Abstract:
A method for applying a viscous material to a lead frame element. A method of the invention includes positioning the lead frame facing downward and bringing the lead fingers into contact with a pool of adhesive material. The contact of the lead fingers to the adhesive material results in a portion of the lead fingers receiving a portion of the adhesive material from the pool of adhesive material. The gravitational forces on the adhesive material on the downward facing lead frame maintain the shape and boundary definition of the adhesive material.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]    This application is a continuation of application Ser. No. 09/405,943, filed Sep. 27, 1999, pending, which is a continuation of application Ser. No. 08/906,673, filed Aug. 5, 1997, now U.S. Pat. No. 6,013,535, issued Jan. 11, 2000. 
     
    
     
       BACKGROUND OF THE INVENTION  
         [0002]    1. Field of the Invention  
           [0003]    The present invention relates to applying viscous materials to lead fingers of a lead frame, such as an adhesive material for the attachment of the lead fingers to a semiconductor die. More particularly, the present invention relates to applying adhesive material to the lead fingers by contacting the lead fingers with a pool of adhesive material.  
           [0004]    2. State of the Art  
           [0005]    Higher performance, lower cost, increased miniaturization of semiconductor components, and greater packaging density of integrated circuits are goals of the computer industry. One way to reduce the overall cost of a semiconductor component is to reduce the manufacturing cost of that component. Lower manufacturing costs can be achieved through faster production and/or reduction in the amount of materials used in fabricating the semiconductor component.  
           [0006]    One area where faster production and reduction in material usage can be achieved is in the area of lead frame attachment to semiconductor dice. U.S. Pat. No. 5,286,679 issued Feb. 15, 1994 to Farnworth et al. (“the ′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 material in a “lead-over-chip” (“LOC”) configuration. The ′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 lead fingers of a lead frame by heating the adhesive layer and pressing the lead fingers 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 ′679 patent is an effective method for attaching leads in a LOC configuration, it is difficult to achieve an adequate profile on the adhesive such that there is sufficient area on the top of the adhesive to attach the lead fingers. The process disclosed in the ′679 patent is illustrated in FIGS.  14 - 20 . FIG. 14 illustrates a side, crosssectional view of a semiconductor substrate  302  with a bond pad  304 , wherein a stencil or a screen print template  306  has been placed over the semiconductor substrate  302 , generally a silicon wafer. The stencil or screen print template  306  is patterned to clear the area around the bond pads  304  and to clear street areas  308  for saw cutting (i.e., for singulating the substrate into individual dice). An adhesive material  310  is applied to the stencil or screen print template  306 , as shown in FIG. 15. Ideally, when the stencil or screen print template  306  is removed, adhesive prints  312  are formed with vertical sidewalls  314  and a planar upper surface  316 , as shown in FIG. 16. However, since the adhesive material  310  must have sufficiently low viscosity to flow and fill the stencil or screen print template  306 , as well as allow for the removal of the stencil or screen print template  306  without the adhesive material  310  sticking thereto, the adhesive material  310  of the adhesive prints  312  will spread, sag, or flow laterally under the force of gravity after the removal of the stencil or screen print template  306 , as shown in FIG. 17. This post-application flow of adhesive material  310  can potentially cover all or a portion of the bond pads  304  or interfere with the singulating of the semiconductor wafer by flowing into the street areas  308 .  
           [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  310  has on the adhesive material upper surface  316 . As shown in FIG. 18, the adhesive material upper surface  316  is the contact area for lead fingers  318  of a lead frame  320 . The gravity-induced flow of the adhesive material  310  causes the once relatively well-defined edges  322  of the adhesive material to curve, resulting in a loss of surface area  324  (ideal shape shown in shadow) for the lead fingers  318  to attach. This loss of surface area  324  is particularly problematical for the adhesive material upper surface  316  at the longitudinal ends  326  thereof. At the adhesive material longitudinal ends  326 , the adhesive material flows in three directions (to both sides as well as longitudinally), causing a severe curvature  328 , as shown in FIGS. 19 and 20. The longitudinal ends of the adhesive print on patch flow in a 180° flow front, resulting in blurring of the print boundaries into a curved perimeter. This curvature  328  results in complete or near complete loss of effective surface area on the adhesive material upper surface  316  for adhering the outermost lead finger closest to the adhesive material longitudinal end  326  (lead finger  330 ). This results in what is known as a “dangling lead.” Since the lead finger  330  is not adequately attached to the adhesive material longitudinal end  326 , the lead finger  330  will move or bounce when a wire bonding apparatus (not shown) attempts to attach a bond wire (not shown) between the lead finger  330  and its respective bond pad  304  (shown from the side in FIG. 20). This movement can cause inadequate bonding or non-bonding between the bond wire and the lead finger  330 , resulting in the failure of the component due to a defective electrical connection.  
           [0009]    LOC attachment can also be achieved by attaching adhesive tape, preferably insulative, to an active surface of a semiconductor die, then attaching lead fingers to the insulative tape. As shown in FIG. 21, two strips of adhesive tape  410  and  410 ′ are attached to an active surface  412  of a semiconductor die  404 . The two adhesive tape strips  410 ,  410 ′ run parallel to and on opposing sides of a row of bond pads  406 . Lead fingers  402 ,  402 ′ are then attached to the two adhesive tape strips  410 ,  410 ′, respectively. The lead fingers  402 ,  402 ′ are then electrically attached to the bond pads  406  with bond wires  408 . Although this method is effective in attaching the lead fingers  402 ,  402 ′ to the semiconductor die  404 , this method is less cost effective than using adhesive since the cost of adhesive tape is higher than the cost of adhesive material. The higher cost of the adhesive tape is a result of the manufacturing and placement steps which are required with adhesive tapes. The individual tape segments are generally cut from a larger tape sheet. This cutting requires precision punches with extremely sharp and accurate edges. These precision punches are expensive and they wear out over time. Furthermore, there is always waste between the segments which are punched out, resulting in high scrap cost. Moreover, once punch out is complete, the tape segments are placed on a carrier film for transport to the die-attach site. Thus, there are problems with placement, alignment, and attachment with film carriers, plus the cost of the film carrier itself.  
           [0010]    LOC attachment can further be achieved by placing adhesive material on the lead fingers of the lead frame rather than on the semiconductor substrate. As shown in FIG. 22, the adhesive material  502  may be spray applied on an attachment surface  504  of lead fingers  506 . However, the viscous nature of the adhesive material  502  results in the adhesive material  502  flowing down the sides  508  of the lead finger  506  and collecting on the reverse, bond wire surface  510  of the lead finger  506 , as shown in FIG. 23. The adhesive material  502 , which collects and cures on the bond wire surface  510 , interferes with subsequent wire bonding, which, in turn, can result in a failure of the semiconductor component. The flow of adhesive material  502  for the attachment surface  504  to the bond wire surface  510  can be exacerbated if the lead fingers  506  are formed by a stamping process rather than by etching, the other widely employed alternative. The stamping process leaves a slight curvature  512  to edges  514  of at least one surface of the lead finger  506 , as shown in FIG. 24. If an edge curvature  512  is proximate the lead finger attachment surface  504 , the edge curvature  512  results in less resistance (i.e., less surface tension) to the flow of the adhesive material  502 . This, of course, results in the potential for a greater amount of adhesive material  502  to flow to the bond wire surface  510 .  
           [0011]    Furthermore, present methods of adhesive material application on a surface (whether the semiconductor die or the lead fingers) tend to waste adhesive material. For example, spray application loses a great deal of adhesive material because not all of the sprayed adhesive material attaches to the target surface. As another example, the patterning of an adhesive layer on a semiconductor die, such as described in the ′679 patent, results in a substantial area of the adhesive pattern not being utilized to attach leads.  
           [0012]    Thus, it can be appreciated that it would be advantageous to develop a method and apparatus for rapidly applying an adhesive material to a lead finger with little waste of adhesive material.  
         SUMMARY OF THE INVENTION  
         [0013]    The present invention relates to a method for applying a viscous material, such as an adhesive material, to lead fingers of a lead frame wherein surfaces of the lead fingers which receive the viscous material face downward to contact a pool of adhesive material. Preferably, the viscous material is an adhesive material which cures with the lead frame in this downward facing position. The advantages of placing viscous material, such as an adhesive material, in a downward facing position are described in U.S. patent application Ser. No. 08/709,182 by Tongbi Jiang and Syed S. Ahmad filed Sep. 6, 1996, assigned to the assignee of the present invention and hereby incorporated herein by reference.  
           [0014]    Rather than gravitational forces causing the viscous material to flow and expand as when on top of the lead frame, the gravitational forces on the inverted lead frame maintain the Do shape and boundary definition of the viscous material.  
           [0015]    It is, of course, understood that the viscous material must be compatible with the lead finger material so as to adhere thereto and must not be of such a low viscosity that it drips when the lead fingers are removed from contact with the viscous material pool. Preferably, the viscous materials have viscosities between about 1000 cps and 500,000 cps.  
           [0016]    Furthermore, with regard to drying or curing an adhesive material, the lead frame need only be inverted until the viscous adhesive material has stabilized sufficiently to maintain its shape and boundary definition. Depending on the particular viscous adhesive material used, the minimum inversion time could be the time required to cure the outer surfaces of the viscous adhesive material such that a film is formed which contains the as yet uncured viscous adhesive material therein, or the minimum inversion time could be the time required to completely dry or cure the viscous adhesive material. 
       
    
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS  
       [0017]    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:  
         [0018]    [0018]FIG. 1 is a top plan view of a typical lead frame strip;  
         [0019]    [0019]FIGS. 2 and 3 are schematic representations of one process of the present invention;  
         [0020]    [0020]FIG. 4 is a schematic representation of an alternate process of the present invention;  
         [0021]    FIGS.  5 - 7  are side views of a process of contacting lead fingers with an adhesive material according to a method of the present invention;  
         [0022]    [0022]FIG. 8 is a side cross-sectional view of a lead finger after adhesive material attachment according to a method of the present invention;  
         [0023]    [0023]FIG. 9 is a cross-sectional view of a lead finger along line  9 - 9  of FIG. 8 after adhesive material attachment;  
         [0024]    [0024]FIG. 10 is a cross-sectional view of a lead finger after adhesive material attachment, wherein the adhesive material exhibits excessive wetting of the lead finger;  
         [0025]    [0025]FIG. 11 is a schematic representation of a mechanical mechanism for maintaining the height of an exposed surface of an adhesive material;  
         [0026]    [0026]FIG. 12 is a schematic representation of a height detection and control loop for maintaining the height of an exposed surface of an adhesive material;  
         [0027]    [0027]FIG. 13 is a schematic representation of another multiple adhesive material attachment process of the present invention;  
         [0028]    FIGS.  14 - 20  are side cross-sectional views of a prior art technique of forming adhesive areas on a substrate for LOC attachment;  
         [0029]    [0029]FIG. 21 is a top view of a prior art technique of LOC attachment using adhesive tape;  
         [0030]    FIGS.  22 - 24  are side cross-sectional views of a prior art technique of forming adhesive areas on lead fingers for LOC attachment;  
         [0031]    [0031]FIG. 25 is a plate-type reservoir design according to the present invention; and  
         [0032]    [0032]FIG. 26 is a spillway-type reservoir design according to the present invention.  
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0033]    [0033]FIG. 1 illustrates a portion of an exemplary lead frame strip  100 . It should be understood that the figures presented in conjunction with this description are not meant to be actual views of any particular portion of an actual semiconducting device or component, but are merely idealized representations which are employed to more clearly and fully depict the process of the invention than would otherwise be possible. Individual lead frames  102 , each including a plurality of lead fingers  104 , are formed in a long, thin strip of conductive material  106 , such as copper, copper alloy, or the like. The lead frames  102  are generally formed by a stamping process or an etching process. The lead frames  102  are formed side-by-side along the conductive material strip  106  wherein the conductive material strip  106  includes a plurality of indexing holes  107 ,  107 ′ on opposing lengthwise edges  109 ,  109 ′, respectively, of the conductive material strip  106 . The indexing holes  107 ,  107 ′ are used to move the lead frame strip  100  and align the lead frames  102  throughout a process of attaching the lead frames  102  to semiconductor dice (not shown).  
         [0034]    [0034]FIGS. 2 and 3 illustrate a schematic of one process of the present invention. Elements common to FIGS. 1, 2, and  3  retain the same numeric designation. The lead frame strip  100 , such as illustrated in FIG. 1, is fed from a source  108 , such as a spool, to an adhesive reservoir  110 . As shown in FIG. 3, the lead fingers  104  (not shown) of the lead frame  102  (not shown) are aligned over the adhesive reservoir  110  and the lead frame strip  100  is biased downward in direction  112 , such as by hydraulic, pneumatic, or electrically-powered biasing mechanisms  116 , to contact an adhesive material  114 . The adhesive material  114  may be any viscous adhesive material including, but not limited to, thermoplastics, thermoset resins, flowable pastes, and B-stage adhesive materials. Preferred adhesive materials  114  include cyanate ester, bismaleimide, epoxy, and polyimide.  
         [0035]    [0035]FIG. 4 illustrates a schematic of another process of the present invention which is similar to the process of FIGS. 2 and 3. Elements common to FIGS. 2 and 3 and FIG. 4 retain the same numeric designation. The only difference between the processes of FIGS. 2 and 3, and FIG. 4 is that the process of FIG. 4 employs an elevator mechanism  117  to move the adhesive reservoir  110  in an upward direction  120  to contact the lead fingers  104  rather than biasing the lead frame strip  100  downward to the adhesive reservoir  110 .  
         [0036]    It is, of course, understood that the biasing and elevator mechanisms  116  and  117  shown in FIGS.  2 - 4  are not required to bring the adhesive material  114  into contact with the lead fingers  104 . Instead, the lead fingers  104  may be brought into close proximity to the adhesive reservoir  110  and additional adhesive material  114  may be delivered by a pump to the adhesive reservoir  110  to raise the level of the adhesive material  114  to contact the lead fingers  104 , or to provide a moving wave or surge of adhesive material traveling across the reservoir  110 .  
         [0037]    FIGS.  5 - 7  illustrate side views of the lead fingers  104  being brought into contact with the adhesive material  114  and being retracted therefrom. Elements common to FIGS.  2 - 4  and FIGS.  5 - 7  retain the same numeric designation. As shown in FIG. 5, the lead fingers  104  are positioned over the adhesive reservoir  110 . The adhesive reservoir  110  has the adhesive material  114  extending above edges  111  of the adhesive reservoir  110 . Due to the forces of adhesion and surface tension inherent in the adhesive material  114 , an exposed surface  122  of the adhesive material  114  will form a meniscus, or convex-shaped configuration, above the reservoir edges em.  111 .  
         [0038]    As shown in FIG. 6, the lead fingers  104  are lowered onto or proximate the exposed surface  122  of the adhesive material  114 . When a bottom surface  124  of the lead fingers  104  comes in contact with the exposed surface  122  of the adhesive material  114 , the adhesive material  114  wets out across the bottom surface  124  of the lead finger  104 . As shown in FIG. 7, when the lead fingers  104  are retracted from the adhesive material  114 , the cohesion of the adhesive material  114  with the lead fingers  104  pulls some of the adhesive material  114  from the bulk of the adhesive material  114  to form an adhesive film  126  on the bottom surface  124  of the lead finger  104 . The thickness of the adhesive film  126  can range from 0.1 to 15 mils, depending on the viscosity of the adhesive material  114 . Changing the shape of the lead finger  104 , changing the rheology of the adhesive material  114 , pre-coating the lead finger  104  with a surfactant, such as NMP, or placing a solvent in the adhesive material  114  to improve wetting and/or adding adhesion promoters, such as silane, siloxane, or polyimide siloxane, to the adhesive material  114  will also change the thickness and/or pattern of the adhesive film  126 . It is, of course, understood that the adhesive material  114  must be capable of adhering to the lead fingers  104  and must not be of such a low viscosity that it drips when the lead fingers  104  are removed from contact with the exposed surface  122  of the adhesive material  114 .  
         [0039]    [0039]FIG. 8 is a side cross-sectional view of a lead finger  104  after adhesive material  114  application. FIG. 9 is a cross-sectional view of the lead finger  104  of FIG. 8 along line  9 - 9 . As shown in FIGS. 8 and 9, by only contacting the bottom surface  124  of the lead finger  104  with the exposed surface  122  of the adhesive material  114 , the adhesive material  114  will not wet sides  128  of the lead finger  104  and, of course, will not collect on a bond wire surface  130  of a lead finger  104  (the bond wire surface  130  is the lead finger surface where a bond wire is subsequently attached during further processing). Since the adhesive material  114  does not collect on the bond wire surface  130 , there will be no adhesive material  114  to interfere with a subsequent wire bonding step subsequent to LOC attachment of the lead fingers  104  to an active surface of a semiconductor die.  
         [0040]    Referring back to FIG. 5, the adhesive reservoir  110  can be shaped such that the exposed surface  122  of the adhesive material  114  is in a precise location. When the lead fingers  104  contact the exposed surface  122  of the adhesive material  114 , the adhesive material  114  attaches to only specific, desired portions of the lead fingers  104 .  
         [0041]    It is very important that the exposed surface  122  be as level as possible. If the exposed surface  122  is not level, the lead fingers  104  may extend too deeply into the adhesive material  114 . When this occurs, the adhesive material  114  may wet the lead finger sides  128  and may even wet the lead finger bond wire surface  130 , as shown in FIG. 10. If the adhesive material  114  wets the bond wire surface  130 , the adhesive material  114  may interfere with a subsequent wire bonding step subsequent to LOC attachment of the lead fingers  104  to an active surface of a semiconductor die, as mentioned above.  
         [0042]    Numerous techniques may be used to keep the exposed surface  122  of the adhesive material  114  level. It is, of course, understood that exposed surface  122  extends from the adhesive reservoir  110  due to a slight excess of adhesive material  114  within the adhesive reservoir  110 . As shown in FIG. 11, the adhesive material  114  is pumped to the adhesive reservoir  110  from an adhesive material source (not shown) by a pump  132 . A desired exposed surface height  134  of exposed surface  122  can be achieved by feeding an excess of adhesive material  114  into the adhesive reservoir  110  such that an initial exposed surface height  136  is higher than the desired exposed surface height  134 . A metering mechanism, such as wiper  138 , can be utilized to meter the adhesive material  114  from the initial exposed surface height  136  to the desired exposed surface height  134 .  
         [0043]    Moreover, a desired exposed surface height  134  of exposed surface  122  can be achieved by feeding an excess of adhesive material  114  into the adhesive reservoir  110  such that an initial exposed surface height  136  is higher than the desired exposed surface height  134 . The adhesive material  114  is then drawn back (e.g., by vacuum), which results in a flattening of the exposed surface  122 .  
         [0044]    Furthermore, a variety of feed back and feed forward control schemes may be used to control the desired exposed surface height  134  of the exposed surface  122 . One such control scheme is shown in FIG. 12. Elements common to FIG. 11 and FIG. 12 retain the same numeric designations. A height detection mechanism, shown as a transmitter  140  and a receiver  142 , is used to determine the height of the exposed surface  122 . A control signal  144  triggers the pump  132  to stop or a valve (not shown) to shut when the desired exposed surface height  134  is achieved. The transmitter  140  and receiver  142  may be a light (preferably a laser) transmitter and receiver. When a light beam (not shown) from the transmitter  140  is altered by the exposed surface  122 , the receiver  142  detects the discontinuation of light transmission and generates the control signal  144 . Additionally, the transmitter  140  and receiver  142  may be an ultrasonic transmitter and receiver. When an ultrasonic sound wave (not shown) from the transmitter  140  is altered by the exposed surface  122 , the receiver  142  detects the change in transit time or phase shifts of the ultrasonic sound wave and generates the control signal  144 .  
         [0045]    It is, of course, understood that precise control of the lead frame position relative to the exposed surface  122  is required to accurately control the depth to which the lead fingers  104  are pressed into the adhesive material  114 .  
         [0046]    It is also understood that multiple reservoirs  110  could be configured as shown in FIG. 13. With such a configuration, the adhesive material  114  can be applied to the lead fingers  104  of multiple lead frames  102  simultaneously. The group of lead frames  102  is then indexed forward and another group is presented to the multiple reservoirs  110  for coating.  
         [0047]    Once the adhesive material  114  has been applied to the lead fingers  104 , the lead frame strip  100  may, optionally, be fed to a curing oven  118 , shown in FIGS. 2, 3,  4 , and  13 , to set the adhesive material  114 . A semiconductor die (not shown) then can be attached to a lead frame  102  and adhesive film  126  by known LOC attach methods.  
         [0048]    It is, of course, understood that the present invention is not limited to the above detailed description. The structures coated are not limited to lead frames and can include TAB structures and the like. The lead frames may not be limited to delivery in strips, but can be delivered individually or in sheets. The viscous material can be applied to a structure under at least a partially-evacuated chamber such that lower-viscosity materials could be used and still form a meniscus wherein the viscous material applied to the structure would be at least partially dried or cured prior to removal from the chamber.  
         [0049]    Furthermore, the present invention is not limited to only applying a viscous material to lead fingers, but it is also contemplated that the viscous material may be applied to bus bars, die attach paddles, or other structures of a lead frame, as well as dipping any semiconductor component which requires a coating of a viscous material. Additionally, the viscous material is not limited to adhesives, but may include various viscous materials for a variety of applications. One such application includes applying a polyimide film to a lead frame in order to eliminate the need for Kapton™ tape.  
         [0050]    Moreover, the reservoir may be any structure which exposes a viscous material pool and may be one of a variety of designs, as shown in FIGS.  25 - 26 . FIG. 25 illustrates a plate-type reservoir  150  in which a very thin layer of viscous material  152  is delivered across plate  154  from an inlet  156  to an opposing outlet  158 . FIG. 26 illustrates a reservoir  160  with a curved-edge spillway  162 . The viscous material  164  is pumped into a chamber  166  and over the spillway  162  at a constant rate. This results in a self-limiting viscous material height  168 . The lead fingers of a lead frame are contacted with the viscous material  164  over the spillway  162  where the viscous material  164  would inherently, due to its viscosity, form a raised area  170  over the spillway  162  into a spill chamber  172 .  
         [0051]    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.