Patent Publication Number: US-6661104-B2

Title: Microelectronic assembly with pre-disposed fill material and associated method of manufacture

Description:
This application is a divisional of pending U.S. patent application Ser. No. 09/651,448, filed on Aug. 30, 2000 now U.S. Pat. No. 6,576,495. 
    
    
     TECHNICAL FIELD 
     The present invention relates to microelectronic substrate packages having a pre-disposed fill material for mounting the package to a supporting member. 
     BACKGROUND 
     Packaged microelectronic assemblies, such as memory chips and microprocessor chips, typically include a microelectronic substrate die encased in a plastic, ceramic or metal protective covering. The die includes functional devices or features, such as memory cells, processor circuits and interconnecting wiring. The die also typically includes bond pads electrically coupled to the functional devices. The bond pads can be coupled to pins or other types of terminals that extend outside the protective covering for connecting to buses, circuits, and/or other microelectronic assemblies. 
     One conventional “flip chip” package  10  shown in plan view in FIG. 1 includes a microelectronic die  20  having a downwardly facing surface  24  with solder ball pads  22 , and an upwardly facing surface  23  opposite the downwardly facing surface  24 . Solder balls  21  are attached to the solder ball pads  22  and dipped in flux. The die  20  is then positioned with the downwardly facing surface  24  facing toward a printed circuit board (PCB)  30  to engage the solder balls  21  with corresponding bond pads  31  on the PCB  30 . The solder balls  21  are partially melted or “reflowed” and solidified to form structural and electrical bonds with the bond pads  31  on the PCB  30 . 
     In one aspect of the arrangement shown in FIG. 1, a gap corresponding roughly to the diameter of the solder balls  21  remains between the upper surface of the PCB  30  and the downwardly facing surface  24  of the die  20  after the die  20  has been attached. The gap can be detrimental to the integrity and performance of the die  20  because it can allow oxidizing agents and other contaminants to attack the solder ball bond between the die  20  and the PCB  30 . Furthermore, the gap can reduce the rate at which heat is transferred away from the die  20 , reducing the life expectancy and/or the performance level of the die  20 . 
     To alleviate the foregoing drawbacks, an underfill material  40  is typically introduced into the gap between the die  20  and the PCB  30 . For example, in one conventional approach, a bead of flowable epoxy underfill material  40  is positioned on the PCB  30  along two edges of the die  20 . The underfill material  40  is heated until it flows and fills the gap by capillary action, as indicated by arrows “A.” The underfill material  40  can accordingly protect the solder ball connections from oxides and other contaminants, and can increase the rate at which heat is transferred away from the die  20 . The underfill material  40  can also increase the rigidity of the connection between the die  20  and the PCB  30  to keep the package  10  intact during environmental temperature changes, despite the fact that the die  20 , the solder balls  21  and the PCB  30  generally have different coefficients of thermal expansion. 
     One drawback with the capillary action approach described above for applying the underfill material  40  is that the underfill material  40  can take up to 20 minutes or longer to wick its way to into the gap between the die  20  and the PCB  30 . Accordingly, the capillary underfill process can significantly increase the length of time required to produce the packages  10 . One approach to addressing this drawback (typically referred to as a “no-flow” process) is to first place the underfill material directly on the PCB  30  and then place the die  20  on the underfill material. For example, as shown in FIG. 2A, a quantity of underfill material  40   a  having an integrated quantity of flux can be disposed on the PCB  30  adjacent to the bond pads  31 . As shown in FIG. 2B, the die  20  can be lowered onto the PCB  30  until the solder balls  21  contact the bond pads  31  of the PCB  30 . As the solder balls  21  approach the bond pads  31 , the die  20  contacts the underfill material  40   a  and squeezes the underfill material  40   a  outwardly around the solder balls  21  and between the downwardly facing surface  24  of the die  20  and the upper surface of the PCB  30 , as indicated by arrows “B”. An encapsulating material  70  is then disposed on the die  20  and the PCB  30 . 
     One problem with the no-flow process described above with reference to FIGS. 2A-2B is that air bubbles can become trapped between the die  20  and the PCB  30 . The air bubbles can reduce the effective bond area between the die  20  and the PCB  30  and can make the die  20  more likely to separate from the PCB  30 . Furthermore, oxygen in the air bubbles can oxidize the connection between the solder balls  21  and the solder ball pads  22  and/or the bond pads  31  to reduce the integrity of the structural and/or electrical connections between the die  20  and the PCB  30 . 
     Another problem with the process described above with reference to FIGS. 2A-2B is that it can be difficult to accurately meter the amount of underfill material  40   a  applied to the PCB  30 . For example, if too little underfill material  40   a  is provided on the PCB  30 , the solder balls  21  may not be adequately covered. Even if the underfill material  40   a  extends beyond the solder balls  21  to the edge of the die  20  (as indicated in dashed lines in FIG. 2B by position P 1 ), it can exert a tensile force on the die  20  that tends to separate the die  20  from the PCB  30 . Conversely, if too much underfill material  40   a  is provided on the PCB  30 , the underfill material can extend over the upperwardly facing surface  23  of the die  20  (as indicated in dashed lines in FIG. 2B by position P 2 ), and can form protrusions  49 . The protrusions  49  can be subjected to high stress levels when the die  20  is encapsulated with the encapsulating material  70 , and can cause the underfill material  40   a  to separate from the die  20 . Still further, the underfill material  40   a  can become trapped between the solder balls  21  and the bond pads  31  and can interfere with the electrical connections between the die  20  and the PCB  30 . 
     SUMMARY 
     The present invention is directed toward microelectronic device packages and methods for forming such packages by bonding microelectronic substrates to support members, such as PCBs. A method in accordance with one aspect of the invention includes disposing a fill material in a fill region defined by a surface of the microelectronic substrate before engaging the fill material with the support member. The fill region can also be defined in part by a bond member (such as a solder ball) or other protrusion projecting away from the surface of the microelectronic substrate. The method can further include engaging the fill material with the support member after disposing the fill material in the fill region, and connecting the bond member and the fill material to the support member. The microelectronic substrate and the fill material can then be at least partially enclosed with an encapsulating material. 
     In one aspect of the invention, the microelectronic substrate is dipped into a vessel of fill material and is then removed from the vessel with a portion of the fill material attached to the surface of the microelectronic substrate. Accordingly, the fill material can have a thixotropic index with a value of from about four to about six. In another aspect of the invention, the surface of the microelectronic substrate can be a first surface and the microelectronic substrate can include a plurality of second surfaces extending away from the first surface, and a third surface facing opposite the first surface. The extent to which the fill material engages the second surfaces of the microelectronic substrate can be controlled so that the fill material engages a portion of the second surfaces extending from the first surface to a point about 60% to about 70% of the distance from the first surface to the third surface of the microelectronic substrate. 
     The invention is also directed toward a microelectronic substrate assembly. In one embodiment, the assembly includes a microelectronic substrate having a substrate surface and at least one bond member extending away from the substrate surface and configured to bond to a support member. A volume of uncured fill material is attached to the substrate surface and to the bond member, with the fill material having an exposed surface to engage the support member. In another aspect of the invention, the microelectronic substrate and the bond member are attached to the support member and the fill material has a thixotropic index of from about four to about six when uncured. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a partially schematic, plan view of a die mounted on a PCB in accordance with the prior art. 
     FIGS. 2A-2B illustrate steps in a process for mounting a die to a PCB in accordance with another prior art method. 
     FIGS. 3A-3D illustrate a process for mounting a microelectronic substrate to a support member in accordance with an embodiment of the invention. 
    
    
     DETAILED DESCRIPTION 
     The present disclosure describes packaged microelectronic devices and methods for manufacturing such devices. Many specific details of certain embodiments of the invention are set forth in the following description and in FIGS. 3A-3D to provide a thorough understanding of these embodiments. One skilled in the art, however, will understand that the present invention may have additional embodiments, and the invention may be practiced without several of the details described below. 
     FIG. 3A is a partially schematic, side elevational view of a microelectronic substrate  120  supported relative to a vessel  150  containing a fill material  140  in accordance with an embodiment of the invention. In one aspect of this embodiment, the microelectronic substrate  120  has an upper surface  123 , a lower surface  124  opposite the upper surface  123 , and four side surfaces  125  extending between the upper surface  123  and the lower surface  124 . The microelectronic substrate  120  further includes solder ball pads  122  on the lower surface  124  that are connected to devices and features (not shown in FIG. 3A) internal to the microelectronic substrate  120 . A plurality of bond members  121 , such as solder balls, are connected to the solder ball pads  122 . The surfaces of the bond members  121  and the lower surface  124  of the microelectronic substrate  120  define a fill region or cavity  126 . Alternatively, the microelectronic substrate  120  can have terminals other than the solder ball pads  122 , and/or the bond members  121  can include conductive epoxy bumps, metal coated polymer studs or other conductive elements. In still another embodiment, the microelectronic substrate  120  can have other protrusions (that are not necessarily electrically conductive) extending away from the lower surface  124 . The fill region  126  of these other embodiments is defined by the lower surface  124  and the other types of protrusions that project from the lower surface  124 . 
     FIG. 3A illustrates a stage of a method in accordance with an embodiment of the invention in which the substrate  120  is partially submerged in the fill material  140 . In one embodiment, a positioning apparatus  160  supports the microelectronic substrate  120  relative to the vessel  150 . In one aspect of this embodiment, the positioning apparatus can include a “pick and place” device conventionally used to pick up a microelectronic substrate for applying flux to solder balls of the substrate. The positioning apparatus  160  can include a suction cup  161  or another mechanism for releasably engaging the microelectronic substrate  120 . The apparatus  160  can further include an actuator (not shown) to move the microelectronic substrate  120  laterally as indicated by arrow M and/or axially as indicated by arrow N. Accordingly, the apparatus  160  can position the microelectronic substrate  120  over the vessel  150 , lower the microelectronic substrate  120  a selected distance into the fill material  140  within the vessel  150 , and remove the microelectronic substrate  120  from the vessel  150 . 
     In one aspect of this embodiment, the microelectronic substrate  120  is partially immersed in the fill material  140  by lowering the microelectronic substrate  120  into the fill material  140  until the lower surface  124  of the microelectronic substrate  120  is positioned beneath a free surface  141  of the fill material  140 . The microelectronic substrate  120  is then withdrawn from the vessel  150 . Referring now to FIG. 3B, a quantity of the fill material  140  remains attached to the microelectronic substrate  120  after the microelectronic substrate  120  has been withdrawn from the vessel  150  (FIG.  3 A). The fill material  140  can have an upper surface  142  adjacent to the microelectronic substrate  120  and an exposed lower surface  143  facing opposite the upper surface  142 . The fill material  140  can also extend partially up the sides  125  of the microelectronic substrate  120 . For example, in one embodiment, the fill material  140  can extend from the lower surface  124  up the sides  125  by a distance S 2  that is from about 60% to about 70% of a distance S 1  between the lower surface  124  and the upper surface  123 . The fill material  140  can also form a thin layer over the lower surfaces of the bond members  121 . For example, when the bond members  121  have a diameter D of about 150 microns, the thickness T of the layer of fill material  140  adjacent to the bond members  121  can be about 25 microns or less. In other embodiments, the thickness T of the fill material layer can have other dimensions, so long as the fill material  140  does not interfere with the electrical connections to the bond members  121 , as described below. 
     Once the microelectronic substrate  120  has been withdrawn from the vessel  150 , the positioning apparatus  160  can move the microelectronic substrate  120  and the attached fill material  140  into position over a support member  130 , which can include a PCB or another suitable substrate. The positioning apparatus  160  then aligns the bond members  121  of the microelectronic substrate  120  with corresponding bond pads  131  of the support member  130 . Referring now to FIG. 3C, the positioning apparatus  160  lowers the microelectronic substrate  120  toward the support member  130  until the fill material  140  contacts the support member  130 . The fill material  140  can also contact the bond pads  131 . The positioning apparatus  160  or another apparatus can optionally drive the microelectronic substrate  120  further downward to press bonding surfaces of the bond members  121  directly against the bond pads  131  and to squeeze out intervening fill material  140  between the bond members  121  and the bond pads  131 . As the fill material  140  stabilizes, it can extend partially up the sides  125  of the microelectronic substrate  120  by the distance S 2 , as described above with reference to FIG.  3 B. 
     When the bond members  121  are solder balls, the bond members  121  can be heated (for example, in a reflow process) to attach the bond members  121  to the bond pad  131 . The fill material  140  can be cured either as part of the reflow process or in a separate heat cycle to harden the fill material  140  and securely fix the fill material  140  to the microelectronic substrate  120  and the support member  130 . Referring now to FIG. 3D, an encapsulating material  170  is then disposed over the assembled microelectronic substrate  120  and support member  130  to form a package  180  that protects the microelectronic substrate  120  and the connections between the microelectronic substrate  120  and the support member  130 . 
     In one embodiment, the fill material  140  can have several characteristics that make it particularly suitable for use with the process described above with reference to FIGS. 3A-3D. For example, the fill material  140  can be in a liquid or gel state at room temperature in one embodiment so that the dipping process can be conducted at room temperature. In another aspect of this embodiment, the fill material  140  can be relatively thick and viscous at room temperature so as to remain attached to the microelectronic substrate  120  when the microelectronic substrate  120  is withdrawn from the vessel  150 . For example, the fill material  140  can have a thixotropic index of from about four to about six, and in one specific embodiment, the fill material  140  can have thixotropic index of about five. In a further aspect of this embodiment, the fill material  140  can include a conventional underfill epoxy material thickened to achieve the desired thixotropic index. For example, the fill material  140  can include FF 2000 epoxy (available from Dexter Labs of City of Industry, Calif.), which has an initial thixotropic index of from about one to about two. The epoxy can be thickened with thickening agents (such as barium sulfate) to increase the thixotropic index to a value of from about four to about six. Alternatively, the fill material  140  can have other suitable compositions and formulations. 
     In still another aspect of an embodiment of the process described above with reference to FIGS. 3A-3D, the fill material  140  can include a small amount of a surfactant, for example, about 1% or less by volume. Accordingly, the fill material  140  can have a reduced tendency (when compared to conventional underfill materials) for forming voids or pockets (a) at the interface between the microelectronic substrate  120  and the fill material  140  when the microelectronic substrate  120  is partially immersed in the fill material  140 , and (b) at the interface between the fill material  140  and the support member  130  when the microelectronic substrate  120  is mounted to the support member  130 . An advantage of this arrangement is that the fill material  140  is more likely to form a secure and hermetically sealed bond between the microelectronic substrate  120  and the support member  130 . 
     Another advantage of an embodiment of the process described above with reference to FIGS. 3A-3D is that the amount of fill material  140  attached to each microelectronic substrate  120  can be controlled. For example, the height S 2  to which the fill material  140  extends up the sides  125  of the microelectronic substrate  120  can be controlled by controlling the thixotropic index and wettability of the fill material  140 , and the depth to which the microelectronic substrate  120  is immersed in the fill material  140 . Furthermore, the thickness T of the fill material  140  adjacent to the solder balls  121  can be controlled by controlling the thixotropic index of the fill material  140 . Still further, the total amount of fill material  140  that adheres to the microelectronic substrate  120  varies with the size of the microelectronic substrate  120 , and in particular, the surface area of the lower surface  124 . For example, as the size of the lower surface  124  increases, the amount of fill material  140  adhering to the lower surface  124  increases correspondingly. Accordingly, the amount of fill material  140  adhering to each microelectronic substrate  120  self-adjusts to the size of the microelectronic substrate  120 . This is unlike some conventional underfill methods described above with reference to FIGS. 1-2B which require changing the amount of underfill material applied to the PCB whenever the size of the microelectronic substrate is changed. 
     Yet another advantage of an embodiment of the process described above with reference to FIGS. 3A-3D is that the relatively high thixotropic index of the fill material  140  can increase the strength of the initial bond between the uncured liquid or gel fill material  140 , the microelectronic substrate  120 , and the support member  130  (i.e., the “green strength” of the bond). For example, the more viscous fill material  140  can more securely support the microelectronic substrate  120  in position on the support member  130  during the interim period between attaching the microelectronic substrate  120  to the support member  130  and curing the fill material  140 . This feature can be advantageous because the microelectronic substrate  120  is expected to be less likely to move relative to the support member  130  during operations that take place before the fill material  140  is cured. Such operations can include moving the microelectronic substrate  120  and support member  130  from one processing station to the next and/or reflowing the bond members  121 . 
     In other embodiments, the processes and materials described above with reference to FIGS. 3A-3D can have other configurations and arrangements. For example, in one alternate embodiment, the temperature of the reservoir  150  can be controlled to control the viscosity of the fill material  140 . In another alternate embodiment, the fill material  140  can be disposed on the microelectronic substrate  120  by processes other than dipping. For example, the fill material  140  can be sprayed onto the microelectronic substrate  120  in one or more coats, or the fill material  140  can be deposited in the fill region using stencil printing or pen-type dispensers known in the surface mounting technology arts. 
     In still further embodiments, the fill material  140  can have suitable configurations other than the configurations described above. The fill material  140 , for example, can be any type of material that can be applied to the lower surface of the microelectronic substrate  120  before attaching the substrate  120  to a support member  130  for filling the gap between the substrate  120  and the support member  130 . Moreover, the bond members  121  need not include solder balls, and/or the microelectronic substrate  120  can have protrusions other than bond members that define a fill region or cavity adjacent to the lower surface  124 . In yet another embodiment, the microelectronic substrate  120  can have no protrusions, so long as the fill material  140  is applied to the lower surface  124  or a portion of the lower surface  124  prior to attaching the microelectronic substrate  120  to the support member  130 . In one aspect of this embodiment, the bond members  121  can first be attached to the support member  130  and then connected to terminals (such as bond pads) of the microelectronic substrate  120  that are flush with the lower surface  124  of the microelectronic substrate  120 . In still another embodiment, the pre-disposed fill material  140  can be supplemented with additional fill material disposed on the support member  130  in a manner generally similar to that described above with reference to FIG. 1 or FIGS. 2A-2B. 
     From the foregoing, it will be appreciated that, although specific embodiments of the invention have been described herein for purposes of illustration, various modifications may be made without from deviating from the spirit and scope of the invention. Accordingly, the invention is not limited except as by the appended claims.