Abstract:
Packages are required to have high heat-dissipating quality. To fulfill this requirement, a highly heat-conductive material has been used for a package&#39;s bottom plate. This results in the use of a material that has a considerable difference in the coefficient of linear expansion between the side plate and bottom plate. Notwithstanding the use of this type of material, the package must be free from reduction in airtightness and degradation in optical coupling. The package of the present invention comprises side plate  1  and bottom plate  2  each having tenons  3 . The tenon portions are combined and bonded. It is desirable that tenons  3  have a width not less than 1 mm and not more than 5 mm.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a package for optical communications modules (hereinafter called a package) for use in optical communications, radio communications, etc. 
     2. Description of the Background Art 
     As shown in FIGS.  4 ( a ) and  4 ( b ), a conventional package  20  comprises a side plate  21 , two alumina members  28  (the alumina member at the other side is not shown.), a bottom plate  22 , and a lid  23 , which are assembled by soldering to hermetically seal the device housed inside. The bottom plate  22  has an extra length on both sides to provide holes  27  for fixing the package  20  to a printed substrate or a heat sink (not shown) by screws. 
     Usually, the bottom plate  22  of the package  20  is made of a Cu—W alloy, having high thermal conductivity, in order to dissipate the heat generated inside the package effectively to the outside. The side plate  21  is usually made of an Fe—Ni—Co alloy (brand name Kovar, for instance), which has a coefficient of linear expansion close to that of the bottom plate  22 . 
     Recently, however, the miniaturization and performance enhancement of electronic equipment have increased the consumption power of ICs used in electronic equipment and the output of light emitting diodes (LEDs) and laser diodes (LDs) used as the device for optical communications. This increase requires the package  20  that houses the device to increase its heat-dissipating power further. In order to meet this requirement, when the bottom plate  22  is made of a material that has higher thermal conductivity than the Cu—W alloy, such as SiC, the difference in the coefficient of linear expansion between the side plate  21  and the bottom plate  22  is increased. This is because whereas SiC has a smaller coefficient of linear expansion than the conventional material, the conventional material is still used as the side plate  21  because a suitable material that has a coefficient of linear expansion comparable to SiC is yet to be found. 
     If the package  20  has a considerable difference in the coefficient of linear expansion between the side plate  21  and the bottom plate  22 , the following problems may arise: 
     (a) Warping of the bottom plate  22 : 
     When the side plate  21  and the bottom plate  22  of the package  20  are soldered, the difference in the coefficient of linear expansion between the two plates produces a thermal distortion, producing a permanent warp of several to several tens of micrometers to the bottom plate  22 . When the package  20  having this warp is fixed to a printed board or a heat sink through the fixing holes  27  by screws, the aforementioned warp is straightened by force. This breaks the optical coupling between the LD  24  and the optical fiber  26  via the lens  25 , so that the light excited by the LD  24  cannot be transmitted satisfactorily to the outside through the optical fiber  26 . 
     (b) Repeated thermal stresses applied to the bonding portion between the side plate  21  and the bottom plate  22 : 
     When the device in the package  20  repeats heat generation by its operation and cooling by the discontinuation of its operation, the bonding portion between the side plate  21  and the bottom plate  22  is subjected to repeated thermal stresses. The repeated thermal stresses cause the bonding portion to produce minute cracks or other abnormalities, which in turn break the hermeticity of the package  20 . As a result, the properties of the device will deteriorate and subsequently lose its reliability. 
     On the other hand, packages housing devices have been required to have further increased heat-dissipating quality. Consequently, this requires he bottom plate of a package to be made of a material that has higher thermal conductivity than the conventional one, increasing the difference in the coefficient of linear expansion between the side plate and the bottom plate. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to offer a package that will not break its hermetic seal or impair the optical coupling between the optical devices even when the package comprises a side plate and a bottom plate that are made of materials having a relatively great difference in the coefficient of linear expansion. 
     The package of the present invention is formed by combining a side plate and a bottom plate, each having a different coefficient of linear expansion, both provided with tenons at their bonding portion. The tenon portions are matched together, and the bonding portion is soldered. When materials having a different coefficient of linear expansion are bonded, the rise and reduction in temperature may cause the material to warp or the bonding portion to generate minute cracks resulting from the repeated thermal stresses. The foregoing tenon structure is to prevent these problems. 
     It is desirable that the tenons have a width not less than 1 mm and not more than 5 mm. The effect will increase when a large number of tenons having a small width are provided. However, it is desirable to provide tenons having a width not less than 1 mm considering the manufacturing difficulty and cost and not more than 5 mm considering the effect. 
     As described above, packages have been required to further increase their heat-dissipating quality. To meet this requirement, the bottom plate of a package must necessarily be made of a material having excellent thermal conductivity. As a result, materials having a considerable difference in the coefficient of linear expansion between the package&#39;s side plate and bottom plate have been used. This may result in permanent warpage to the bottom plate after the soldering and generate cracks at the bonding portion resulting from repeated thermal stresses. 
     In order to solve these problems, the present invention offers a package that has a tenon structure at the bonding portion between the side plate and bottom plate. The package is assembled by soldering the combined tenon portions. The package suppresses the generation of the aforemeetioned warp, is free from cracks caused by repeated thermal stresses, has excellent performance, and therefore is highly reliable. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In the drawings: 
     FIG. 1 is a perspective view illustrating the tenons provided at the bottom plate and the side plate of the package of the present invention; 
     FIG. 2 is a perspective view illustrating the soldered bottom plate and side plate, each having the tenons, of the present invention; 
     FIG. 3 is a perspective view illustrating the soldered bottom plate and side plate, having no tenons, of the prior art; and 
     FIG. 4 is a diagram illustrating the conventional package. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The present invention&#39;s superiority to the conventional product is explained below. The conventional method produces the package by soldering a side plate  21  and a bottom plate  22  as shown in FIGS.  4 ( a ) and  4 ( b ). Only one side of the bottom plate  22  (the upper surface in the figure) has the soldering surface across the entire length of L. Consequently, the difference in the coefficient of linear expansion between the side plate  21  and the bottom plate  22  causes the bottom plate  22  to generate a permanent warp after the soldering or applies repeated thermal stresses of tension and compression to the one side of the bottom plate  22  as the package temperature rises or falls. 
     As opposed to the conventional method, the present invention has the following advantages: 
     (a) As shown in FIG. 1, the soldering portions between the side plate  1  and the bottom plate  2  are the hatched portions and the dotted portions. The total length of the hatched portions on one side of the bottom plate  2  (the upper surface in the figure) is less than half that of the conventional product. It can be seen dearly that, the hatched portions are provided at specific intervals. 
     (b) As shown in FIG. 2, the side plate  1  and the bottom plate  2  are mutually constrained at the tenon portions. 
     (c) As shown by the dotted portions in FIG. 1, bonding surfaces are also provided at the side faces of the bottom plate  2  in order to suppress the warp. 
     The above advantages not only reduce considerably the warpage produced on the bottom plate  2  after the soldering but also reduce to a negligible level the repeated thermal stresses of tension and compression applied to the one side of the bottom plate  2  as the package temperature rises or falls. 
     EXAMPLE 1 
     FIG. 2 shows an embodiment of the present invention. The package has a side plate  1  made of Mo (coefficient of linear expansion: 5.1×10 −6 /K) having a thickness of 0.5 mm, two alumina members  5  (coefficient of linear expansion: 6.7×10 −6 /K) having a thickness of 0.5 mm, and a bottom plate  2  made of SiC (coefficient of linear expansion: 3.7×10 −6 /K) having a thickness of 0.6 mm. The dimensions of the package are 21 mm length, 10 mm depth, and 10 mm height. The tenons provided in the longitudinal direction have a width of 3 mm; the tenons provided in the depth direction, 2 mm. 
     Four segments of the side plate  1  were cut from a molybdenum plate with a thickness of 0.5 mm. The segments were provided with tenons. The four segments of the side plate  1  and the two pieces of the alumina members  5  were assembled and silver-soldered to form the frame shown in FIG.  1 . The side plate&#39;s surfaces for bonding with the bottom plate  2  were plated with Ni (0.1 μm) and Au (0.5 μm) in order to increase the strength of bonding with the solder. 
     The package&#39;s electrode portions  4  were formed by bonding leads made of an Fe—Ni—Co alloy to the gold-plated portions  6  (streaked portions in FIG. 2) on the alumina members  5 . 
     The bottom plate  2  was produced by filling and sintering a material powder in a metal mold provided by considering the shrinkage ratio of SiC at the time of sintering. The surface was plated with Ti (0.1 μm), Pt (0.2 μm) and Au (0.5 μm) for bonding with the side plate  1  in order to increase the strength of bonding with the solder. 
     Application of Au—Ge solder to the bonding portion between the side plate  1  and the bottom plate  2  and subsequent heating completed the assembly as shown in FIG.  2 . The heating for the gold soldering was carried out at 400° C. for 30 minutes in a hydrogen gas furnace. 
     EXAMPLE 2 
     The package has a side plate  1  made of an Fe—Ni—Co alloy (coefficient of linear expansion: 5.3×10 −6 /K) having a thickness of 0.5 mm, two alumina members  5  (coefficient of linear expansion: 6.7×10 −6 /K) having a thickness of 0.5 mm, and a bottom plate  2  made of SiC (coefficient of linear expansion: 3.7×10 −6 /K) having a thickness of 0.6 mm. The package&#39;s overall dimensions and the tenon&#39;s widths in the longitudinal and depth directions are the same as in Example 1. 
     Four segments of the side plate  1  were produced by heating a plate of an Fe—Ni—Co alloy, pouring the molten alloy into a mold, removing the formed plate from the mold, and giving it precision machining (cutting). The segments were provided with tenons. As with Example 1, the four segments of the side plate  1  and the two pieces of the alumina members  5  were assembled and silver-soldered to form a frame. The side plate&#39;s surfaces were plated with Ni (0.1 μm) and Au (0.5 μm) for bonding with the bottom plate  2 . The package&#39;s electrode portions  4  were also formed. The Fe—Ni—Co alloy was superior in machinability in Example 1. 
     The bottom plate  2  was produced by processing an SiC plate having a thickness of 0.6 mm with a cutter and a laser. As with Example 1, the surface was plated with Ti (0.1 μm), Pt (0.2 μm) and Au (0.5 μm) for bonding with the side plate  1  Similarly, Au—Ge solder was applied to the bonding portion. The bottom plate  2  was coupled with the frame-shaped side plate  1  and then heated to complete the assembly work. 
     COMPARATIVE EXAMPLE 1 
     FIG. 3 is a perspective view for Comparative Example 1. The package has a side plate  1  made of an Fe—Ni—Co alloy (coefficient of linear expansion: 5.3×10 −6 /K) having a thickness of 0.5 mm, two alumina members  5  (coefficient of linear expansion: 6.7×10 −6 /K) having a thickness of 0.5 mm, and a bottom plate  2  made of a Cu—W alloy (coefficient of linear expansion: 6.5×10 −6 /K) having a thickness of 0.6 mm. The package&#39;s overall dimensions are the same as in Examples 1 and 2. The side plate  1  and the bottom plate  2  were processed without providing tenons as shown in FIG.  3 . They were assembled in a manner similar to that in Examples 1 and 2. The combination of a side plate of this type and a bottom plate of this type has been conventionally used. 
     COMPARATIVE EXAMPLE 2 
     The package has a side plate  1  made of an Fe—Ni—Co alloy (coefficient of linear expansion: 5.3×10 −6 /K) having a thickness of 0.5 mm, two alumina members  5  (coefficient of linear expansion: 6.7×10 −6 /K) having a thickness of 0.5 mm, and a bottom plate  2  made of SiC (coefficient of linear expansion: 3.7×10 −6 /K) having a thickness of 0.6 mm. The package&#39;s overall dimensions and shape (having no tenon) are the same as in Comparative Example 1. The assembly work was carried out by a manner similar to that in Examples 1 and 2 and Comparative Example 1. 
     Table 1 shows the materials and the structure of the bonding portion employed in Examples 1 and 2 and Comparative Examples 1 and 2. Table 2 shows physical properties of the materials used. 
     
       
         
               
               
               
               
               
             
               
               
               
               
               
             
           
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                   
                   
                 Comparative 
                 Comparative 
               
               
                   
                 Example 1 
                 Example 2 
                 Example 1 
                 Example 2 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                 Material 
                   
                   
                   
                   
               
               
                 Side 
                 Mo 
                 Fe—Ni—Co 
                 Fe—Ni—Co 
                 Fe—Ni—Co 
               
               
                 plate 
               
               
                 Bottom 
                 SiC 
                 SiC 
                 Cu—W 
                 SiC 
               
               
                 plate 
               
               
                 Structure of 
                 With tenons 
                 With tenons 
                 Without 
                 Without 
               
               
                 bonding 
                   
                   
                 tenons 
                 tenons 
               
               
                 portion 
               
               
                   
               
             
          
         
       
     
     
       
         
               
               
               
             
               
               
               
               
               
             
               
               
               
               
               
             
           
               
                   
                 TABLE 2 
               
             
             
               
                   
                   
               
               
                   
                 Side plate 
                 Bottom plate 
               
             
          
           
               
                   
                 Mo 
                 Fe—Ni—Co 
                 SiC 
                 Cu—W 
               
               
                   
                   
               
             
          
           
               
                 Coefficient of linear 
                 5.1 × 10 −6   
                 5.3 × 10 −6   
                 3.7 × 10 −6   
                 6.5 × 10 −6   
               
               
                 expansion (/K) 
               
               
                 Thermal 
                 140 
                 17 
                 270 
                 170 
               
               
                 conductivity 
               
               
                 (W/(m.K)) 
               
               
                   
               
               
                 Fe—Ni—Co: an Fe—Ni (29 wt. %)-Co (16 wt. %) alloy.  
               
             
          
         
       
     
     The packages produced in the Examples and Comparative Examples were subjected to the measurement of the magnitude of warp and the hermetic seal test using He gas. 
     The magnitude of warp was measured by placing a package bottom-side up (the bottom plate being at the top) on a surface plate. The magnitude was determined by the difference between a maximum height and a minimum height measured by a precision stylus-type height meter (capable of measuring a magnitude of the order of micrometer). 
     Samples for the hermetic seal test were prepared by providing a lid to the packages produced in Examples 1 and 2 and Comparative Examples 1 and 2 by the following method: The lid was a plate made of alumina. The portion to be in contact with the side plate and the alumina members was plated with Ti, Pt, and Au in this order. Next, Au—Sn solder was sandwiched between the lid and the side plate and between the lid and the alumina members. Finally, the samples were heated at 300° C. for 10 minutes to complete the preparation. 
     The hermetic seal test was carried out by using a helium-pressurizing vessel and a helium leak detector. First, a package was left for one hour in the He-pressurizing vessel (with a helium gas atmosphere at 5 atm). Next, the package was removed from the He-pressurizing vessel. Then, the amount of He gas leakage from the package was measured by the He leak detector. In other words, the He gas that penetrated into the package while the package was in the He-pressurizing vessel was detected if the hermetic seal of the package was not completely. 
     Before the hermetic seal test, the packages were subjected to the following heat cycle process: A package was first kept at −65° C. for 30 minutes. Next, the temperature was increased to 150° C. in 10 minutes and maintained at 150° C. for 30 minutes. Then, the temperature was reduced to −65° C. in 10 minutes. This cycle was repeated 10 times. 
     The obtained test results of warp and leak rate of He gas are shown in Table 3. 
     
       
         
               
               
               
               
               
             
               
               
               
               
               
             
           
               
                   
                 TABLE 3 
               
               
                   
                   
               
               
                   
                   
                   
                 Comparative 
                 Comparative 
               
               
                   
                 Example 1 
                 Example 2 
                 Example 1 
                 Example 2 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                 Warp (μm) 
                 20 
                 22 
                 20 
                 50 
               
               
                 Leak Rate of He 
                 Less than 
                 Less than 
                 Less than 
                 About 
               
               
                 Gas (atm.cm 3 /s) 
                 1 × 10 −8   
                 1 × 10 −8   
                 1 × 10 −8   
                 5 × 10 −6   
               
               
                   
               
               
                 Note: the value in the table is an average value of five samples.  
               
             
          
         
       
     
     The materials of the side plate and bottom plate of Example 2 are the same as those of Comparative Example 2. Example 2, however, has a tenon structure at the bonding portion. Example 2 showed not only a warp as small as about 40% of that of Comparative Example 2 but also a smaller amount of leakage in the airtightness test than that of Comparative Example 2. These results demonstrate that the tenon structure in a package is notably effective in preventing warpage and in increasing leak rate of He gas. 
     Comparative Examples 1 and 2 have the same structure at the bonding portion; i.e., both have no tenon structure. The difference in the coefficient of linear expansion between the side plate and bottom plate of Comparative Example 1 is smaller than that of Comparative Example 2. The test results that this is attributable to Comparative Example 1 having less warpage and leakage than Comparative Example 2. 
     Heat generated by an LD is mostly conducted by the bottom plate to dissipate it to the outside of the package. As shown in Tables 1 and 2, the packages in Examples 1 and 2 with the bottom plate made of SiC have a thermal conductance as high as about 1.6 times that of Comparative Example 1, the bottom plate of which is made of a Cu—W alloy with a thickness of 0.6 mm, the same thickness as in Examples 1 and 2. Comparative Example 2 having the same bottom plate made of SiC with a thickness of 0.6 mm as in Examples 1 and 2 has no tenon structure. As a result, it is poor in leak rate as shown in Table 3. Hence, it cannot be used in actual application. The warpage and hermetic seal obtained on the package in Comparative Example 1, which has been conventionally used, are the practical criteria. The packages obtained in Examples 1 and 2 have the qualities equivalent to these criteria as shown in Table 3.