Abstract:
A method and apparatus for enabling z-axis offset of narrow metal ties straps in lead frames used for packaging integrated circuits to prevent bowing or distortion. Simultaneous offsetting of the tie strap and stress relief mechanisms are provided on both the front and back sides of the lead frame. Those mechanisms include indentations along the long or primary axis of each tie strap, coupled with depressions across the top surface both at the center of the lead frame and between the base of the off set and the chip attach locations to prevent bowing in small pad and no pad lead frames, in particular.

Description:
This application is a Division of application Ser. No. 09/671,083, filed Sep. 27, 2000, now U.S. Pat. No. 6,755,069, which claims benefit of 60/157,780, filed Oct. 5, 1999. 

   FIELD OF THE INVENTION 
   This invention relates generally to the field of metal forming, and more particularly to the forming of lead frames used in the assembly of micro electronic devices. 
   BRIEF DESCRIPTION OF PRIOR ART 
   Integrated circuit devices, having an integrated circuit chip and a lead frame which are sealed within a protective enclosure find wide use in products, among which are consumer electronics, computers, automobiles, telecommunications and military applications. A means to electrically interconnect an integrated circuit chip to circuitry external to the device frequently takes the form of a lead frame. The lead frame is formed from a highly electrically and thermally conductive material, such as copper or copper alloys. The lead frame is stamped or etched into a plurality of leads, and a central area, called a chip pad, on which the integrated circuit chip is attached. The chip is electrically connected to the leads, usually by wire bonding, and the device is encapsulated to provide mechanical and environmental protection. 
   Lead frames typically include a solid chip pad somewhat larger than the chip to which the integrated circuit chip is attached by an adhesive or alloy. However, currently many lead frames  101  are fabricated with a single or multiple small circular pads  102  as shown in  FIG. 1 , or simply to strips of metal where the chip is attached, and the large chip pad is eliminated. The chip pads  102  are connected to outer support rails  105  by thin etched or stamped extensions of the metal, called tie straps  106 . Support rails  105  also hold together one or more lead frames in a strip until encapsulation is completed. 
   Those lead frames having one or more small pads  102  as in  FIG. 1  are typically referred to as S-pad or small pad lead frames. A small, circular pad is positioned approximately mid-way from the edge of the tie strap to the center of the lead frame where the tie straps intersect  108 . The chip is positioned atop the frame and the unpatterned side of the chip attached to the pads by an adhesive. An outline of a chip position is represented by the dashed line  103 . 
   As shown in  FIG. 2   a , lead frames  201  may be attached to the active patterned surface of the chip  203 , as in devices referred to as LOC or lead-on-chip, and illustrated in  FIGS. 2   a  and  2   b.  A chip is attached to a flat portion of the lead frames itself, and most often to a down-set or offset portion  209  of the frame. 
   Lead frames having a reduced chip pad area were developed in response to a failure mechanism in surface mount packages often referred to as “pop corning”. Moisture ingress into the plastic package is trapped between the chip and the metal chip pad, and when subjected to a rapid thermal excursion, such as solder attachment to a printed wiring board, the vapor pressure causes the plastic package to bulge and sometimes crack. This failure mechanism can be avoided by eliminating the large solid metal die pad. 
   In the process of lead frame down-setting a selected strip of metal is elongating in a die under pressure from either a hydraulic or electrically driven press while the support metal remains planar. The metal in two or more tie straps metal is forced downward to form angled bends and is pressed toward the center of the lead frame by using a forming punch to press the tie straps against the die surface. The more ductile metal, following the path of least resistance moves toward the center where the tie straps converge, and lacking a relief mechanism, the metal strip bows.  FIG. 3   a  is a schematic of a lead frame  301  with bowed tie strap  308  at the center where the tie straps converge. The semiconductor chip  302  is attached to the small pads  303  by adhesive  304  only at localized area owing to the non-planar attach area. The schematic in  FIG. 3   a  demonstrates a device bowed in a concave direction. Convex bowing is equally as big an issue for the assembly of semiconductor devices. 
   In conventional lead frames having a rigid or solid chip attach pad in the center, stress is relieved in the large pad, and bowing is much less of a problem. However, in the case of small pad frames where only small relief areas are provided, the narrow metal frequently is distorted at the location where pressure converges. 
   However, small or no pad lead frames are not without significant manufacturing challenges. One of the more difficult issues has been warping or distortion of the long thin tie straps which occurs during offsetting the chip mount area. Either convex or concave bowing of the tie strap and chip attach areas prevent the chip from seating correctly. 
   Owing to the reduced contact area for chip attachment, it is imperative that the chip attach area be planar and allow the small amount of adhesive on the pads to contact and firmly seat the chip. Losses resulting from bowed or distorted lead frames result in not only yield loss, but also present a reliability concern for both mechanical stresses on wire bonds, as well as diminished thermal transport path. A solution to this issue has been sought by the industry since the inception of small pad and no pad packaged devices. 
   SUMMARY OF THE INVENTION 
   As thinner integrated circuit packages, and consequently thinner lead frames are demanded by the industry, warping and distortion of the lead frame chip attach area has becomes a more prevalent issue. Further, the heavily favored lead frame materials are alloys of copper because of its excellent thermal conductivity and ease of processing, but the malleability and ductility of these copper alloys allows greater warping than other more rigid metallic alloys. 
   It is an object of this invention to provide an essentially no cost, permanent, and consistent means of eliminating bowing and distortion of lead frame tie straps resulting from the z-axis down-setting process. 
   It is an object of the invention to provide tooling which is common to a large family of devices, and the methodology for altering the tooling design to be applicable to many other lead frames sizes and shapes. 
   The chip attach area of lead frames is offset from the plane of the lead frame in order to minimize the length and looping height of bonding wires by positioning the chip surface at more nearly the same level as the lead flame bond fingers to which bond wires are attached. Offset or z-axis down-set is accomplished by positioning the stamped or etched lead flame in a press having a down-set die and a forming tool. The material to be offset is elongated by the contact and pressure from the forming tool, thereby stretching the metal and increasing its length with respect to the surrounding planar portion of the lead frame. The metal is being elongated or stretched along the surface of the forming die from opposite sides of the lead frame and converges toward the center of the lead frame. This results in the central area bowing in either a concave or convex direction, either of which is unacceptable. 
   The present invention provides a set of inserts to be positioned in the down-set die which have a protrusion above the die surface in the center of each tie strap. In addition, a series of protrusions are fabricated on the down-set forming punch. The insert tooling forms an indentation on the backside of the tie strap which allows the lead frame material to be pushed upward, and to move to the outer edge of the tie strap. In the same operation, protrusions on the forming punch create small lateral impressions on the top surface of the tie strap which control the flow of material being pushed toward the center of the lead frame during down-setting. Simultaneously creating small controlled indentations in the top and bottom surfaces, as well as along both the longitudinal and horizontal axis allows relief for the deformed lead frame material, and results in a flat, planar chip attach area. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a plane view of a prior art “S” or small pad lead frame. 
       FIG. 2   a  is across section of a known LOC package. 
       FIG. 2   b  is a plane view of a known LOC type lead frame. 
       FIG. 3   a  is cross sectional view of a prior art down-set “S” pad lead frame. 
       FIG. 3   b  is a cross sectional view of a down-set “S” pad lead frame fabricated using the method of the current invention. 
       FIG. 4   a  is a schematic view of a die insert of the current invention. 
       FIG. 4   b  is a schematic view of the die surface with insert locations. 
       FIG. 5  provides a schematic of forming punch tool with protrusions and locations. 
       FIG. 6  is a plane view of the top surface of a “S” pad lead frame having impressions from forming punch protrusions. 
       FIG. 7  is a plane view of the bottom surface of a “S” pad lead frame having indentations from the die inserts. 
       FIG. 8  shows the relative positions of stress relief mechanisms on an “S” pad lead frame. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   In accordance with the present invention, there is provided a method to eliminate bowing or distortion of lead frames resulting from offsetting the tie straps, including the first step of providing the necessary forming tools 
   Detailed descriptions of the preferred embodiment are provided herein. It is to be understood, however, that the present invention may be embodied in various forms. Therefore, specific details disclosed herein are not to be interpreted as limiting, but rather as a basis for teaching one skilled in the art to employ the present invention in virtually any appropriate detailed system, structure or manner. 
   A preferred embodiment of the present invention uses conventional hydraulic or electrically driven presses for offsetting lead frames, in combination with a novel offset tooling set. The improved tooling set of the current invention simultaneously relieves stress on the bottom surface and the top surface of the lead frame tie strap during the down-setting process and avoids build-up and bowing of the metal toward the center of the work-piece.  FIG. 3   b  illustrates the chip  312  attached to small chip pads  313  in a lead frame  311  processed using the forming tooling of the current invention, wherein the tie strap, the small pads  313  and the central area  318  where tie straps converge is flat after offset forming. 
     FIGS. 4   a  and  4   a′  are planar and cross sectional schematic representations of one of a set of tools to be insert into the surface of the down-set die. Each insert tool  401  comprises a steel device having a rib  402  which protrudes about 0.0005 inches above the surface of the die, and corners  404  shaped to conform to those of a cavity in the die.  FIG. 4   b  shows the locations of cavities  411  in the die surface  410 . One rib shaped tool  401  is provided to be positioned in each cavity  411  between the angled down-set areas  412 , and the small pad of the lead frame (not shown). The preferred embodiment provides a cavity  411  in the die for each pad of a four (4) small circular pad device. 
   Length and width of the protrusions  402  on the insert tools  401  are determined by the specific lead frame dimensions. The length is derived from the location of chip pads and the base of the downs-set, and will extend inside those boundaries. In the preferred embodiment the tool is in about 0.12 inches long. Width of the protruding rib is dependent upon the width of lead frame tie strap, and the width  403  of the protrusion is about 0.0002 inches wide for a tie strap of nominal width 0.012 inches. 
   Protrusions  402  on the inserts force the ductile lead frame metal to create an indentation on the bottom side along the primary axis of the tie strap, and to flow toward the outer edge of the lead frame metal, rather than simply moving toward the center of the device. 
   The surface of the forming punch, as shown in  FIG. 5  has a series of down set angles  504  which press the lead frame tie straps onto the die surface, thereby forming a down-set area out of plane with the support straps. The downs-set angle is 30 degrees and depth of the down-set in the preferred embodiment is about 0.009 inches. The forming punch of the current invention includes five (5) protrusions  502 / 503  of about 0.0005 inches height on the surface of the tool. There are 4 small protrusions  502 , each of which contacts one of the tie straps, and a centrally located large protrusion  503  which contacts all of the tie straps. These protrusions correspond to the locations where impressions are formed on the top surface of the tie strap. An array of four (4) protrusions  502 , about 0.015 to 0.016 inches on a side create a horizontal depression, about 0.015 inches long in the tie strap near the edge of each small circular chip pad, on the side toward the angled down-set. These impressions extend the full width of the tie strap, and are perpendicular to the backside indentations. The fifth protrusion  503  on the forming tool is a square positioned at the center of the tool so that one side intersects each of the tie straps, and makes a depression from a location near the edge of the circular chip pad across the center of the lead frame to near the pad on the opposite tie strap. Protrusions on the forming punch are about 0.0003 inches in height, and spacing is related to the location and size of the circular die pads. In the preferred embodiment, the square protrusion is about 0.13 inches, and will form an impression extending radially about 0.065 inches inward from the center of the tie strap intersection. 
   The impressions on the top surface of the lead frame horizontal to the length of the tie strap, coupled with the longitudinal indentation on the backside of the tie strap allow the metal to be moved laterally toward the edges on the back side, and provide a horizontal relief stop for the metal as it moves toward the center on the top side. 
     FIG. 6  illustrates the top surface  610  of a small pad lead frame having depressions  602 / 603  as created by the forming tool protrusions  502 / 503  in FIG.  5 . Each impression extends the width of the tie strap, and is about 0.0002 to 0.0003 inches deep. Shaded areas  602 / 603  represent the depressed area caused by protrusions  502 / 503  on the forming tool. The outline of protrusions  502 / 503  is represented by dashed lines  502   a / 503   a  in FIG.  6 . 
     FIG. 7  is a plane view of the back surface of the lead frame  710 . Indentations  711  along the primary axis of the tie strap  712  result from pressing the tie straps against ribs  402  on the form die insert as shown in  FIG. 4   a . Indentations  711  are in the range of 0.00035 to 0.0005 inches in depth, and are located between the termination of the down-set angle termination  715  and the circular chip pad  713 . Indentations are designed to terminate prior to the onset of impressions  602  on the top surface in FIG.  6 . 
     FIG. 8  illustrates the relative locations of the top surface depressions  802 / 803  and the bottom surface indentations  811  with respect to the termination of the down-set angle  815  and the circular chip pad  813  in one half of a lead frame  820  cross section. The center of the lead frame where tie straps converge is noted by the dashed lines  821 . 
   It should be understood that within a standardized lead pitch and for a specified number of leads on a lead frame, the small pad lead frame accommodates a large range of chip sizes. For example, a 132 pin quad flat pack lead frame may be used for many chip sizes and types, and therefore, the embodiment of this invention is applicable to a large number of Integrated circuit devices. 
   It should further be understood that precise dimensions of the indentations and impressions are dependent upon the lead frame dimensions, and that tooling dimensions are different for different lead frames, but the relative locations are similar, and therefore the invention is applicable to the entire family of small pad lead frames. 
   It should further be understood that the preferred embodiment described a tooling set and application including four (4) small circular chip pads, but that the invention is applicable to devices having a single circular or other shaped chip pads by omitting the center protrusion on the forming press. It is further applicable to LOC (lead-on-chip) or COL (chip-on-lead) lead frames where there are no chip pads, or to conventional pad devices where distortion of the ties strap is a problem. Such devices include those having a deep down-set angle, long tie straps or very thin or ductile material. 
   It should also be understood that because the stress relief mechanisms of forming depressions in both the front and back side of the lead frame are created during the forming process, the solution is permanent, as opposed to a solution where the stress corrected, but may return as a function of memory in the metal. 
   Because the stress relief mechanisms are created by hard tooling with limited tolerance ranges, the relief mechanisms are reproducible and limited by tooling tolerances.