Glass fiber with solderability enhancing heat activated coating

A protective layer or film applied to the metal surface of metal-clad optical fibers or metal-clad glass capillaries protects the metal surface during storage and provides a suitable fluxing surface. The protective layer or film comprises a mixture of a dicarboxylic acid fluxing agent and a chosen protective material which is unreactive with the dicarboxylic acid and which forms a film with the dicarboxylic acid that alters at a chosen soldering temperature to release the dicarboxylic acid. Prior to soldering, the chosen protective material maintains the metallic layer in an oxide-free state and the protective layer protects the metallic layer from external contamination to thereby enhance the solderability of the metallic surface when exposed to the soldering temperature at which the fluxing agent is released.

CROSS-REFERENCE TO RELATED PATENT 
The present application is related to U.S. Pat. No. 5,145,722, issued 
September 8, 1992, which is directed to a method and composition for 
protecting and enhancing the solderability of metallic surfaces. 
BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates generally to the soldering of metal-coated 
glass fibers. More specifically, the present invention relates to a 
coating or film which protects the metallic coating f rom oxidation and 
contamination during storage of metal-coated optical fibers and 
metal-coated glass capillaries and which provides at the same time for 
proper fluxing of the metal coating to enhance solderability at the solder 
temperature. 2. Description of Related Art 
Optical fibers, which comprise a glass core and a glass cladding to provide 
waveguiding of light, are coated with a metal or alloy in order to 
facilitate connections in electro-optic circuits and to other optical 
fibers and to provide increased strength and durability to the optical 
fiber. 
The metal coating of the optical fibers is described in, for example, U.S. 
Pat. Nos. 4,407,561 and 4,418,984, both assigned to the same assignee as 
the present application. As disclosed in U.S. Pat. No. 4,407,561, the 
metal (or alloy) that is used is one that (a) is substantially chemically 
inert with respect to the material comprising the glass fiber at the 
melting point of the metal or alloy, (b) has a recrystallization 
temperature greater than room temperature or the contemplated working 
temperature, whichever is greater, and (c) forms a hermetic seal around 
the outer surface of the glass cladding. Examples of suitable metals 
include vanadium, chromium, iron, cobalt, nickel, copper, zirconium, 
niobium, and palladium. Other metals which have been used in the art to 
coat optical fibers are disclosed in U.S. Pat. No. 4,407,561 and include 
aluminum, antimony, bismuth, cadmium, silver, gold, zinc, lead, indium, 
tin, and their alloys. Optionally, a plastic coating may be provided on 
top of the metallic coating to provide added mechanical protection and 
electrical insulation. 
U.S. Pat. No. 4,418,984 discloses the formation of at least two metallic 
coatings on the glass waveguide structure. The metallic coatings may be 
the same or different compositions. 
The metal-coated optical fibers are connected to electro-optic circuits or 
to other optical fibers by soldering. The solder operation requires proper 
fluxing agents, for without them, reliable electrical and/or mechanical 
connection may not be made even on apparently clean surfaces. In 
particular, the presence of metal oxides on the surface to be soldered 
prevents adequate wetting of the surface with the solder and results in a 
poor bond. Such oxides are referred to as "interfering oxides" and are 
often difficult to remove. 
Metal-coated optical fibers and circuit boards and components to which the 
metal-coated optical fibers are to be soldered, as received, are not 
necessarily clean and may carry various kinds of surface contaminants. 
Subsequent storage of metal-coated optical fibers is often done in an 
inert environment in order to prevent the surface from oxidizing. However, 
inert environment storage has been found to suffer from two disadvantages: 
(1) the cost of the inert gas used to provide the inert environment and of 
the sealed cabinets, and (2) the hazard of an oxygen-free environment to 
operators of the storage system. 
Metal-coated glass capillaries are also known as described, for example, in 
European Pat. No. 0 063 580, granted Jan. 21, 1987, for "Metallic Clad 
Capillary Tubing" and assigned to the present assignee. Many of the metals 
and alloys listed above are used as the metal coating. The same 
considerations involved in soldering metal-coated optical fibers also 
apply for soldering metal-coated capillaries. 
Thus, there is a need for a protective coating on the metal-coated glass 
fibers to protect the metal against oxidation during storage and to 
provide a suitable fluxing surface without the need for precleaning. As 
used herein, the term "metal-coated glass fibers" is intended to be 
generic to both metal-coated optical fibers and metal-coated glass 
capillaries. 
SUMMARY OF THE INVENTION 
In accordance with the invention, a surface coating or film is applied to 
the metal surface of metal-clad glass fibers in order to protect the 
surface against oxidation during storage and to provide a suitable fluxing 
surface without the need for precleaning. The surface coating or film 
comprises a mixture of a dicarboxylic acid fluxing agent and a chosen 
protective material which is unreactive with the dicarboxylic acid and 
which forms a film with the dicarboxylic acid that alters at a chosen 
soldering temperature to release the dicarboxylic acid. Prior to 
soldering, the chosen protective material maintains the metallic layer in 
an oxide-free state and the protective layer protects the metallic layer 
from external contamination to thereby enhance the solderability of the 
metallic surface when exposed to the soldering temperature at which the 
fluxing agent is released. 
The glass fiber may comprise an optical fiber, comprising a core having a 
first refractive index and a clad surrounding the core and having a second 
refractive index. Alternatively, the glass fiber may comprise a capillary. 
In either case, the outer surface of the glass fiber is provided with a 
metallic coating, which is coated with the protective layer of the 
invention. 
The protective layer or film is formed by providing the foregoing mixture 
and forming a film from the mixture on the metallic coating wherein the 
film provides protection of the metallic coating from contamination and 
against oxidation during storage prior to a soldering process, and, upon 
subsequent exposure to the chosen soldering temperature, releases the 
fluxing agent to thereby enhance the solderability of the metallic 
coating. 
The mixture comprises a solution in which the protective material and 
dicarboxylic acid fluxing agent are dissolved in a suitable solvent. The 
solution may be applied in a variety of ways to the metal-coated optical 
fiber, such as by spraying the solution on the fiber, dipping the fiber in 
the solution, or otherwise passing the fiber through the solution. 
The teachings of the invention (a) eliminate the need for the prior art 
environmental storage and (b) improve pretinning by providing a more 
solderable oxide-free surface and by supplying flux in the coating rather 
than by a separate application prior to tinning. 
DESCRIPTION OF THE PREFERRED EMBODIMENTS 
A novel glass fiber structure is formed which has a 
thermo-degradable/heat-activated coating that durably preserves and 
protects the solderability of the metallized optical fiber or capillary 
surface during storage and acts as a flux during the soldering operation. 
The coating comprises mixtures of protective materials and fluxing agents. 
The present invention has wide applicability to protecting and enhancing 
the solderability of a variety of metallic coatings on glass fibers. 
The present invention is particularly well-suited for treating copper-clad 
optical fibers. However, the invention is also useful in treating other 
metal-clad optical fibers having a metal coating that must be joined to 
electro-optic circuits or to other metal-clad optical fibers, and is 
useful for treating glass capillaries (i.e., tubular structures). 
The present invention may be used with a wide variety of solder types. 
Although lead and lead alloy solders are the preferred type of solder, the 
film produced in accordance with the present invention has use in treating 
metal-clad glass fibers prior to soldering wherein any of the other 
well-known solder materials are to be used. 
The films or coatings of the present invention are applied as a mixture to 
the metallic surface. The mixture includes a protective material and a 
dicarboxylic fluxing agent. 
The protective material is essentially unreactive with the dicarboxylic 
fluxing agent and the metallic surface to be soldered and is capable of 
forming, in the presence of the fluxing agent, the film which protects the 
metallic surface from contamination in accordance with the present 
invention. Further, the protective agent must alter its form when 
subjected to soldering temperatures, for example, between 500.degree. and 
700.degree. F. (260.degree. to 371.degree. C.), in such a manner that the 
fluxing agent is released. Thus, the heat at the soldering temperature 
activates the film to release the fluxing agent which enhances the 
solderability of the metallic surface. 
While not limiting the present invention to a particular theory of 
operation, it is believed that the protective material melts, although 
other forms of alteration may be possible. The particular protective 
material is chosen to be compatible with the particular dicarboxylic acid 
used and the particular soldering temperature used. Suitable protective 
materials include, but are not limited to, cellulose and derivatives of 
cellulose; vinyl polymers, particularly vinyl chloride copolymers alone or 
in combination with vinyl acetate or vinyl alcohol; acrylic copolymers; 
polyether glycols; and thermoplastic elastomers. Suitable cellulosic 
protective materials include cellulose; cellulose esters such as cellulose 
acetate, cellulose nitrate, cellulose acetate butyrate, cellulose acetate 
propionate, and the like; cellulose ethers such as ethyl cellulose, methyl 
cellulose, carboxy methyl cellulose, and the like; and other similar 
cellulose derivatives. Preferably, the cellulose is partially, but not 
totally, esterified. Since the protective material protects the surface to 
be soldered from contamination and oxidation, no presoldering cleaning 
step is required, and thus the use of undesirable chemicals is avoided. 
The dicarboxylic acid fluxing agent must be a dicarboxylic acid or a 
derivative thereof. Suitable dicarboxylic acids include adipic acid, 
succinic acid, fumaric acid, maleic acid, glutaric acid, their alkyl 
derivatives, and their aromatic derivatives. It is advantageous to use 
such a dicarboxylic acid fluxing agent which, when exposed to soldering 
temperatures, forms volatile by-products which leave no residue on the 
metallic surface. Thus, in accordance with the present invention, no 
post-soldering cleaning step is required. This feature of the present 
invention is particularly significant, since such cleaning to remove rosin 
flux as practiced in the art typically uses chlorofluorocarbon materials, 
which have an undesirable environmental impact. 
The exact ratio of fluxing agent to protective material will depend upon 
the particular application. Where it is desired to provide increased 
fluxing action or increase in the energy of low-energy surface sites, then 
the amount of fluxing agent should be increased. However, increases in the 
amount of fluxing agent tend to reduce the adherence of the film to the 
metallic surface. Accordingly, it is preferred to reduce the amount of 
fluxing agent to as low a level as possible to provide the desired 
enhancement of solderability while still insuring that adherence of the 
film or coating to the metallic surface is maximized. For a mixture 
comprising adipic acid and cellulose acetate butyrate, a preferred ratio 
by weight of the protective material to the fluxing agent has been found 
to be about 1:1. 
The above-described mixture may be applied to the metallic surface in a 
number of different ways. Preferably, the protective material is dissolved 
in a suitable solvent. For a cellulose derivative, suitable solvents are 
acetone, methyl ethyl ketone, other ketone solvents, or tetrahydrofuran. 
The fluxing agent is likewise dissolved separately in an appropriate 
solvent, such as methanol, isopropanol, acetone, or tetrahydrofuran. The 
two resulting solutions are then combined to form a coating solution in 
which the required ratio range of protective material and fluxing agent is 
provided. Alternatively, the fluxing agent and protective material may be 
dissolved in predetermined amounts in a common solvent or solvent blend. 
For example, adipic acid is dissolved in a solvent comprising isopropyl 
alcohol and acetone in a ratio of 40:60 to 60:40 isopropyl 
alcohol:acetone, and preferably 50:50. The concentration of the adipic 
acid in the solvent mixture is no more than about 20 percent by weight. 
A preferred coating solution is made by dissolving the protective material 
in a suitable solvent to form a protective material solution, and 
dissolving the dicarboxylic acid in a suitable solvent to provide a 
fluxing agent solution. These two solutions are then mixed together in 
equal proportions to form the final mixture which is preferably stored at 
room temperature for several hours, typically about twenty-four hours, 
prior to use. This storage time may be reduced if the storage temperature 
is raised slightly. However, it is preferred to store the mixture at room 
temperature to allow complete mixture of the solutions. The mixture may be 
diluted as required with an appropriate solvent or solvent blend in order 
to obtain the desired viscosity for applying the mixture, such as by 
dipping the substrate containing the metallic surface into the solution, 
to form a good quality coating. 
After the setting period, the solution can be applied to the metallic 
surface by spraying or brushing, or the surface can be dipped into the 
solution. In a dipping process, the structure supporting the metallic 
surface, as well as the metallic surface itself becomes coated. 
Silk-screening of the solution onto the surface is also possible. The 
solution is quick drying and results in a tough and well-adherent thin 
film. The solution has an indefinite shelf life and one sample has been 
stored for over a year and a half without loss in effectiveness. 
In order to increase the adherence of the film to the metallic surface, it 
was discovered that heating of the film to temperatures between 
120.degree. and 150.degree. C. resulted in a more adherent film having an 
improved appearance. Other methods for achieving this improved adherence 
and appearance of the film include hot spraying of the solution at 
temperatures between 120.degree. and 150.degree. C. and/or preheating of 
the metallic surface within this temperature range. 
The thickness of the coating formed in accordance with the present 
invention depends on the particular fluxing agent and protective material 
used and their relative proportions in the coating solution, as well as on 
the particular method used to apply the coating and the particular solder 
reflow technique used. The coating must be of sufficient thickness to 
provide an adequate amount of adipic acid (or other dicarboxylic acid 
fluxing agent) to be released at soldering temperatures so that 
solderability is enhanced. In addition, the coating must be sufficiently 
thick to provide a barrier to contamination. On the other hand, if the 
coating is too thick, the protective material may char or produce 
contamination when subjected to soldering temperatures. The preferred 
thickness may be readily determined by experimentation. For the embodiment 
of the present invention described in the example herein, the preferred 
thickness was within the range of about 0.4 to 1.2 mils (0.001 to 0.003 
cm). 
Another method for preparing the mixture which is to be applied to the 
metallic surface involves melting the protective material and fluxing 
agent in the appropriate weight ratios to form a liquid having the 
appropriate composition. The liquid is cooled and the resulting solid is 
dissolved in a suitable solvent, such as acetone or methyl ethyl ketone. 
The resulting solution is then allowed to set for several hours at room 
temperature prior to application to the metallic substrate by any of the 
methods previously mentioned. The concentration of the protective material 
and fluxing agent in the solvent can be varied widely depending on the 
method of application used, such as spraying, brushing, dipping, etc. 
Preferably, the combined concentration of the protective material and 
fluxing agent in the solvent should be below about seventy percent. Higher 
concentrations result in the solution becoming exceedingly viscous and 
difficult to apply evenly in thin coatings. 
The coating of the present invention is preferably applied to the metallic 
surface as soon as possible after the metallic surface is formed, in order 
to avoid any possible contamination of the surface. 
If desired, the coating can be softened after application by subjecting it 
to a suitable solvent such as a halocarbon or a ketone such as acetone. 
The softened coating is tacky and is especially useful in fabricating 
electronic circuit boards where it is desired to stick the fiber onto the 
board prior to soldering. The film should be treated with the additional 
solvents for only as long as it takes to make the surface softened and 
tacky. Unduly long exposure of the film to solvents may deteriorate the 
film. 
The protective layer provided in accordance with the teachings of the 
present invention will maintain solderability of a metallized optical 
fiber or capillary surface so that restoration of solderability by use of 
chemical and mechanical treatments can be precluded. 
The advantages provided by the protective layer of the present invention 
are: 
a. Protection against oxidation. During storage, the dicarboxylic acid 
serves as a pro-active flux to maintain the metal surface in an oxide-free 
state, which provides a clean surface for forming an improved solder 
connection. 
b. Protection against contamination. The coating protects the bare metal 
surface during storage against contamination from the environment. 
c. No additional solder flux required. The coating acts as a flux during 
the soldering operation. 
d. No solder coating during storage. Because the structure does not have to 
be solder-coated to maintain solderability during storage, harmful 
intermetallics which degrade solderability cannot form prior to actual 
preparation for component use. 
e. Fewer operational steps and expenditures. 
f. No post-solder cleaning. The use of restricted solvents is avoided. 
The structure provided by the teachings of the present invention should 
reduce the cost of soldering both optical fibers and cables made therefrom 
and glass capillaries by providing a surface that is easily soldered even 
after prolonged storage. 
The structure protects and preserves the solderability of the metal-coated 
glass fibers. Upon application of heat, the protective layer is vaporized 
and the solderability of the metallization of the glass fibers is 
enhanced. Thus, the fiber can be easily soldered to other solderable 
surfaces particularly to many other fibers to form an integral 
hermetically joined cable.

EXAMPLES 
Two lengths of copper-clad optical fiber were divided into 8 equal pieces 
to provide one data point for each of the parameter combinations listed 
below. The test results for the thin and thick samples of each type were 
averaged. 
1) Cable thickness 
a) Thin (0.015 to 0.019 inch, or 0.038 to 0.048 cm, dia.) 
b) Thick (0.020 to 0.025 inch, or 0.051 to 0.064 cm, dia.) 
2) Flux or coating used 
a) RMA (applied just before testing) (RMA is rosin mildly activated) 
b) NTC 1500-041088 (this invention) (NTC comprised cellulose acetate 
butyrate, comprising 2 to 3% acetyl and 50% butyryl, as determined by 
Fourier Transform Infrared Spectrometry; this material is no longer 
commercially available) 
c) 531-1 with adipic acid and IPA/acetone (this 
invention) (531-1 comprised cellulose acetate butyrate having a high 
butyryl content (average of 50 percent) and a melting range of 135.degree. 
to 150.degree. C.; the ratio of 531-1 to fluxing agent was 1:1) 
3) Storage conditions 
a) Fresh ("As Rec'd"); and 
b) 60 hours at 70.degree. C. in a circulation oven without humidity 
controls ("Accel Age"). 
The solderability test data is presented in the Table below. In addition to 
the three flux coatings used above, a fourth condition was also studied: 
no flux applied to the metal coating. The time to cross buoyancy line was 
measured under the two conditions: fresh and after aging. 
TABLE 
______________________________________ 
Solderability Test Data. 
Zero Time (sec) 
______________________________________ 
No Coating 
As Rec'd &gt;5.00 
Accel Age 2.65 
RMA 
As Rec'd 3.05 
Accel Age 1.60 
NTC 
As Rec'd 0.55 
Accel Age 1.40 
531-1 
As Rec'd 1.95 
Accel Age &gt;5.00 
______________________________________ 
The zero time, expressed in seconds, indicates how long it took the part 
being submerged to a specific depth in the solder to achieve neutral 
buoyancy. In other words, the buoyancy of the immersed part has been 
overcome by wetting action between the molten solder and the submerged 
portion of the part. In addition, the solder surface is perpendicular to 
the surface of the part. Non-wetting occurs when the part remains buoyant 
in the solder and the angle between the part and the solder remains 
obtuse. Positive wetting occurs when an acute angle is formed between the 
part and the solder that contacts the part. Further, the buoyancy of the 
part has been completely overcome and the solder is actually pulling down 
on the part. The smaller the zero time number, the more solderable the 
part. 
In the case of the results shown in the Table, a set of common parts was 
used, so the results indicate relative capabilities of the fluxes tested. 
The NTC formulation is the best performer; in both cases, it had the 
lowest zero time value. The data further show that the two versions of 
this invention (NTC and 531-1) are as good or better than RMA in achieving 
the same solderability level. In both cases, the advantages of protection 
against metal oxidation and contamination are added to the achievement of 
good solderability. 
Thus, there has been disclosed a structure for protecting metal-clad glass 
fibers during storage and for enhancing solderability. It will be readily 
apparent to those skilled in this art that various changes and 
modifications of an obvious nature may be made, and all such changes and 
modifications are deemed to fall within the scope of the invention, as 
defined by the appended claims.