Abstract:
Fluxless soldering processes use pressure variations and vented cavities within large-area solder joints to reduce void volumes and improve the properties of the large-area solder joints. The vents can be sealed after soldering if closed cavities are desired. A cavity can also improve hermeticity of a solder joint by providing an additional solder fillet around the cavity in addition to the solder fillet around the perimeter of the solder joint.

Description:
BACKGROUND  
         [0001]    Soldering is an effective method for joining metallic components and can even join many non-metallic components provided that the faying surfaces are suitably metallized. Accordingly, many types of solders, soldering processes, and solder joint designs are known. Of the many types of solder joints, one of the most difficult to make is a large-area joint that provides a hermetic seal and is free of internal voids. In this context, a large-area solder joint is a solder joint covering an area having a smallest dimension more than 2 mm long.  
           [0002]    Hermetic seals are difficult to form with large-area joints because the most reliable method of guaranteeing joint hermeticity is to ensure the formation of a continuous edge fillet around the entire perimeter of the joint. Meeting this requirement becomes progressively more difficult for joints having larger areas and perimeters.  
           [0003]    Voids are a problem for a large-area solder joint because at dimensions greater than 2 mm, gas bubbles that are trapped between components or evolve internally on heating to the soldering temperature cannot overcome the hydrostatic pressure of the molten solder and escape via the edges of the joint. The bubbles thus remain trapped in the solder and form voids when the solder solidifies. FIG. 1 illustrates the dependence of the percentage of voids in a solder joint on the minimum joint dimension for some conventional solders. As can be readily seen from FIG. 1, the problem of voids in solder joints increases with the dimensions of the joints. These voids generally impair the electrical, thermal, and mechanical properties of a solder joint.  
           [0004]    Making a large-area joint without flux further increases the difficulty of making void free joints. In general, a flux helps to remove surface oxides and thereby promotes wetting and spreading of molten solder. Without flux, making good quality solder joints is inherently more difficult, but avoiding the need for flux can simplify a soldering process. Accordingly, fluxless processes and technologies have been devised for making solder joints that are thin, large-area, and void-free. Some of these techniques include pre-applying solder, the “pressure variation” process, and applying compressive stress during the thermal cycle of the soldering. For best results, all three methods can be combined.  
           [0005]    Pre-applying solder applies solder to the surface of one or both of the components being soldered, thereby decreasing the number of surfaces in the joint and hence sources of voids.  
           [0006]    The pressure variation process reduces void levels in solder joints by compressing the trapped gas bubbles so that the gas bubbles and resulting voids occupy a much smaller fraction of the joint volume. The pressure variation process generally uses external gas pressure in a way that has many analogies with hot isostatic pressing. A typical pressure variation process involves placing the assembly of components to be soldered in a chamber at reduced pressure (P 1 ) and heating the assembly to the peak process temperature. The pressure in the enclosure is then increased several orders of magnitude to a higher pressure (P 2 ), and the assembly is allowed to cool under the high pressure P 2 . To the extent that the bubbles behave as an ideal gas, an initial volume V 1  of voids at pressure P 1  decreases to a volume V 2  of voids at pressure P 2 , where volumes V 1  and V 2  are related as indicated in Equation 1. 
       Equation                 1        :               V   2     =       V   1     ·     (       P   1       P   2       )                             
 
           [0007]    Equation 1 illustrates that the greater pressure P 2  is in relation to pressure P 1  the more effective the process is at reducing voids. Practical work has shown that a pressure ratio of 10:1 can typically achieve a void level of about 15%, and a ratio of 30:1 can reduce void levels to as low as 5%.  
           [0008]    Difficulties arise with the pressure variation method when the solder joint is required to form a hermetic seal around a closed cavity. If the molten solder seals a closed cavity by wetting all of the joint surfaces, any variation in internal or external gas pressure can blow the solder off the joint line, thereby breaking the seal. Thus, the pressure variation process cannot be used with parts including solder seals around closed cavities.  
           [0009]    In view of the limitations of known soldering techniques and solder joints, soldering processes and joints are sought that are able to provide thin, large area joining that is essentially void free and capable of hermetically sealing a cavity.  
         SUMMARY  
         [0010]    In accordance with an aspect of the invention, a vented cavity is formed between surfaces of components being joined with a large-area solder joint. The cavity reduces the distance that gas bubbles in molten solder must travel to escape during formation of the large area solder joint. Accordingly, fewer gas bubbles are trapped, resulting in fewer voids in the solder joint. Additionally, since the cavity is vented, a pressure variation process can be applied during soldering to improve the fill and hermeticity of the solder joint. The vent can be sealed after forming the solder joint to hermetically seal the cavity, if desired.  
           [0011]    The vented cavity with or without the pressure variation process can be applied not only to solder joints but also to joints formed using a braze or an adhesive.  
           [0012]    One embodiment of the invention is a process for attaching components. The process begins by forming an assembly including a first component and a second component with a joining material such as a solder, a braze, or an adhesive sandwiched between the first and second components. The first and second components form a vented cavity that the joining material surrounds. Heating the assembly activates or melts the joining material and gas bubbles in the joining material during heating can escape from the joining material via the cavity and the vent to the surroundings of the assembly. Sealing the vent after the joining material solidifies can hermetically seal the cavity.  
           [0013]    After heating of the assembly, pressure surrounding the assembly can be increased to compress gas bubbles that may still remain trapped in the joining material. The increased pressure is maintained while cooling the assembly to solidify the joining material, so that any voids corresponding to the gas bubbles are smaller than they would be in a process that did not increase the pressure. Since the cavity is vented, pressure inside the cavity is same as the pressure outside the assembly and the increased pressure does not disturb hermeticity of the seal.  
           [0014]    Another embodiment of the invention is a joined structure including first and second components made of materials such as a metal (e.g., molybdenum), a semiconductor (e.g., silicon), a glass, or a ceramic with a joining material such as a solder, a braze, or an adhesive sandwiched between the first and second components. The first and second components form a cavity that the joining material surrounds, and a vent leads away from the cavity. The vent can be sealed after the first and second components are joined so that the joining material and the vent together hermnetically seal the cavity. The joint structure can further include a series components and solder joints forming a series of vented cavities that share a common vent, and/or a set of components that have individually vented cavities. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0015]    [0015]FIG. 1 is a bar graph illustrating the effect that the size of a conventional solder joint has on the percentage volume of voids in the solder joint.  
         [0016]    [0016]FIG. 2A and 2B are cross-sectional views of solder joints including vented cavities in accordance with embodiments of the invention.  
         [0017]    [0017]FIGS. 3A and 3B are cross-sectional views of assemblies including multiple solder joints in accordance with embodiments of the invention.  
         [0018]    Use of the same reference symbols in different figures indicates similar or identical items. 
     
    
     DETAILED DESCRIPTION  
       [0019]    In accordance with an aspect of the invention, one or more vented cavities can be provided within the area of a large-area solder joint to improve the release of gas bubbles and reduce the volume of voids in the solder joint. The pressure variation process for solder can further reduce the volume of voids in the solder joint without disturbing the seal that the solder forms. Use of the pressure variation process in this manner is counter-intuitive in that the solder joint deliberately includes cavities, the very feature that normally precludes application of the pressure variation process.  
         [0020]    In accordance with another aspect of the invention, a cavity to be hermetically sealed by a solder joint is provided with a vent to permit a pressure variation process that reduces voids in the solder joint. After the solder joint is complete, the vent can be sealed to create a closed cavity.  
         [0021]    In accordance with yet another aspect of the invention, providing a cavity at the location of aligned vent holes in components being soldered substantially improves yield of hermetic joints through modification of the peripheral fillet, which is now also at an interior perimeter of the cavity in addition to the exterior perimeter of the joint.  
         [0022]    [0022]FIG. 2A illustrates a cross-section of an assembly  200  in accordance with an embodiment of the invention. Assembly  200  includes an upper component  210  that is joined to a lower component  220  by a solder joint  230 . Components  210  and  220  can be made of any material suitable for soldering. For example, either component  210  or  220  can be made of a metal, a semiconductor, a glass, or a ceramic having a planar surface to which a solder will adhere. Generally, if component  210  or  230  is a semiconductor, a glass, or a ceramic, the surface of the component must be coated with a metal, which can be accomplished using a conventional technique such as electroplating, vapor deposition, or sputtering.  
         [0023]    In assembly  200 , a cavity  240 , which is between components  210  and  220  and ringed by solder joint  230 , results from one component  220  including a depression in an otherwise planar surface. Cavity  240  has a vent or outlet  250  that provides fluid communications between cavity  240  and the surroundings of assembly  200 . As an example of one vent system, FIG. 2A shows a configuration where vent  250  includes a hole passing from cavity  240 , through lower component  220 , to an opening at the bottom of lower component  220 . FIG. 2B shows an alternative assembly  205  in which a vent  255  passes from cavity  240 , through a lower component  235 , to an opening on a side of lower component  235 . As will be understood, many other alternative vented cavity systems can be fabricated in components  210  and  220  and serve the same functions as vents  250  and  255 .  
         [0024]    A process for fabricating assembly  200  can begin with fabrication of components  210  and  220 . Each component  210  or  220  has a bonding surface (typically a planar surface) that matches a bonding surface of the other component and is metal or metallized. Solder  230  can be pre-applied to the bonding surface of component  210  or  220 , or both. Equally, solder  230  could be a freestanding solder perform that is inserted between components  210  and  220 . One or both of components  210  and  220  are further shaped so that placing the bonding surfaces of components  210  and  220  in contact leaves cavity  240  between the components and provides a vent  250  or  255  from cavity  240  to the surroundings. Components  210  and  220  can be placed together with or without a flux between them.  
         [0025]    The assembly  200  or  205  is then place in a chamber that has facilities to change the internal temperature and pressure in a controlled manner. The chamber then is evacuated to provide a low pressure (e.g., about 10 mPa) surrounding the assembly. While the chamber pressure is low, the assembly is heated to a peak temperature (e.g., the soldering temperature). When the assembly is at the peak temperature, pressure in the chamber is raised to a high pressure (e.g., 200 kPa), while assembly  200  or  205  is allowed to cool and solder  230  solidifies.  
         [0026]    Employing a vented cavity in soldered structures such as illustrated in FIGS. 2A and 2B provides for several benefits: The vented cavity reduces the breadth of solder joint  230 , which minimizes the tendency for voids to form in solder joint  230 . The reduction in joint breadth also means that a pressure variation process is more effective at reducing the volume of voids in solder joint  230  because the effect of the hydrostatic pressure of the solder is decreased. Furthermore, the probability that solder joint  230  provides a hermetic seal is greatly increased because a second, internal fillet forms around the short joint periphery of cavity  240 .  
         [0027]    After joining components  210  and  220  or  210  and  225  to form assembly  200  or  205 , cavity  240  can be filled or evacuated via vent  250  or  255 , and then vent  250  or  255  can be sealed to provide a hermetically sealed structure. A variety of known methods such as welding, crimping and mechanically plugging are known and suitable for sealing a vent.  
         [0028]    [0028]FIG. 3A illustrates another exemplary embodiment of the present invention where a soldered assembly  300  is part of an optical switch. Assembly  300  includes three components  310 ,  320 , and  330  that are joined using two solder joints  315  and  325 .  
         [0029]    In the optical switch, upper component  310  is a quartz waveguide. In an exemplary embodiment of the invention, components  310  and  320  and solder joint  315  form a cavity  340  that measures about 18 mm in diameter by 5 μm deep. Optical switching in this device requires a fluid in cavity  340 , and a pipe  350  to cavity  340  is used to fill cavity  340  with fluid after assembly of optical switch  300 .  
         [0030]    Component  320  is a silicon chip that locally heats the liquid in cavity  340  to create a gas bubble that redirects a selected light beam during optical switching. In an exemplary embodiment, the active area of silicon chip  320  is a roughly square and about 250 mm 2  in area.  
         [0031]    Because silicon is intrinsically brittle and must otherwise have an attached pipe  350  for filling cavity  340 , the third component  330  is a metal backing plate to which silicon chip  320  and pipe  350  are attached. Backing plate  330  supports and dissipates heat from silicon chip  320 . In the exemplary embodiment of the invention, backing plate  330  is made of molybdenum to provide a reasonably good match between the coefficient of thermal expansion (CTE) of silicon chip  320  and the CTE of metal backing plate  330 .  
         [0032]    A variety of considerations dictate that solder joint  325  between silicon chip  320  and metal backing plate  330  must be thin, hermetic, and free of voids. In particular, since solder is generally a relatively poor heat conductor, solder joint  325  needs to be thin, typically thinner than 20 μm to conduct heat away from silicon chip  320 . Additionally, solder joint  325  needs to be relatively free of voids to maximize adhesion between backing plate  330  and silicon chip  320  and to maximize the metal area for heat conduction. Solder joint  325  also needs to be hermetic to prevent the liquid in cavity  240  from leaking out between silicon chip  320  and metal backing plate  330 . These requirements on solder joint  325  indicate the pressure variation process is desirable during joint formation.  
         [0033]    The soldering process can exploit the presence of pipe  350  by using pipe  350  as a vent for cavity  340 . In an exemplary embodiment, a solder such as an indium solder is pre-applied to a top surface of metal backing plate  330 . Quartz waveguide  310 , silicon chip  320 , and metal backing plate  330  are then brought into contact inside a chamber. The chamber is evacuated to a pressure of about 10 mPa while the assembly is heated to a peak temperature of 175° C. At the peak temperature, the pressure in the chamber is raised to about 200 kPa, which is maintained for approximately 1 minute. The chamber and assembly cools while still maintaining the pressure of 200 kPa.  
         [0034]    After completing assembly  300 , cavity  340  is filled with liquid via pipe  350 . Crimping then seals pipe  350 , which is made of thin-wall nickel and pre-attached to metal backing plate  330  by high temperature brazing.  
         [0035]    [0035]FIG. 3B shows an alternative embodiment of an optical switch assembly  305  that differs from optical switch assembly  300  of FIG. 3A primarily in the addition of a cavity  345  within solder joint  325  between silicon chip  320  and metal backing plate  330 . As described above, cavity  345  improves the effectiveness of the pressure variation process on solder joint  325  and also enhances the likelihood of solder joint  325  being hermetic. The improved likelihood of successfully forming a hermetic solder joint  325  is believed to result from a second, interior solder fillet around the perimeter of cavity  345 . In contrast, the exterior solder fillet around silicon chip  320  extends around a much larger perimeter, and the chance of a defect occurring along the larger perimeter is proportionally larger.  
         [0036]    In an exemplary embodiment of invention, cavity  345  measures about 6.5 mm long by 1.25 mm wide by 2.8 mm deep. The openings in silicon chip  320  and metal backing plate  330  for venting and filling cavities  340  and  345  are about 1 mm in diameter.  
         [0037]    Although the invention has been described with reference to particular embodiments, the description is only an example of the invention&#39;s application and should not be taken as a limitation. For example, although the above embodiments are primarily described as employing solder as a joining material between components, other joining materials such as brazes and some adhesives similarly suffer from trapped gas bubbles and would benefit from use of vented cavities as described above. Various other adaptations and combinations of features of the embodiments disclosed are within the scope of the invention as defined by the following claims.