Patent Document

CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]    This application is a continuation of application Ser. No. 09/944,233, filed Aug. 30, 2001, pending, which is a continuation of application Ser. No. 08/906,578, filed Aug. 5, 1997, now U.S. Pat. No. 6,336,973. 
     
    
     
       BACKGROUND OF THE INVENTION  
         [0002]    1. Field of the Invention  
           [0003]    The present invention relates to achieving a level surface on an exposed surface of a viscous fluid. More particularly, the present invention relates to maintaining a level surface on a pool of adhesive material for applying the adhesive material to the lead fingers by contacting the lead fingers with the 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 &#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 material 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 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 &#39;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 &#39;679 patent is illustrated in FIGS.  23 - 29 . FIG. 23 illustrates a side, cross-sectional 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. 24. 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. 25. 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. 26. 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. 27, 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 to. 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. 28 and 29. 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 wirebonding 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. 29). 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. 30, 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 step 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. 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. 31, 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. 32. The adhesive material  502  which collects and cures on the bond wire surface  510  interferes with subsequent wirebonding 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. 33. 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 .  
           [0010]    Furthermore, present methods of adhesive material application on a surface (whether of 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 &#39;679 patent, results in a substantial area of the adhesive pattern not being utilized to attach leads.  
           [0011]    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.  
         BRIEF SUMMARY OF THE INVENTION  
         [0012]    The present invention relates to a method for applying an adhesive material to lead fingers of a lead frame wherein surfaces of the lead fingers which receive the adhesive material face downward to contact a pool of adhesive material. Preferably, the adhesive material 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 is 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. An adhesive reservoir retaining the adhesive material can be shaped such that the exposed surface (pool) of the adhesive material is in a precise location. When the lead fingers contact the exposed surface of the adhesive material, the adhesive material attaches to only specific, desired portions of the lead fingers.  
           [0013]    Rather than gravitational forces causing the adhesive material to flow and expand as when on top of the lead frame, the gravitational forces on the inverted lead frame maintain the shape and boundary definition of the adhesive material. It is, of course, understood that the viscous adhesive 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 adhesive material pool. Preferably, the viscous materials have viscosities between about 1000 cps and 500,000 cps.  
           [0014]    Of critical importance to the application of the adhesive material to the lead fingers in the method described above is the levelness of the exposed surface of the adhesive material of the pool. If the exposed surface is not level, the lead fingers may extend too deeply into the adhesive material. When this occurs, the adhesive material may wet sides of the lead finger and may even wet a bond wire surface of the lead finger. If the adhesive material wets the bond wire surface, the adhesive material may interfere with a wirebonding step subsequent to LOC attachment of the lead fingers to an active surface of a semiconductor die.  
           [0015]    A preferred method of controlling the levelness of the exposed surface is by attaching a coating stencil having small apertures, such as a screen or a plate with slots, to the adhesive reservoir, such that the only outlet for the adhesive material is through the apertures in the coating stencil. The adhesive material is thus forced through the coating stencil. The surface tension between walls of the small apertures and the adhesive material flattens out the exposed surface of the adhesive material. This allows a larger area to be printed with a more uniform thickness layer than if the coating stencil is not used. It is, of course, understood that the flatness or shape of the adhesive material can be controlled by the design of the apertures of the coating stencil. Thus, the present invention is an efficient way to use the surface tension of the adhesive material to control surface area and thickness of the adhesive material available for application to lead fingers. 
       
    
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS  
       [0016]    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:  
         [0017]    [0017]FIG. 1 is a top plan view of a typical lead frame ribbon;  
         [0018]    [0018]FIGS. 2 and 3 are schematic representations of one process of the present invention;  
         [0019]    [0019]FIG. 4 is a schematic representation of an alternate process of the present invention;  
         [0020]    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;  
         [0021]    [0021]FIG. 8 is a side cross-sectional view of a lead finger after adhesive material attachment according to a method of the present invention;  
         [0022]    [0022]FIG. 9 is a cross-sectional view of a lead finger along line  9 - 9  of FIG. 8 after adhesive material attachment;  
         [0023]    [0023]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;  
         [0024]    [0024]FIG. 11 is a schematic representation of a mechanical mechanism for maintaining the height of an exposed surface of an adhesive material;  
         [0025]    [0025]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;  
         [0026]    [0026]FIG. 13 is a plan view of a coating stencil of the present invention;  
         [0027]    [0027]FIG. 14 is a plan view of an alternate coating stencil of the present invention;  
         [0028]    [0028]FIG. 15 is a side cross-sectional view of an adhesive reservoir of the present invention;  
         [0029]    [0029]FIG. 16 is a top plan view of the adhesive reservoir of the present invention shown in FIG. 15 along line  16 - 16 ;  
         [0030]    [0030]FIG. 17 is a side plan view of stenciled and non-stenciled adhesive material profiles;  
         [0031]    [0031]FIG. 18 is a side plan view of a stenciled adhesive material profile after the induction of a vacuum;  
         [0032]    FIGS.  19 - 21  are side cross-sectional views of a technique of forming an adhesive film on lead fingers according to the present invention;  
         [0033]    [0033]FIG. 22 is a schematic representation of another multiple adhesive material attachment process of the present invention;  
         [0034]    FIGS.  23 - 29  are side cross-sectional views of a prior art technique of forming adhesive areas on a substrate for LOC attachment;  
         [0035]    [0035]FIG. 30 is a top view of a prior art technique of LOC attachment using adhesive tape; and  
         [0036]    FIGS.  31 - 33  are side cross-sectional views of a prior art technique of forming adhesive areas on lead fingers for LOC attachment.  
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0037]    [0037]FIG. 1 illustrates a portion of an exemplary lead frame ribbon  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 semiconductor 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 ribbon  100  and align the lead frames  102  throughout a process of attaching the lead frames  102  to semiconductor dice (not shown).  
         [0038]    [0038]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 ribbon  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 ribbon  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, thermoses resins, flowable pastes, and B-stage adhesive materials. Preferred adhesive materials  114  include cyanate ester, bismaleimide, epoxy, and polyimide.  
         [0039]    [0039]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 ribbon  100  downward to the adhesive reservoir  110 .  
         [0040]    It is, of course, understood that the biasing and elevator mechanisms  116  and  117  shown in FIGS. 2 and 3 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 .  
         [0041]    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  111 .  
         [0042]    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 AMP, 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 .  
         [0043]    [0043]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 wirebonding step subsequent to LOC attachment of the lead fingers  104  to an active surface of a semiconductor die.  
         [0044]    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 .  
         [0045]    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 wirebonding step subsequent to LOC attachment of the lead fingers  104  to an active surface of a semiconductor die, as mentioned above.  
         [0046]    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 .  
         [0047]    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 .  
         [0048]    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 light (preferably a laser) transmitter  140  and a light receiver  142 , is used to determine the height of the exposed surface  122 . The control signal  144  from control system  200  triggers the pump  132  to stop or a valve (not shown) to shut when the desired exposed surface height  134  is achieved.  
         [0049]    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 .  
         [0050]    A preferred method of controlling the levelness of the exposed surface  122  is by forcing or extruding the adhesive material  114  through a coating stencil having small apertures, such as a screen or a plate with slots. Such a coating stencil  150  is shown in FIG. 13. The coating stencil  150  is a flat plate  152  having a plurality of slots  154 . The coating stencil  150  shown has twenty-three parallel slots  154  approximately 0.260 inches in length  158  and approximately 0.010 inches in width  160 , with the slots  154  being on parallel centerline pitch  162  of approximately 0.020 inch from one another. An alternate coating stencil  156  is shown in FIG. 14. The coating stencil  156  is a screen comprising a flat plate  157  having a plurality of square or rectangular apertures  159 . It is, of course, understood that the apertures may be of any size (depending on the viscosity of the adhesive material) and any shape, including triangles, rectangles, squares, circles, ovals, or the like.  
         [0051]    The coating stencil  150  is attached to an adhesive reservoir  180 . The exemplary adhesive reservoir  180 , shown in FIGS. 15 and 16 without the coating stencil attached, comprises a housing  164  having an adhesive inflow chamber  166  in fluid communication with a pool chamber  168 . The coating stencil  150  is attached proximate an upper surface  170  of the pool chamber  168 , such that the only upward outlet for the adhesive material is through the apertures in the coating stencil. It is, of course, understood that the adhesive reservoir  180  may include an adhesive circulation mechanism to circulate the adhesive material to maintain the uniformity thereof.  
         [0052]    The cohesion between the aperture (slot) walls (not shown) and the adhesive material  114  flattens out the exposed surface  122  of the adhesive material  114 . This allows a larger area to be printed with a more uniform thickness of the adhesive material  114  than if the coating stencil  150  is not used. Put another way, the cohesion between the aperture walls and the adhesive material  114  basically pulls the adhesive material  114  down to the screen surface, which counteracts the force caused by the surface tension of the adhesive material  114 . As a result, the adhesive material  114  is pulled to the coating stencil  150 , thus flattening out. The mathematical formulation for the phenomena is Δp=2γ/R where Δp is the difference between the pressure within the adhesive material and the ambient air, γ is the surface tension of the adhesive material, and R is the radius of curvature when the adhesive material is extruded through the apertures in the coating stencil. R will be about the same for all openings, since Δp and γ are generally constant for most operations. Since the apertures are small, the extruded material is “flat” with about the same R.  
       EXAMPLE 1  
       [0053]    An example of the difference between a non-stenciled adhesive material exposed surface  172  and stenciled adhesive material exposed surface  174  is shown in FIG. 17. For this example, the adhesive material  114  was Ablestik XR-041395-9™ Polyimide LOC Adhesive (Ablestik Laboratories, Rancho Dominguez, Calif.) and the coating stencil  150  was as described above for FIG. 13. Ablestik XR-041395-9™ has a viscosity of 62,000 cps at 25° C. and a thixotropic index of 3.5. It is, of course, understood that the width, length, pitch and shape of the apertures in the coating stencil will vary for different viscosities of adhesive materials. A rule of thumb for determination of aperture size is that, for every viscosity increase of 25%, the aperture size should decrease by 50%.  
         [0054]    The illustration in FIG. 17 is an AutoCad™ program rendering of a digitized measurement of the non-stenciled adhesive material exposed surface  172  and stenciled adhesive material exposed surface  174 . The maximum height  176  of the non-stenciled adhesive material exposed surface  172  was approximately 0.07 inches above an upper surface  175  of the coating stencil  150  and the effective adhesion surface  178  of the non-stenciled adhesive material exposed surface  172  was approximately 0.26 inches wide. The maximum height  181  of the stenciled adhesive material exposed surface  174  was approximately 0.05 inches and the effective adhesion surface  182  of the stenciled adhesive material exposed surface  174  was approximately 0.33 inches wide. Thus, the use of a coating stencil  150  resulted in an increase of effective adhesion surface of about 21.2%. The effective adhesion surfaces  178 ,  182  are determined as the area from the maximum height  176 ,  181  of the stenciled adhesive material exposed surface  172 ,  174 , to a position about 5 mils below the maximum height  176 ,  181 .  
         [0055]    It has also been found that an even more uniform profile for the exposed surface can be achieved by inducing a slight vacuum on a bottom side of the coating stencil  150  by any known technique. FIG. 18 illustrates such a profile using the same adhesive material  114  and coating stencil  150  described in FIG. 17, wherein a vacuum of between about 2 and 3 inches of H 2 O is applied. The vacuum method provided a very uniform coating at between about 0.02 and 0.03 inch in adhesive material height.  
       EXAMPLE 2  
       [0056]    An example of one preferred embodiment of the coating process is illustrated in FIGS.  19 - 21 . Elements common to FIGS.  19 - 21  and previous FIGs. retain the same designations. As shown in FIG. 19, the lead fingers  104  are brought into close proximity to the adhesive material exposed surface  122 . Sufficient adhesive material  114  is then delivered to the adhesive reservoir  110  until the adhesive material exposed surface  122  comes in contact with the bottom surface  124  of the lead fingers  104 . At this point, additional adhesive material  114  is delivered to the adhesive reservoir  110  to raise the adhesive material exposed surface  122  about an additional 0.02 to 0.06 inches so that the lead fingers  104  are submerged past a top surface  184  of the adhesive material exposed surface  122 , as shown in FIG. 20. The lead fingers  104  remain in this position for a time sufficient to allow the adhesive material  114  to wet the bottom surface  124  of the lead fingers  104 , preferably approximately 10 to 25 milliseconds. As shown in FIG. 21, the adhesive material exposed surface  122  is then lowered, thereby forming the adhesive film  126  from the bulk of the adhesive material  114  on the bottom surface  124  of the lead finger  104 . The lead frame ribbon  100  is then indexed to the next site that requires coating. Before the adhesive material  114  is raised again, more adhesive material  114  is delivered, as required, to replenish the amount used in the previous coating cycle.  
         [0057]    It is also understood that multiple reservoirs  110  could be configured as shown in FIG. 22. With such a configuration, the adhesive material  114  can be applied to the lead fingers  104  of multiple lead frames  102  simultaneously.  
         [0058]    Once the adhesive material  114  has been applied to the lead fingers  104 , the lead frame ribbon  100  may, optionally, be fed to a curing oven  118 , shown in FIGS. 2, 3,  4 , and  22 , 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.  
         [0059]    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 Category: h