Abstract:
An apparatus and method for adhesively bonding a back surface of a substrate to an active surface of at least one semiconductor die having adhesive tape interposed therebetween. A wetting agent layer is provided on at least one of the back surface of the substrate and the active surface of at least one semiconductor die. The wetting agent layer interacts with an adhesive on the adhesive tape when the substrate is heated so that the substrate is adhesively bonded to at least one semiconductor die. The interaction of the wetting agent layer allows the adhesive tape to bond thereto at a lower temperature than that of the conventional bonding methods, and more importantly, enhances adhesion thereto.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a divisional of application Ser. No. 09/591,144, filed Jun. 9, 2000, now U.S. Pat. No. 6,501,170, issued Dec. 31, 2002. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a “chip-over-board” semiconductor assembly, a type of “chip-on-board” (COB) semiconductor assembly. More specifically, the present invention relates to a method and apparatus for attaching a semiconductor device to a substrate for the subsequent wire bonding of the bond pads on the active surface of the semiconductor device to circuits of the substrate. 
     2. State of the Art 
     Definitions: 
     The following terms and acronyms will be used throughout the application and are defined as follows: 
     BGA—Ball Grid Array: 
     An array of minute solder balls disposed on an attachment surface of a semiconductor die wherein the solder balls are refluxed for simultaneous attachment and electrical communication of the semiconductor die to a printed circuit board. A BGA may also be disposed on the printed circuit board. Conductive polymer balls or bumps may also be employed. 
     COB—Chip-On-Board: 
     The techniques used to attach semiconductor die to a printed circuit board or other suitable substrate. In this instance, COB also refers to a semiconductor die attached to a printed circuit board or substrate having an aperture therein for wire bonds to extend therethrough from the bond pads on the semiconductor die to circuits of the printed circuit board or substrate. 
     Flip-Chip: 
     A semiconductor chip or semiconductor die having a pattern or array of terminations spaced around the active surface of the chip or die for face-down mounting of the chip or die to a substrate. 
     Flip-Chip Attachment: 
     A method of attaching a semiconductor die to a substrate in which the die is inverted so that the connecting conductor pads on the face of the device are set on mirror-image pads on the substrate and bonded by solder reflux or a conductive polymer curing. 
     Wire Bonding: 
     Conductive wires attached between the bond pads on the active surface of a semiconductor die and the circuits of a circuit board or lead frame to form an electrical connection therebetween. 
     TAB—Tape-Automated-Bonding. 
     Conductive traces are formed on a dielectric film such as a polyimide (the structure also being termed a “flex circuit”). The film is precisely placed to electrically connect a die and a circuit board or lead frame through the traces. Multiple connections are simultaneously effected. 
     Glob Top: 
     A glob of encapsulant material (usually epoxy or silicone or a combination thereof) surrounding a semiconductor die in a COB assembly. 
     PGA—Pin Grid Array: 
     An array of small pins extending substantially perpendicular from the major plane of a semiconductor die, wherein the pins conform to a specific arrangement on a printed circuit board or other substrate for attachment thereto. 
     SLICC—Slightly Larger than Integrated Circuit Carrier: 
     An array of minute solder balls disposed on an attachment surface of a semiconductor die similar to a BGA, but having a smaller solder ball pitch and diameter than a BGA. 
     State-of-the-Art COB Technology Generally Consists of Three Techniques for Attaching Semiconductor Die to a Substrate: 
     flip-chip attachment, wire bonding, and TAB. 
     Flip-chip attachment consists of attaching a semiconductor die, generally having a BGA, a SLICC or a PGA, to a printed circuit board. With the BGA or the SLICC, the solder ball arrangement on the semiconductor die must be aligned with the connecting bond pads on the printed circuit board such that precise connection is made. After proper alignment, the semiconductor die is bonded to the printed circuit board by reflowing the solder balls. With the PGA, the pin arrangement of the semiconductor die must be a mirror-image of the pin recesses on the printed circuit board. After insertion, the semiconductor die is generally bonded by soldering the pins into place. An underfill encapsulant is then generally disposed between the semiconductor die and the printed circuit board for environmental protection and to enhance the attachment of the die to the board. 
     Wire bonding, unlike flip-chip attachment, generally begins with attaching either the active surface or the backside of a semiconductor die to the surface of a printed circuit board with an appropriate adhesive, such as an epoxy. In wire bonding, a plurality of bond wires are attached, one at a time, to each bond pad on the semiconductor die and extend to a corresponding lead or trace end of a circuit on the printed circuit board. The bond wires are generally attached through one of three industry-standard wire bonding techniques: ultrasonic bonding—using a combination of pressure and ultrasonic vibration bursts to form a metallurgical cold weld; thermocompression bonding—using a combination of pressure and elevated temperature to form a weld; and thermosonic bonding—using a combination of pressure, elevated temperature, and ultrasonic vibration bursts. The semiconductor die may be oriented either face up or face down (with its active surface and bond pads either up or down with respect to the circuit board) for wire bonding, although face-up orientation has been more common. 
     In TAB semiconductor assemblies, ends of metal leads carried on an insulating tape, such as a polyimide, are attached to the bond pads on the semiconductor die and to corresponding lead or trace ends on the printed circuit board. An encapsulant, or plastic resin, is generally used to cover the bond wires and metal tape leads to prevent contamination, aid mechanical attachment of the assembly components, and increase long-term reliability of the electronics with reasonably low-cost materials. 
     A common manner of forming the encapsulant or plastic package about a semiconductor die assembly is molding and, more specifically, transfer molding. In this process (and with specific reference to COB die assemblies), after the semiconductor die is attached to the substrate (i.e., FR-4 printed circuit board), the semiconductor die assembly is placed in a mold cavity in a transfer molding machine. The semiconductor die assembly is thereafter encapsulated in a thermosetting polymer which, when heated, reacts irreversibly to form a highly cross-linked matrix no longer capable of being re-melted. In addition, another common manner of forming encapsulants for COB assemblages is “glob top” polymeric encapsulation. Glob top encapsulation can be applied by dispensing suitably degassed material from a reservoir through a needle-like nozzle onto the semiconductor die assembly. 
     The thermosetting polymer of transfer molding generally is comprised of three major components: an epoxy resin, a hardener (including accelerators), and a filler material. Other additives such as flame retardants, mold release agents and colorants are also employed in relatively small amounts. Furthermore, glob top encapsulation can comprise a nonlinear thixotropic material that also includes fillers to achieve the desired degree of thixotropy. 
     As previously set forth, bonding a semiconductor die to the surface of a substrate for wire bonding chip-on-board and TAB chip-on-board semiconductor assemblies is well known in the art. However, there are problems in the bonding process thereof, and specifically, problems associated with heating the adhesive between the semiconductor die and the substrate. In particular, the substrate is heated to a temperature so that the adhesive flows and then cures to thereby bond the die to the substrate with the cured adhesive therebetween. Some of the problems associated with heating the substrate at the temperature required to flow the adhesive are generally as follows: first, thermal stress exists in the interface between the adhesive and the semiconductor die and the adhesive and the substrate; and second, undesirable outgassing of a solder mask material layering a BGA on the substrate. These problems may result in health and contamination issues, and more importantly, potential detachment of the semiconductor die. 
     In U.S. Pat. No. 5,972,739, there is illustrated a semiconductor chip attached to a printed circuit board using a sheet of B-stage thermosetting resin having a filler therein, such as particles, fibers, or nonwoven fabric, wherein the active surface, or other surfaces, of the semiconductor chip is treated with a solution of a silane-coupling agent. The bumps on the active surface of the semiconductor chip extend through holes in the B-stage thermosetting resin to contact circuits on the printed circuit board. 
     In U.S. Pat. No. 5,348,607, a mixture for affixing dice to a substrate is illustrated. The mixture includes a thermoplastic polyimide, a solvent for the polyimide, and a solvent which does not dissolve the polyimide but adds thixotropicity to the mixture. The mixture is applied to the substrate, the dice are placed thereon, and the solvents are evaporated to bond the dice to the substrate. A poor solvent for the polyimide, spread over the dice and exposed portions of die attach material, causes some polyimide to precipitate out of the solution in the exposed portions of the die attach material to form a grid that extends between the dice and prevents the dice from “swimming together” during the high temperature processing. In a solvent die attachment method, the substrate is first coated with a mixture of die attach material and then the mixture is dried. Spraying a solvent over the die attach material causes the material to soften so that the dice applied thereto may adhere. The die attach material is then dried to form the bond. 
     U.S. Pat. Nos. 5,990,545 and 5,866,949 illustrates a semiconductor die in a chip-scale ball grid array having a nonpolymer layer or support structure positioned between a semiconductor die and a substrate. 
     U.S. Pat. Nos. 5,977,226 and 5,914,186 illustrate a vacuum dispensable silicone composition having low outgassing properties suitable for use in chip-scale packages and a high temperature antistatic pressure-sensitive adhesive tape. 
     In U.S. Pat. Nos. 5,994,772, 5,977,632, 5,929,521, and 5,892,277, various types of semiconductor attachments for attaching and electrically connecting a semiconductor die to a substrate are illustrated. 
     Based on the foregoing, it would be advantageous to develop a method for attaching a semiconductor die to a substrate at lower temperatures than that done conventionally, and thus, prevent the problems as discussed previously. 
     SUMMARY OF THE INVENTION 
     The present invention relates to a method and apparatus for bonding an active surface of a semiconductor die to the back surface of a substrate, the substrate having an aperture therethrough for the wire bonds between the bond pads on the active surface of the semiconductor die and the circuits on the front surface to pass through the aperture. Subsequently, the aperture is filled with an encapsulant material, as well as the portions of the wires of the wire bonds being encapsulated. The present invention is directed to a method and apparatus for bonding the active surface of the semiconductor die to the substrate using an adhesive tape therebetween. The method and apparatus includes pretreating the back surface of the substrate and/or the active surface of the semiconductor die with a wetting/adhesion promotion agent layer. The wetting/adhesion promotion agent layer is applied to at least a portion of the back surface of the substrate and/or portions of the active surface of the semiconductor die. 
     In the present invention, the wetting/adhesion promotion agent layer is directed to increase the surface tension thereof to promote adhesion of the substrate and/or semiconductor die. In addition, by increasing the surface tension, the substrate may be heated at a lower temperature in order for an adhesive on the adhesive tape to flow. In this manner, the present invention provides a decrease in thermal stress in the interface between the semiconductor die and substrate. Further, the lower temperature provides a decrease in the potential for out-gassing from the adhesive used on the tape for attaching the semiconductor die to the substrate during BGA attachment of the substrate and semiconductor die to another substrate. Accordingly, the present invention provides enhanced adhesion between a semiconductor die and a substrate than that provided conventionally. 
     In addition, the wetting/adhesion promotion agent layer in the present invention may comprise a silane-coupling agent, titanate-coupling agent, and/or a solvent. The wetting/adhesion promotion agent layer may comprise one or more layers and may be applied using a dispensing method, a brushing method, a spraying method, and/or a screen printing method. 
     In a preferred embodiment, the present invention is directed to bonding a semiconductor die to a substrate for the use of wire bonding chip-on-board semiconductor assemblies. 
     Alternately, the method and apparatus of the present invention may be directed to pretreating any type of semiconductor device, bare or packaged, and any type of substrate with a wetting agent layer to enhance bonding between multiple substrates, semiconductor devices, and/or a semiconductor die to a substrate, particularly where adhesive tape is used to secure the semiconductor device to a substrate. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     While the specification concludes with claims particularly pointing out and distinctly claiming that which is regarded as the present invention, the advantages of this invention can be more readily ascertained from the following description of the invention when read in conjunction with the accompanying drawings in which: 
     FIG. 1 is a cross-sectional side view of an intermediate semiconductor assembly of the present invention; 
     FIG. 2 is a cross-sectional side view of inset  8  in FIG. 1, illustrating a wetting agent layer with adhesive tape therebetween, according to the present invention; 
     FIG. 3 is a cross-sectional side view of an intermediate semiconductor assembly, illustrating bond wire connections, according to the present invention; 
     FIG. 4 is a cross-sectional side view of an intermediate semiconductor assembly, illustrating a glob top encapsulation, according to the present invention; 
     FIG. 5 is a cross-sectional side view of an intermediate semiconductor assembly, shielded by an encapsulation material, according to the present invention; and 
     FIG. 6 is a cross-sectional side view of an intermediate semiconductor assembly, illustrating TAB connections, according to the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to drawing FIG. 1, there is illustrated an intermediate semiconductor die assembly  100  according to the present invention, wherein a semiconductor die  102 , such as those known in the art, extends over and is attached to a semiconductor substrate  104  having an opening  106  defined therein. The semiconductor substrate  104  may comprise a BT (Bismaleimide Triazine) substrate, FR-4 printed circuit board, a ceramic substrate or other known substrates including multiple layered substrates. In addition, the present invention is not intended to be limited to the use of one substrate per die assembly. 
     For purposes of illustration, the semiconductor die  102  may comprise memory devices, such as dynamic random access memory (DRAM) and static random access memory (SRAM), and any other type of semiconductor die. 
     The adhesive tape  108  is positioned between the semiconductor die  102  and the semiconductor substrate  104  on opposing sides of the semiconductor substrate opening  106 , thereby attaching an active surface  112  of the semiconductor die  102  to a back surface  114  of the substrate  104 . The adhesive tape  108  includes a carrier film  116 , preferably a planar dielectric or insulative carrier film  116  (See FIG.  2 ), such as UPILEX® (UBE Industries, LTD., Ube City, Japan), KAPTON® (E.I. du Pont de Nemours and Co., Midland, Mich., USA), or other such films. The adhesive tape  108  includes a first adhesive layer  118  on a first planar surface  122  of the carrier film  116  and a second adhesive layer  124  on a second planar surface  126  of the carrier film  116 , as shown in drawing FIG.  2 . 
     According to the present invention, the first and second adhesive layers  118  and  124  are preferably an adhesive known as DF-400 made by Hitachi Chemical Co. However, it should be noted that the present invention is not limited to the previously mentioned adhesive and that any suitable adhesive may be used to attach the semiconductor die to the substrate. Further, it should be noted that the first and second adhesive layers  118 ,  124  may be different types of adhesives or the same type thereof. 
     In the present invention, prior to interposing the adhesive tape  108  between the semiconductor die  102  and substrate  104 , a wetting agent layer  4  is formed on the back surface  114  of the substrate  104  and/or the active surface  112  of the semiconductor die  102 . The wetting agent layer  4  can be formed thereon by any suitable method, such as spray methods, brush methods, dispense methods, and/or screen printing methods, although spraying the wetting agent layer  4  is the preferable method in order to provide a substantial uniform layer thereon. The wetting agent layer  4  is most preferably formed as a monolayer thickness, but may be formed as one or more multiple layers or formed in addition to other layers promoting a wetting effect on the surface of either the back surface  114  of the substrate  104 , the active surface  112  of the semiconductor die  102 , or both. The wetting agent layer  4  may be provided to the active surface  112  of the semiconductor die  102  while in its wafer form prior to or after burn-in testing, or after the wafer has been diced into multiple individual dice or an individual die. As to the substrate  104 , the wetting agent layer  4  may be provided thereon at any stage prior to the semiconductor die  102  being attached thereto. In addition, the wetting agent layer  4  may be comprised of any silane-coupling agent, titanate-coupling agent, solvent, and/or any material promoting a wetting effect, so long as there is a sufficient wetting effect on the back surface of the substrate and/or the active surface of the semiconductor die. However, the wetting agent layer  4  utilizing a silane-coupling agent is preferable. 
     Referring back to drawing FIG. 1, the adhesive tape  108  (shown generally as a width) preferably extends proximate an edge  128  of the semiconductor die  102  and proximate an edge  132  of the substrate opening  106 , but may also be positioned to extend past edges  128  and  132  so that the adhesive tape  108  may be narrower or longer in length than that illustrated in drawing FIG.  1 . In the preferred configuration, the adhesive tape  108  maximizes the contact area between the semiconductor die  102  and the substrate  104 . The increased contact area assists in preventing the semiconductor die  102  from flexing, twisting, or bending away from the substrate  104 , thus reducing or eliminating localized stress failures occurring during subsequent molding processes. 
     In addition, according to the present invention, the adhesive tape  108  is positioned so that it abuts with the wetting agent layer  4  formed on the back surface  114  of the substrate  104  and/or the active surface  112  of the semiconductor die  102  as shown in drawing FIG.  2 . It should be noted that drawing FIG. 2 illustrates the wetting agent layer  4  as being provided on both the back surface  114  of the substrate  104  and the active surface  112  of the semiconductor die  102 . However, the wetting agent layer  4  may be provided only on the back surface  114  of the substrate  104  or the active surface  112  of the semiconductor die  102 . 
     The process of adhesively attaching the semiconductor die  102  to the substrate  104  with the adhesive tape therebetween is accomplished by heating the substrate  104 . This allows the first and second adhesive layers  118  and  124  on the adhesive tape  108  to flow or melt. Next, the heat is eliminated or the temperature is lowered to allow the adhesive layers to cure, thereby adhesively bonding the semiconductor die  102  to the substrate  104 . According to the present invention, it has been found that by providing the wetting agent layer  4  to the surface or surfaces as previously described, the wetting agent layer  4  increases a surface tension thereon. The increased surface tension thereby enhances the adhesion between the substrate  104  and semiconductor die  102 . In addition, the adhesive layers abutting the wetting agent layer  4  with an increased surface tension will flow at a lower temperature. 
     Thus, it can be well appreciated that by lowering the temperature necessary to adhesively bond the semiconductor die  102  to the substrate  104 , the problems associated with having to heat the semiconductor assembly at a higher temperature will be prevented. Specifically, there will be a reduction of thermal stress between the semiconductor die  102  and substrate  104  and further, a reduction of outgassing from the adhesive on the adhesive tape  108  during the reflow soldering of BGA on the substrate  104  to attach the substrate  104  to another substrate. 
     More importantly, it can be well appreciated that by increasing the surface tension with the wetting agent layer  4  on the back surface  114  of the substrate  104  and/or the active surface  112  of the semiconductor die  102  with the adhesive tape  108  therebetween, adhesion thereto is enhanced. As a result, the present invention provides enhanced adhesion, further resulting in a more efficient and cost-effective method for adhesively bonding a semiconductor die  102  to a substrate  104 . 
     In one embodiment, the semiconductor die  102  is electrically connected to the substrate  104  via the wire bonding chip-on-board technique as shown in drawing FIG.  3 . As such, the active surface  112  of the semiconductor die  102  is aligned so that at least one bond pad  134  is aligned with the substrate opening  106 . Electrical connections  136  in the form of bond wires are then attached between the semiconductor die bond pads  134  and traces  138  (which are in electrical communication with electrical components either internal or external to the substrate  104 ) on an active surface  142  of the substrate  104  through the substrate opening  106 . 
     The substrate opening  106  is filled and the electrical connections  136  are covered with a glob top material  144  injected into the opening  106 , as shown in drawing FIG.  4 . Thus, the electrical connections  136  are protected from bond wire sweep and connection detachment by the glob top material  144 . 
     As shown in drawing FIG. 5, an encapsulant material  146  is molded over the semiconductor die  102 . It is, of course, understood that the encapsulant material  146  could be a glob top material applied over the semiconductor die  102  and that the encapsulant material could also be molded to encase the glob top material  144 . Again, the adhesive tape  108  extending proximate the semiconductor die edge  128  substantially prevents encapsulant material  146  from residing between semiconductor die active surface  112  and the substrate back surface  114 , thereby eliminating problems associated with filler particles used in the encapsulant material  146 . 
     In a second embodiment, the semiconductor die  102  is electrically connected to the substrate  104  via the tape-automated-bonding (TAB) chip-on-board technique as shown in drawing FIG.  6 . As in the previously discussed embodiment, the active surface  112  of the semiconductor die  102  is aligned so that at least one bond pad  134  is aligned with the semiconductor substrate opening  106 . Electrical connections  136  (shown as TAB) are then attached between the semiconductor die bond pads  134  and traces  138  (which are in electrical communication with electrical components either internal or external to the substrate  104 ) on an active surface  142  of the semiconductor substrate  104  through the substrate opening  106 . 
     Having thus described in detail preferred embodiments of the present invention, it is to be understood that the invention defined by the appended claims is not to be limited by particular details set forth in the above description, as many apparent variations thereof are possible without departing from the spirit or scope thereof. For example, the use of a wetting agent may be used to promote adhesion between any type of substrate and any type of semiconductor device, whether a bare die type device or packaged semiconductor device. Further, the use of a wetting agent may be used to promote adhesion between multiple substrates and/or multiple semiconductor devices.