Abstract:
Stenciling machines and methods for forming solder balls on microelectronic workpieces are disclosed herein. In one embodiment, a method for depositing and reflowing solder paste on a microelectronic workpiece having a plurality of dies includes positioning a stencil having a plurality of apertures at least proximate to the workpiece. The method further includes placing discrete masses of solder paste into the apertures and reflowing the discrete masses of solder paste while the stencil is positioned at least proximate to the workpiece and while the discrete masses are in the apertures. In another embodiment of the invention, a stenciling machine for depositing and reflowing solder paste on the microelectronic workpiece includes a heater for reflowing the solder paste, a stencil having a plurality of apertures, and a controller operatively coupled to the heater and the stencil. The controller has a computer-readable medium containing instructions to perform the above-mentioned method.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   This application is a continuation of U.S. application Ser. No. 11/012,584 filed Dec. 14, 2004, now U.S. Pat. No. 7,347,348, which is a divisional of U.S. patent application Ser. No. 10/226,509 filed Aug. 22, 2002, now U.S. Pat. No. 6,845,901, both of which are incorporated herein by reference in their entireties. 

   TECHNICAL FIELD 
   The present invention relates to an apparatus and method for depositing and reflowing solder paste on a microelectronic workpiece. 
   BACKGROUND 
   Microelectronic devices are used in cell phones, pagers, personal digital assistants, computers and many other products. A packaged microelectronic device can include a microelectronic die, an interposer substrate or lead frame attached to the die, and a molded casing around the die. The microelectronic die generally has an integrated circuit and a plurality of bond-pads coupled to the integrated circuit. The bond-pads are coupled to terminals on the interposer substrate or lead frame. The interposer substrate can also include ball-pads coupled to the terminals by traces in a dielectric material. An array of solder balls is configured so that each solder ball contacts a corresponding ball-pad to define a “ball-grid” array. Packaged microelectronic devices with ball-grid arrays generally have lower profiles and higher pin counts than conventional chip packages that use a lead frame. 
   Packaged microelectronic devices are typically made by (a) forming a plurality of dies on a semiconductor wafer, (b) cutting the wafer to singulate the dies, (c) attaching individual dies to an interposer substrate, (d) wire-bonding the bond-pads to the terminals of the interposer substrate, and (e) encapsulating the dies with a molding compound. It is time consuming and expensive to mount individual dies to interposer substrates. Also it is time consuming and expensive to wire-bond the bond-pads to the interposer substrate and then encapsulate the individual dies. Therefore, packaging processes have became a significant factor in producing semiconductor and other microelectronic devices. 
   Another process for packaging devices is wafer-level packaging. In wafer-level packaging, a plurality of dies is formed on a wafer and then a redistribution layer is formed on top of the dies. The redistribution layer has a dielectric layer, a plurality of ball-pad arrays on the dielectric layer, and traces coupled to individual ball-pads of the ball-pad arrays. Each ball-pad array is arranged over a corresponding die, and the ball-pads in each array are coupled to corresponding bond-pads on a die by the traces in the redistribution layer. After forming the redistribution layer on the wafer, a highly accurate stenciling machine deposits discrete blocks of solder paste onto the ball-pads of the redistribution layer to form solder balls. 
   The stenciling machine generally has a stencil and a wiper mechanism. The stencil has a plurality of holes configured in a pattern corresponding to the ball-pads on the redistribution layer. The wiper mechanism has a wiper blade attached to a movable wiper head that moves the wiper blade across the top surface of the stencil. In operation, a volume of solder paste is placed on top of the stencil along one side of the pattern of holes. A first microelectronic workpiece is then pressed against the bottom of the stencil and the wiper blade is moved across the stencil to drive the solder paste through the holes and onto the first microelectronic workpiece. The solder paste deposited on the microelectronic workpiece forms small solder paste bricks on each ball-pad. The first microelectronic workpiece is then removed from the bottom of the stencil, and the process is repeated for other microelectronic workpieces that have the same pattern of ball-pads. 
   After forming the solder paste bricks on the ball-pads, the microelectronic workpiece is transferred to a reflow oven. The entire microelectronic workpiece is heated in the oven to reflow the solder (i.e., to vaporize the flux and form solder balls from the solder paste bricks). The reflow process creates both a mechanical and electrical connection between each solder ball and the corresponding ball-pad after the reflowed solder has cooled and solidified. 
   Conventional solder printing equipment and processes, however, have several drawbacks. For example, after the microelectronic workpiece is removed from the stencil, residual solder paste may remain in the holes of the stencil. The residual solder paste can cause inconsistencies in the size and shape of the deposited solder paste bricks. For example, when the process is repeated with residual solder paste in the holes, an insufficient volume of solder paste may be placed onto the ball-pads of the subsequent microelectronic workpiece. This may create solder balls that are too small for attachment to another device. Additionally, the volume of the residual solder paste may vary across the stencil. This results in different sizes of solder paste bricks across the workpiece, which produces different sizes of solder balls. 
   Another drawback of conventional processes is that solder paste can be smeared while the microelectronic workpiece is moved from the stenciling machine to the reflow oven. Even if the solder paste is not smeared, when the pitch between the solder paste bricks is small, the solder paste on several ball-pads may bridge together after the microelectronic workpiece is removed from the stencil. Accordingly, a new stenciling machine and a new method for applying solder paste to microelectronic workpieces is needed to improve wafer level packaging processes. 
   SUMMARY 
   The present invention is directed to stenciling machines and methods for forming solder balls on microelectronic workpieces. One aspect of the invention is directed to a method for depositing and reflowing solder paste on a microelectronic workpiece having a plurality of microelectronic dies. In one embodiment, the method includes positioning a stencil having a plurality of apertures at least proximate to the workpiece and placing discrete masses of solder paste into the apertures. The method further includes reflowing the discrete masses of solder paste while the stencil is positioned at least proximate to the workpiece and while the discrete masses are in the apertures. In one aspect of this embodiment, the discrete masses of solder paste can be placed into the apertures and proximate to bond-pads of the dies or ball-pads in or on a redistribution layer of the microelectronic workpiece. In a further aspect of this embodiment, reflowing the solder paste can include heating the solder paste with infrared light, a laser, a gas, or another device to reflow the solder paste. The heating device can be movable relative to the stencil or stationary, such as a heating device having heating elements in the stencil or in a microelectronic workpiece holder. 
   In another embodiment of the invention, a method for forming solder balls on the microelectronic workpiece includes placing solder paste into the plurality of apertures in the stencil. The apertures in the stencil are aligned with corresponding ball-pads or bond-pads of the microelectronic workpiece. The method further includes forming solder balls within the apertures and on the ball-pads or bond-pads. In a further aspect of this embodiment, forming solder balls can include heating the solder paste in the apertures through convection. In another aspect of this embodiment, placing solder paste can include wiping solder paste across the stencil in a first direction to press discrete portions of the solder paste into the apertures. In a further aspect of this embodiment, the method can also include separating the microelectronic workpiece from the stencil after forming the solder balls. 
   Another aspect of the invention is directed to a stenciling machine for depositing and reflowing solder paste on the microelectronic workpiece. In one embodiment, the stenciling machine includes a heater for reflowing the solder paste, a stencil having a plurality of apertures, and a controller operatively coupled to the heater and the stencil. The controller has a computer-readable medium containing instructions to perform any one of the above-mentioned methods. In one aspect of this embodiment, the heater can include an infrared light source, a laser source, or a gas source. In another aspect of this embodiment, the heater can be movable relative to the stencil, such as movable laterally over the top surface of the stencil. Moreover, the heater can include elements that are stationary, such as heating elements that are positioned in the workpiece holder or in the stencil. In another aspect of this embodiment, the machine can also include a wiper to force solder paste into the apertures in the stencil. 
   In another embodiment, a stenciling machine includes a stencil having a plurality of holes and a moveable wiper configured to move a mass of solder paste across the stencil. The moveable wiper is also configured to press discrete portions of the mass of solder paste into the holes and onto the microelectronic workpiece. The machine further includes a heating means for reflowing the discrete portions of solder paste in the plurality holes and on the microelectronic workpiece. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1A  is a schematic cross-sectional view of a stenciling machine depositing solder paste onto a microelectronic workpiece in accordance with one embodiment of the invention. 
       FIG. 1B  is a schematic cross-sectional view of the stenciling machine of  FIG. 1A  having a heat source in accordance with one embodiment of the invention. 
       FIG. 1C  is a schematic cross-sectional view of the microelectronic workpiece including the attached solder balls after removing the stencil. 
       FIG. 2  is a schematic cross-sectional view of a stenciling machine having a heat source in accordance with another embodiment of the invention. 
       FIG. 3  is a schematic cross-sectional view of a stenciling machine having a heat source in accordance with yet another embodiment of the invention. 
       FIG. 4  is a schematic cross-sectional view of a stenciling machine depositing solder paste onto a microelectronic workpiece in accordance with another embodiment of the invention. 
       FIG. 5  is a schematic view of a stenciling machine in accordance with another embodiment of the invention. 
   

   DETAILED DESCRIPTION 
   The following description is directed toward microelectronic workpieces and methods for forming solder balls on microelectronic workpieces. The term “microelectronic workpiece” is used throughout to include substrates upon which and/or in which microelectronic devices, micromechanical devices, data storage elements, and other features are fabricated. For example, microelectronic workpieces can be semiconductor wafers, glass substrates, insulative substrates, or many other types of substrates. Many specific details of several embodiments of the invention are described below with reference to microelectronic workpieces having microelectronic dies and in some applications redistribution layers to provide a thorough understanding of such embodiments. Those of ordinary skill in the art will thus understand that the invention may have other embodiments with additional elements or without several of the elements described in this section. 
   A. Environment 
     FIG. 1A  is a schematic cross-sectional view of a stenciling machine  180  for depositing solder paste  140  onto a microelectronic workpiece  100  in accordance with one embodiment of the invention. The microelectronic workpiece  100  can include a substrate  108  having a plurality of microelectronic devices and a redistribution layer  120  formed on the substrate  108 . In the illustrated embodiment, the microelectronic devices are microelectronic dies  110 . Each microelectronic die  110  can have an integrated circuit  111  (shown schematically) and a plurality of bond-pads  112  coupled to the integrated circuit  111 . The redistribution layer  120  provides an array of ball-pads for coupling the bond-pads  112  on the microelectronic die  110  to another type of device such as a printed circuit board. The redistribution layer  120  has a dielectric layer  121  with a first surface  126  facing away from the dies  110  and a second surface  127  adjacent to the dies  110 . The redistribution layer  120  also has a plurality of ball-pads  122  and a plurality of traces  124  in or on the dielectric layer  121 . The ball-pads  122  are arranged in ball-pad arrays relative to the dies  110  such that each die  110  has a corresponding array of ball-pads  122 . The traces  124  couple the bond-pads  112  on the microelectronic dies  110  to corresponding ball-pads  122  in the ball-pad arrays. 
   The stenciling machine  180  in the illustrated embodiment includes a stencil  130 , a wiper assembly  150 , and a controller  102  operatively coupled to the stencil  130  and the wiper assembly  150 . The stencil  130  has a plurality of apertures  132  arranged in a pattern to correspond to the ball-pads  122  on the microelectronic workpiece  100 . More specifically, each aperture  132  in the stencil  130  is arranged so as to align with a particular ball-pad  122  in the redistribution layer  120 . The stencil  130  also includes a first surface  134 , a second surface  136  opposite the first surface  134 , a first end  137 , and a second end  138  opposite the first end  137 . The stencil  130  has a thickness T from the first surface  134  to the second surface  136  that corresponds with a desired thickness of a solder paste brick on each ball-pad. The wiper assembly  150  can include an actuator  152  and a blade  154  coupled to the actuator  152 . In the illustrated embodiment, the actuator  152  moves the blade  154  across the stencil  130  from the first end  137  to the second end  138  to drive a solder paste  140  into the apertures  132 . In other embodiments, other stenciling machines can be used, such as machines that use print heads or pins to deposit the solder paste into apertures in a stencil. 
   B. Depositing Solder Paste 
   In operation, the controller  102  moves the microelectronic workpiece  100  to press the first surface  126  of the redistribution layer  120  against the second surface  136  of the stencil  130 . Each aperture  132  in the stencil  130  is positioned over a corresponding ball-pad  122  on the microelectronic workpiece  100 . A large volume of the solder paste  140  is on the first surface  134  at the first end  137  of the stencil  130 . Next, the wiper assembly  150  moves across; the first surface  134  of the stencil  130  in a direction D 1  from the first end  137  to the second end  138 . The wiper blade  154  presses a portion of solder paste  140  into the apertures  132  to form solder paste bricks  142  on the ball-pads  122 . The wiper  154  sweeps the remaining solder paste  140  to the second end  138  of the stencil  130 . 
   C. Forming Solder Balls 
     FIG. 1B  is a schematic cross-sectional view of the stenciling machine  180  of  FIG. 1A  having a heat source  290  in accordance with one embodiment of the invention. The heat source  290  is operatively coupled to the controller  102  to reflow the solder paste  140  in the apertures  132  of the stencil  130  before separating the stencil  130  from the workpiece  100 . In the illustrated embodiment, the heat source  290  moves laterally in the direction D 1  across the stencil  130  over the first surface  134  from the first end  137  to the second end  138 . As the heat source  290  moves over each aperture  132 , the solder paste  140  is reflowed in the aperture  132 . More specifically, the heat source  290  heats the solder paste  140 , vaporizes the flux, and melts the solder. In one aspect of this embodiment, the heat source  290  heats the solder to at least approximately 200° C. In other embodiments, the heat source  290  heats and melts the solder at a temperature less than 200° C. The molten solder naturally forms into spherically shaped balls on the ball-pads  122  of the microelectronic workpiece  100  because of the surface tension of the molten solder. After the heat source  290  moves past the apertures  132 , the molten solder cools and solidifies into solder balls  240 . The wetting characteristics between the molten solder and the ball-pads  122  causes the solder balls  240  to form on top of the ball-pads  122  creating a mechanical and electrical connection between the solder balls  240  and the ball-pads  122 . 
   In one embodiment, the stencil  130  can be made of a nonwettable material, such as Kapton® manufactured by DuPont, so that the molten solder does not stick to the sidewalls  233  of the apertures  132 . The non-vetting aspect of the stencil  130  further forces the molten solder into sphere-like balls or other solder elements on top of the ball-pads  122 . The particular material for the stencil, therefore, should be selected so that the stencil resists wetting by a liquid state of the solder material. As such, materials other than Kapton® can be used for the stencil, such as any material that repels the liquid state of the solder material. 
   In other embodiments, the heat source  290  can follow the wiper assembly  150  ( FIG. 1A ) as it moves from the first end  137  of the stencil  130  to the second end  138 , or the heat source  290  can be stationary relative to the stencil  130 . In any of the foregoing embodiments, the heat source  290  can be a laser, an infrared light, a radiating element or other suitable heat sources. In other embodiments, the heat source  290  can heat the solder paste  140  by convection, such as by blowing a hot gas onto the solder paste  140 . 
     FIG. 1C  is a schematic cross-sectional view of the microelectronic workpiece  100  including the attached solder balls  240  after separating the workpiece  100  from the stencil  130 . After the solder balls  240  are formed on the ball-pads  122  in the reflow process, the microelectronic workpiece  100  is moved in a direction D 2  and released by the stencil  130 . Alternatively, the stencil  130  can be raised relative to the workpiece  100 . In either circumstance, the solder-balls  240  remain on the ball-pads  122  because the cross-sectional dimension of the solder-balls  240  is less than that of the apertures  132  in the stencil  130 . The solder-balls  240  are smaller than the apertures  132  because the flux in the solder paste bricks  142  ( FIG. 1A ) vaporizes during the reflow stage. 
   One advantage of the illustrated embodiments is that reflowing the solder paste  140  before disengaging the microelectronic workpiece  100  from the stencil  130  eliminates the problems that occur when residual solder paste remains in the apertures  132  of the stencil  130 . In the illustrated embodiments, no residual solder paste remains in the stencil  130  after reflow because the stencil  130  repels the molten solder, the reflow process reduces the volume of the solder by vaporizing the flux, and the molten solder naturally forms into the solder elements. Moreover, the solder-balls  240  are typically allowed to harden and adhere to the ball-pads  122  before the microelectronic workpiece  100  is separated from the stencil  130 . As such, neither the solder paste bricks  142  nor the solder-balls  240  remain attached to the stencil  130  after separating the stencil  130  from the workpiece  100 . 
   Another advantage of the illustrated embodiments; is that solder paste bricks  142  will not be smeared or bridged on the workpiece  100 . In the illustrated embodiment, the solder paste  140  is formed into hardened solder balls  240  before the microelectronic workpiece  100  is removed from the stencil  130 . As such, no smearing or bridging occurs on the workpiece  100 . A further advantage of the illustrated embodiments is that stencil machines and reflow equipment are combined in a single machine to reduce the floor space for forming solder balls. 
   D. Alternate Embodiments 
     FIG. 2  is a schematic cross-sectional view of a stenciling machine  380  having a heat source in accordance with another embodiment of the invention. The stenciling machine  380  can include the controller  102 , the stencil  130 , and the wiper assembly  150  described above with reference to  FIG. 1A . The stenciling machine  380  of the illustrated embodiment also includes a workpiece holder  382  having a plurality of heating elements  390 . The workpiece holder  382  is operatively coupled to the controller  102  and configured to secure the microelectronic workpiece  100  during the deposition and reflow of the solder paste. The heating elements  390  are positioned in the workpiece holder  382  proximate to the microelectronic workpiece  100  to heat and reflow the solder paste in the apertures  132  of the stencil  130 . The heating elements  390  heat the microelectronic dies  110 , which in turn heat the ball-pads  122  of the redistribution layer  120 . The heat is transferred from the ball-pads  122  to the solder paste to reflow the solder paste and form the solder balls  240 . The heating elements  390  can be resistance heaters, heat exchangers, or other devices to heat the workpiece holder  382 . 
     FIG. 3  is a schematic cross-sectional view of a stenciling machine  480  having a heat source in accordance with another embodiment of the invention. The stenciling machine  480  can include the controller  102  and the wiper assembly  150  described above with reference to  FIG. 1A  The stenciling machine  480  of the illustrated embodiment also includes a stencil  430  having a plurality of apertures  132  and a plurality of heating elements  490  positioned proximate to the apertures  132  to reflow the solder paste  140 . Heat is transferred from the heating elements  490  to the solder paste through the sidewalls  233  of the apertures  132  by conduction and convection to reflow the solder paste and form solder balls  240  on the microelectronic workpiece  100 . 
     FIG. 4  is a schematic cross-sectional view of a stenciling machine  580  for depositing solder paste  140  onto a microelectronic workpiece  500  in accordance with another embodiment of the invention. The microelectronic workpiece  500  can include a substrate  508  having a plurality of microelectronic dies  510  which can be similar to the microelectronic dies  110  described above with reference to  FIGS. 1A-3 . For example, each microelectronic die  510  can have an integrated circuit  511  (shown schematically) and a plurality of bond-pads  512  electrically coupled to the integrated circuit  511 . 
   The stenciling machine  580  of the illustrated embodiment can include the controller  102 , the wiper assembly  150 , and the heat source  290  described above with reference to  FIGS. 1A-1C . In other embodiments, other heat sources can be used, such as those described in  FIGS. 2-3 . The stenciling machine  580  also includes a stencil  530  having a plurality of apertures  532  arranged in a pattern to correspond to the bond-pads  512  of the microelectronic workpiece  500 . In operation, the wiper assembly  150  of the stenciling machine  580  presses a portion of the solder paste  140  into the apertures  532  of the stencil  530  to form solder paste bricks  542  on the bond-pads  512 . Next, the heat source  290  can move over each aperture  532  to reflow the solder paste bricks  542  and form solder balls on the bond-pads  512 . 
   One advantage of the illustrated embodiments is that forming solder balls within the apertures of the stencil allows the microelectronic workpiece to have a fine pitch between the bond-pads or ball-pads. A fine pitch is permitted because the stencil separates the solder paste bricks on adjacent bond-pads or ball-pads and thus prevents smearing and bridging between the adjacent bricks before and during reflow. Accordingly, the fine pitch between the bond-pads or ball-pads of the microelectronic workpiece reduces the size of the microelectronic devices formed from the workpiece. 
     FIG. 5  is a schematic view of a stenciling machine  680  in accordance with another embodiment of the invention. The stenciling machine  680  includes a housing  682 , a stencil  630  in the housing  680 , and a heat source  690  in the housing  680 . The stencil  630  and the heat source  690  can be similar or identical to any one the stencils  130  and  430  and the heat sources  290 ,  390  and  490  described above with reference to  FIGS. 1A-4 . For example, the stencil  630  can have a plurality of apertures arranged to align over the ball-pads of the microelectronic workpiece  100 , and the heat source  690  can heat and melt the solder paste bricks within the apertures of the stencil  630  to produce spherically shaped balls on the ball-pads of the microelectronic workpiece  100 . In other embodiments, the solder balls can be formed on the bond-pads of the microelectronic workpiece  500  described above with reference to  FIG. 4 . In the illustrated embodiment, the stencil machine  680  also includes a conveyor  650  having a first end  651  and a second end  652  opposite the first end  651  to move the microelectronic workpiece  100  within the housing  682  to and from the stencil  630  and the heat source  690 . In other embodiments of the invention, the housing  682  may not include the conveyor  650 . 
   From the foregoing, it will be appreciated that specific embodiments of the invention have been described herein for purposes of illustration, but that various modifications may be made without deviating from the spirit and scope of the invention. Accordingly, the invention is not limited except as by the appended claims.