Abstract:
A titanium alloy strip has a reduced cross section in the central region of the strip. By concentrating heat in this central region the process of bonding laser devices to a substrate is greatly improved. Furthermore, the present invention allows for the possibility of removing the laser device from the substrate without destroying the laser device.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     The present application is a divisional patent application of U.S. patent application Ser. No. 10/195,057 filed on Jul. 12, 2002. This application hereby claims priority of United Kingdom Patent Application No. 0116990.3 filed on Jul. 12, 2001, which is hereby incorporated by reference in its entirety and U.S. patent application Ser. No. 10/195,057 filed on Jul. 12, 2002, which is incorporated by reference in its entirety. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to an improved diebond strip. More specifically, the present invention relates to a Titanium alloy diebond strip which can be used to both attach and remove a laser die from a substrate. 
     BACKGROUND OF THE INVENTION 
     A known diebond strip is shown in  FIGS. 1 and 2 . The strip  5  is typically an elongated rectangular shaped thin piece of graphite with a centrally located hole  20  and two further holes  22 ,  24  located at opposite edges of the strip. The diebond strip is typically 1 mm thick approximately 15 mm wide and 54 mm long. Typically a component to be attached to a substrate is placed over central hole  20  and held in place with vacuum pressure. The component is placed on the substrate using various known designs of ceramic tooling attached around the strip. The strip is heated by connecting a low voltage, high current supply to holes  22  and  24 . These holes also function as a means of retaining the strip in place during the diebond process. 
     Currently, semiconductor laser die are mounted to a substrate by heating up an assembly in order to re-flow a preform of solder and allow the semiconductor laser die to be attached to the substrate. This process generates heat by passing a low voltage, high current supply along the die bond strip. With reference to  FIG. 3 , the semiconductor laser die  1 , also know as a chip, in attached to a submount  2 . The entire assembly  10  is heated via graphite strip  5  to a point where the solder preform  4  between the submount and the substrate reflows and attaches the chip-on-submount  2  to the substrate  3 . However, a technical problem exits with this know process in that the temperature must not be sufficient to compromise the joint between the chip and its submount. Thus, accurate and consistent temperature control is required. 
     Current methods of attaching the chip-on-submount to the substrate do not use any active control on the graphite diebond strip. Instead, the system uses thermocouples  31 ,  32  as seen in  FIG. 4 , and periodically checks to ensure the temperature remains within set parameters. 
     Another technical problem associated with the current system is maintaining the contact between the thermocouple and the diebond strip, as the thermocouple cannot be attached reliably to the diebond strip. The position and contact of the thermocouple are maintained by using springs  33 ,  34 . As a result, the control is based on the perceived temperature at the junction of the strip and thermocouple. Any variation in this junction can result in a variation of as much as 50° C., without any apparent change in the system. 
     Furthermore, the current system incorporates a parallel thermocouple system. Thus, if both thermocouples showed similar results, the system is considered to be “in balance” and ready for use. Any imbalance between the two thermocouples required the system to be re-balanced. 
     Yet another technical problem associated with the current diebond strip is that, due to the nature of graphite, it is difficult to press a thermocouple against the strip without the strip being flexed in some way. This reduces the effectiveness of the vacuum for the retention of the substrate on the strip, and compromises the transfer of heat energy from the strip to the substrate. Pressing the thermocouple against the strip also reduces efficiency of the thermocouple. 
     In addition, from the first time that the graphite strip is used, the strip begins to deteriorate. This is partially due to the processing of parts on the strip and causes an increase in the resistance of the strip. As the strip&#39;s effectiveness is reduced, and the process time gradually increases. This can be slightly compensated for by increasing the power to the strip. However, the current diebond strip must still be replaced approximately every 3 months. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to overcome or at least mitigate the above mentioned technical problems. 
     According to the present invention there is provided a strip for use in diebonding a component to a substrate, the strip comprising a central region having a reduced cross section compared to extremities of the strip. 
     The strip may have a stepped cross section. Alternatively, the strip may have a tapered cross section. 
     The strip is preferably made of an alloy, such as Titanium alloy. 
     According to the present invention, there is also provided a method for bonding a component to a substrate using a strip comprising a central region having a reduced cross section compared to the extremities of the strip, the method comprising the steps of disposing the substrate proximate the central region of the strip, disposing the component on the substrate, heating the strip to a predetermined temperature sufficient to cause solder disposed between the component and the substrate to reflow, and cooling the strip thereby causing said solder to bond the component to the substrate. 
     According to the present invention, there is also provided a method for removing a component bond to a substrate using a strip comprising a central region having a reduced cross section compared to the extremities of the strip, the method comprising the steps of disposing the substrate having the component bonded thereto proximate the central region of the strip, heating the strip to a predetermined temperature sufficient to cause solder disposed between the component and the substrate to reflow and break the bond, and removing the component from the substrate. 
     While the principle advantages and features of the invention have been described above, a greater understanding and appreciation of the invention may be obtained by referring to the drawings and detailed description of a preferred embodiments, presented by way of example only, in which; 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a top planar view of a diebond strip according to the prior art; 
         FIG. 2  is a cross-sectional side view of the diebond strip according to  FIG. 1 ; 
         FIG. 3  is a side view of a semiconductor laser die according to the prior art; 
         FIG. 4  is a schematic representation of the mounting of the semiconductor laser die according to  FIG. 3  to a substrate via thermocouples; 
         FIG. 5  shows a top view of the diebond strip according to a preferred embodiment of the present invention, 
         FIG. 6  shows a side view of the strip shown in  FIG. 5 , 
         FIG. 7  shows the chip-on-submount and substrate located on the diebond strip shown in  FIGS. 5 and 6 , 
         FIG. 8  shows a more detailed view of  FIG. 7 , 
         FIG. 9  shows an alternative embodiment of the present invention, and 
         FIG. 10  shows yet another alternative embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     In  FIG. 5  the diebond strip  50  is shown comprising a centrally located hole  51  to which a vacuum can be applied in order to temporarily hold a substrate in place. Electrical connection is made to the strip at electrical connection points  52 ,  53 . As in the prior art, these points also function as a means for retaining the strip in place during processing. The strip is preferably made of an alloy and most preferably made of Titanium alloy. The strip is heated by passing electrical current through the strip in much the same manner as with known graphite strips. 
     However, as can be clearly seen in  FIG. 6 , the diebond strip according to the present invention has a reduced cross section at the central region  70  of the strip then at the edge region  56 . This cross sectional reduction leads to an increase in the resistance of the strip, and consequently, and increase in the temperature for a given power supply. 
     Located at the center of the strip is a thermocouple  60 . The thermocouple is preferably welded to the strip. The weld is used, rather then a spring contact, as it allows for more accurate reading of the temperature of the strip due to an improved and more permanent contact being made between the strip and the thermocouple. As a result, better control of the system is achieved. 
     As shown in  FIGS. 7 and 8 , this central region of the strip labeled  70  is where the chip-on-submount and substrate are located. The chip on submount and substrate are essentially the same as those used in the prior art and shown in  FIGS. 3 and 4  above. Thus parts appearing in  FIGS. 7 and 8  which also appear in  FIGS. 3 and 4  bear identical numerical designation. 
     Advantageously, heat applied to the diebond strip according to the present invention is now concentrated in the central area  70  and is not distributed throughout the strip, thus increasing the efficiency of the strip. The diebond strip according to the present invention does not have a complex setup, as was associated with the known graphite strip. Thus better confidence can be placed in the control loop to maintain the required temperatures. 
     Furthermore, Titanium is hard wearing material. Thus, whereas the graphite strip typically lasts a few months, the Titanium diebond strip will last years. 
     The Titanium strip has a faster response than the graphite strip. This allows for a reduced process time, as the strip does not take as long to reach temperature and, as a result, will heat the substrate to soldering temperatures in a fraction of the time than was possible with the previously known strip. Thus, advantageously, less power is required to run the strip at the required temperatures. 
     The stepped design of the present invention allows for a variation in the resistance along the strip. This allows the extremities of the strip to remain comparatively cool, and the central “working” area  70  to be running at the required temperature. 
     The system setup is simplified due to the use of only one thermocouple. This means that greater confidence can be placed on the measured temperature, and consequently, greater control, and stability can be expected from the process. 
     The thermocouple  60  is spot welded to the strip as opposed to being held in place by spring pressure, meaning greater confidence can be placed in the displayed temperature. Also, by welding the thermocouple in place, the thermocouple does not deform the strip. 
     Preferably, the strip is wire EDM (Electro-Discharge Machining) machined, and is produced with no curvature. This results in good contact being made between the substrate  3  and the strip  50  yielding better thermal conduction, and retention by the vacuum. 
       FIG. 9  shows an alternative to the stepped strip design shown in the preferred embodiment. Here the diebond strip  90  only has a single step  91  from the thicker edge region  56  to the thinner central region  70 . 
       FIG. 10  shows yet a further alternative, in which the diebond strip  100  is tapered from the thicker edge region  56  towards the thinner central region  70 . 
     Due to an increase in the control and reliability of the strip, a new process has been developed. The diebond strip according to the above described embodiments allows not only a better method of placing the components onto a substrate, but also for a method of removing a component already bonded to a substrate. 
     Thus, a laser chip can be removed from the substrate using the diebond strip according to the present invention. Currently production yield rate for mounting a chip-on-submount to a substrate is low. This low yield is due to a variety of factors including laser chip failure and problems with other components in the final assembly. It is desirable to be able to reclaim any functioning components from the failed assembly and reuse them in another assembly. Thus, the ability to reclaim laser chips or other components which are still functional from a failed assembly can result in a significant production cost savings. 
     It is not intended that the present invention be limited to the above embodiments and other modifications and variations are envisaged within the scope of the claims.