Abstract:
A reflow heating system includes a housing assembly defining an internal thermal processing chamber that encapsulates at least a microelectronic assembly on a substrate. A first heating source is coupled to the housing assembly and within the thermal processing chamber. The first heating source is biased by a force-applying assembly into engagement with the microelectronic assembly. The first heating source comprises one or more heating platens adapted to engage the microelectronic assembly for applying direct heat sufficient to melt solder. A vacuum assembly is incorporated in the heating platen for allowing application of at least a partial vacuum to the microelectronic assembly to permit withdrawal thereof from the substrate. A radiant heating source is applied beneath the substrate and a directional heating source is applied to the microelectronic assembly.

Description:
BACKGROUND  
       [0001]     The present invention relates generally to an improved heating system and method of controlling a solder reflow process.  
         [0002]     Printed circuit boards are commonly fabricated using the reflow solder technique. Many modern semiconductor devices are constructed to be attached to a higher level of assembly by means of solder balls. Solder balls are formed on the surface of the substrate forming the semiconductor device. Typically, those solder balls are formed on contact pads on the surface of a substrate, as those contact pads form the external connection with the internal circuitry of the substrate.  
         [0003]     However, some semiconductors are not satisfactorily bonded in the process and/or might be otherwise be damaged. Therefore, a need exists for a method of replacement or remounting. As a result, they are removed and re-soldered to the board. Because of environmental concerns regarding the use of lead in conventional soldering materials, there is a continuing trend to utilize lead-free materials for joining electronic devices to a printed wiring board and the like. While lead based solder has a melting temperature of about 185° C., the lead-free approaches tend to have significantly higher temperatures, for example in the order to about 220-250° C. It will be appreciated, therefore, that it is extremely important that a high degree of control be maintained in order to affect the successful transfer of solder to the contact pads without the significantly higher temperatures damaging the surrounding components. Accordingly, higher temperatures require different approaches than those used conventionally for effecting reflow.  
         [0004]     Without the ability to effectively heat the lead-free solder-based materials, and at the same time minimize the damage to surrounding board components, the desirability of effective and controlled heating of lead-based materials may not be effectively and efficiently met.  
       SUMMARY OF THE INVENTION  
       [0005]     The present invention is related to a method and system for effecting an improved reflow process without negative effect and that overcome many of the disadvantages of the prior art.  
         [0006]     In one exemplary embodiment there is provided a reflow heating system that includes a heating housing assembly defining an internal thermal processing chamber. The thermal processing chamber is adapted to generally encapsulate at least a microelectronic assembly that is to be solder mounted on a substrate. A first heating source may comprise a heating platen or element adapted to engage a surface of the microelectronic assembly in order to apply direct heat on the microelectronic assembly sufficient to melt solder.  
         [0007]     In another exemplary embodiment, provision is made for a heating system wherein a heating platen is biased into engagement with a surface of the microelectronic assembly, and a source of vacuum allows establishment of suction between the platen, whereby the microelectronic assembly may be removed from the substrate in response to application of the vacuum source.  
         [0008]     In an illustrated embodiment there is further included a radiant or second heat source positionable beneath the microelectronic assembly and the substrate.  
         [0009]     In another illustrated embodiment, provision is made for a directional heating source that is movable to preselected areas of the mounted microelectronic assembly. In addition, a source of vacuum is also movable to preselected areas of the mounted microelectronic assembly to allow selective soldering.  
         [0010]     An aspect of this invention is that it satisfactorily addresses problems of controlling solder reflow of microelectronic assemblies while preventing degradation to components surrounding the microelectronic assemblies on a substrate.  
         [0011]     Another aspect of this invention is that it enhances versatility of controlling solder reflow of microelectronic assemblies in a variety of environments.  
         [0012]     These and other aspects of the present invention will be more fully understood from the following detailed description of the preferred embodiments that should be read in light of the accompanying drawings. It should be understood that both the foregoing description and the following detailed description are exemplary and not restrictive. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0013]      FIG. 1  is a diagrammatic illustration of a cross-sectional view of one preferred embodiment of a heating system according to the present invention  
         [0014]      FIG. 2  illustrates a plan view of certain components of a diagrammatic illustration in  FIG. 1  of the present invention. 
     
    
     DETAILED DESCRIPTION  
       [0015]     Reference is made is made to  FIGS. 1 &amp; 2  for illustrating one preferred embodiment of a solder reflow heating system  100  that is adapted for use in implementing a solder reflow process. The solder reflow heating system  100  operates to efficiently effect solder reflow without the permitting degradation to microelectronic assemblies subject to the reflow process and those objects immediately surrounding the former.  
         [0016]     The solder reflow heating system  100  is versatile in that it may adapted to encapsulate one or more microelectronic assemblies or microelectronic assemblies  102  (only one is shown) that are solder mounted via solder balls  105  on a substrate  104 . In this embodiment, the substrate  104  is a printed circuit board  104  that may be any conventional board on which the heat-producing electronic/electrical microelectronic assemblies  102  are mounted. For instance, the circuit board  104  may be a printed wiring board (“PWB”) of the type commonly used in a PC. In addition, the electronic/electrical microelectronic assemblies  102  may be any type of microelectronic assembly that does not function properly if overheated. The microelectronic assemblies  102  are mechanically coupled to the circuit board  104  by conventional soldering techniques and are electrically coupled to, among other things, a power source (not shown). The microelectronic assemblies may be arranged in any suitable manner.  
         [0017]     While the present embodiment is described in terms of effecting reflow with lead-free solder, it should be understood that the scope and teachings of the present invention are not limited to the context presented in the discussion of the preferred and alternate embodiments. Indeed, the teachings of the present invention may be applied to any sort of object for which selective and controlled heating is desired.  
         [0018]     Included in the solder reflow heating system  100  is a portable modular type of heating housing assembly or furnace  106 . To provide and enhance portability of the system of the present invention, the heating housing assembly  106  is mounted, preferably, releaseably on the circuit board  104  replacement.  
         [0019]     The heating housing assembly  106  is particularly adapted to substantially encapsulate one or more of the microelectronic assemblies  102  within an internal thermal processing chamber  108 . The heating housing assembly  106  is constructed of suitable thermal insulating material and is thereby adapted to thermally insulate surrounding microelectronic assemblies on the circuit board from the adverse affects of heating the microelectronic assembly  102 . In an exemplary embodiment, as will be described, the thermal processing chamber  108  is adapted to have a variable volume. Accordingly, one or more microelectronic assemblies may be encapsulated for enhanced versatility of the present invention. In this regard, the housing assembly  106  is comprised of a pair of mating housing portions  112 ,  114  that are slidably arranged with respect to each other. The mating housing portions  112 ,  114  are, preferably, made of thermal insulation glass to facilitate a user handling and observing the reflow process consistent with the teachings of the present invention. Each of the mating housing portions  112 ,  114  includes an elongated slot  116 . The pair of mutually cooperating and axially aligned slots  116  on the housing portions  112 ,  114  cooperate with each other to expand and contract in length as the housing portions  112  and  114  move away or towards each other. This allows external access to the heating chamber by external heating and vacuuming tools as will be described, whereby the entire length of the of the microelectronic assembly  102  may be treated. While a variable volume furnace is depicted, other embodiments envision fixed volume chambers as well.  
         [0020]     In the illustrated embodiment, a first direct heating source  120  may be comprised of a generally rectangular and flat heating platen or element  122 . The heating platen  122  has a flat surface  124  that is adapted to engage a surface(s) of the microelectronic assemblies  102  for imparting heat. A controlled source of electrical power  126  is coupled to the heating platen  122  by means of appropriate leads  128 . The leads  128  extend from the exterior of the housing assembly through a central supporting assembly  130  for the heating platen  122  as illustrated in  FIG. 1 . Electrical power may be applied to the heating platen  122  to generate the kind of heat to be transferred for reheating the solder. In one embodiment, temperatures may be, preferably, in the range of 225-250° F. for heating lead-free solder. Other temperature ranges are envisioned depending on the materials being treated. In this embodiment, an aluminum-based material is selected for the heating platen  122 . This is in order to provide heat transfer to the microelectronic assemblies at a rate sufficient to effect reflow. A wide variety of other materials may be used including those yet to be developed. In one embodiment, the heating platen  122  may be a single unit, but a plurality of units are contemplated including a variety of heat engaging surfaces that may have a variety of surface configurations. The heating platen  122  within the chamber generates the most heat in this process. In the present embodiment, provision is made for an integral series of suction passages  129  that allow the application of a partial vacuum therein. The partial vacuum facilitates retention of the microelectronic assemblies to the heating platen. In this manner, whenever it is desired to remove the microelectronic assemblies from the printed circuit board the vacuum is applied. Thereafter, the heating platen may be lifted following the solder reflow by the temperature applying mechanisms. The openings are appropriately spaced apart from each other to effect a firm suction gripping action over a significant area.  
         [0021]     Within the central supporting assembly  130  of the heating platen  122  is, preferably spring-biased or loaded by a force-applying assembly  140 . The force-applying assembly  140  is adapted to urge the heating platen  122  into firm engagement with the microelectronic assembly  102 . The force-applying assembly  140  may include a compression spring  142  inserted between a bottom plug  144  and a threaded spring retainer member  146 . The spring retainer member  146  is threadedly mounted within a central support tube  148  of the central supporting assembly  130 . The central support tube  148  is located within and coaxial with an outer support tube  150 . An annular space  152  is located between the support tubes  148 ,  150  and defines a partial vacuum conduit by which a partial vacuum from a partial vacuum source  154  may be applied to the microelectronic assembly through the platen. In this regard, at the proximal end of the support tube  150  provisions is made for a plurality of radially extending openings (not shown) in the support tube  150  that communicate with a plurality of vacuum tubes  158  made of suitable material. The vacuum tubes  158  are inserted and sealed within the upper portions of the suction passages  129 . As noted, the suction passages  129  are effective in distributing a vacuum hold-down force over the microelectronic assembly  102 . As a result, there is provided a vacuum communication between the vacuum source and the microelectronic assembly. As noted, the vacuum is applied when the microelectronic assembly has had the solder completely reflow thereby facilitating removal of the microelectronic assembly. The central supporting assembly  130  is mounted of selective vertical movement relative to the housing assembly. When the vacuum is released, the microelectronic assembly may be released from the heating platen.  
         [0022]     The heating system  100  may include a second heating source  160 , such as a burner  160 , preferably, positioned exterior to and beneath the microelectronic assembly  102  that is being treated by the first heating source. The burner  160  is for applying convective heat to a bottom surface of the substrate that is beneath the mounted microelectronic assembly  102 . The purpose of the second heating source  160  is to further assist in ensuring that the reflow process is completed in a timely manner. The burner  160  provides additional heating and in the process reduces the amount of time that is necessary for effecting the reflow. In the illustrated embodiment, various temperatures may be applied and in the illustrated embodiment the temperature is about 225-250° F. While the second heating source is preferable, it is not mandatory. Other known or yet to be developed equivalent heat sources are contemplated. While the second heating source is a radiant heat source, the present invention is not so limited.  
         [0023]     A third heating source  170  is contemplated by the present invention. The third heating source  170  may be a wand  172  emitting a jet of heated gas at a temperature of about 225-250° F. for reflow. The wand  172  is adapted to pass thru the slots  116  and access the microelectronic assembly  102  for heating solder around the latter during the reflow process. It will be appreciated that a user will hold the wand  172  outside the heating housing assembly and move it relative to the elongated slots  116 . The wand  172  allows selective heating to occur to zones of the microelectronic assembly  102  that are not completely reflowed. This ensures a complete solder reflow. The wand  172  may be a commercial type. Other sources of directional heating may be applied. Other known or yet to be developed equivalent heat sources are contemplated.  
         [0024]     The present illustrated embodiment includes the use of an external vacuum source  180 . A suction hose  182  is connected the vacuum source  180  and is particularly adapted to extend into the housing assembly through the slots  116 , whereby it functions to remove melted solder from the housing assembly. In the present embodiment, the vacuum source  180  and the third heating source  170  are coupled as part of a suitable single commercial unit. Clearly, separate sources of heat and vacuum may be applied. A jet  190  of localized cooling air may be applied to the exterior of the housing assembly.  
         [0025]     After having described one preferred construction of the heating system according to the present invention, its operation is believed to be self-evident. To supplement such a description, however, a brief description of the method as implemented by the foregoing reflow heating system is set forth below.  
         [0026]     Basically, the heating housing assembly is arranged to generally encapsulate one or more microelectronic assemblies within the internal thermal processing chamber that are mounted on the printed circuit board. The volume of the thermal processing chamber may be varied depending on the situation. The first heating source is selectively coupled to the housing assembly and positioned within the thermal processing chamber. Preferably, a force-applying mechanism is for resiliently urging the heating platen into engagement with the microelectronic assembly  102 . A vacuum is applied to the microelectronic assembly thru the heating platen to retain the latter in contact with the former. As a result, the removal of microelectronic assemblies from the circuit board following complete reflow is expeditiously accomplished. The second and/or third heating assemblies may be applied as necessary to complete the reflow process for the microelectronic assembly in a timely and known manner.  
         [0027]     The embodiments and examples set forth herein were presented to explain best the present invention and its practical applications, thereby enabling those skilled in the art to make and use the invention. However, those skilled in the art will recognize that the foregoing description and examples have been presented for the purposes of illustration and example only. The description set forth is not intended to be exhaustive or to limit the invention to the precise forms disclosed. In describing the above-preferred embodiments illustrated in the drawings, specific terminology has been used for the sake of clarity. However, the invention is not intended to be limited to the specific terms selected. It is to be understood that each specific term includes all technical equivalents that operate in a similar manner to accomplish a similar purpose. Many modifications and variations are possible in light of the above teachings without departing from the spirit and scope of the appended claims.