Abstract:
A thermally curable material (including a conductive adhesive or solder) is formed on an electronic device support structure such as a die on a wafer or a printed wiring board (PWB). The support structure is placed on a heatable surface or placed in a process chamber having an infrared radiation (IR) mechanism. The thermally curable material is formed using a dispensing technique such as screen printing or pneumatic dispensing. To heat the curable material to a temperature sufficient to cure, the heatable surface can be heated or the IR mechanism can be activated. The heatable surface can be located in a screen printing machine.

Description:
BACKGROUND 
     The invention relates to forming thermally curable materials on a support structure in an electronic device. 
     Electronic packaging (including packaging of one or more dies into an integrated circuit housing; packaging chips into multichip modules; and attaching chips onto a printed wiring board or PWB) involves forming thermally curable materials on the underlying support structure (which can be a wafer, PWB, leadframe, or other support surface). After formation of the thermally curable material, such as by screen printing, the material is cured to properly bond the material to the surface of the support structure. 
     For example, to attach chips to a PWB or other support surface, solder or a conductive adhesive can be deposited onto the support surface through a screen in a screen printing machine. After the solder or conductive adhesive has been formed, the support surface is removed from the screen printing machine and placed in an oven to cure the solder or conductive adhesive at high temperatures. 
     Another process that involves formation of a thermally curable material onto a support surface is flip-chip bonding, in which an unpackaged die is mounted directly onto a support structure (also referred to as an interconnecting substrate). The unpackaged die is turned upside down and bonded to the interconnecting substrate by some connecting medium, including solder bumps and conductive adhesives that contains polymers such as metal-filled epoxies or conductive thermoplastic compounds. 
     Flip-chip bonds provide a high density, low inductance direct electrical path between the die and the interconnecting substrate. Referring to FIG. 1, a typical flip-chip assembly is shown. An interconnecting substrate  10  has a surface  18  that provides conductive terminals  12 . To prevent bonding material from flowing in the region between conductive terminals  12 , solder dams  14  are mounted on the interconnecting substrate  10  between adjacent terminals  12 . 
     A die  16  for mounting to the interconnecting substrate  10  is flipped over so that its active surface  21  faces the top surface  18  of the interconnecting substrate. Bumps  20  (solder or adhesives) are formed on bond pads  22  (typically made of aluminum) on the surface  21  of the die  16 . As shown by the enlarged cross-sectional portion of the die  16 , a window is cut through a passivation layer  24  over each bond pad  22  so electrical contact can be made. One or more thin circular layers  26  are formed over the exposed surface of the bond pad  22 . Next, solder bumps or conductive polymer bumps  20  are formed over the circular layers  26 , using such dispensing techniques as screen printing or pneumatic dispensing. 
     Typically, the formation of the layers  26  and bumps  20  is performed on an entire wafer, which includes multiple dies. After the layers  26  and bumps  20  are formed, one or more wafers are placed inside an oven to enable the bumps  20  to bond to the layers  26 . Solder bump connections are made using a reflow process, which takes the plated solder through a solid-liquid-solid transition, allowing the solder to bond with the layers  26 . If thermoset conductive polymers such as metal-filled epoxy are used, then the oven heating cures the thermoset polymer so bonding can occur. 
     Next, the wafer is sawed into individual dies. Each die  16  is placed onto the interconnecting substrate  10  such that the bumps  20  contact corresponding conductive terminals  12  on the interconnecting substrate  10 . 
     In the typical manufacturing processes described above, support surfaces (wafers, PWBs, etc.) are physically removed from a screen printing or pneumatic dispensing machine and loaded into an oven to cure the deposited thermally curable material, which can be labor-intensive and time-consuming. 
     SUMMARY 
     Generally, the invention is directed to formation of a thermally curable material on an electronic device support structure that is placed on a heatable surface. Curing of the thermally curable material can then be performed by heating the heatable surface. 
     The invention has one or more of the following advantages. Using an in-line process to form thermally curable material on a support structure followed by thermally curing the material, steps in the manufacturing process are reduced. By eliminating the need to physically move support structures from one system to another system, first to deposit a thermally curable material and subsequently to cure the material, time and labor can be saved. 
     In general, in one aspect, the invention features a method of forming a thermally curable material on a support structure in an electronic device. The method includes placing the support structure on a heatable surface, and depositing the thermally curable material onto a surface of the support structure. The heatable surface is then heated to a temperature sufficient to cure the deposited material. 
     In general, in another aspect, the invention features a method of forming bumps on bond pads on a die. The die is placed on a heatable surface. Thermally curable bumps are formed on the bond pads. The heatable surface is heated to a temperature sufficient to cure the bumps. 
     In general, in another aspect, the invention features a method of fabricating a flip-chip assembly. A thermally curable material is deposited onto bond pads on a die placed on a heatable surface. The heatable surface is then heated to a sufficient temperature to cure the thermally curable material. The die is face mounted onto an interconnecting structure by connecting the cured material on the die bond pads to corresponding terminals on the interconnecting structure. 
     In general, in another aspect, the invention features a method of forming structures on a semiconductor die. A thermally curable material is screen printed onto bond pads of the die using a screen printing machine. The thermally curable material is heated in the screen printing machine to a temperature sufficient to cure the material. 
     In general, in another aspect, the invention features a screen-printing system including a member for holding an electronic device support structure, the member capable of being heated. The screen printing system further includes a screen through which a thermally curable material can be deposited onto portions of the support structure. The heated member is adapted to cure the material after deposition of the thermally curable material onto the support structure. 
     In general, in another aspect, the invention features an apparatus for forming a predetermined pattern on an electronic device support structure using a thermally curable material. A device is configured to deposit the material onto the support structure. A heatable surface on which the support structure is positioned is heated to a temperature sufficient to cure the material. 
     Other features and advantages will become apparent from the following description and from the claims. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is an enlarged, cross-sectional view of a flip-chip assembly. 
     FIG. 2 is a flow diagram of the process for manufacturing a flip-chip assembly according to the invention. 
     FIG. 3 is a block diagram of a screen printing machine for performing in-line formation and curing of thermally curable materials on a support structure. 
     FIG. 4 is a perspective view of a printed wiring board. 
    
    
     DETAILED DESCRIPTION 
     Referring to FIG. 2, an improved process is illustrated for building a flip-chip assembly (shown in FIG.  1 ). First, one or more plated layers  26 , including, e.g., a palladium (Pd) layer, are formed on the exposed area (through a window in the passivation layer  24 ) of aluminum bond pads  22  on the die  16  (step  102 ). The Pd plating can be used to enhance bonding to bumps  20  (which can be solder bumps or adhesive bumps) deposited on the layers  26  by use of a dispensing process such as screen printing or pneumatic dispensing (step  104 ). According to one embodiment of the invention, the machine used to deposit the thermally curable material includes a heatable chuck providing a surface on which the wafer is placed. 
     The bumps  20  are formed of a thermally curable material. After the bumps  20  have been printed or otherwise deposited onto the die bond pads  22 , the chuck is heated to cure the deposited material (step  106 ). Thus, using one system (e.g., screen printing system or pneumatic dispensing system), the thermally curable material can both be deposited on the support structure and heated to cure. This is referred to as in-line formation and curing of thermally curable material on a support structure. 
     Next, after curing, the wafer is removed from the system and the wafer is sawed to form multiple dies (step  108 ). A wet adhesive is then screen printed onto the interconnect terminals  12  on the interconnecting substrate  10  (step  110 ). The die is next face mounted onto the interconnecting substrate  10  such that the bond pads contact corresponding terminals  12  and the wet adhesives are cured (step  112 ) to bond to the corresponding bumps  20 . 
     The bumps  20  can be deposited onto the bond pads on each die on the wafer using a screen printing machine  400 , such as the exemplary system shown in FIG.  3 . The screen printing machine  400  can be used with other support structures as well, such as PWBs and other types of interconnecting substrates. The screen printing machine  400  includes a screen  402  (which can be a mesh screen or a metal mask stencil) and a squeegee  404 . During screen printing, the squeegee  404  is moved along the direction generally indicated by X over the screen  402 . A paste is inserted between the squeegee  404  and the screen  402 . The squeegee  404  applies a downward pressure to force the paste through the openings of the screen  402 . 
     The screen printing machine  400  includes a loading surface  410  that provides support for a heatable chuck  406 . A wafer  408  can be placed on the chuck  406 , which is then loaded into the screen printing machine  400  and positioned under the screen  402 . The locations at which paste material is deposited on the wafer is determined by the positions of the openings in the screen  402 . Bumps can thus be selectively deposited onto bond pad sites on each die on the wafer  408 . 
     The chuck  406  used in the screen printing machine  400  can be thermally controlled to achieve temperatures within a predetermined range. For example, the AirCool® Thermo Control Chuck from ERS Elektronik GmbH has a temperature range between −10° C. and 200° C. Another exemplary chuck that can be used is the ARCTIC TC Temperature Control Chuck from TRIO-TECH of San Fernando, Calif., which has a temperature range from −65° C. to 300° C. 
     Once the desired bumps are screen printed onto the bond pad sites on the wafer  408 , the chuck  406  is heated to uniformly heat the wafer (step  106  in FIG. 2) to cure the bump material. An exemplary adhesive material used to form the bump is a snap cure adhesive including silver-filled adhesive, such as the QMI  505  from Quantum Materials, Inc., of San Diego, Calif. Using this type of bump, after the chuck  406  and wafer  408  reach 200° C., the bumps on the wafer  408  can be cured within approximately 2 seconds. 
     Other types of conductive adhesives can be used, including bismaleimide, or conductive thermoplastics and thermoset materials (e.g., epoxy or polyimide) filled with a conductive filler. Solder bumps can also be used, in which case heating the chuck  406  causes the solder bumps to reflow to the desired configuration. As used in this application, thermal curing includes both chemical or physical alteration of the curable material, including altering the chemical characteristics of an adhesive material or reflowing solder. 
     An in-line curing process in described that uses a heatable chuck in a screen printing system. Alternatively, an infrared radiation (IR) process can also be used to cure the curable materials. Mechanisms to perform IR curing can be included in the screen printing system. 
     Although the screen printing machine  400  is shown with one chuck, multiple chucks can be provided onto which multiple wafers can be placed. After the bumps are screen printed onto the wafers (using one screen or multiple screens for parallel screen printing), all the chucks can be heated at the same time to perform in-line curing of the bumps deposited onto the die bond pads on the wafers. 
     The in-line material formation and curing process can be extended to other support structures, including PWBs. Portions of a PWB  300  are shown in FIG. 4. A conductive paste  306  that is thermally curable (e.g., Ag-filled epoxy) can be screen printed onto the bonding pads  312  of the PWB  300  to bond to bonding pads  308  of an integrated circuit device  304 . A chuck or other heatable surface sized to receive the PWB is used in the screen printing or pneumatic dispensing machine. The conductive material is then cured using the chuck or other surface heating method. After curing, joints  306  are formed, which connect to conductor lines  310  extending through a layer in the multilayer PWB  300 . Alternatively, an IR process can be used to cure the thermally curable material. 
     Other embodiments are within the scope of the following claims. For example, although specific materials are mentioned in the process described, other materials can be used and still achieve desirable results. The steps of the described process can also be varied. Other support structures that can be used with the described in-line process include interconnecting substrates used to receive chip-on-board devices and ball grid array (BGA) devices.