Patent Abstract:
A method of forming high definition elements for electrical and electronic devices, substrates, and other components from or including viscous material. The method includes inverting the electrical components after the viscous material is applied and maintaining the inverted orientation until the viscous material dries or cures enough to maintain definition of its perimeter and edge characteristics.

Full Description:
CROSS-REFERENCE TO RELATED APPLICATION  
       [0001]    This application is a divisional of application Ser. No. 09/295,709, filed Apr. 21, 1999, pending, which is a divisional of application Ser. No. 08/709,182, filed Sep. 6, 1996, now U.S. Pat. No. 6,083,768, issued Jul. 4, 2000.  
     
    
     
       BACKGROUND OF THE INVENTION  
         [0002]    1. Field of the Invention  
           [0003]    The present invention relates to maintaining the structure of viscous materials applied to semiconductor components. More particularly, the present invention relates to inverting electrical components formed from viscous materials or which include viscous materials in order to maintain the material boundary definition during baking, curing, and/or drying.  
           [0004]    2. State of the Art  
           [0005]    Higher performance, lower cost, increased miniaturization of components, and greater packaging density of integrated circuits are goals of the computer industry. As components become smaller and smaller, tolerances for all semiconductor structures (circuitry traces, printed circuit board and flip chip bumps, adhesive structures for lead attachment, encapsulation structures, and the like) become more and more stringent. However, because of the characteristics of the materials (generally viscous materials) used in forming the semiconductor structures, it is becoming difficult to form smaller circuitry traces, conductive polymer bumps with closer pitches, adequate adhesive structures for leads attachment, and adequate encapsulation structures.  
           [0006]    U.S. Pat. No. 5,286,679 issued Feb. 15, 1994 to Farnworth et al. (“the &#39;679 patent”), assigned to the assignee of the present invention and hereby incorporated herein by reference, teaches attaching leads to a semiconductor device with adhesive in a “lead-over-chip” (“LOC”) configuration. The &#39;679 patent teaches applying a patterned thermoplastic or thermoset adhesive layer to a semiconductor wafer. The adhesive layer is patterned to keep the “streets” on the semiconductor wafer clear of adhesive for saw cutting and to keep the wire bonding pads on the individual dice clear of adhesive for wire bonding. Patterning of the adhesive layer is generally accomplished by hot or cold screen/stencil printing or dispensing by roll-on. Following the printing and baking of the adhesive layer on the semiconductor wafer, the individual dice are singulated from the semiconductor wafer. During packaging, each adhesive coated die is attached to leadfingers of a lead frame by heating the adhesive layer and pressing the leadfingers onto the adhesive. If the adhesive layer is formed of a thermoset material, a separate oven cure is required. Furthermore, the adhesive layer may be formulated to function as an additional passivating/insulating layer or alpha barrier for protecting the packaged die.  
           [0007]    Although the teaching of the &#39;679 patent is a substantial advancement over previous methods for attaching leads in a LOC configuration, the miniaturization of the circuitry makes it difficult to achieve an adequate profile on the adhesive, such that there is sufficient area on the top of the adhesive to attach the leadfingers. The process disclosed in the &#39;679 patent is illustrated in FIGS.  23 - 29 . FIG. 23 illustrates a side, cross-sectional view of a semiconductor substrate  602  with a bond pad  604 , wherein a stencil or a screen print template  606  has been placed over the semiconductor substrate  602 . The semiconductor substrate  602  is generally a wafer, although the term as used herein is not so restricted, and other substrate structures including silicon-on-insulator (“SOI”) and printed circuit boards (“PCB”) are specifically included. The stencil or screen print template  606  is patterned to clear the area around the bond pads  604  and to clear street areas  608  for saw cutting (i.e., for singulating the substrate into individual dice). An adhesive material  610  is applied to the stencil or screen print template  606 , as shown in FIG. 24. Ideally, when the stencil or screen print template  606  is removed, adhesive prints  612  are formed with vertical sidewalls  614  and an adhesive material upper surface  616 , as shown in FIG. 25. However, since the adhesive material  610  must have sufficiently low viscosity to flow and fill the stencil or screen print template  606 , as well as allow for the removal of the stencil or screen print template  606  without the adhesive material  610  sticking thereto, the adhesive material  610  of the adhesive prints  612  will spread, sag, or flow laterally under the force of gravity after the removal of the stencil or screen print template  606 , as shown in FIG. 26. This post-application flow of adhesive material  610  can potentially cover all or a portion of the bond pads  604  or interfere with the singulating of the semiconductor wafer by flowing into the street areas  608 .  
           [0008]    Furthermore, and of even greater potential consequence than bond pad or street interference is the effect that the lateral flow or spread of adhesive material  610  has on the adhesive material upper surface  616 . As shown in FIG. 27, the adhesive material upper surface  616  is the contact area for leadfingers  618  of a lead frame  620 . The gravity-induced flow of the adhesive material  610  causes the once relatively well-defined edges  622  of the adhesive material to curve, resulting in a loss of surface area  624  (ideal shape shown in shadow) for the leadfingers  618  to attach. This loss of surface area  624  is particularly problematical for the adhesive material upper surface  616  at the adhesive material end  626  thereof. At the adhesive material end  626 , the adhesive material flows in three directions (to both sides as well as longitudinally), causing a severe curvature  628 , as shown in FIGS. 28 and 29. Stated are three ways the longitudinal ends of the adhesive print on patch flow in a 180E° flow front, resulting in blurring of the print boundaries into a curved perimeter. This curvature  628  results in complete or near complete loss of effective surface area on the adhesive material upper surface  616  for adhering the outermost leadfinger closest to the adhesive material end  626  (leadfinger  630 ). This results in what is known as a “dangling lead.” Since the leadfinger  630  is not adequately attached to the adhesive material end  626 , the leadfinger  630  will move or bounce when a wirebonding apparatus (not shown) attempts to attach a bond wire (not shown) between the leadfinger  630  and its respective bond pad  604  (shown from the side in FIG. 28). This movement can cause inadequate bonding or non-bonding between the bond wire and the leadfinger  630 , resulting in the failure of the component due to a defective electrical connection.  
           [0009]    LOC attachment can also be achieved by placing adhesive material on the leadfingers of the lead frame rather than on the semiconductor substrate. The adhesive material  702  is generally spray applied on an attachment surface  704  of leadfingers  706 , as shown in FIG. 30. However, the viscous nature of the adhesive material  702  results in the adhesive material  702  flowing down the sides  708  of the leadfinger  706  and collecting on the reverse, bond wire surface  710  of the leadfinger  706 , as shown in FIG. 31. The adhesive material  702 , which collects and cures on the bond wire surface  710 , interferes with subsequent wirebonding which can result in a failure of the semiconductor component. The flow of adhesive material  702  from the attachment surface  704  to the bond wire surface  710  can be exacerbated if the leadfingers  706  are formed by a stamping process, rather than by etching, the other widely employed alternative. The stamping process leaves a slight curvature  712  to edges  714  of at least one surface of the leadfinger  706 , as shown in FIG. 32. If an edge curvature  712  is proximate the leadfinger attachment surface  704 , the edge curvature  712  results in less resistance (i.e., less surface tension) to the flow of the adhesive material  702 . This, of course, results in the potential for a greater amount of adhesive material  702  to flow to the bond wire surface  710 .  
           [0010]    Material flow problems also exist in application of encapsulation materials. After a semiconductor device is attached to a printed circuit board (“PCB”) by any known chipon-board (“COB”) technique, the semiconductor device is usually encapsulated with a viscous liquid or gel insulative material (e.g., silicones, polyimides, epoxies, plastic, and the like). This encapsulation (depending on its formulation) allows the semiconductor device to better withstand exposure to a wide variety of environmental conditions such as moisture, ions, heat and abrasion.  
           [0011]    One technique used in the industry is illustrated in FIGS.  33 - 35 . A stencil  802  is placed on a conductor-carrying substrate or PCB  804  such that an open area or stencil cavity  806  in the stencil  802  exposes a semiconductor device  808  to be encapsulated and a portion of the substrate or PCB  804  surrounding the semiconductor device  808 , as shown in FIG. 33. An encapsulant material  810  is then extruded from a nozzle  812  into the stencil cavity  806 , as shown in FIG. 34. However, when the stencil  802  is removed, the encapsulant material  810  sags or flows laterally under the force of gravity, as shown in FIG. 35. This flowing thins the encapsulant material  810  on the top surface  814  of the semiconductor device  808 , which may result in inadequate protection for the semiconductor device  808 . Using a thicker encapsulant material would help minimize the amount of flow; however, thicker encapsulant materials are difficult to extrude through a nozzle and are subject to the formation of voids/air pockets. These voids/air pockets can cause delamination from the PCB  804  or the semiconductor device  808 , and if the voids/air pockets contain water condensation, during subsequent processing steps the encapsulant material can be heated to the point at which the condensed water vaporizes, causing what is known as a “popcorn effect” (i.e., a small explosion) which damages (i.e., cracks) the encapsulation material, resulting in at least contamination and usually irreparable damage, effectively destroying the semiconductor device. Furthermore, using encapsulant materials with high thixotropic indexes may result in a concave shape which thins the encapsulant material  810  on the top surface  814  of the semiconductor device  808 , which may result in inadequate protection for the semiconductor device  808 , as shown in FIG. 36.  
           [0012]    In an effort to cope with the encapsulant flow problem, the damming technique shown in FIGS.  37 - 40  has been used. A high viscosity material  902  is extruded through a nozzle  904  directly onto a substrate or PCB  906  to form a dam  908  around a semiconductor device  910 , as shown in FIG. 37, or a stencil  912  can be placed on the substrate and PCB  906 , such that a continuous aperture  914  in the stencil  912  exposes an area around the semiconductor device  910  to be dammed, as shown in FIG. 38. The high viscosity material  902  is then disposed in the stencil aperture  914  to form the dam  908 . A low viscosity encapsulation material  916  is then extruded into the area bounded by the dam  908  by a second nozzle  918 , as shown in FIG. 39. The dam  908  prevents the low viscosity encapsulation material  916  from flowing, to form the dammed encapsulated structure  920  shown in FIG. 40 after curing. The dam  908  can be made with high viscosity material without adverse consequences since it does not directly contact the semiconductor device  910  or form any part, other than a damming function, of the encapsulation of the semiconductor device  910 . Although this damming technique is an effective means of containing the low viscosity encapsulation material  916 , it requires additional processing steps and additional equipment, which increase the cost of the component.  
           [0013]    Material flow problems further exist in forming conductive line and trace materials. As discussed in Liang et al., “Effect of Surface Energies on Screen Printing Resolution,” IEEE Transactions on Components, Packaging, and Manufacturing Technology Part B, Vol. 19, No. 2, May 1996 (“the Liang article”), miniaturization of semiconductor packages results in increased circuit densities which require a proportionate reduction of the width of printed lines and traces on semiconductor substrates. However, there are two conflicting requirements for the conductive material applied in screen printing the printed lines and traces. The first requirement is that the conductive material should have sufficiently low viscosity to remove mesh marks and surface imperfections induced during the printing process. The conflicting requirement is that the conductive material should be sufficiently high in viscosity such that it does not flow excessively (i.e., spread). If the conductive material spreads, parallel lines could contact one another, resulting in a short. The Liang article investigates the influences of surface energies of the substrates and the conductive material on screen printing resolution. The conclusion of the Liang article is to use substrates with low surface energies, such as polymer-based substrates, to decrease the wettability of the conductive material to improve screen printing resolution. However, this approach limits the flexibility of using different substrate material for applications demanding different performance parameters. Furthermore, using polymer-based substrates may not be acceptable in certain applications such as high surface energy ceramic substrate.  
           [0014]    Material flow problems further exist in forming conductive bumps on printed circuit boards and flip chips. Solder bumps, also termed “C4” bumps, for Controlled Collapse Chip Connection, are a conventional means for attaching and forming an electrical communication between a flip chip and a substrate or PCB, wherein the solder bumps are formed on the flip chip as a mirror-image of the connecting bond pads on the PCB, or vice versa. The flip chip is bonded to the PCB by reflowing the solder bumps.  
           [0015]    State-of-the-art solder bumps are generally made of multiple layers of various metals or metal alloys (e.g., lead, tin, copper), which will achieve an effective, strong and controlled-boundary bond between the substrate/PCB and the flip chip. However, the formation of these layered solder bumps requires a substantial number of processing steps which increase the cost of the component. Furthermore, the solder bumps require a high temperature to reflow during the attachment of the flip chip to the substrate/PCB, which may damage temperature-sensitive components on the semiconductor device. Thus, solder bumps are being replaced by conductive polymer bumps.  
           [0016]    As shown in FIG. 43, conductive polymer bumps  1002  are formed on bond pads  1004  on a semiconductor device substrate  1006 . Alternatively, the bumps may be applied to a carrier substrate, such as a PCB. The bond pads  1004  are in electrical communication with circuitry (not shown) on or in the semiconductor substrate  1006  via electrical traces  1008  (shown in shadow) in or on the semiconductor substrate  1006 . The conductive polymer bumps  1002  are generally formed either by screen printing or stenciling. As shown in FIG. 41, a print screen or stencil  1010  is placed over the semiconductor substrate  1006  with openings  1012  over and aligned with each bond pad  1004 . A conductive polymer  1007  is deposited in the openings  1012 , as shown in FIG. 42. The print screen or stencil  1010  is then removed to form the conductive polymer bumps  1002 , as shown in FIG. 43. The conductive polymer bumps  1002  are generally made from material which is sufficiently viscous that minimal material flow occurs when the print screen or stencil  1010  is removed. However, this self-minimization of flow is only applicable to specific limited ratios of height to width of the conductive polymer bumps  1002 . If the height of the conductive polymer bump  1002  is too great relative to the width, the weight of the conductive material will cause the conductive polymer bump  1002  to collapse on itself and flow laterally. Thus, height-to-width ratios approaching the preferred target of 3:1 or greater obtainable with solder bumps are unattainable with present methods. In short, to attain a satisfactory height of the conductive polymer bump  1002 , the width of the conductive polymer bump  1002  must be increased proportionately. However, when the conductive polymer bump  1002  width is increased, for a given minimum pitch in spacing between adjacent conductive polymer bumps  1002 , bond pad pitch also increases, which takes up more space on the semiconductor substrate  1006 , limiting the number and arrangement of the die-to-carrier substrate connections. This is, of course, in conflict with the goal of miniaturizing semiconductor devices of ever-increasing circuit density.  
           [0017]    Thus, it can be appreciated that it would be advantageous to develop a technique to control viscous material flow in the formation of semiconductor components while using commercially-available, widely-practiced semiconductor device fabrication techniques.  
         BRIEF SUMMARY OF THE INVENTION  
         [0018]    The present invention relates to a method for maintaining viscous material boundary definition by inverting electrical components formed from viscous materials or which include viscous materials during drying or curing.  
           [0019]    The present invention comprises using standard techniques for applying viscous materials (e.g., spin on, spray on, roll on, screen printed, and the like) which form semiconductor device elements, such as circuitry traces, printed circuit board and flip chip bumps, adhesive structures for lead attachment, encapsulation structures, and the like. After application of the viscous materials on a semiconductor or carrier structure, the entire structure is flipped to an inverted position, followed by ambient or elevated temperature drying or curing. Rather than gravitational forces causing the viscous material to flow and expand as when upright and supported from below, the gravitational forces on the inverted semiconductor or carrier structure maintain the shape and boundary definition of the original viscous material formation. It has been found that inverting the semiconductor results in a substantial improvement for wall angles and improvement in the shape and boundary definition of the elements made from the viscous materials.  
           [0020]    As a general matter, the entire structure is inverted immediately or as quickly as practical after the application of the viscous material to prevent any substantial spreading of the viscous material. This immediate inversion maximizes the benefit of the present invention by preserving the shape and boundary definition of the viscous material as applied. It is, of course, understood that the viscous material must be capable of adhering to the semiconductor or carrier structure and must not be of such a low viscous that it drips when inverted.  
           [0021]    Furthermore, with regard to drying or curing, the structure need only be inverted until the viscous material has stabilized sufficiently to maintain its shape and boundary definition. Depending on the particular viscous material used, the minimum inversion time could be the time required to cure the outer surfaces of the viscous material such that a film is formed which contains the viscous material therein, or the minimum inversion time could be the time required to completely dry or cure the viscous material element.  
       
    
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS  
       [0022]    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:  
         [0023]    FIGS.  1 - 5  are a top plan and side cross sectional views of adhesive prints formed by the method of the present invention;  
         [0024]    FIGS.  6 - 8  are schematic and graphical representations of experimental results comparing the lateral edges of an adhesive print formed by a prior art method and the method of the present invention;  
         [0025]    FIGS.  9 - 11  are schematic and graphical representations of experimental results comparing the trailing edge of an adhesive print formed by a prior art method and the method of the present invention;  
         [0026]    FIGS.  12 - 14  are schematic and graphical representations of experimental results comparing the leading edge of an adhesive print formed by a prior art method and the method of the present invention;  
         [0027]    FIGS.  15 - 17  are cross-sectional views of a prior art wirebonded LOC semiconductor assembly;  
         [0028]    [0028]FIG. 18 is a cross-sectional view of an encapsulated semiconductor device formed by the inversion method of the present invention;  
         [0029]    FIGS.  19 - 21  are oblique views of the formation of traces on a semiconductor substrate by the method of the present invention;  
         [0030]    [0030]FIG. 22 is a side cross-sectional view of a conductive polymer bump formed by the method of the present invention;  
         [0031]    FIGS.  23 - 29  are side cross-sectional views of a technique of forming adhesive areas on a substrate for LOC attachment;  
         [0032]    FIGS.  30 - 32  are side cross-sectional views of a technique of forming adhesive areas on leadfingers for LOC attachment;  
         [0033]    FIGS.  33 - 35  are side cross-sectional views of a technique of forming an encapsulant layer on a semiconductor device;  
         [0034]    [0034]FIG. 36 is a side cross-sectional view of an encapsulated semiconductor device with a concave shaped cured encapsulant;  
         [0035]    FIGS.  37 - 40  are oblique views of techniques of forming an encapsulant layer on a semiconductor device using high viscosity material dams; and  
         [0036]    FIGS.  41 - 43  are side cross-sectional views of a technique of forming conductive polymer bumps on a substrate.  
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0037]    FIGS.  1 - 5  illustrate forming a rectangular adhesive print  102  on a semiconductor substrate  104 . FIG. 1 shows several rectangular adhesive prints  102  uniformly distributed on the semiconductor substrate  104 , such as a silicon wafer or SOI substrate. The spaces between the rectangular adhesive prints  102  can have a plurality of bond pads  108  disposed between a pair of rectangular adhesive prints  102 . The spaces may also be void of any circuitry or structures to form vertical streets  110  and horizontal streets  112  along which a cutting saw proceeds to sever or singulate the semiconductor substrate  104  into individual semiconductor dice.  
         [0038]    The rectangular adhesive prints  102  are generally formed in the manner discussed above for the &#39;679 patent illustrated in FIG. 23- 28 . Referring to FIG. 24, when the adhesive material  610 , such as thermoplastic adhesive materials including polyimides and thermosetting adhesive materials including phenolic resins, is applied to the stencil or screen print template  606 , an adhesive material dispensing means, such as a spray nozzle, moves across the stencil or screen print template  606 . Thus, as shown in FIG. 2, the adhesive material dispensing means moves in direction  114  forming the adhesive print  102  with two lateral edges  116  parallel with direction  114 , and a trailing edge  118  and a leading edge  120  which are perpendicular with respect to direction  114 .  
         [0039]    As shown in FIG. 3, when the stencil or screen print template (shown in FIG. 24) is removed, the adhesive prints  102  are ideally formed with vertical sidewalls  122  and a planar upper surface  124 . However, as previously discussed, the material forming the adhesive prints  102  must have sufficiently low viscosity to flow and fill the stencil or screen print template, as well as to allow for the removal of the stencil or screen print template without the material forming the adhesive print  102  sticking to the stencil or screen print template and thus being lifted off the semiconductor substrate  104 . Thus, the adhesive print  102  will flow laterally under the force of gravity after the removal of the stencil or screen print template, as shown in FIG. 4. This flow of the adhesive print  102  can potentially cover a portion of the bond pads  108  or interfere with the singulating of the semiconductor wafer by flowing into the street areas  110 ,  112 . This results in shortening street width W and decreasing gravity-reduced wall angle (α G ), which eventually creates problems with dicing the wafer, inference with bond pads, and dangled leadfingers (due to loss of surface area on a leadfinger attachment surface  128  on the adhesive print  102 ), as previously discussed.  
         [0040]    The present invention inverts the semiconductor substrate  104  shortly after removal of the stencil or screen print template, as shown in FIG. 5. The inversion of the semiconductor substrate  104  results in gravitational force assisting in containing the flow and expansion of the adhesive prints  102  during drying or curing. The inversion of the semiconductor substrate  104  results in higher, inversion-contained wall angles (α I ) (also known as the “angle of repose”), wider street width W, and a greater surface area on the leadfinger attachment surface  128 .  
         [0041]    Experimental results have demonstrated that angles of the leading edge, trailing edge and lateral edges of printed adhesives were increased and the top surface area was also increased. FIGS.  6 - 8  illustrate the profile of the lateral edges  116 . FIG. 6 illustrates the scan direction across two adjacent adhesive prints, a first adhesive print  130  and a second adhesive print  132 . The scan  134  for the profiles shown in FIGS. 7 and 8 starts near lateral edge  136  of the first adhesive print  130 , extends across the gap  138  between the first adhesive print  130  and the second adhesive print  132 , and ends after a lateral edge  140  of the second adhesive print  132 . It is noted that the z-axis (height) scales of FIGS. 7 and 8 have been expanded in a twenty (20) to one (1) ratio from the x-axis (scan length) scales to better show the details of the profiles. FIG. 7 shows a profile of the scan  134  of the first adhesive print  130  and the second adhesive print  132  formed by a conventional non-inversion method. FIG. 8 shows a profile of the scan  134  of the first adhesive print  130  and the second adhesive print  132  which were formed by the inversion method of the present invention. FIGS. 7 and 8 show that the lateral edge angles of repose have increased from α G  of 18.4 degrees (lateral edge  136 ) and 18.0 degrees (lateral edge  140 ) for the non-inversion method to α I  of 22 degrees (lateral edge  136 ) and 20.6 degrees (lateral edge  140 ) for the inversion method of the present invention.  
         [0042]    FIGS.  9 - 11  illustrate the profile of the trailing edge  118 . FIG. 9 illustrates the scan direction across the adhesive print  102 . The scan  142  for the profiles shown in FIGS. 10 and 11 starts prior to the trailing edge  118  of the adhesive print  102  and ends on the leadfinger attachment surface  128  of the adhesive print  102 . It is noted that the z-axis (height) scales of FIGS. 10 and 11 have been expanded in a ten (10) to one (1) ratio from the x-axis (scan length) scales to better show the details of the profiles. FIG. 10 shows a profile of the scan  142  of the trailing edge  118  formed by a conventional non-inversion method. FIG. 11 shows a profile of the scan  142  of the trailing edge  118  formed by the inversion method of the present invention. FIGS. 10 and 11 show that the trailing edge angle of repose has increased from α G  of 9.0 degrees for the non-inversion method to α G  of 13.5 degrees for the inversion method of the present invention.  
         [0043]    FIGS.  12 - 14  illustrate the profile of the leading edge  120 . FIG. 12 illustrates the scan direction across the adhesive print  102 . The scan  144  for the profiles shown in FIGS. 13 and 14 starts on the leadfinger attachment surface  128  of the adhesive print  102  and ends past the leading edge  120  of the adhesive print  102 . It is noted that the z-axis (height) scales of FIGS. 13 and 14 have been expanded in a ten (10) to one (1) ratio from the x-axis (scan length) scales to better show the details of the profiles. FIG. 13 shows a profile of the scan  144  of the leading edge  120  formed by a conventional non-inversion method. FIG. 14 shows a profile of the scan  144  of the leading edge  120  formed by the inversion method of the present invention. FIGS. 13 and 14 show that the leading edge angle of repose has increased from 15.9 degrees for the non-inversion method to 22.6 degrees for the inversion method of the present invention.  
         [0044]    From these scans it was also determined that the level surface length within the adhesive print between the lateral edges  116  increased 2 to 4 mils. Although the angles and definition increases from these scans are specifically for Ablestick® XR-41395-10 with a viscosity of 40,000 cps, thixotropic index of 3.6, and a baking profile of 30 minutes at 125° C., 30 minutes at 200° C., and 30 minutes ramping from 200° C. to 245° C., comparable results have been achieved for OxyChem® 2421-A6-sp 7495-128B with a viscosity of 46,000 cps, thixotropic index of 1.35, and a baking profile of 60 minutes at 120° C. and 180 minutes at 190° C. Thus, the graphs shown in FIGS.  6 - 14  illustrate the general improvement trend which will be achieved through the use of the present invention.  
         [0045]    As shown in FIGS.  15 - 17 , adhesive coated leadfingers for LOC attachment can be formed by the inversion method of the present invention. An adhesive material  202  is applied, generally by spray application, on an attachment surface  204  of a leadfinger  206 , as shown in FIG. 15. After application of the adhesive material  202 , the leadfinger  206  is inverted, as shown in FIG. 16. By inverting the leadfinger  206 , the adhesive material  202  will not flow down the sides  208  of the leadfinger  206  and, of course, will not collect on the bond wire surface  210  of the leadfinger  206 , as shown in FIG. 17. Since the adhesive material  202  does not collect on the bond wire surface  210 , there will be no adhesive material  202  to interfere with the wirebonding step subsequent to LOC attachment of the active surface of the die to the leads.  
         [0046]    [0046]FIG. 18 illustrates an encapsulated semiconductor device  302  made by the inversion method of the present invention. As discussed above and illustrated in FIGS.  33 - 36 , a stencil  802  is placed on a conductive-carrying substrate, such as a PCB  804 , such that a cavity  806  in the stencil  802  exposes a semiconductor device  808  to be encapsulated and a portion of the substrate or PCB  804  surrounding the semiconductor device  808 , as shown in FIG. 33. An encapsulant material  810 , such as silicone, polyimide, urethane, acrylic, epoxy, plastic, and the like, is then extruded from a nozzle  812  into the stencil open area  806 , as shown in FIG. 34. When the stencil  802  is removed, the substrate or PCB  804  is inverted to prevent the encapsulant material  810  from spreading or flowing laterally under the force of gravity. By preventing the flow of the encapsulant material  810 , the encapsulant material  810  on the top surface  814  of the semiconductor device  808  remains thick enough to provide adequate protection for the semiconductor device  808 .  
         [0047]    FIGS.  19 - 21  illustrate the formation of traces on a semiconductor substrate by the method of the present invention. A stencil or print screen  402  with an appropriate trace design is placed over a semiconductor substrate  404 , as shown in FIG. 19. A conductive material  406  is applied to the stencil or print screen  402 , as shown in FIG. 20. The stencil or print screen  402  is then removed leaving conductive traces  408 , and the semiconductor substrate  404  is inverted during the drying or curing of the conductive traces  408 , as shown in FIG. 21. Since the conductive material  406  is prevented from flowing laterally by the inversion of the semiconductor substrate  404 , the distance between parallel conductive traces  408  can be reduced, resulting in a reduction of the size of the semiconductor substrate.  
         [0048]    [0048]FIG. 22 illustrates conductive polymer bumps  502  formed by the method of the present invention. As previously discussed and illustrated in FIGS.  41 - 43 , the conductive polymer bumps  1002  are generally formed on bond pads  1004  on the surface of a semiconductor substrate  1006 . The bond pads  1004  are in electrical communication with integrated circuitry (not shown) on or in the semiconductor substrate  1006  via electrical traces  1008  in or on the semiconductor substrate  1006 . As shown in FIG. 41, a print screen or stencil  1010  is placed over the semiconductor substrate  1006  with openings  1012  over each bond pad  1004 . The conductive polymer  1007  is deposited in the openings  1012 , as shown in FIG. 42. The print screen or stencil  1010  is removed and the semiconductor substrate  1006  inverted to maintain the definition of the conductive polymer bumps  502 , as shown in FIG. 22. With the present invention, the conductive polymer bumps  502  can achieve height to width ratios of the preferred target of 3:1 or greater, since the weight of the polymer material causing the conductive polymer bump  502  to collapse on itself and flow or spread is no longer an issue. It is also understood that the inversion method of the present invention could also be used in the formation of metallic conductive bumps.  
         [0049]    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.

Technology Classification (CPC): 7