Abstract:
A method and apparatus for fabricating a three dimensional array of semiconductor chips is disclosed. The method uses a multiple step fabrication process that automates the surface mounting of semiconductor chips with unique chip carriers to achieve the three dimensional array of chips. The method includes a step of depositing solder on a multitude of chip carriers at one time, placing the chip carriers with chips on a printed circuit board and then running the board with chips and carriers arranged in a three dimensional array through a single reflow oven to complete a single reflow process to permanently connect all of the components. The apparatus includes a unique chip carrier pallet and print fixture pedestal that work in combination to position the chip carriers for the automatic deposition of solder on a multitude of carriers at once and then position them for addition to the circuit board.

Description:
RELATED APPLICATIONS 
     The present application claims priority under 35 USC §119(e) from U.S. provisional application Ser. No. 60/275,843, filed Mar. 14, 2001 and entitled “A Method and Apparatus for Fabricating a Circuit Board with a Three Dimensional Surface Mounted Array of Semiconductor Chips”. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to a semiconductor manufacturing process and more particularly to a method and system for fabricating semiconductor chips in a three-dimensional array on a printed circuit board. 
     BACKGROUND OF THE INVENTION 
     Semiconductor chips are typically connected to a printed circuit board that in turn interconnects the chip into the rest of circuitry with which the chip will operate including other chips on the printed circuit board. In the past the chips were spread out across the printed circuit board on their large flat sides in a simple two-dimensional array. Over the years the trend in the computer industry has been towards more densely packed printed circuit boards. Among the causes for this are the increasing demand for larger random access computer memories, demand for faster computers, demand for more compact computers and a push to decrease costs of printed circuit boards by increasing the circuit density on the printed circuit board. In the mid to late 1980&#39;s the industry switched over from a technology that attached computer chips to a printed circuit board through holes in the printed circuit board to one that used a surface mounting technology. With the advent of surface mount technology, conventional through-holes on printed circuit boards have been replaced with conductive mounting pads on the surface of the printed circuit board. This allows for multiple layered circuit boards with a complex network of interconnect lines running between the layers of the board. In turn this has allowed for the increase in the density of chips on a printed circuit board which not only decreases the size of the board but increases the operating speed of the computer by reducing the distance signals have to travel between chips on the board. 
     The move to surface mount technology has consequently resulted in the practice of positioning the chips on the printed circuit board in a variety of configurations to increase chip density on the circuit board and thereby decrease the distance between the chips to speed up operation of the to overall system. Generally, conventional configurations stack the chips on one another to increase density. The practice of stacking the chips on one another is particularly adaptable to memory chips given the redundancies in their circuits. Up until the present, in order for the chips to be stacked on one another to increase chip density and achieve a three-dimensional array on the circuit board, computer makers had to send the chips to a third party manufacturer that specialized in the technique of permanently bonding chips in a stacked fashion. Stacking the chips generally consisted of soldering them together. This in turn created a variety of problems including time delays inherent in having to rely on an outside manufacturing facility and potential damage to the chip as a result of directly soldering the chips together. 
     Recent developments, in particular those of the applicant of the present invention have resulted in new and much more efficient means for stacking chips on a printed circuit board in a three dimensional array. These developments are described in detail in copending patent applications owned by the applicant herein, they being: United States patent application for a “Circuit Board Assembly Having A Three Dimensional Array of Integrated Circuit Packages” Ser. No. 09/285,354 filed Apr. 4, 1999, and United States patent application for a “Electronic Module Having a Three Dimensional Array of Carrier-Mounted Integrated Circuit Packages” Ser. No. 09/524,324 filed Mar. 3, 2000. Both of these applications are incorporated herein by reference and made part hereof as if set forth herein at length. The two referenced applications describe a unique electronic module that in effect provides a platform that is placed over a chip on a circuit board and connects to contact pads on the circuit board that the platform shares with the chip underneath it. The second chip is then connected to the top of the platform to achieve a stacked three-dimensional array as more fully described in the above referenced patent applications.  21 A and  21 B of  FIG. 1  depict two different variations of the chip carriers described and claimed in the two above referenced applications.  21 A depicts a chip carrier made in the form of a printed circuit board and  21 B depicts a chip carrier made in a molded packaging. 
     However, in order to maximize the advantages of the electronic chip carrying modules described in the two above mentioned copending applications what is needed is a manufacturing process and apparatus which will automate and optimize their installation. Additionally, it should be a manufacturing process and apparatus that can be used in house by a computer or circuit board manufacturer without the need to use the services of a third party manufacturer. 
     SUMMARY 
     It is an object of the present invention to provide an efficient and cost effective manufacturing process and apparatus that utilize new developments that allow the placing chips in a three dimensional array on a printed circuit board. It is a further object of the present invention to provide an apparatus and method that can be utilized with current methods and semiconductor manufacturing machines used in the assembly of printed circuit boards. 
     These and other objects are achieved by providing a method for populating a circuit board with a three dimensional array of semiconductor chips with the following steps: a) verifying electrical contacts on a plurality of chip carriers are properly aligned for an assembly process; b) preparing a chip receiving side of said chip carriers to receive a chip and passive components; c) preparing a circuit board for a chip assembly process; d) populating said circuit board with a first layer of chips and passive components said chips and passive components being positioned to make contact with preselected predetermined electrical contact points; e) positioning said chip carriers over said first layer of chips so that said chip carriers make contact with pre-selected electrical contact points on said circuit board; f) placing on each of said chip carriers a semiconductor chip with passive components; and g) interconnecting in a permanent fashion said chips, passive components and chip carriers to said circuit board. 
     In another aspect of the invention it provides a system for populating a circuit board with a three dimensional array of semiconductor chips comprising: a) a plurality of chip carriers attachable to a circuit board with space for a chip to be positioned directly on the circuit board beneath of each chip carrier as well as for positioning a chip on top of the chip carrier to thereby create a three dimensional array of chips on the circuit board; b) a pallet for holding and moving a plurality of chip carriers during a circuit board assembly process, the pallet having a matrix of chambers in a frame like form with the chambers being open at least at a top side of the pallet each chamber being formed to hold a chip carrier during the circuit board assembly process, the chip carriers being positioned in each of the chambers of the pallet with a top, chip receiving side, of the chip carrier facing out from the top of the pallet to thereby make the top side of the chip carrier accessible during the circuit board assembly process; and c) a mechanism to move and position the pallet during the assembly process so that the plurality of chip carriers held by the pallet can be prepared to receive a chip during the assembly process and easily accessed, removed from the pallet and positioned on the circuit board over chips positioned directly on the circuit board with chips positioned on each chip carrier to thereby create a three dimensional array of chips on the circuit board. 
     In still another aspect of the invention it provides an apparatus for positioning and securely but detachably holding a chip module during a semiconductor fabrication process comprising: a) a pallet for holding chip modules the pallet having a two dimensional matrix of chambers, the chambers being open at a first and second opposing parallel sides of the pallet, the chambers having at a base adjacent to the opening on the second side of the pallet a flange around the inside of the chamber to allow the chamber to retain a chip module of approximately the same dimensions as the chamber when the first side of the pallet faces up; b) a print fixture pedestal with a two dimensional matrix of raised portions that match the matrix of the chambers of the pallet such that the raised portions are sized such that the raised portions fit on a one for one basis into the chambers of the pallet from the second side of the pallet; and c) wherein when the chambers are filled with chip modules and a the print fixture pedestal is joined with the pallet at the pallets second side the raised portions elevate chip modules located in the chambers to a work position from which they can be worked on from the first side of the pallet. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention will be better understood by an examination of the following description, together with the accompanying drawings, in which: 
         FIG. 1  depicts two different versions of the electronic chip carrying modules; 
         FIG. 2  is a raised perspective view of the pallet that holds the electronic chip carrying modules during the manufacturing process of the present invention; 
         FIG. 2A  is a cross sectional view of the pallet depicted in  FIG. 2  along line I—I; 
         FIG. 3  is a raised perspective view of a print fixture pedestal of the present invention used in the stenciling step of the chip carrying modules; 
         FIG. 3A  is a cross sectional view of the print fixture pedestal depicted in  FIG. 3  along line II—II; 
         FIG. 4  is a view of the bottom the print fixture pedestal of the present invention; 
         FIG. 5  is top view of a stencil used in the manufacturing process of the present invention; 
         FIG. 6  is a side view of the pallet and print fixture pedestal joined together for one of the stages of the manufacturing process of the present invention; 
         FIG. 6A  is a cross sectional view from  FIG. 6  at III of one chamber of the pallet with a raised portion inserted therein supporting a chip carrier; 
         FIG. 7  is a view of the stenciling step of the present invention; 
         FIG. 8  is a view of the circuit board assembly step of the present invention; and 
         FIG. 9  is a side view of an oven used in the single reflow process of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The method of the preferred embodiment of the present invention uses a three step fabrication process that automates the surface mounting on a printed circuit board of the chip carriers similar to those depicted as  21 A and  21 B in FIG.  1  and as described in the above two copending applications referenced above that have already been incorporated herein by reference. The first step involves the stenciling (the depositing of solder paste) on a large number of chip carriers  21  at one time. The second step involves moving the chip carriers to an assembly stage where they are placed on the circuit board over the chips and passive devices that have been placed directly onto the circuit board. Chips together with appropriate passive devices are then placed on the chip carriers. In the third and final stage the circuit board with components attached is passed through a single reflow process to complete permanent interconnection with solder of the various components on the board and chip carriers. 
     The preferred embodiment of the present invention uses two new devices to aid in the movement and stenciling of a large number of chip carriers at one time. During the fabrication process the chip carriers  21  are held by a chip carrier pallet  23  as depicted in FIG.  2 . The preferred embodiment of pallet  23  as depicted has a total of fifty-four chambers  25  each one of which holds a chip carrier  21 . In  FIG. 2  only chambers  25 A,  25 B and  25 C have chip carriers  21 . Each of the chambers  25  in pallet  23  are open at the top  23 A and bottom  23 B of the pallet. The size of each of the chip carriers are approximately the same size as each chamber  25  in pallet  23  with the exception of four corner projections or flanges  22  on chip carriers  21 . Each chip carrier is thus sized such that when a chip carrier is positioned in a chamber  25  the flanges  22  project beyond the chamber and rest on the top surface  23 A of the pallet as demonstrated by  25 A,  25 B and  25 C in FIG.  2 . Thus, each chip is prevented from falling through chamber  25  when placed in the chamber. 
     Each chamber  25  in the preferred embodiment has four abutments  27  around the top outside edge as depicted in FIG.  2 . The abutments  27  are designed to hold the chip carriers  21  and protect the chip carriers when positioned in a chamber  27 . The abutments  27  provide protection because a chip carrier  21  when positioned in a chamber  25  is located in a recessed position below the top of the surrounding abutments  27 . The pallet  23  can be made of a variety of materials including durable plastic, aluminum or any other suitable material.  FIG. 2A  is a cross sectional view of the pallet along line I—I that clearly shows the open tops  23 A and bottoms  23 B of chambers  25 . The abutments  27  protect the top of each chip carrier  21  during the fabrication process while the chip carriers are in the chambers  25  of pallet  23 . The tops of the chip carriers  21  are protected so that after solder paste is deposited on top of the carrier, as will be described below the pallets can be stacked and moved about without disturbing the solder paste on top of the chip carriers. 
     The second new device is the print fixture pedestal  31  shown in a raised perspective view in FIG.  3 . The print fixture pedestal  31  has series of raised block like areas  33 . The block like areas  33  are in a nine by six matrix that matches the nine by six matrix of chambers  25  of pallet  23 . The matrix of raised areas  33  are designed and sized to fit into the bottom of the corresponding chamber  25  in the pallet  23 . Print fixture pedestal  31  is hollow inside and each raised block area  33  has a top opening  35  that opens into the hollow interior of print fixture pedestal  31 .  FIG. 4  provides a bottom view of the print fixture pedestal  31 . Some of the top openings  35  can be seen through circular opening  39  of the bottom plate  41  of the print fixture pedestal  31 .  FIG. 3A  is a cross sectional view of print fixture pedestal  31  along line II—II of FIG.  3 . The hollow interior  36  can be seen in FIG.  3 A. The bottom plate  41  secures to a movable hollow shaft, not shown. The hollow portion of the shaft opens into the hollow interior  36  of print fixture pedestal  31 . Print fixture pedestal  31  can be made of cast aluminum or any other of a number of suitable materials. 
     Print fixture pedestal  31  is designed to fit like a glove into the bottom of pallet  23  and raise and secure the chip carriers  21  in the chambers  25  of pallet  23 . When print fixture pedestal  31  is joined with pallet  23  (FIG.  6 ), top openings  35  are flush against the bottoms of the chip carriers located in chambers  25  and a sealed space  36  is created in print fixture pedestal  31  to create a slight vacuum to hold chip carriers  21  during the stenciling process as will be explained in more detail below.  FIG. 6A  is a cross section along line III of  FIG. 6  of one chamber  25  with a raised portion  33  inserted therein supporting a chip carrier  21 . As can be seen raised area  33  positioned in a chamber  25  has its top opening  35  flush against the bottom  29  of a chip carrier  21 . 
       FIG. 5  provides a top view of a stencil  47  used in the stenciling process. Stencil  47  is a template used to deposit solder on the connector pads  46  ( FIGS. 1 and 2 ) of the chip carriers  21  when they are positioned in the chambers  25  of pallet  23  and chip carriers  21  are securely held by the print fixture pedestal  31 . As will be shown below, stencil  47  is placed over the pallet  23  when it is filled with chip carriers  21  and the matrix of rows of holes  49  on stencil  47  match up with the contact pads  46  on top of the chip carriers in each of the chambers  25  of the pallet  23 . Stencil  47  is usually a made of a sheet of stainless steel or some other similarly suitable material. 
     The preferred embodiment of the present invention uses an automated stencil printer for the stenciling process. As depicted in  FIG. 6  pallet  23  filed with chip carriers  21  with print fixture pedestal  31  positioned underneath it is positioned on the work nest  51 . Also, as noted above when print fixture pedestal is joined to pallet  23  this raises chip carriers  21  partially out of the top of chambers  25  of pallet  23 . Referring next to  FIG. 7  work nest  51  with the joined print fixture pedestal  31  and pallet  23  are positioned below the stencil  47 . Stencil  47  is positioned by an appropriate positioning apparatus with the aid of a special bi-directional camera not shown that is inserted between the stencil  47  and pallet  23  in a process well known in the art. The camera lines up the stencil and pallet by keying off of fiducials located on the stencil  47  and pallet  23 . Once the machine assures correct alignment, print fixture pedestal  31  and pallet  23  are raised by work nest  51  towards stencil  47  until the tops of the chip carriers  21 , positioned in pallet  23 , abut up against stencil  27 . Work nest  51  has pneumatic means to raise and lower the enter structure. At this point solder deposition mechanism  59  is lowered onto the top of stencil  47  and the solder is deposited through the array of matrix holes  49  of the stencil. Matrix holes  49  expose the contact pads  46  of chip carriers  21  so that mechanism  59  can precisely deposit the solder on the contact pads. Once completed, mechanism  59  is raised from stencil  47  and stencil  47  is retracted upward. Since the chip carriers are being securely held by the vacuum created between raised area  33  of print fixture pedestal  31  and the bottom of each of chip carriers  21  (see  FIG. 6A ) none of the chip carriers  21  will inadvertently stick to the stencil  47  pallet  23  is retracted or lowered. 
     Once the stenciling process has been completed pallet  23  is disengaged from print fixture pedestal  31  and the pallet  23  with stenciled chip carriers  21  is moved onto the next stage, the circuit board assembly process as depicted in FIG.  8 . In the preferred embodiment of the circuit board assembly process a standard Pick and Place machine  60  is used. During the assembly process a gantry  61  with various pick and place nozzles  63  first places semiconductor computer chips  73  onto circuit board  65  together with various passive devices. The chips in  FIG. 8  are being taken from tray  67  or alternatively a tape in a manner standard to the industry. Also, passive devices, i.e. resistors, capacitors, etc. are coming from rolls  69  in a manner standard to the industry. Once circuit board  65  is populated with the first layer of chips and passive devices gantry  61  then begins placement of the chip carriers  21  that it takes from pallet  23 . Upon completion of placement of all of the chip carriers  21  on circuit board  65  it begins placing the chips on the chip carriers  21  together with appropriate passive devices.  FIG. 9  depicts a portion of a completed circuit board  65  with first layer of chips  73 , passive devices  75 , chip carrier  21  and second layer of chips  77  and passive devices  79  on the chip carriers  21 . During the placement process each chip, chip carrier and passive device placed on circuit board  65  is momentarily present to camera  81  ( FIG. 8 ) for inspection. Any of these devices that appear on their surface to be defective are then discarded. 
     The third and final step is a single reflow process during which the circuit board  65  with components attached is passed through an oven  87  to permanently attach the components to the board by melting the solder previously placed on the board and chip carriers.  FIG. 9  depicts board  65  entering oven  87 . Use of a single reflow process avoids unduly stressing the circuit board and components attached to it since circuit board  65  has all of the components placed on it before going through oven  87 . In the preferred embodiment a standard reflow oven is used. As is well known in the art the single reflow oven causes the solder paste to melt and thus fuse the various components placed on the board to the board including the chip carriers. Naturally, the components placed on each chip carrier to fuse to that chip carrier at the same time. One of the advantages of the present invention is that it allows all of the components, chips, chip carriers and passive components to be positioned in one step on the printed circuit board. Once in place the board is sent through an oven to melt the solder paste holding the components on the board and permanently to the board. This avoids the need to put the board through an oven a multiple number of times. However, those skilled in the art once they have reviewed this specification and understand the concepts of this invention will be able to adapt any number of commercially available fabrication machines for the stenciling, assembly or single reflow process. 
     While the invention has been particularly shown and described with reference to a preferred embodiment thereof, it will be understood by those skilled in the art that various changes in form and detail may be made to it without departing from the spirit and scope of the invention.