Abstract:
A method and apparatus are provided for attaching a semiconductor device to a substrate. One end of the substrate is elevated to position the substrate and the coupled semiconductor device on an inclined plane. An underfill material is introduced along a wall of the semiconductor device located at the elevated end of the inclined substrate with the underfill material being placed between the substrate and the semiconductor device. An optional but preferred additional step of the invention includes coupling a barrier means to the substrate at a point on the substrate adjacent to a sidewall of the semiconductor device located at the lowest point of the slope created by the inclined substrate. The barrier means prevents the underfill material from spreading beyond the sidewalls of the semiconductor device, particularly in instances where the substrate is inclined at a steep angle.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of application Ser. No. 09/991,010, filed Nov. 16, 2001, now U.S. Pat. No. 6,455,933, issued Sep. 24, 2002, which is a continuation of application Ser. No. 09/619,595, filed Jul. 19, 2000, now U.S. Pat. No. 6,376,918 B1, issued Apr. 23, 2002, which is a divisional of application Ser. No. 09/304,502, filed May 3, 1999, now U.S. Pat. No. 6,194,243 B1, issued Feb. 27, 2001, which is a continuation of application Ser. No. 08/910,299, filed Aug. 11, 1997, now U.S. Pat. No. 5,973,404, issued Oct. 26, 1999, which is a divisional of application Ser. No. 08/612,125, filed Mar. 7, 1996, now U.S. Pat. No. 5,766,982, issued Jun. 16, 1998. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a method and apparatus for underfilling a semiconductor device. More specifically, the present invention relates to a method and apparatus for uniformly underfilling a bumped or raised semiconductor chip to be essentially void free. 
     2. State of the Prior Art 
     Flip-chip and bumped die technology is well known in the art. A flip-chip or bumped (raised) die is a semiconductor chip (die) having bumps on the bond pads formed on the active surface or front side thereof, the bumps being used as electrical and mechanical connectors, which is inverted (flipped) and bonded to a substrate by means of the bumps. Several materials are typically used to form the bumps on the die, such as conductive polymers, solder, etc. Typically, if the bumps are solder bumps, the solder bumps are reflowed to form a solder joint between the so-called flip-chip and the substrate, the solder joint forming both electrical and mechanical connections between the flip-chip and substrate. In any event, due to the presence of the bumps on the flip-chip, a gap exists between the substrate and the bottom surface of the flip-chip. 
     Typically, since the flip-chip and the substrate have different coefficients of thermal expansion and operate at different temperatures and also have different mechanical properties with differing attendant reactions to mechanical loading and stresses, stress develops in the joints formed by the bumps between the flip-chip and substrate. Therefore, the bumps must be sufficiently robust to withstand such stressful conditions to maintain the joint between the flip-chip and the substrate. To enhance the joint integrity formed by the bumps located between the flip-chip and the substrate, an underfill material comprised of a suitable polymer is introduced in the gap between the flip-chip and the substrate. The underfill also serves to equalize stress placed on the flip-chip and substrate, helps transfer heat from the flip-chip and helps protect the bump connections located between the flip-chip and the substrate from contaminants such as moisture, chemicals, and contaminating ions. 
     In practice, the underfill material is typically dispensed into the gap between the flip-chip and the substrate by injecting the underfill along two or more sides of the flip-chip with the underfill material flowing, usually by capillary action, to fill the gap. For example, U.S. Pat. No. 5,218,234 to Thompson et al. discloses a semiconductor device assembly whereby an epoxy underfill is accomplished by applying the epoxy around the perimeter of the flip-chip mounted on the substrate and allowing the epoxy to flow underneath the chip. Alternatively, the underfill can be accomplished by backfilling the gap between the flip-chip and the substrate through a hole in the substrate beneath the chip. 
     However, the traditional method of underfilling by way of capillary action has a serious disadvantage. The small gap formed between the flip-chip and substrate to which it is connected prevents filling the gap in a uniform manner. Such non-uniform underfilling is particularly prevalent in the areas surrounding the bumps interconnecting the flip-chip to the substrate. When underfilling a flip-chip on a substrate situated in a substantially horizontal plane, the underfill material will generally be non-uniform in character and contain bubbles, air pockets, or voids therein. This non-uniform underfill decreases the underfill material&#39;s ability to protect the interconnections between the flip-chip and substrate and environmentally compromises the flip-chip itself, thereby leading to a reduction in the reliability of the chip. 
     A different method of bonding a semiconductor chip to a substrate is disclosed in U.S. Pat. No. 5,385,869 to Liu et al. whereby the gap between the semiconductor chip and substrate is underfilled utilizing a substrate having a through hole formed therein which is centrally located below the semiconductor chip mounted thereon. The through hole has gates or notches formed at each corner thereof which extend beyond the semiconductor chip, which is mounted thereover. Underfilling the gap between the semiconductor chip and the substrate is accomplished by blocking one side of the through hole, applying an encapsulation material on top of and around the chip, and allowing the encapsulation material to flow into the through hole by way of the gates or notches in the substrate. 
     As disclosed in U.S. Pat. No. 5,203,076 to Banerji et al., a vacuum chamber is used to underfill the gap between a semiconductor chip and a substrate. A bead of underfill polymeric material is provided on the substrate about the periphery of the chip. Next, the semiconductor chip and substrate are placed within a vacuum chamber with a vacuum being subsequently applied to the chip and the substrate to evacuate the gap therebetween. Air is then slowly allowed to re-enter the vacuum chamber to force the underfill material into the gap between the semiconductor chip and the substrate. 
     Although the underfill methods disclosed in the Liu and Banerji patents attempt to address the problem of underfilling in a non-uniform manner, those references present solutions that require specialized substrates, use additional equipment in the underfilling process and increase the cost of production. For example, implementation of the underfilling method illustrated in the Liu reference requires the use of a specialized substrate having a through hole therein. Similarly, the underfilling method illustrated in the Banerji et al. reference requires the use of specialized equipment in the form of a vacuum chamber. 
     Therefore, it would be advantageous to develop a method for performing underfill of semiconductor chips which results in underfill material that is uniform and substantially free of voids or air. It would be a further improvement to develop a method for performing uniform underfilling of the gap between the flip-chip and substrate that is cost effective and utilizes standard substrates. 
     BRIEF SUMMARY OF THE INVENTION 
     The present invention is directed to an improved method and apparatus for underfilling the gap between a semiconductor device (flip-chip) and substrate. The improved method of attaching a semiconductor device to a substrate begins with the step of electrically connecting the semiconductor device to the substrate. Next, one end of the substrate is elevated to a position where the substrate and semiconductor device are located on an inclined or tilted plane. Finally, an underfill material is introduced along the sidewall of the semiconductor device located at the elevated end of the inclined substrate with the underfill flowing into and filling the gap formed between the substrate and the semiconductor device. 
     The improved method of the present invention may include the step of using a suitable dam or barrier located adjacent to the lower edge of the inclined substrate at the lowest point of the inclined substrate. The suitable dam or barrier prevents the underfill material from spreading beyond the sidewalls of the semiconductor device, particularly in instances where the substrate is inclined at a steep angle with respect to a horizontal plane. 
    
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
     The advantages, as well as other novel and important features of the present invention will be better understood when the following description is read along with the accompanying drawings of which: 
     FIG. 1 is a cross-sectional view of a preferred underfill dispensing step for a semiconductor device, a bumped flip-chip, and an inclined substrate in accordance with the present invention; 
     FIG. 2 is a cross-sectional view of another preferred underfill dispensing method, which illustrates the placement of a suitable barrier, a dam, located adjacent the substrate; 
     FIG. 3 is a side view illustrating the placement of the semiconductor device, bumped flip-chip, and substrate of FIG. 1 on top of a support member having a vibrator attached thereto; 
     FIG. 4 is a top view of a semiconductor device, bumped flip-chip, and an inclined substrate illustrating the use of two suitable barriers, dams, to perform the underfill step; 
     FIG. 5 is a cross-sectional view of another embodiment of the present invention, illustrating a backfill method of underfilling the gap formed between a semiconductor device, bumped flip-chip, and a substrate; 
     FIG. 6 is a cross-sectional view of another embodiment of the present invention, illustrating a backfill method of underfilling the gap formed between a semiconductor device, a bumped flip-chip, and a substrate without the use of dams; and 
     FIG. 7 is a cross-sectional view of another embodiment of the present invention, illustrating a backfill method of underfilling the gap formed between a semiconductor device, a bumped flip-chip, and a substrate wherein the substrate is inverted during the underfilling process. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to FIG. 1, a substrate or chip carrier  10  is shown for connecting a semiconductor device or flip-chip  12  by conventional direct chip bonding techniques. Substrate  10  is typically made of ceramic, silicone, glass, and combinations thereof. Substrate  10  is preferably comprised of a printed circuit board (PCB) or other carrier, which is used in flip-chip technology, such as an FR4 PCB. Substrate  10  has a front end  14 , a rear end  16 , and a top surface  18 , the top surface  18  having contact pads thereon. 
     Flip-chip  12  has a front sidewall  30 , a rear sidewall  32 , and an active surface  20 . The active surface  20  comprises integrated circuitry and a plurality of contact pads  22 . The contact pads  22  have bumps  24  thereon, which provide both electrical and mechanical connection to substrate  10 . 
     An electrical assembly is produced by placing and securing the flip-chip  12  on the top surface  18  of substrate  10  having active circuitry thereon. Specifically, the bumps  24  are aligned with the contact pads of the active circuitry located on top surface  18  of substrate  10 . The flip-chip  12  is then electrically and mechanically connected to the substrate  10  by curing or reflowing the bumps  24 , depending upon the type of material comprising the bumps  24 . Alternatively, the bumps  24  may be formed on the substrate  10  prior to attachment of the flip-chip  12 . In other words, either surface may bear the bumps thereon. Although bumps  24  are typically formed with solder, it is understood that any other materials known in the art (e.g. gold, indium, tin, lead, silver or alloys thereof) that reflow to make electrical interconnects to substrate  10  can also be used. Additionally, the bumps  24  may be formed of conductive polymeric and epoxy materials and may include various metals being plated thereon. 
     After reflowing of the bumps  24 , a space or gap  26  is formed between the active surface  20  of flip-chip  12  and the top surface  18  of substrate  10 . The size of the gap  26  is controlled by the size of the reflowed solder bumps and typically varies from approximately 3 to about 10 mils. 
     Next, an underfill material  28  is applied to fill the gap  26  between the flip-chip  12  and the substrate  10 . As previously stated, the purpose of the underfill material  28  is to environmentally seal the active surface  20  of the flip-chip  12  and the bumps  24 , to help provide an additional mechanical bond between the flip-chip  12  and the substrate  10 , to help prevent and/or distribute stress on the flip-chip  12  and bumps  24 , and to help transfer heat from the flip-chip  12 . The underfill material is typically a polymeric material, such as an epoxy or an acrylic resin, and may contain inert filler material therein. The underfill material  28  typically has a thermal coefficient of expansion that approximates that of the flip-chip  12  and/or the substrate  10  to help minimize stress placed on either the flip-chip  12  or the substrate  10  during the operation of the flip-chip caused by the heating of the underfill material  28 . To promote filling of the gap between the substrate  10  and flip-chip  12 , the viscosity of the underfill material  28  is controlled, taking into account the flow characteristics of the underfill material, the material characteristics of the substrate  10 , the material characteristics of the flip-chip  12 , and the size of the gap  26 . 
     As shown, the underfill process is started by elevating or inclining the front end  14  of the substrate  10  in order to position the substrate  10  on an inclined plane  2  with respect to a horizontal plane  1 . The angle of elevation or inclination of the inclined plane  2  and the attendant substrate  10  and flip-chip  12  is dependent on the viscosity or the rate of dispensing of the underfill material  28 . The viscosity of the underfill material  28  should be adjusted to allow facile flow of the underfill material  28  but should be left low enough to readily prevent the flow of the underfill material  28  beyond the perimeter of the flip-chip  12 . It should also be understood that the substrate  10  may be inclined by placing the substrate  10  on a support member  44 , such as a tilted table or conveyor belt, as is shown in FIG.  3  and further described below. Alternately, the substrate  10  may be inclined by placing the substrate  10  below a support member or horizontal plane  1  as described hereinbelow. 
     Underfilling is accomplished by applying the underfill material  28  under the front sidewall  30  of flip-chip  12  and allowing it to flow between the flip-chip  12  and the substrate  10  and around the bumps  24 . The underfill material  28  is applied with an underfill dispenser  34 , such as a syringe having a suitable nozzle thereon or any other suitable dispensing means known in the art. 
     As shown, since the substrate  10  having flip-chip  12  thereon is placed on an incline, in addition to any fluid pressure used to inject the underfill material and any capillary action force acting on the underfill material, a gravitational force also acts on the underfill material, causing the underfill material  28  to readily flow from front sidewall  30  toward rear sidewall  32 . Due to the additional action of the gravitational force to that of the injection pressure and capillary action, air pockets, bubbles, and voids found within the underfill material  28  are displaced by the denser underfill material  28  as it flows toward the rear sidewall  32  of flip-chip  12 . The ability to displace and the speed of displacement of the voids is dependent on the inclined angle of the substrate  10  having flip-chip  12  thereon, the viscosity of the underfill material  28 , the injection rate of the underfill material  28 , and the uniformity of the injection of the underfill material  28  into the gap between the substrate  10  and the flip-chip  12  to form a substantially uniform flow front of underfill into and through the gap. If desired, the process of underfilling the gap may be repeated by inclining the substrate  10  in the opposite direction and subsequently dispensing another amount of underfill material  28  from an opposing side of the flip-chip  12  into the gap to improve the uniformity of the underfill material  28  filling the gap. 
     After application of the underfill material  28 , the material is cured either by heat, ultraviolet light, radiation, or other suitable means in order to form a solid mass. 
     Referring now to FIG. 2, a second embodiment of an interconnected flip-chip  12  and substrate  10  is shown. As shown, a dam or barrier  40  is used on the top surface  18  of the substrate  10  to help contain the flow of the underfill from the gap at the rear sidewall  32  of the flip-chip  12 . Conventional molding equipment and techniques (e.g. pour molding, injection molding, adhesive bonding, etc.) can be used to form the dam  40  on the substrate  10 . The dam  40  is typically formed from any suitable epoxy resin material compatible with the substrate  10 . 
     The dam  40  extends upwards from and is substantially perpendicular to the top surface  18  of the substrate  10 . As shown, the dam  40  may be seen to lay substantially parallel and slightly aft the rear sidewall  32  of the flip-chip  12 . 
     The dam  40  limits the expansion or gravitational flow of the underfill material  28  beyond the position of the dam  40 . During the underfill procedure, the underfill material  28  coats and spreads out onto the surfaces of the flip-chip  12  and substrate  10 . The dam  40  prevents the spread of underfill material  28  beyond the rear sidewall  32  of the flip-chip  12  by means of surface tension. 
     Additionally, use of the dam  40  (as opposed to using no dam) permits use of lower viscosity underfill materials, if so desired, during the underfill procedure. The underfill  28  material may be easily controlled and a wider range of viscosities may be used by controlling the depth of the dam  40  and by controlling the width between the rear sidewall  32  of the flip-chip  12  and the dam  40 . Use of the dam  40  also permits tilting the substrate  10  at a greater angle of elevation with respect to the horizontal plane  1  in order to accelerate the underfill process or to permit the use of higher viscosity underfill materials should such a need arise. Furthermore, if desired, a dam  40  may be used on all three sides of the flip-chip  12  located on the substrate  10  except the side of the flip-chip  12  from which the underfill material  28  is being dispensed. 
     Referring to FIG. 3, a side view of a flip-chip  12  and substrate  10 , interconnected via bumps  24 , of a third embodiment of the invention, is shown. The substrate  10  is inclined with respect to a horizontal plane  1  by placing the substrate  10  onto a support member  44 . Support member  44  can be a tilt table, a tilted conveyor belt, or any other means of support suitable for holding the substrate  10  of the present invention. Preferably, support member  44  can be positioned and locked at various angles and can also be elevated or lowered from front to back, as well as side to side. 
     Attached to the support member  44  is a vibrator  48 . The vibrator  48  facilitates and hastens the displacement of air pockets and voids by the underfill material  28  during the previously described underfill process. The action of the vibrator  48  also permits the use of higher viscosity underfill materials and/or permits underfilling with the support member  44  positioned at a gradual slope. 
     Referring to FIG. 4, a top view of an interconnected solder-bumped  24  flip-chip  12  and substrate  10  of a fourth embodiment of the present invention is shown similar to that of the second embodiment as shown in FIG.  2 . However, this particular embodiment illustrates the use of two dams  40  and  40 ′, which are oriented transversely with respect to one another. The two dams  40  and  40 ′ lie in substantially parallel orientation with respect to two mutually perpendicular and abutting sidewalls  50  and  52  of the flip-chip  12 . 
     The method of this embodiment permits underfilling along two sidewalls  54  and  56  simultaneously. Dams  40  and  40 ′ prevent the spread and overflow of underfill material  28  beyond sidewalls  50  and  52  of the flip-chip  12 . The underfill material may be easily controlled and a wider range of viscosities may be used by controlling the depth of the dams  40  and  40 ′, by controlling the width between the sidewalls  50  and  52  of the flip-chip  12  and the dams  40  and  40 ′, and by controlling the distance between the corners  60  and  60 ′ of the dams  40  and  40 ′. 
     An alternative method comprises tilting the substrate  10  so as to elevate sidewall  54  and applying the underfill material  28  under sidewall  54  via the underfill dispenser  34 ′. The substrate  10  is then tilted so as to elevate sidewall  56  and the underfill material  28  is dispensed along sidewall  56  via underfill dispenser  34 . This alternating underfill technique can be repeated until the underfill material  28  is free of air pockets and voids. 
     Referring to FIG. 5, a cross-sectional view of an interconnected solder-bumped  24  flip-chip  12  and substrate  10  of a fifth embodiment of the present invention is shown midway through the underfill process. In this particular embodiment, the substrate  10  has a suitable shaped opening  160  situated near the center of the substrate  10  through which underfill material  28  can be applied via the underfill dispenser  34 . Additionally, dams  40  and  40 ′ located on each side of the flip-chip  12  are molded or suitably attached to top surface  18  of the substrate  10 , as described hereinbefore, being positioned to lay slightly beyond first and second sidewalls, rear sidewall  32 , and front sidewall  30 , respectively. It should also be understood that other dams  40 ′ (not shown) are located on the first and second lateral sidewalls of the flip-chip  12  to confine the underfill. 
     Referring to drawing FIG. 6, a cross-sectional view of an interconnected solder-bumped  24  flip-chip  12  and substrate  10  of a sixth embodiment of the present invention is shown midway through the underfill process. In this particular embodiment, the substrate  10  has a suitable shaped opening  160  situated near the center of the substrate  10  through which underfill material  28  can be applied via the underfill dispenser  34 . In this instance, there is no dam used to confine the underfill material  28 . Additionally, if desired, the substrate  10 , having flip-chip  12  located thereon, may be tilted in each direction to enhance the flow of the underfill material  28  in the gap  26  between the substrate  10  and the flip-chip  12  during the underfilling process. 
     Referring to drawing FIG. 7, a cross-sectional view of an interconnected solder-bumped  24  flip-chip  12  and substrate  10  of a seventh embodiment of the present invention is shown midway through the underfill process. In this particular embodiment, the substrate  10  has a suitable shaped opening  160  situated near the center of the substrate  10  through which underfill material  28  can be applied via the underfill dispenser  34 . Additionally, dams  40  and  40 ′ located on each side of the flip-chip  12  are molded or suitably attached to top surface  18  of the substrate  10 , as described hereinbefore, being positioned to lay slightly beyond first and second sidewalls, rear sidewall  32 , and front sidewall  30 , respectively. It should also be understood that other dams  40 ′ (not shown) are located on the first and second lateral sidewalls of the flip-chip  12  to confine the underfill. In this instance, the substrate  10 , having flip-chip  12  located thereon, is inverted during the underfill process so that the underfill material  28  is dispensed through the opening  160  into the gap  26  between the substrate  10  and flip-chip  12 . As in the previous embodiments, the substrate  10  is located at an angle with respect to horizontal plane  1 , although located therebelow and inclined with respect thereto. 
     In operation, the present method is initiated by elevating or inclining front end  14  of the substrate  10 . As the underfill material  28  is added, in this case by means of an opening  160  through the substrate  10 , the underfill material  28  flows towards the dam  40  and fills the lowered portion of the gap  26  between the flip-chip  12  and the substrate  10 . The front end  14  of the substrate  10  is then lowered and the rear end  16  of the substrate  10  is elevated. The backfill method is then repeated with the underfill material  28  now flowing towards the dam  40 ′ to complete the filling of the gap  26  between the flip-chip  12  and the substrate  10 . The underfill material  28  is then cured, as previously described. Alternately, the underfill material  28  may be cured after the partial filling of the gap between the substrate  10  and flip-chip  12 , then the remainder of the gap filled and subsequently cured. 
     While the present invention has been described in terms of certain methods and embodiments, it is not so limited, and those of ordinary skill in the art will readily recognize and appreciate that many additions, deletions and modifications to the embodiments described herein may be made without departing from the scope of the invention as hereinafter claimed.