CHEM.,25, 891-4 (1933); Washburn, E. W., Brunn, J. H., and Hicks, M. M., Bur. Standards J. Research, 2, 467-88 (1929). (7) Waterman, H. I., and Rijks, H. J., 2. deut. 01- u. Fett-lnd., 46, 177-8 (1926); Waterman, H. I., and Nijholt, J. A., Chem. Weekblad, 24, 268-9 (1927); Waterman, H. I., and Elabach, E. B., Ibid., 26, 469 (1929). (1) (2) (3) (4)
~
Constant-yield oil has been used extensively in this laboratory for the analytical distillation of vitamins A and D and for testing the purity of appropriate substances, such as dyes. It may be safely used in vitamin studies because its absorption in the ultraviolet is small and it contains no toxic substances to interfere with feeding experiments.
RECBWED June 1. 1937. Communication 632 from the Kodak Research Laboratories.
Soft Solder Fluxes Practice and Theory CLIFFORD L. BARBER Kester Solder Company, Chicago, Ill.
But first some of the practical phases of the flux problem should be discussed.
Types and Properties of Fluxes
T
H E literature on soft solder fluxes is sketchy, incomplete, generally unreliable, and seldom applicable to the real problems of soldering. For this reason the solder user is mired in that atmosphere of doubt, dogma, and confusion which has long enveloped the subject of solder fluxes. The purpose of this article is to relieve and clarify some of this existing confusion and at the same time to provide the user with information which is sound, practical, comprehensive, and directly applicable to his real needs.
Nature and Purpose of a Soldering Flux A soldering flux may be defined independently of theory as some agent which promotes or accelerates the wetting of a metal by molten solder. Soldering involves an alloy action between bare metals and it is the function of the flux to ensure a completely metallic contact; naturally, if metals cannot touch each other, they cannot alloy. To say that a soldering flux is a substance which “dissolves metallic oxides” is, after all, an assumption with respect to the theory of the fluxing mechanism rather than a statement of an observed fact. The exact mechanism of the flux reaction is not fully understood, and a satisfactory theory has yet to be advanced.
There is a general impression that soldering fluxes are active acids or that they owe their activity to an acid character; in fact, the terms “soldering flux” and “soldering acid” are used interchangeably by the trade. It is true that some fluxes are very mild acids; it is further true that certain others react acid by hydrolysis; however, some fluxes are not acid under any conditions. The general practice of attempting to activate a flux by the addition of mineral or other acids is of doubtful value. At most, the acidity of a soldering flux is very slight; the exact relation between the acidity of a flux and its fluxing effectiveness has never been definitely established. For practical purposes fluxes may be classified as follows: 1. The salt type, such as the chloride of zinc, ammonia, calcium, magnesium, aluminum, and certain other metals. Solutions of one or more of these salts are popularly known as acids. 2. The carboxylic acid type, such as stearic, oleic, palmitic, benzoic, tartaric, furoic, phthalic, and similar organic acids; these are often popularly classified as waxes. 3. The weak organic base type, such as aniline, urea, ethylene, diamine, acetamide, and certain other amines and amides. 4. Rosin.
OCTOBER,1937
INDUSTRIAL AND ENGINEERING CHEMISTRY
111s
(Left) A GOODSOLDERING PASTE( X 110) SHOWING FINE DISPERSION OF AQUEOUSCHLORIDE SOLUTION (DISPERSE PHASE)IN GREASE(DISPERSING PHASE) (Right) A COMMERCIAL SOLDERINQ PASTE ( X 110) SHOWINQ POORDISPERSION OF AQUEOUS SOLUTION AND PRECIPITATION OF DRY CRYSTALS FROM SATURATED DISPERSEPHASE
The acid flux of commerce is really an aqueous solution of one or more inorganic chlorides. Since it is the salt of a weak base and a strong acid, the aqueous solution is slightly acid by hydrolysis; this is the source of the acidity of acid fluxes. Although these are among the most active of soldering fluxes, they owe their chief value more to their stability under operating conditions than to their chemical activity. Their inorganic character renders them valuable where it is necessary to apply rather prolonged heat during the solder operation; under these conditions they are not charred or volatilized as are the organic fluxes. To the acid flux, however, there are inany objections. It is highly corrosive and its application is somewhat limited, since it can be used only where corrosion of the flux residue is not a factor. More important perhaps than corrosion of the residue is its high electrical conductivity; these two factors render acid fluxes unfit for radio or other electrical assembly. Not only is the salt alone corrosive and electrically conductive, but the corrosive and conductive features are further augmented by the hygroscopic character of the fused residue. One of the worst features of an acid flux is its tendency to absorb moisture from the air, enabling it to increase tremendously its total volume and thereby flow and spread to adjacent parts or other points remote from the original application. Often a soldered assembly, which tests satisfactorilywhen newly soldered, will show electrical leakage after being placed in storage or put into use, because of this increase in the activity range of the flux residue after soldering. The efficiency of a chemical as a fluxing agent is determined not alone by its activity and stability, but also by its mobility, or its ability to flow ahead of the molten solder to points somewhat remote froin the soldering iron and prepare a path for the flow of molten metal in “sweating” a joint. I n this respect the aqueous salt fluxes are ineffective. The volatile solvent rapidly boils out of the salt solution, leaving a viscous, immobile residue which can be reactivated only by the addition of more solvent. Dry, immobile, grainy mixtures are inefficient fluxes for soft soldering. The eutectic (3 to I) mixture of zinc chloride and ammonium chloride, which has been widely recommended because of its “low melting point” ( 2 ) , is particularly objectionable
for the reasons mentioned. As pointed out, a salt flux must be a solution and is most inert when it has lost too much solvent. The precipitation of ammonium salt from a solution of the eutectic mixture leads to an undue proportion of grainy residue which impedes the flow of solder. Fortunately zinc chloride, even under extreme conditions, retains a small amount of moisture and can be concentrated under use without precipitation of solid residue. Such fluxes therefore retain a better order of mobility when not loaded too heavily with ammonium chloride or other salts which are easily dehydrated or whose precipitation imparts a mushy or semisolid character to the flux. A final objection to the fluxes of class 1 is that the physical character of the flux residue does not readily lend itself to removal by washing. Although the salts are all soluble in water, the residue after soldering is a hard fused mass which dissolves very slowly. For the removal of such residues, advantage is taken of their hygroscopic character. Although hard and difficultly soluble a t first, the residue gradually absorbs moisture from the air which softens and dissolves it, after which the entire residue can be removed by hot water or steam. Any discussion of the salt or acid type of flux would be incomplete without reference to the soldering paste. A paste is an emulsion of one or more of the salt solutions; the properties of a soldering paste are therefore identical in all respects with those of the acid flux because the paste is itself an acid flux. The emulsifying agent is usually a grease which takes no part in the fluxing action but serves only as a convenient medium for utilizing an active agent. The second class of soldering flux-the carboxylic acidis quite different froin class 1. I n the first place, the fluxes are organic; they decompose, volatilize, sublime, or carbonize depending on the time and temperature of solder application. Their temperature range of effectiveness is therefore relatively short, and judicious variation of solder temperature is required to secure greatest flow of solder. This instability feature, which is a disadvantage in some respects, is an advantage in others since it provides the user with a semiautomatic control of solder flow. In general, these fluxes are not stable enough for those types of solder application which require long and continued heat application.
1116
INDUSTRIAL AND ENGINEERING CHEMISTRY
The instability of these fluxes should not, however, be confused with their activity. I n this respect these fluxes vary over a considerable range. I n general, the short-chain carboxylic acids are the most active and the long chain acids the least. For instance, benzoic acid is one of the most active fluxes known to soldering, whereas palmitic acid is fairly inactive. All the carboxylic acids are corrosive, and their residues are electrically conductive. Although somewhat less corrosive than the salt type flux, they are not recommended for fine electrical work where corrosion and electrical leakage are vital factors. The third type of flux-namely, that characterized by the amines, amides, and certain weak organic bases-is similar in some ways to the carboxylic acid type with respect to physical behavior. In general, these substances are more active than the wax fluxes and are employed where the work requires a flux action more rapid and efficient than is afforded by the slower acting waxes and greases. They are even less stable than this type, and they decompose and volatilize more readily; they are therefore not very effective for “sweat” operations. Rather they resist the flow of solder and are most useful in restraining or confining the molten metal to a given desired point. The instability of these fluxes, therefore, particularly adapts them to “spot” work or similar operations, because the operator can control solder flow by deliberate carbonization of residue. Most important, however, is the fact that these fluxes afford a means for reducing or limiting corrosion. Because of their volatile or unstable character, the heat of the solder operation either volatilizes much of the residue or converts it to a less active derivative. These somewhat carbonized residues are not hygroscopic as are the acid residues, nor are they pasty or waxy as are the long-chain aliphatic acids. The amine bases and their hydrochlorides are protected by patents which utilize these special properties. The fourth class of soldering flux is composed of but one flux-namely, rosin. This substance is put in a class by itself, not because it is chemically dissimilar to other fluxes, but because rosin possesses certain important physical properties which completely differentiate it from any other flux. I n particular, rosin is the only noncorrosive %ux known and also the only one whose residue is electrically nonconductive. Less important, but still a practical point worth noting, is the fact that the flux residue does not catch and retain sawings, clippings, or other miscellaneous debris liberated in the course of electrical manufacture. For these same reasons, rosin is a less active and less effective fluxing agent than the other fluxes; the value of rosin lies, not in its fluxing e%ciency, but in the unequaled character of its flux residue.
Corrosion of Solder Fluxes The popular mind generally has long felt that somewhere there exists or should exist a perfect flux-a substance that is highly active and yet truly noncorrosive; this impression has been aggravated by both popular and scientific literature. Some have divided fluxes into “water-soluble” and “waterinsoluble” classes, claiming that the latter type does not corrode “after the heat of the iron has been removed” (3); others have introduced chlorine into an aromatic nucleus with the expectation that the resulting “chloride” would be noncorrosive ( I ) ; others assume that the introduction of ammonia, aniline, etc., which “neutralizes” the acid, ensures a noncorrosive flux (4); still others contend that the soldering paste is absolutely noncorrosive through some highly mystifying property imparted to it by the grease. It has been pointed out that a soldering flux is an active chemical which promotes the wetting of a metal by molten solder. That any substance could exert such an action on a hot metal
VOL. 29, NO. 10
covered, as it must be, with oxides, is ample evidence of high chemical activity. Certainly if the flux is highly active one moment, it cannot be completely inert the next, and the chemical action which occurs during soldering must continue to some extent after soldering has ceased. Long, careful, thorough experimentation involving every conceivable class of chemical, including not only all the known fluxes but many additional ones as well, has completely demonstrated that, with the sole exception of rosin, there is no noncorrosive soldering flux. In the case of soldering salts, corrosion appears to be electrolytic or galvanic. Although these salts are slightly acid because of hydrolysis, it is doubtful whether the resulting acidity plays an important practical role. Although acidity may speed the electrolytic action, it does not alter the final result, and in practice the weakly acid solutions are just as objectionable as the stronger ones. Equilibrium is reached in either case only when all the flux residue has been converted to a corrosion product. In the case of the carboxylic acids and the amine bases electrochemical action is less important only to the extent that the final corrosion result is delayed for a longer period. Since their resistance to the entrance of oxygen or other accelerating agents is greater, the rate but not the final result of corrosion is altered. With regard to the amines, less is known; suffice to say that their action does not cease with the solder operation. Most important from a practical point of view is that such terms as “little corrosive,” “less corrosive,” etc., apply more strictly, not to the final result, but rather to the speed with which it is reached; a soldering flux residue is corrosive, or else it is rosin. Rosin owes its noncorrosive character to its unique physical rather than chemical properties. Rosin in the cold solid state presents a surface which is impervious and resistant to the entrance of external agents. Corrosion of rosin would involve an action between solids which at most offer only limited contact. Perhaps there is an equilibrium relation between a metal surface and the thin film of rosin with which it is in contact-a condition which is maintained static because further reactant material is not physically accessible. Rosin, in short, “starves out” a chemical reaction by its physical inability to supply the necessary materials for continued action.
Neutralizing a Soldering Flux Much has been said in popular literature about neutralizing a solder flux to prevent corrosion by the flux residue. The fallacy of such a procedure will be readily obvious from a consideration of the properties of fluxes. I n the first place, the most corrosive fluxes are not acids but salts which owe their corrosive action to electrolytic action. Although it is true that these salts react slightly acid from hydrolysis, such acidity contributes little to the final corrosive result. Furthermore, the acidity resulting from the hydrolysis of a salt is not capable of complete neutralization according to the law of mass action. The addition of ammonia, sodium cyanide, carbonates, etc., and other neutralizing agents is inadvisable from both practical and theoretical considerations, since they not only dilute and spread the flux, but are equally as harmful as the “acid” they pretend to neutralize. Even alcohol is used widely as a neutralizing agent; its action appears to be associated with a great deal of mystery.
Theory of a Soldering Flux One theory that has been advanced to explain the action of soft soldering fluxes is that the flux LLlowersthe surface tension of molten solder.” This statement appears to be largely a definition rather than an explanation since wetting
OCTOBER. 1937
INDUSTRIAL AND ENGINEERING CHEMISTRY
is a surface tension phenomenon. It would he more proper to say that the flux exerts some action which serves to keep the metal surface clean in order that the normal surface tension inherently present may completely assert itself; the action of the flux in cleaning the surface is, however, left unexplained. That flux activity on oxides is a solution or coagulation action agrees in many respects with the behavior of fluxes; nevertheless, such a statement is inadequate as an explanation of the fluxing mechanism. Some chemicals which are known to have a pronounced solution action on certain oxides are totally useless as fluxes for the corresponding metals. For instance, the sodium carbonate fusion mixture used in analysis for the decomposition of difficultly soluble oxides is an utterly ineffective fluxing agent. Again certain mineral acids, such as sulfuric and nitric acids, which are used for the decomposition of insoluble substances, are not only totaUy ineffectivefluxes, but the addition of even small amounts of these or of similar acids renders a flux useless. The simple statement that a soldering flux "dissolves the oxide" does not appear to cover all the facts. TABLE1. OROANIC ACIDSIN THE ORDER01"DECREASING A cIniTY Acid Trieliloroscetio OXSliO Maid0 Pbthsiio Tartaric saiieyiio citrio Manuohloraaoetio Sulfanilic M"& LhCtio BWVSOio S"cOini0 Boric Carboiio Quinol
Dissociation Constsnt 2 x 10-1 3 x 10'' 1.5 X 10-2 1.2 x 10.8 1 x 10-1 1 x 10-2
s
x
1.5 X 6 x 0 x 1.4 X 6 x 6 x 6 x 6 X I x
10-4
1 0 . 10-4
10-1
lo-: 1010-1 10."
io-"
Flux Activity very poor Good Good Good Good Good Good POW
very poor
Good Good
God
Good
P",,
POOX
Poor
The role of acidity, as previously indicated, looms somewhat uncertainly in the background of solder flux theory. Table I shows a number of organic acids arranged in the order of decreasing acidity. Although it may be contended that with a few exceptions this order is ako the approximate order of decreasing flux activity, it is even more notable to consider the exceptions. It is significant that these excep tions (trichloroacetic, monochloroacetic, and sulfanilic acids) are the only ones which contain an oxidizing group. Furthermore, sulfates, nitrates, chlorates, permanganates, eta., in any form, whether as acids or as salts, are totally ineffective fluxes. Not only must a flux be free from such groups, hut it must he incapable of yielding chlorine, iodine, or other oxidizing vapors on thermal dissociation. Therefore, it seems evident that fluxingis directly related to reduction, and that n soldering flux must be free from oxidizing groups. It is a mistake to assume, as did Dean and Wilson ( I ) , that the flux action of the inorganic chlorides is associated with the evolution of chlorine gas and that the chlorination of an organic nucleus would result in a flux. It is important to distinguish here between polar and nonpolar compounds and, particularly, between positive and negative chlorine. Strictly speaking, chlorination does not yield a chloride in the chemical sense; chlorinated organic compounds are not similar, physically or chemically, to the inorganic chlorides. Had Dean and Wilson contended that a good soldering flux is a substance capable of yielding a reducing gas instead of an oxidizing gas, their theory would have had a more sound theoretical basis and would have been in better agreement with facts. That oxide formation is prevented by the a p plication of an oxidizing agent is not based on sound theory.
1117
That soldering may he carried out in a reducing atmosphere without the addition of conventional fluxes was easily demonstrated. Small metal strips of oxidized copper and rusted iron containing trapped magnetic oxide were placed in a horizontal Pyrex glass tube. On each strip were placed small pellets of clean solder. Pure hydrogen was then passed through the tube and the latter was heated. Perfect soldering results were secured in each case, the molten solder be having exactly as it does when conventional fluxes are applied. That fluxing action is closely related to reduction is further indicated by the fact that the electronegative metals are the easiest to solder and the electropositive ones the hardest. Such metals as copper, antimony, hismuth, silver, gold, etc., solder most easily; nickel, iron, cadmium, and zinc less readily; chromium and aluminum very difficultly; and magnesium not at all. In the hydrogen soldering test imperfect results were obtained when the solder pellets were not cleaned, indicating that conventional soldering fluxes exert a coagulation action which is capable of removing traces of extraneous insoluble matter OD the surface of molten solder. This removal of insoluble matter is, however, confined strictly to the molten metal; the flux will not remove visible amounts of inert matter siich a8 paint, shellac, carbonized debris, or other gross forms of dirt from the solid metal during soldering. All surface matter other than oxides must be removed from a metal before i t can be soldered. The theory of flux action which best fits the facts is that the fluxing agent, depending on its activity, first penetrates the oxide film and attacks the raw metal, liberating small amounts of hydrogen or reducing vapors which prevent further oxidation &? long as oxidizing agents are not present in the flux itself; this attack in conjunction with the reducing agent serves further to loosen the remaining oxide film until, being wetted by the flux, it is finally coagulated and floated off into the main body of the flux. It is unlikely that the flux dissolves much of the oxide; it is more probable that the flux wets, suspends, and coagulates the oxide loosened by a penetrating and rcducing action.
Literature Cited (1) Dean end Wilson, IND.ENG.C ~ E M .1.9,1312 (1927). (2) Intern. Tin Research and Developmant Council, Bull. 2, 10 (Sept., 1935). (3) Wagner. Radio E w . , 16, 8-9 (April. 1936); 10-11, 14 (May, ,"*S\
(4) Willstrop, Sidery, and Sutton, J . Inst. M e o l a . 59, 175 (1936) (advenoe copy 732).
HELIUXAND HYDROGEN COMPRE~SOR~
