Abstract:
A method of creating a solder joint between a flexure platform and an optical glass fiber, including pre-treating the flexure platform to be solderable, metalizing the optical fiber at the location to be soldered, engaging the pre-treated flexure platform to the metalized area of the optical fiber, and subjecting the engaged flexure and fiber to induction energy to create the desired solder joint therebetween without harming the optical fiber with excessive heat.

Description:
REFERENCE TO RELATED APPLICATION 
     This application is based on U.S. Provisional Application No. 60/365,087, filed Mar. 18, 2002. 
    
    
     FIELD OF THE INVENTION 
     This invention relates generally to the soldering of flexures to optical fibers for use in optical telecommunications applications, and more particularly to induction soldering of a pre-treated flexure member to a metalized optical fiber. 
     BACKGROUND OF THE INVENTION 
     So-called flexures, i.e., small solderable metallic platforms, are used at the ends of optical fibers, such as in manufacturing optical telecommunications components, for support, alignment, and connection purposes. For example, a flexure platform, such as in the form of a flexible four-legged clip member, is connected to an optical fiber and used to orient and align the fiber end to other components, such as to lenses, receiver diodes, and transmitter devices. Prior methods used to connect flexures to optical fibers included hot air or hot gas soldering, as well as hot plate soldering. Such prior soldering methods would require the use of lower melting point solders due to the exposure of the entire optical fiber to excessive heat from the soldering process. However, problems are created when using such lower melting point solders, because over the life of the electronic component, the soldered connection of components can undergo unwanted metallurgical creep, inasmuch as the solder connection is not sufficiently strong. Further, hot plate soldering was disadvantageous because of the difficulty in fixturing and clamping the flexure member and optical fiber relative to the hot plate without at the same time causing undue heat concentration in the optical fiber. Further, there are always safety concerns present for the operator when using a hot plate soldering approach. Also, such prior soldering methods took excessive lengths of time for both preparation and completion, thereby substantially reducing production economies. 
     There has been an ongoing need for an improved soldering technique that permits rapid soldering of flexures to optical fibers without heat damage of the fiber, and which soldering technique can be rapidly achieved. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is an exploded perspective assembly view of the flexure member and an associated optical fiber; 
         FIG. 2  is a top plan view of the flexure member of  FIG. 1 ; 
         FIG. 3  is a side elevation view of the flexure member of  FIG. 1 ; 
         FIG. 4  is a perspective view depicting the components of  FIG. 1  as fixtured, and the presence of an induction soldering coil, with the holding pocket fixture removed for clarity; 
         FIG. 5  is an enlarged view of the soldered assembly of the flexure and optical fiber; 
         FIG. 6  is a cross-sectional view taken along lines  6 — 6  of  FIG. 5 ; 
         FIG. 7  is a perspective view of a holding pocket fixture for clamping the optical fiber, via non-metallic, non-heat conducting holder pins (shown in solid lines in their open, unclamped position, and in phantom lines in their clamped position), to the flexure member; and 
         FIG. 8  is a flow chart of certain exemplary embodiments of the present inventive process. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Having reference to the drawings, wherein like reference numerals indicate corresponding elements, there is shown in  FIG. 1  an illustration of a so-called flexure member  20 . A series of four flexible legs  22  extend downwardly from the respective outer corner edges of the main body portion  24  of flexure  20 . Legs  22  are used to secure the flexure  20  in place, and at the correct orientation and alignment, when fixedly mounted within an electronic module (not shown). In use, the flexure  20  carries and supports the operating end of an optical fiber  25 . The flexure  20  is oriented generally horizontally with the legs  22  extending outwardly and downwardly. It is preferably formed of KOVAR® material, but it can also be formed of Iron  43  and 17-7 ph steel. Depending upon the application, the flexure  20  can be formed to have a width of approximately 0.150 inch and a length of approximately 0.300 inch. However, it is recognized that the width can range up to as much as approximately 0.150 inch, and the length can vary as much as 0.600 inch. Typically, the flexure  20  is approximately 4 mils thick. The main body portion  24  of flexure  20  is formed so as to have a center valley section  26  on an upper surface of the flexure  20 , down the middle of which is formed an etched channel  28 . 
     Before being ready to accept an optical fiber  25  for subsequent mounting in a telecommunications component, e.g. optical module, the flexure  20  is first pre-treated. This is done in order to permit successful soldering by the induction soldering method of the present invention. More specifically, such pre-treating of the flexure  20  can comprise pre-tinning with a layer of so-called “80/20” solder (comprising 80% gold and 20% tin) into and along the etched channel  28  of flexure  20 . In any event, such pre-tinning is preferably to be perfomed at the location where the soldering to an optical fiber is to occur. Such a pre-tinning with solder can be achieved by applying a layer of 80/20 solder paste atop the flexure  20 , and then heating the flexure with solder paste in an oxygen free environment to re-flow, i.e. evenly spread and adhere, the solder to the flexure. Alternatively, one could employ a specially configured solder preform member. In that alternate method, a thin foil strip, i.e., solder preform, approximately 10 mils long and 2 mils wide, can be placed in the channel  28  of flexure  20 , and then the preform and flexure are heated to re-flow the solder. In either method, the result is a flexure  20  with a solderable channel  28  ready to accept a mating optical fiber. Any remaining solder flux should be removed from the flexure after pre-tinning. 
     Turning to  FIG. 2 , there is shown, besides the now pre-treated flexure  20 , a glass optical fiber  25 . Typically, when used for optical telecommunications applications, such a fiber  25  can be approximately 250 microns in overall diameter (D″), with an outer polymeric coating of approximately 125 microns in thickness, a secondary glass cover layer of approximately 125 microns in diameter (D′), and a central core glass strand of approximately 5 microns in diameter (D) (see FIG.  6 ). In any event, the flexure  20  and optical fiber  25  are preferably appropriately sized for the particular application at hand. 
     When used with the present invention, the optical fiber  25  is first pre-treated and prepared, preferably at just the local area where soldering is to occur. In commercially available form, the two-part glass fiber strand making up the optical fiber  25  is typically pre-coated with a polymeric coating. That polymeric coating is stripped away (e.g., mechanically or thermally) at the location along its length where soldering is required; that location is typically at the outer end  27  of the optical fiber  25 , and extends for approximately 14 to 18 mm, and preferably for approximately 16 mm. The resulting stripped length L of the optical fiber  25  (see  FIG. 1 ) is then metalized for purposes of soldering. metalizing can be achieved by applying a thin adhesive layer coating of Titanium-Tungsten, or Chromium-Gold. Other suitable metal materials to use for such metalization include Chrome or Nickel-Chromium. Thereafter, as a solderable layer, a Nickel layer is applied on top of the thin adhesion layer. It is recognized that Platinum could also be used. That Nickel solderable layer is present to facilitate a satisfactory solder joint of the metalized optical fiber  25  with the flexure  20 . Thereafter, as a protective layer, a thin Gold layer is placed on top of the Nickel or other solderable metal layer, so as to protect the same prior to its soldering. All of these metalizing layers can be applied, for example, through known metal evaporation techniques. 
     Instead of pre-treating the flexure platform, such as by the above-described pre-tinning methods, with solder paste, or a solder preform, the metalized end of the optical fiber  25  may alternatively be pre-tinned itself, such as with a suitable 80/20 solder coating. 
     Turning next to  FIG. 5 , there is shown the engaged physical assembly of metalized optical fiber  25  and flexure  20 , as engaged against one another and now ready for the non-contact induction soldering in accordance with the present invention. To assure proper contact during soldering, there is seen in  FIG. 7  how the flexure  20  is fitted into a small holding pocket fixture  34 . The optical fiber  25  is seen as clamped (via non-metallic, non-heat conducting, e.g. ceramic, holder pins,  36 ,  38 , preferably under downward pressure, as shown in their clamped position in phantom lines in  FIG. 7 ) in place over the pre-tinned solder area  27  on the flexure  20 . In this way, the metalized area of the optical fiber  25  is maintained in direct contact with the pre-tinned solder area  27  of the pre-treated flexure  20  and is ready for soldering. 
     Then, the water-cooled induction coil  30  of an associated induction soldering wand machine (not shown) is placed in position (see coil  30  shown in solid lines in  FIG. 4 ) over the engaged and clamped assembly of the optical fiber  25  and the flexure  20 . The fixture  34  has been omitted from  FIG. 4  for clarity. A suitable induction soldering machine for use with the present invention is made by Seit Electronics, of Italy, as sold under the Minimax model name, operating at 900 KHZ. 
     Thereafter, a short induction cycle is triggered by switching on the induction energy source (not shown). Depending on the amount of energy utilized and the size and design of the induction coil  30 , a typical cycle of some 5 to 9 seconds can be utilized for the present induction soldering method. Preferably, the induction soldering cycle is approximately seven and one-half (7½) seconds, with an initial four (4) seconds of a pre-gas, e.g. Nitrogen, is directed by local jets onto the solder site (to purge the area of oxygen), then one and one-half (1½) seconds of induction coil heating time is applied, followed by an additional nitrogen gas flow time of two (2) seconds to assure that no solder flux residue results. 
     Thereafter, the induction energy source is deactivated, and the induction coil  30  is removed. Then, the soldered parts comprising the joined assembly of flexure  20  and optical fiber  25 , with solder fillets  32 , can be removed from the fixture  34  to complete the soldering cycle. 
     The above-described method of the present induction solder operation is reflected in flow-chart form in FIG.  8 . 
     Advantageously, the non-contact induction soldering process of the present invention does not create any heat damage to the optical fiber  25 . This is because the optical fiber  25  is formed of glass, which does not undergo any induction energization because the induction soldering process only heats the associated metals involved, and thus, does not directly heat the glass itself, exploiting the non-reactivity of glass to the electromagnetic field of this induction soldering process. This lack of heating of the glass fiber is a significant advantage over the known prior art soldering methods, where great care had to be taken to assure the associated glass fiber member and its protective polymeric coating was not damaged by the hot gas or hot plate soldering apparatus conventionally used. 
     Further, the present induction soldering technique is comparatively very quick. For example, a hot gas soldering technique would often take some 20 seconds to achieve a suitable heat and melt of the solder, primarily because of the low heat transfer rate of hot gas directed on a metal. However, the present induction soldering technique can achieve a satisfactory finished solder joint in only 7½ seconds, thereby giving an approximately 62.5% increase in speed over the prior art soldering techniques. The present induction soldering technique also results in a substantial improvement in solder joint quality, since higher melting temperature solder can be used, with better creep results. 
     Another advantage of the present invention is the substantial reduction in the amount of Nitrogen gas required, since the solder site is so localized, and is done more quickly, as compared to when a large hot plate is used for soldering. Further, the present invention advantageously results in a compact solder apparatus, instead of the large solder stations required in hot gas and hot plate solder techniques. 
     The optical fibers with soldered flexure as resulting from the techniques of the present invention can be used in fiber optic module applications, such as in so-called butterfly transmitters, butterfly receivers, and miniature modules, for example. Also, besides being used with solderable flexure platforms, such as flexure  20 , the present induction soldering process can also be used to solder metalized optical fibers to yet other type electronic components, such as to platforms, metalized ceramic components, and fiber feed through channels.