Patent Publication Number: US-2003235663-A1

Title: Method and apparatus for packaging microelectronic substrates

Description:
TECHNICAL FIELD  
       [0001] This invention relates to methods and apparatuses for packaging microelectronic substrates.  
       BACKGROUND OF THE INVENTION  
       [0002] Packaged microelectronic devices, such as memory chips and microprocessor chips, typically include a microelectronic substrate die encased in an epoxy protective covering. The die includes functional features, such as memory cells, processor circuits, and interconnecting circuitry. The die also typically includes bond pads electrically coupled to the functional features. The bond pads are coupled to pins or other types of terminals that extend outside the protective covering for connecting to buses, circuits and/or other microelectronic devices.  
       [0003] In one conventional arrangement shown in FIG. 1, a mold or cull tool  40  simultaneously encases a plurality of microelectronic substrates  30 . The cull tool  40  can include an upper plate  42  removably positioned on a lower plate  41  to define a plurality of substrate chambers  45 , an upright pellet cylinder  60 , and a plurality of channels  46  connecting the substrate chambers  45  to the cylinder  60 . A narrow gate  44  is positioned between each channel  46  and a corresponding substrate chamber  45 . A cylindrical pellet  20  formed from an epoxy mold compound is positioned in the cylinder  60 , and a plunger  50  moves downwardly within the cylinder  60  to transfer heat and exert pressure against the pellet  20 . The heat and pressure from the plunger liquifies the mold compound of the pellet  20 . The liquified mold compound flows through the channels  46  and into the substrate chambers  45  to surround the microelectronic substrates  30  and drive out air within the cull tool  40  through vents  43 .  
       [0004] The mold compound in the substrate chambers  45  forms a protective covering around each microelectronic substrate  30 . The residual mold compound in the channels  46  and in the lower portion of the cylinder  60  forms a “cull.” The cull has thin break points corresponding to the location of each gate  44 . After the upper plate  42  is separated from the lower plate  41 , the encapsulated microelectronic substrates  30  and the cull are removed from the tool  40  as a unit. The encapsulated microelectronic substrates  30  are then separated from the cull at the break points.  
       [0005] The mold compound that forms the pellet  20  is typically a high temperature, humidity-resistant, thermoset epoxy. One drawback with this compound is that it can be brittle and accordingly the comers of the pellet  20  can chip. One approach to addressing this drawback is to provide a shallow chamfer at the corners  21 , as shown in FIG. 1. Another drawback with this compound is that it must be elevated to a relatively high temperature before it will flow through the cull tool  40 . Accordingly, the cull tool  40  and the plunger  50  can be heated to improve the heat transfer to the pellet  20 . Furthermore, the lower plate  41  of the cull tool  40  can include one or more protrusions  47  that can improve the flow of the mold compound within the cull tool  40 .  
       [0006] Still another drawback with the molding process discussed above is that the cull cannot be easily recycled because it is formed from a thermoset material that does not “re-liquify” upon re-heating. Accordingly, the cull is waste material that must be discarded, which increases the materials cost of producing the packaged microelectronic devices. One approach to address this drawback is to reduce the volume of the pellet  20  and, correspondingly, the channels  46  that define the shape and volume of the cull. For example, one conventional approach includes reducing the length and/or the diameter of the pellet  20 . However, such pellets are not compatible with existing handling machines. For example, if the pellet length is decreased substantially, the length and diameter of the pellet will be approximately equal. The sorting and handling machines (not shown) that orient the pellets  20  for axial insertion into the cylinder  60  cannot properly orient the shorter pellets because the machines cannot distinguish between the length and diameter of the pellet. Furthermore, the handling machines are typically calibrated to reject undersized pellets on the basis of pellet length and accordingly would likely reject all or none of the reduced-length pellets.  
       SUMMARY OF THE INVENTION  
       [0007] The present invention is directed toward methods and apparatuses for packaging microelectronic substrates. A method in accordance with one aspect of the invention includes forming a pellet of uncured thermoset mold compound to have a first end surface, a second end surface opposite the first end surface, and an intermediate surface between the first and second end surfaces. The method further includes forming at least one cavity in the pellet and at least partially enclosing the microelectronic substrates by pressurizing the pellet and flowing the pellet around the microelectronic substrate.  
       [0008] A method in accordance with another aspect of the invention includes forming a pellet suitable for use with a pellet-handling apparatus configured to handle cylindrical pellets having a selected length, a selected radius less than the selected length, and a selected volume approximately equal to pi times the selected length times the square of the selected radius. The method includes forming a pellet material into a pellet body having a first end surface, a second end surface opposite the first end surface, and an intermediate surface between the end surfaces. The pellet body has a maximum length approximately equal to the selected length, a maximum cross-sectional dimension approximately equal to twice the selected radius, and a volume less than the selected volume by at least about 5%.  
       [0009] The invention is also directed to a pellet for packaging at least one microelectronic substrate. The pellet can include a pellet body formed from an uncured thermoset mold material. The pellet body has a first end surface, a second end surface facing opposite the first end surface, and an intermediate surface between the first and second end surfaces. The first end surface, the second end surface and the intermediate surface define an internal volume, and at least one of the surfaces and/or the internal volume has at least one cavity. In one aspect of this invention, the cavity has a generally spherical shape. In another aspect of this invention, the cavity can include a slot in the first end surface arranged transverse to the side surface. In still another aspect of this invention, the pellet body can have a generally right-cylindrical shape with a chamfered corner forming angles of approximately 45 degrees between the first end surface and the side surface.  
       [0010] The invention is also directed to an apparatus for packaging a microelectronic substrate. The apparatus can include a mold body having a chamber with a first portion configured to extend at least partially around the microelectronic substrate and a second portion coupled to the first portion. A plunger is positioned in the second portion of the chamber and is moveable within the second portion of the chamber in an axial direction. The plunger has a side wall aligned with the axial direction and an end wall transverse to the axial direction. At least a portion of the end wall extends axially away from the side wall. In one aspect of this embodiment, the plunger is configured for use with a pellet having a cylindrical side surface and two end surfaces. Each end surface can have a cavity defining a cavity shape, and the end wall of the plunger can be shaped to be received in the cavity of the pellet. 
     
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
     [0011]FIG. 1 is a partially schematic cross-sectional view of a molding apparatus for encapsulating microelectronic substrates in accordance with the prior art.  
     [0012]FIG. 2 is a partially schematic cross-sectional view of a molding apparatus and pellet for encapsulating microelectronic substrates in accordance with an embodiment of the invention.  
     [0013]FIG. 3 is a top isometric view of a pellet having a slotted end surface for encapsulating a microelectronic substrate in accordance with another embodiment of the invention.  
     [0014]FIG. 4 is a side cross-sectional view of a pellet having an end surface with conical indentations in accordance with still another embodiment of the invention.  
     [0015]FIG. 5 is a side cross-sectional view of a pellet having beveled corners in accordance with another embodiment of the invention.  
     [0016]FIG. 6 is a side elevation view of a pellet having a hollow internal cavity in accordance with still another embodiment of the invention.  
     [0017]FIG. 7 is a top isometric view of a pellet having a cavity extending therethrough in accordance with yet another embodiment of the invention.  
     [0018]FIG. 8 is a top isometric view of a pellet having a side surface with a plurality of cavities in accordance with still another embodiment of the invention. 
    
    
     DETAILED DESCRIPTION  
     [0019] The present disclosure describes methods and apparatuses for encapsulating microelectronic substrates. Many specific details of certain embodiments of the invention are set forth in the following description and in FIGS.  2 - 8  to provide a thorough understanding of these embodiments. One skilled in the art, however, will understand that the present invention may have additional embodiments, or that the invention may be practiced without several of the details described below.  
     [0020]FIG. 2 is a partially schematic cross-sectional view of a portion of an apparatus  110  for encapsulating a microelectronic substrate  130  in accordance with an embodiment of the invention. In one aspect of this embodiment, the apparatus  110  includes a mold or cull tool  140  configured to receive a pellet  120 , with both the tool  140  and the pellet  120  configured to reduce the volume of waste pellet material when compared to conventional arrangements. In one aspect of the invention, the tool  140  includes an upper portion  142  positioned above a lower portion  141 . The upper and lower portions  142  and  141  have recesses which, when aligned as shown in FIG. 2, form an internal chamber  170  for encapsulating the microelectronic substrate  130 . The microelectronic substrate  130  can be a die, such as a DRAM die or a processor die, or alternatively, the microelectronic substrate  130  can include other electronic components.  
     [0021] The internal chamber  170  can include a substrate portion  145  that houses the microelectronic substrate  130 , a cylinder portion  160  that houses the pellet  120 , and a channel portion  146  connecting the cylinder portion  160  to the substrate portion  145 . The chamber  170  can also include a vent  143  for exhausting air and/or other gases from the tool  140  as the pellet  120  fills the channel portion  146  and the substrate portion  145 . For purposes of illustration, one channel portion  146  and one substrate portion  145  are shown in FIG. 2; however, the tool  140  can include additional channel portions  146  and substrate portions  145  radiating outwardly from the cylinder portion  160  so that a single pellet  120  can be used to encapsulate several (e.g., two-six, or even more) microelectronic substrates  130 .  
     [0022] The portions of the internal chamber  170  that fill with waste pellet material (i.e., the pellet material that extends from the cylinder portion  160  to the substrate portion  145 ) define the cull volume as discussed above. These portions of the internal chamber  170  have a volume less than that of conventional chambers configured to encapsulate the same number and type of microelectronic substrates  130 . For example, the channel portions  146  can be smaller than the channels of conventional molds. Furthermore, the upper portion  142  of the tool  140  can include a protrusion  147  aligned with a central portion  148  of the chamber  170 . The protrusion  147  can further reduce the volume of the chamber  170 .  
     [0023] The volume of the pellet  120  is also less than the volume of conventional pellets; however, the maximum external dimensions of the pellet  120  are approximately identical to those of conventional pellets configured to encapsulate the same number and type of microelectronic substrates  130 . For example, the overall length L and diameter D of the pellet  120  are identical to or nearly identical to the length and diameter, respectively, of a conventional pellet used for the same application. Accordingly, the pellet  120  can be used with conventional pellet handling and sorting machines without changing the design, configuration or settings of the conventional machines. In one embodiment, the pellet  120  can have an overall diameter D of approximately 13 millimeters to 16 millimeters and an overall length L greater than the diameter D. For example, when the diameter D is about 13 millimeters, the length L can be about 17 millimeters. In other embodiments, the pellet  120  can have other dimensions so long as the length L exceeds the diameter D by an amount sufficient to allow the pellet  120  to be used with conventional pellet handling machines that properly orient the pellets  120  in the chamber  160  by distinguishing the length L from the diameter D.  
     [0024] In one embodiment, the volume of the pellet  120  is less than that of conventional pellets having the same maximum external length and diameter because the external surfaces of the pellet  120  include one or more cavities. For example, the pellet  120  can include a cylindrical side surface  125  positioned between two circular end surfaces  124 , and each end surface  124  can include a cavity  122 . In one aspect of this embodiment, the cavities  122  reduce the volume of the mold compound forming the pellet  120  by from about 5% to about 20% when compared to a conventional pellet with the same maximum external dimensions for the length and width. Conventional pellets have a volume of approximately π R 2  L, where R (radius)=½ D. Alternatively, the pellet  120  can have a greater than 20% volume reduction when compared to conventional pellets. In another aspect of this embodiment, the cavities  122  can be defined by a hemispherical or partially hemispherical cavity wall  123 . Alternatively, the cavities  122  can have other shapes that reduce the volume of the pellet  120  without reducing the overall external dimensions of the pellet  120 , as will be described in greater detail below with reference to FIGS.  3 - 8 .  
     [0025] The pellet  120  can be formed from a mold compound that includes a high temperature, humidity resistant thermoset material, such as an epoxy resin. The epoxy resin can have a variety of suitable formulations and can include biphenyl compounds, di-cyclo pentadiene compounds, ortho-cresole novolak compounds and/or multifunctional compounds, all of which are available from Nitto Denko Co. of Fremont, Calif. In other embodiments, the pellet  120  can have other formulations suitable for encapsulating the microelectronic substrates  130 .  
     [0026] In all the foregoing embodiments described with reference to FIG. 2, the pellet  120  is sized to fit within the cylinder  160  of the cull tool  140  and above a plunger  150 . The plunger  150  is axially movable within the cylinder  160  between a first position (shown in FIG. 2) to receive the pellet  120  and a second position with the plunger  150  moved axially upwardly to compress the pellet  120 . Accordingly, the plunger  150  can force the mold compound forming the pellet  120  into the channel portion  146  and the substrate portion  145  of the chamber  170 .  
     [0027] In one aspect of this embodiment, the plunger  150 , the walls of the cylinder  160 , and/or the other surfaces of the cull tool  140  that define the chamber  170  are heated to liquefy the pellet  120 . In still a further aspect of this embodiment, the plunger  150  can include a side wall  151  adjacent the walls of the cylinder  160 , an end wall  152  transverse to the side wall  151  and a protrusion  153  that extends axially away from the end wall  152  and the corner between the end wall  152  and the side wall  151 . The protrusion  153  can have a width less than or equal to the width of the end wall  152 . In still a further aspect of this embodiment, the protrusion  153  is sized to fit within the cavity  122  at the end of the pellet  120 . Accordingly, when the plunger  150  is heated, the protrusion  153  can increase the rate of heat transfer to the pellet  120  (relative to a conventional plunger having a flat end surface) because more surface area of the plunger  150  contacts the pellet  120 . Similarly, when the upper portion  142  of the cull tool  140  is heated, the protrusion  147  can increase the heat transferred to the pellet  120  by engaging the walls  123  of cavity  122  at the opposite end of the pellet  120 .  
     [0028] In operation, the microelectronic substrate  130  is positioned in the substrate portion  145  of the chamber  170  and the pellet  120  is positioned in the cylinder portion  160 . The plunger  150  and/or the surfaces defining the chamber  170  are heated, and the plunger  150  is moved upwardly to compress and liquify the pellet  120 . The plunger accordingly forces the liquified pellet  120  through the channel portion  146  and into the substrate portion  145  around the microelectronic substrate  130 . The encapsulated microelectronic substrate  130  and the cull (which occupies the channel  146  and the central portion  148  of the chamber  170 ) are removed as a unit, and then the encapsulated microelectronic substrate  130  is separated from the cull, in a manner generally similar to that discussed above.  
     [0029] One feature of an embodiment of the apparatus  110  and the method described above with reference to FIG. 2 is that the pellet  120  has the same maximum length and width as a conventional pellet to be compatible with existing pellet handling machines, but the pellet  120  has a reduced volume. Accordingly, the culls formed from the pellet  120  have a lower volume than conventional culls to reduce the cost of the pellets and the waste material left over after encapsulating the microelectronic substrates  130  with the pellets.  
     [0030] Another feature of an embodiment of the apparatus  110  and method described above with reference to FIG. 2 is that the size of the cavities  122  can be selected to match the size of the internal chamber  170  and/or the size of the microelectronic substrate  130 . For example, pellets  120  having relatively large cavities  122  can be used with cull tools  140  having relatively small internal volumes  170 , and pellets  120  having relatively small cavities  122  (or no cavities) can be used with cull tools  140  having relatively large internal volumes  170 . Similarly, pellets  120  having relatively large cavities  122  can be used to encapsulate relatively large microelectronic substrates  130  and pellets  120  having relatively small cavities  122  (or no cavities) can be used to encapsulate relatively small microelectronic substrates  130 . Accordingly, pellets  120  having the same overall external dimensions can be used with different cull tools  140  to encapsulate different microelectronic substrates  130  without requiring different pellet handling equipment.  
     [0031] FIGS.  3 - 8  depict other pellets having the same overall external dimensions as conventional pellets (but reduced volumes) in accordance with alternate embodiments of the invention. For example, FIG. 3 is a top isometric view of a pellet  220  having a generally cylindrical side surface  225 , circular end surfaces  224 , and a slot  222  in each end surface  224 . Each end surface  224  can include a single slot  222 , or alternatively, each end surface  224  can include a plurality of slots  222 . In either embodiment, the pellet  220  can be used in conjunction with an apparatus generally similar to the apparatus  110  shown in FIG. 2, but having tab-shaped protrusions that match the shape of the slots  222  instead of the hemispherical protrusions  147  and  153  shown in FIG. 2. Accordingly, the rate of heat transfer to the pellet  220  can be increased when compared to conventional devices in a manner generally similar to that described above with reference to FIG. 2.  
     [0032] Referring now to FIGS. 2 and 3, the pellet  220  can be compressed with a plunger  150  having a flat end wall  152  and a cull tool  140  having a flat central portion  148  opposite the end wall in an alternate embodiment. In this alternate embodiment, the volume of the cull can be reduced by an amount equal to the volume of the cavities  222  by reducing the volume of the channels  146  and/or other portions of the cull tool  140 . Accordingly, the slots  222  in pellet  220  may have certain advantages over the spherical cavities  122  in the pellet  120  described above with reference to FIG. 2. For example, when the plunger  150  has a flat end wall  152 , the slot  222  will not entrap air as the plunger  150  engages the pellet  220 . Instead, air in the slot  222  will tend to flow laterally around the side surface  225  of the pellet  220  as the plunger  150  compresses the pellet  220 .  
     [0033]FIG. 4 is a side cross-sectional view of a pellet  320  having frustro-conical cavities  322  each end surface  324 . FIG. 5 is a side cross-sectional view of a cylindrical pellet  420  having a side surface  425 , end surfaces  424  and a chamfered or beveled corner  421  at the intersection between the side surface  425  and each end surface  424 . In one aspect of this embodiment, the chamfered corner  421  can form an angle of approximately 45 degrees with the side surface  425  and each of the end surfaces  424 . In alternate embodiments the chamfered corner  421  can form other angles with the side surface  425  and end surfaces  424 , so long as the pellet  420  has a reduced volume of at least 5% (and between 5% and 20%, in one embodiment) when compared to a conventional pellet having the same maximum length and width.  
     [0034]FIG. 6 is a side elevation view of a pellet  520  having a side surface  525  and end surfaces  524  that completely enclose an internal cavity  522 . Alternatively, the side surface  525  and/or the end surfaces  524  can have one or more apertures that extend into the cavity  522  to provide a vent. An advantage of this alternate arrangement is that the apertures can reduce the likelihood for entrapping air as the pellet  520  is compressed by the plunger  150  (FIG. 2).  
     [0035]FIG. 7 is a top isometric view of a pellet  620  having a side surface  625 , opposite-facing end surfaces  624 , and a cavity  622  extending entirely through the pellet  620  from one end surface  624  to the other. FIG. 8 is a top isometric view of a pellet  720  having round end surfaces  724  and a cylindrical side surface  725  with a plurality of cavities  722 . In one aspect of this embodiment, the cavities  722  extend part-way into the side surface  725 . Alternatively, the cavities  722  can extend entirely through the side surface  725 .  
     [0036] In each of the foregoing embodiments discussed above with reference to FIGS.  2 - 8 , the pellets have the same overall external dimensions as conventional pellets, but are formed from a volume of mold compound that is less than the volume used for conventional pellets having the same maximum length and width. In one aspect of these foregoing embodiments, the volume is at least 5% less than the volume of the conventional pellets. In another aspect of these foregoing embodiments, the density of the mold compound used to form the pellets is approximately the same as the mold compound density of the corresponding conventional pellets. Alternatively, the mold compound density can be increased or decreased. In any of the foregoing embodiments, the volume occupied by the cull is reduced by an amount approximately equal to the volume of the cavity or other volume-reducing feature of the pellet, for example by providing protrusions in the plunger  150  and/or the upper plate  142  and/or by reducing the volume of the channels  146  extending between the cylinder  160  and the substrate portion  145 . Accordingly, reducing the volume of the pellet will not result in the mold material failing to fill the substrate portion  145  of the cavity  170 , which could result in incomplete encapsulation of the microelectronic substrate  130 .  
     [0037] 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 deviating from the spirit and scope of the invention. For example, the cavities and other volume-reducing features described individually with respect to a particular embodiment can be combined in other embodiments. Accordingly, the invention is not limited except as by the appended claims.