Patent Publication Number: US-7220622-B2

Title: Method for attaching a semiconductor die to a substrate and heat spreader

Description:
FIELD OF THE INVENTION 
     The invention relates generally to the manufacture of semiconductor devices and, more particularly, to the attachment of a semiconductor die to a substrate and a thermal component, such as a heat spreader. 
     BACKGROUND OF THE INVENTION 
     An integrated circuit (IC) device may include a semiconductor die upon which integrated circuitry has been formed, and this die may be mounted on a package substrate or other die carrier. The die may be both electrically and mechanically coupled to the package substrate. By way of example, an array of solder bumps (or other electrical leads) extending from the die may be coupled (e.g., by a reflow process) to a corresponding array of lands (or other electrical leads) on the substrate to form electrical connections between the die and substrate. In addition, an underfill material may be disposed between the die and substrate to secure the die onto the substrate, as well as to protect the electrical connections extending between the die and substrate. The use of an array of solder bumps to provide electrical connections, as described above, is often referred to as Controlled Collapse Chip Connect (or “C4”). 
     The IC device may further include one or more thermal components coupled with the die, these thermal components functioning to transfer away or otherwise dissipate heat generated by the integrated circuitry during operation. For example, a heat spreader may be thermally coupled with the die, and a heat sink may, in turn, be thermally coupled with the heat spreader. Typically, a first thermal interface is disposed between the die and heat spreader, and a second thermal interface is disposed between the heat spreader and heat sink. The first thermal interface mechanically and thermally couples the die with the heat spreader, with the second thermal interface performing a similar role between the heat spreader and heat sink. The thermal interfaces may comprise any thermally conductive material capable of providing the requisite mechanical attachment, such as a solder material. 
     The assembly of the above-described IC device may require multiple heating, or reflow, operations. For example, a first heating process may be performed to both cure (or at least partially cure) the underfill material and reflow the C4 solder bumps, this first heating operation mechanically and electrically coupled with die with the package substrate. A second heating step may then be performed to reflow a solder thermal interface material disposed between the die and heat spreader, this second heating step reflowing the solder thermal interface material and forming a mechanical connection between the die and heat spreader. Each heating operation can increase manufacturing time and reduce through-put, while also increasing costs. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a block diagram illustrating an embodiment of a method for attaching a semiconductor die to a substrate and heat spreader. 
         FIGS. 2A–2D  are schematic diagrams illustrating embodiments of the method shown in  FIG. 1 . 
         FIGS. 3A–3B  are schematic diagrams illustrating further embodiments of the method shown in  FIG. 1 . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Disclosed are embodiments of a method for attaching a semiconductor die to a substrate and a heat spreader (or other thermal component). According to one embodiment, an assembly comprises a substrate, an IC die placed over the substrate, a layer of underfill material disposed between the die and substrate, a heat spreader disposed over the die, and a thermal interface disposed between the die and heat spreader. In one embodiment, the underfill material is cured and the thermal interface reflowed in a single process step. In a further embodiment, a plurality of conductive bumps (or other leads) extending from the die are reflowed during this single process step. By performing underfill cure and solder reflow in a single heating operation, process time and costs may be reduced and through-put may be increased. 
     Turning now to  FIG. 1 , illustrated is an embodiment of a method  100  for attaching a semiconductor die to a substrate and heat spreader (or other thermal component). Embodiments of the method  100  shown in  FIG. 1  are further illustrated in the schematic diagrams of  FIGS. 2A through 2D , as well as  FIGS. 3A and 3B , and reference should be made to these figures as called out in the text. 
     Referring first to block  110 , an underfill material is disposed on a substrate. This is illustrated in  FIG. 2A , which shows a quantity of underfill material  220  which has been dispensed on a substrate  210 . Substrate  210  may comprise any suitable type of package substrate or other die carrier. In one embodiment, the substrate  210  comprises a multilayer substrate including a number of alternating layers of metallization and dielectric material. Each layer of metallization comprises a number of conductors (e.g., traces), and these conductors may comprise any suitable conductive material, such as copper. Further, each metal layer is separated from adjacent metal layers by the dielectric layers, and adjacent metal layers may be electrically interconnected by conductive vias. The dielectric layers may comprise any suitable insulating material—e.g., polymers, including both thermoplastic and thermosetting resins or epoxies, ceramics, etc.—and the alternating layers of metal and dielectric material may be built-up over a core layer of a dielectric material. 
     The underfill material  220  will ultimately function to mechanically secure an IC die to the substrate  210 , as well as to fill voids between the die and substrate and to provide structural support for a number of electrical leads extending between the die and substrate. Typically, the underfill material comprises an insulating substance, such as an epoxy, although in other embodiments the underfill material may exhibit anisotropic electrical properties (e.g., an anisotropic electrically conductive epoxy). Examples of suitable underfill materials include STAYCHIP NUF-2076E, available from Cookson Semiconductor Packaging Materials, and LR-9000, available from Advanpack Solutions Ltd. In one embodiment, as shown in  FIG. 2A , the underfill material  220  comprises a paste or other highly viscous material that is dispensed onto the substrate  210  as a glob. In another embodiment, which is illustrated in  FIG. 3A , an underfill material  320  is dispensed onto the substrate  210  as a preform sheet of material. 
     Referring now to block  120 , a die is placed on the substrate. This is illustrated in  FIG. 2B , where a die  230  has been placed on the substrate  210 . Die  230  may comprise any type of IC device, such as a microprocessor, a network processor, an application specific integrated circuit (ASIC), a field programmable gate array (FPGA), or other type of logic device, as well as a memory device. In one embodiment, the die has a thickness of approximately 200 μm, or less. According to another embodiment, an array of conductive bumps  235  (e.g., solder bumps) or other conductive leads extends from a lower surface  231  of die  230 , each of the conductive bumps being electrically connected to a bond pad (not shown in figures) on the die. The array of conductive bumps  235  mates with a corresponding array of conductive lands (not shown in figures) formed on an upper surface  211  of the substrate  210 . When the conductive bumps  235  are connected with their respective lands on substrate  210  (e.g., by a reflow process, as described below), electrical communication can be established between the die and substrate. 
     When the die  230  is placed on and compressed against the substrate  210 , the underfill material is forced to flow underneath the die and outwardly toward the die&#39;s periphery to form a uniform and perhaps void-free layer between the die and substrate, as shown in  FIG. 2B . When fully cured, this layer of underfill material will both secure the die to the substrate, as well provide structural support for the conductive bumps  235 . In an alternative embodiment, as set forth in block  130 , the underfill material  220  may be partially cured after die placement in order to “hold” the die  230  on the substrate  210  for subsequent processing (such as movement by a pick-and-place head, etc.). For example, the underfill material may be partially cured by heating to a temperature of between 100 and 130 degrees C., for time period of between 10 and 30 minutes. To partially cure the underfill material  220 , heat (and pressure to compress the underfill material) may be applied by the head of a pick-and-place machine or the head a of thermo-compression bonding machine. 
     Referring to  FIG. 3B , where the underfill material is dispensed as a preform sheet  320 , the preform sheet may include a pattern of through holes (not shown in figures) matching the pattern of conductive bumps  235 . The underfill preform  320  and die  230  are placed on substrate  210  such that the conductive bumps  235  are aligned with the holes in the underfill preform  230 , as well as the lands on substrate  210 . The preform sheet of underfill material  320  does not “flow” in the same manner as the paste material (see  FIGS. 2A–2B ), although the underfill preform may deform during die placement. Again, as set forth in block  130 , the underfill material may be partially cured to hold the die  230  on the substrate  210  for subsequent processing. However, in another embodiment, the preform sheet of underfill  320  may comprise a tacky or sticky substance, and this tackiness may be sufficient to hold the die on the substrate for additional processing, in which case a partial cure of the underfill would not be needed. 
     In the discussion that follows, for ease of illustration, only the embodiments of  FIGS. 2A–2B  are further illustrated (see  FIGS. 2C and 2D ). However, as the reader will appreciate, the text and embodiments that follow are also applicable to the embodiments of  FIGS. 3A–3B . More generally, the disclosed embodiments are applicable to any type of underfill material, as well as to any method of dispensing the underfill material. 
     As set forth in block  140 , a thermal interface and a heat spreader are placed over the die to form an assembly. This is illustrated in  FIG. 2C , which shows a thermal interface  240  and a heat spreader  250  that have been placed over an upper surface  232  of die  230  to form an assembly  200 . In the illustrated embodiment, the assembly  200  comprises substrate  210 , underfill  220 , die  230 , thermal interface  240 , and heat spreader  250 . However, it should be understood that such an assembly may include additional components (e.g., a heat sink, a second thermal interface, a liquid cooling system, etc.) 
     The function of the heat spreader  250  is to conduct heat away from the die  230  (in the z-direction, as denoted by arrow  5 ), as well as to spread the heat laterally and outwards toward the periphery of the heat spreader. Ultimately, the heat spreader may be thermally coupled to a heat sink (e.g., a multi-fin heat exchanger), and the heat sink can dissipate the heat to the surrounding environment. The heat spreader  250  may be constructed from any suitable conductive material, such as, for example, copper and copper alloys, other thermally conductive metals, thermally conductive non-metals (e.g., diamond), as well as composite materials (e.g., an array of carbon nanotubes disposed in a matrix material). 
     The function of the thermal interface  240  is to thermally couple the heat spreader  250  with the die  230  and to conduct heat from the die to the heat spreader. The thermal interface  240  may comprise any suitable conductive material. In one embodiment, the thermal interface  240  comprises a layer of a solder material. However, the disclosed embodiments are not limited to a solder thermal interface, and it is contemplated that other materials (e.g., thermally conductive epoxies) may also be used as the thermal interface material. According to one embodiment, the thermal interface  240  comprises a preform sheet of solder (or other material) that is placed between the die  230  and heat spreader  250 . According to another embodiment, the thermal interface  240  may comprise a layer of solder that has be formed on a lower surface  251  of the heat spreader  250 . Where the thermal interface  240  comprises a solder material, a backside  232  of the die  230  may include a layer of material (e.g., gold) to promote adhesion with the solder thermal interface. 
     Referring to block  150  in  FIG. 1 , a force is applied to the heat spreader and/or substrate. This is also illustrated in  FIG. 2C , where a force F has been applied to the assembly  200  in order to compress the heat spreader  250 , thermal interface  240 , die  230 , and substrate  210  against one another for bonding. In one embodiment, a spring clamp is used to apply a force to the assembly. This is illustrated in  FIG. 2D , which shows the assembly  200  disposed in a clamp  290 . Clamp  290  includes a spring  295  to apply a compressive force to the assembly  200 . Of course, the reader will appreciate that other types of clamps and alternative devices may be employed to compress the assembly for bonding. 
     Referring next to block  160 , the assembly is heated to cure the underfill material and to reflow the solder thermal interface, as well as to reflow the conductive bumps, such that the assembly is bonded together. According to one embodiment, the underfill material  220  is cured simultaneously with reflowing of the solder thermal interface  240 . In another embodiment, reflowing of the solder bumps  235  occurs simultaneously with underfill curing and reflowing of the solder thermal interface. When bonded together, the die  230  is both electrically and mechanically&#39;secured to the substrate  220 , and the heat spreader  250  is both mechanically and thermally coupled with the die  210 , and in one embodiment, this bonding together of the assembly  200  occurs during a single process step. Heating and bonding of the assembly  200  may, in one embodiment, be accomplished by placing the assembly (and perhaps a clamp  290 ) in an oven that is heated to a peak temperature of between approximately 230 and 250 degrees C. In a further embodiment, the assembly is heated to the peak temperature in multiple stages (e.g., by moving the assembly on a belt through a multi-zone oven, etc.) over a time period of several minutes. 
     In an alternative embodiment, as set forth in block  170 , a post curing operation may be performed to further cure the underfill material. By way of example, post curing may be achieved by heating the underfill material to a temperature of between 150 and 175 degrees C. for a time period of approximately 1 to 3 hours. 
     The foregoing detailed description and accompanying drawings are only illustrative and not restrictive. They have been provided primarily for a clear and comprehensive understanding of the disclosed embodiments and no unnecessary limitations are to be understood therefrom. Numerous additions, deletions, and modifications to the embodiments described herein, as well as alternative arrangements, may be devised by those skilled in the art without departing from the spirit of the disclosed embodiments and the scope of the appended claims.