PERFLUOROHEPTAGLYME - AN IMPROVED VAPOR-PHASE SOLDERING FLUID

Abstract: A method of vapor-phase soldering employing perfluoroheptaglyme as vapor phase soldering fluid is disclosed.

Description: PERFLUOROHEPTAGLYME - AN IMPROVED VAPOR-PHASE SOLDERING FLUID Background of the Invention A preferred method for soldering a series of joints is to immerse' the pre-positioned components into a vapor bath which is sufficiently hot to melt the solder. The latent heat of vaporization (con¬ densation) causes the solder to melt as the vapors condense on the cool surfaces. Preferabl , solders 10 having melting points below about 200 * C are used. Several advantages are realized over more tradi¬ tional soldering methods, most notably, very rapid heating can be achieved. For example, an entire circuit board can be heated evenly since the vapor ■ j _5 condenses primarily on the cooler components having a large thermal capacity. Even heating minimizes polymer degradation and produces a finished product that is clean and has minimal metal oxidation. Vapor phase soldering, for these reasons, is becom- 20 ing widely used in the electronics industry. Two of the most commonly used vapor phase soldering fluids are perfluorotripentylamine (FC 70) and perfluorotetradecahydrophenanthrene (U.S. Patent No.4,549,686) . 25 Briggs (Brazing Soldering, 1_: 6-Z , 12 (1984)) describe use of perfluoropolyetherε for vapor phase reflow soldering fluids. One specific fluid has the structure: CF_ [ (OCF(CF 3 )CF 2 ) n " (OCF 2 ) m ^ OCF 3 ; where the ratio of n/m is 40. Dishart describe a vapor phase heat transfer process which uses low molecular weight hexafluoro- propylene oxide oligomers having the formula: F[CF(CF 3 )CF 2 0] n CF 2 CF 3 ; where n is 1 to 9. Calini and Modena (European Patent Application 269,029) describe several perfluoroethers which are suitable for vapor phase soldering, such as perfluoropentaglyme. Summary of the Invention This invention pertains to a novel perfluoro- polyether, perfluoroheptaglyme, and an improved vapor-phase soldering method using perfluoro¬ heptaglyme as a vapor-phase soldering fluid. Detailed Description of the Invention This invention relates to the technique of condensation soldering, which is also known as "vapor-phase soldering" and "vapor-phase solder reflow" . In order to make a satisfactory soldering fluid, the fluid must have a sufficiently high boiling point to melt the solder. Since the fluid is refluxing in the bath, it must be oxidatively stable at its boiling point and must be chemically inert in the vapor phase in the presence of metals, polymeric materials, and the flux which is typically present on the printed circuit board. The fluid must be recoverable by condensation and should be nonflammable. The fluid should have good dielectric properties and must have a melting point below room temperature. And finally, an ideal fluid would have a high heat of vaporization, should be nontoxic and not leave a residue after evaporation. It has now been discovered the perfluoropoly- ether, perfluoroheptaglyme, CF,0(CF_CF 2 0) 7 CF_ can be used as a vapor-phase soldering fluid and offers several advantages in this capacity over the prior art vapor-phase soldering fluids. The most notable advantages are: (1) Because perfluoroheptaglyme does not have more than two sequential difluoro ethylene units, the likelihood of producing deadly perfluoroiso- butylene while refluxing is greatly diminished (time-weighted average threshold limit for an 8-hour day is 0.01 ppm). No evidence of perfluoroisobuty- lene was found using a state-of-the-art gas chromatographic technique. This compares favorably to the greater than 0.1 ppm level sometimes produced from FC 70. (2) The cost of manufacturing perfluorohepta¬ glyme will probably be less than that of producing the prior art fluids. A cost savings is expected since heptaglyme is an inexpensive starting material while the yield of the direct fluorination reaction is comparatively high. (3) With the higher molecular weight of perfluoroheptaglyme (966 vs. 624 for perfluorotetra- decahydrophenanthrene and 821 for FC 70) the heat transfer is more efficient than with the prior art fluids having similar boiling points with lower molecular weights. (4) Perfluoroheptaglyme is superior to low molecular weight oligo ers of hexafluoropropylene oxide such as those described by Dishart and the copolymers of hexafluoropropylene oxide and difluoromethylene oxide described by Briggs since it is a unimolecular fluid. The composition of a fluid consisting of a mixture of components having different molecular weights is always changing as the more volatile components evaporate thus giving a fluid with a higher boiling point with time. (5) Perfluorheptaglyme is superior to the perfluoropentaglyme described by Calini and Modena since its boiling point of 205"C is better suited for solders presently being used in the industry. In order to prepare perfluoroheptaglyme by the method of this invention heptaglyme is first pre¬ pared. Heptaglyme can be made in a variety of ways, two of which are described below: (1) Diethylene glycol monomethyl ether can be reacted with sodium metal to form an alcoxide which can then be reacted with 1,2-bis(2-chloroethoxy)- ethane to form heptaglyme. (2) Alternatively, triethylene glycol mono- methyl ether can be reacted with thionyl chloride to replace the hydroxyl group with chlorine which can then be reacted with the alkoxide formed in the reaction of tetraethylene glycol monomethyl ether with sodium. Regardless of the method used, the heptaglyme can be easily distilled at reduced pressure to obtain a pure product. The heptaglyme is then perfluorinated preferab¬ ly by direct fluorination techniques. Because of the reactive nature of elemental fluorine, it is beneficial to dilute the fluorine with an inert gas such as nitrogen or 'helium. Typically, the fluorine is diluted with nitrogen and as higher degrees of fluorination are achieved, the concentration of fluorine is usually increased. Due to the extreme exothermicity of the reaction, the fluorination must be carried out slowly unless provisions have been made for rapidly removing the heat of reaction. Submersion of the reactor in a cooled liquid bath is usually adequate for achieving commercially ac¬ ceptable rates of reaction. Fluorine gas is the preferred fluorinating agent and is commercially available at sufficiently high purity levels and at an acceptable cost. Other fluorinating agents such as chlorine trifluoride can be used, however, some chlorine substitution may occur. The fluorination reaction is generally carried out at a temperature between about -80 and about +200"C, preferably between -20 and +20 * C. It can be carried out in a reactor containing an ultraviolet radiation source or in the dark. Using the pre¬ ferred temperature range, it is not necessary to have an ultraviolet light source since the fluorine is sufficiently reactive. If an ultraviolet light source is used, a wavelength between 250 and 350 nm is preferred. When the reactor is radiated with an external light source, a transparent window is needed which does not react with either fluorine or hydrogen fluoride. A quartz lens coated with a thin film of fluorinated ethylene-propylene copolymer works well. The fluorination reaction can be carried out in a variety of ways . The heptaglyme can be coated on an inert solid such as sodium fluoride powder to give a free-flowing powder which can be fluorinated in either a stationary tube, in a rotating drum-type reactor, or in a fluidized bed. The yields associ¬ ated with such reactions are generally poor due to the abundance of side reactions which give primarily higher molecular weight branched products. Much higher yields can be obtained by dissol¬ ving the heptaglyme in a solvent which is inert to fluorine. See U.S. Patent Application Serial No. 07/250,376, entitled "Liquid Phase Fluorination", by Bierschenk, Juhlke, Kawa and Lagow, filed September 28, 1988, and U.S. Patent Application Serial No. , entitled "Liquid Phase Fluorination", Attorney's Docket No. EXF86-06A, filed September 28, 1989, the teachings of which are incorporated by reference. For example, 1,1,2-trichlorotrifluoroethane (Freon 113 TM), does not react appreciably with fluorine when the preferred temperature range is used and functions as a good solvent for the reaction. Other suitable solvents are perfluorocarbons, perhalogenated σhlorofluorocarbons and perhalogenated chloro- fluoroethers. The reaction can be carrier out either in a batch mode where all of the heptaglyme is dissolved in the solvent prior to fluorination or in a continuous mode where heptaglyme is continuously being pumped into the solvent as fluorine is being bubbled through solution. Generally, the continuous operation gives a prefer- red yield, better product quality, and improved rates. When carrying out the reaction in a solvent, a hydrogen fluoride scavenger such as sodium fluoride may or may not be present in the solution to scav- enge the by-product hydrogen fluoride. However, the preferred mode for carrying out the reaction is with a sufficient quantity of sodium fluoride being present to complex with all of the hydrogen fluoride formed. When fluorinating ethers in the presence of sodium fluoride, improved yields are obtained while chain cleavage and rearrangements are minimized. See U.S. Patent No. 4,755,567, and U.S. Patent Application Serial No. 07/198,154, filed May 24, 1988, the teachings of which are incorporated herein by reference. In summary, perfluoroheptaglyme is preferably prepared by continuously introducing heptaglyme into a stirred reactor containing a solvent, such as 1, 1 ,2-trichlorotrifluoroethane, which is maintained between about -20 and about +20'C. Dilute fluorine gas is the preferred fluorinating agent and reacts well in the absence of ultraviolet radiation. The fluorine is diluted so that it is below the flamm¬ able limits of the liquid medium (in fluorine) . Superior yields are obtained when sufficient quanti¬ ties of sodium fluoride are present to complex all of the by-product hydrogen fluoride. The product produced in this manner generally has a purity between 90 and 95% with a residual hydrogen content of 0.001% or less. In order to obtain a fluid which is essentially free of residual hydrogen and void of any reactive terminal groups, such as acyl fluoride groups resulting from chain degradation reactions, a final fluorination near 175"C with 50% fluorine for several hours works well. The resulting inert fluid can then be distilled to give perfluoroheptaglyme of sufficient purity for most applications. The reactions are best summarized by the following equations: CH 3 OCH 2 CH 2 OCH 2 CH 2 OH + a->NaOCH 2 CH 2 OCH 2 CH 2 OCH 3 + %H 2 2 aOCH 2 CH 2 OCH 2 CH 2 OCH 3 + ClCH 2 CH 2 OCH 2 CH 2 OCH 2 CH 2 Cl → CH 3 (CH 2 CH 2 0) 7 CH 3 Freon 113/N2 CH 3 0(H 2 CH 2 ) ) ? CH 3 + 34NaF + 34F 2 > O * C CF 3 0(CH 2 CF 2 0) 7 CF 3 + 34NaHF In the vapor phase soldering method of this invention, a component to be soldered is immersed in a perfluoroheptaglyme vapor bath to melt the solder. Soldered components are then removed from the bath. A preferred solder is 60/40 tin/lead (Sn/Pb) solder, but generally any solder which has a melting point below about 200"C can be used. The method is particularly useful for soldering electrical com¬ ponents to printed circuit boards. The invention is illustrated further by the following examples. Example 1: Synthesis of Heptaglyme Into a dry 3 liter flash containing a mechani¬ cal stirrer and flushed with nitrogen was placed 1500 ml of diethylene glycol methyl ether. The temperature of the flask was held below 70 * C as 138 g of sodium metal (6 mol) was added intermittently as small chunks over a 2 hour period. After 24 hours all the sodium had reacted and 561 g of l,2-bis(2-chloroethoxy)ethane (3 mol) was added slowly over a 2 hour period. The mixture was kept approximately 50 * C with a water bath as the contents of the flask reacted. The mixture was stirred overnight at room temperature, then at 70 * C for 1 day. Recovery of the product, heptaglyme, was accomplished by first adjusting the pH of the mixture to 6 with hydrochloric acid followed by filtration and distillation of the filtrate. 552 g of a light yellow liquid, heptaglyme, was recovered in 52% yield (b.p. 200-213'C @ 7 mm Hg) . Example 2: Fluorination of Heptaglyme in a Rotating Drum Reactor Into a 750 ml rotating drum reactor was placed 17.5 g heptaglyme coated on 175 g of finely ground sodium fluoride. The freely flowing mixture was purged with nitrogen for 2 hours as the reactor rotated at a rate of 45 revolutions per minute. The reactor was cooled to -13'C using a low temperature bath while the reactant gas flows were set at 17.5 cσ/min and 200 cc/min for fluorine and nitrogen, respectively. The conditions were maintained for 28 hours before the reactor was slowly heated to 35 * C over a 12 hour period. Upon achieving a reactor temperature of 35 * C, the nitrogen flow was turned off and the contents of the reactor were exposed to pure fluorine for several hours. The crude product mixture was removed from the reactor following purging with several volumes of nitrogen. Extrac¬ tion of the product with approximately 1 liter of 1, 1,2-trichlorotrifluoroethane gave 38.9 g of fluid after the 1, 1,2-trichlorotrifluoroethane was flashed off using a rotary evaporator. Distillation of the fluid gave 9.2 g of perfluoroheptaglyme with the remainder being a less volatile material consisting primarily of dimers and tri ers of per¬ fluoroheptaglyme (19% yield). Example 3: Fluorination of Heptaglyme in a Batch- Type Solvent Reactor A mixture of 200.6 g of heptaglyme, 4 liters of 1, 1,2-trichlorotrifluoroethane and 800 g of sodium fluoride powder was placed in a 2 foot long aluminum reactor which was constructed from 10" pipe. The reactor was rotated at approximately 60 revolutions per minute in a water/ethylene glycol bath which was held at -10'C. The reactor was flushed with nitro¬ gen for several hours prior to beginning the fluorination. A gas flow of 400 cc/min fluorine and 800 cc/min nitrogen was maintained for 14 hours, then the fluorine and nitrogen flows were reduced to 300 and 600 cc/min, respectively, for an additional 5 hours during which time the reactor was slowly heated to 70"C. The contents of the reactor were exposed to pure fluorine at 70 * C overnight (12 hours) before cooling to room temperature and flushing with several volumes of nitrogen. The contents of the reactor were filtered and the NaF was washed several times with approximately 3 liters of 1, 1 ,2-trichlorotrifluoroethane. The combined filtrates were distilled to remove the 1, 1,2-tri¬ chlorotrifluoroethane 401.5 g of a fluid was re¬ covered (50%) of which approximately 50% distilled over as perfluoroheptaglyme (b.p. 205 * C). Example : Fluorination of Heptaglyme Using a Continuous-Addition Solvent Reactor Into a 22 liter stirred, cylindrical aluminum tank being approximately 10 inches wide and 2 feet tall was placed 5.5 liters of 1, 1,2-trichlorotri- fluoroethane and 900 g of sodium fluoride powder. The fluorination of hydrocarbons in a continuous addition solvent reactor is more fully described in U.S. Patent Application Serial Number 07/250,376, entitled "Liquid Phase Fluorination", supra. The reactor was placed in a water/ethylene glycol bath which was held at approximately 0 * C. Attached to the reactor was a condensor kept at -30 * C which condensed and returned to the reactor the 1,1,2- trichlorotrifluoroethane which was swept from the reactor by the gas mixture used to fluorinate the heptaglyme. Prior to beginning the fluorination, the reactor was purged with several volumes of nitrogen. 253 g of heptaglyme, diluted to a total volume of 660 ml with 1, 1,2-trichlorotrifluoro¬ ethane, was pumped into the reactor over a 22 hour period. Gas flows of approximately 450 to 525 cc/min fluorine and 1500 cc/min nitrogen were used during the reaction. The exit gas of the reactor was monitored throughout the reaction to ensure that an adequate amount of fluorine was being delivered to completely fluorinate the incoming heptaglyme. Typically, the exit gas contained approximately 5% fluorine which indicated that a slight excess of fluorine was being used. The contents of the reactor were filtered following the reaction. The insoluble portion of the mixture was washed with several liters of 1, 1 ,2-trichlorotrifluoroethane. The filtrates were combined and distilled to give 428 g of a fluorocarbon fluid. Treatment of the fluid at 175 * C with 50% fluorine gave a fluid of similar weight which was essentially free of any hydrogen. A second distillation resulted in the isolation of 396 g of perfluoroheptaglyme (57% yield) . Example 5: Perfluoroheptaglyme Co patability with Solder Flux Perfluoroheptaglyme was poured into a vapor phase soldering tank. The liquid was heated to its boiling point and a vapor layer was established. A glass slide coated with Solder Flux 611 (Alpha- metals), a representative flux used in the industry was dried at 100'C and subsequently placed into the vapor zone of the perfluoroheptaglyme. A thirty second exposure to the vapors caused essentially no removal of the flux from the slide. Example 6; Test Soldering of Electronic Components with Perfluoroheptaglyme An evaporation tank was filled to its normal operating level with perfluoroheptaglyme. After achieving a controlled reflux, a circuit board containing three integrated circuits was lowered into the vapor zone for 25 seconds. The following chips were successfully soldered: (1) A twelve connector chip inserted in a tin-lead through-plated board containing 60/40 Sn/Pb solder. (2) A fourteen connector surface mount chip containing gull connectors was placed on a surface-mount board containing 60/40 Sn/Pb solder. (3) n eighteen connector surface mount chip containing sidemount connectors was placed on a surface-mount board containing 60/40 Sn/Pb solder. Equivalents Those skilled in the art will recognize, or be able to ascertain using no more than routine experimentation, many equivalents to the specific embodiments of the invention described herein. Such equivalents are intended to be encompassed by the following claims.

Claims: CLAIMS 1. A method of soldering a component, comprising immersing the component and solder in a vapor bath to melt the solder, the vapor bath com- prising perfluoroheptaglyme; and removing the soldered component from the vapor bath. 2. The method of Claim 1 , wherein the solder has a melting point below 200 * C. 3. The method of Claim 2, wherein the solder is a 60/40 tin/lead solder. 4. The method of Claim 1, wherein the component is an electrical component. 5. The use of perfluoroheptaglyme as a vapor phase soldering fluid.

Inventors: BIERSCHENK THOMAS R, JUHLKE TIMOTHY, KAWA HAJIMU, LAGOW RICHARD J, BIERSCHENK, THOMAS, R., JUHLKE, TIMOTHY, KAWA, HAJIMU, LAGOW, RICHARD, J.
Applicants: EXFLUOR RES CORP, EXFLUOR RESEARCH CORPORATION
Application year: 1989
Main classification: B23K35
IPC codes: B23K1, B23K35, C07C43, H05K3