Patent Publication Number: US-2004053447-A1

Title: Leadframe having fine pitch bond fingers formed using laser cutting method

Description:
BACKGROUND  
       [0001] 1. Field of the Invention  
       [0002] The present invention relates to packages for semiconductor chips or other electronic devices.  
       [0003] 2. Background Information  
       [0004] A typical package for a semiconductor chip includes an internal metal leadframe, which functions as a substrate for the package. The leadframe includes a central die pad and a plurality of leads that radiate outward from the die pad. A hardened, insulative encapsulant material covers the die, die pad, and an inner portion of each of the leads.  
       [0005] The semiconductor chip is mounted on the die pad and is electrically connected to the leads. In particular, the chip includes a plurality of bond pads, each of which is electrically connected by a bond wire or the like to a bond finger that is at an inner end of one of the leads. An outer portion of each lead extends outward from the encapsulant, and serves as an input/output terminal for the package. The outer portion of the leads may be bent into various configurations, such as a J lead configuration or a gull wing configuration.  
       [0006] In the market for semiconductor packaging today, there is a trend toward decreasing the bond finger pitch and/or the size of the die pad. This is driven by semiconductor die size reductions that accompany each new generation of fabrication processes. As the die size shrinks, so must the bond finger pitch, otherwise wire lengths get too long and mold yield suffers due to wire sweep. Reducing bond finger pitch allows the bond fingers to extend further into the package, which allows for shorter wire lengths. This in turn increases quality and yields, enhances electrical performance, and increase productivity.  
       [0007] In keeping with these trends, ever finer leads and bond finger pitches are required. It can be difficult to meet this industry need while also keeping the cost of the package within reason. Limitations on known methods for making leads and bond fingers, such as chemical etching or mechanical stamping, also makes meeting industry needs difficult, as these methods have inherent limitations as to how fine and dense the leads and the bond fingers can be made. At the same time, the bond fingers must be wide enough to serve as a site for electrical connection to a wire or some other conductor that electrically connects the respective bond finger to the chip. Accordingly, an improved method of making a leadframe is desirable.  
       SUMMARY  
       [0008] The present invention provides leadframes having a minimal space between the bond fingers of adj acent leads, thereby reducing the bond finger pitch. Correspondingly smaller die pads can be made with such bond fingers than is achievable by conventional methods.  
       [0009] In accordance with one embodiment of the invention, a method of making a leadframe comprises providing a metal sheet; patterning the metal sheet to form a plurality of leads that are integrally joined in an end block at an inner end of the leads; and cutting the end block with a laser to singulate the inner end portion of each lead from the end block. The patterning of the metal sheet to form the leads and end block can be carried out using a masking and etching process, or a stamping process. This method can further comprise reducing a thickness of the end block relative to an initial thickness of the metal sheet prior to laser-forming the inner end portion of the leads, which can facilitate the lasering step.  
       [0010] These and other aspects of the present invention will be more apparent in view of the following detailed description of the exemplary embodiments and the accompanying drawings thereof.  
     
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
     [0011]FIG. 1 is a top plan view of a leadframe.  
     [0012]FIG. 2 is a plan view of a portion of a leadframe at an intermediate stage of manufacture, wherein an inner end portion of a plurality of leads are integrally joined in an end block.  
     [0013]FIG. 3 is a plan view of the leadframe of FIG. 2, wherein the end block has been laser cut to singulate the bond fingers of the leads.  
     [0014]FIG. 4 is a cross-sectional side view of a semiconductor package.  
     [0015]FIG. 5 is a plan view of a portion of a leadframe wherein two leads are tied together.  
     [0016]FIG. 6 is a plan view of a portion of a leadframe with an alternative bond finger shape. 
    
    
     [0017] In the drawings, like features are typically labeled with the same reference numbers across the various drawings.  
     DETAILED DESCRIPTION  
     [0018]FIG. 1 is a top plan view of a portion of a leadframe  100  that will provide context for the discussion below. Practitioners will appreciate that the techniques of the present invention may be used to make leadframes having a wide variety of configurations. Accordingly, the overall configuration of leadframe  100  is exemplary only.  
     [0019] Leadframe  100  is formed from a metal, such as copper. Other metals also can be used, including, but not limited to, copper alloys, plated copper, plated copper alloys, copper plated steel, Alloy  42 , Alloy  37 , or any other material that is conductive and can be used for making leadframes. Typically, a plurality of leadframes are formed in a contiguous metal sheet, and the leadframes of the sheet are processed through package assembly in strip form.  
     [0020] Leadframe  100  includes a closed internal frame, denoted as dam bar  102 , that supports a plurality of leads  104  and a planar rectangular die pad  106 . Although not shown in FIG. 1, leads  104  may extend outward beyond dam bar  102 . In such a case, the portion of leads  104  inside of dam bar  102  would be called “inner leads,” and the portion of leads  104  outside of dam bar  102  would be called “outer leads.” The portion of leads  104  within dam bar  102  is encapsulated later in the assembly process.  
     [0021] Die pad  106  is at a central region of leadframe  100  and serves as a base upon which a semiconductor chip is ultimately mounted. Each of the four corners of die pad  106  is connected by a tie bar  108  to dam bar  102 . A downset  110  is provided in tie bars  108  so that die pad  106  is vertically below leads  104 . Dam bar  102  will be severed from leads  104  and tie bars  108  after an encapsulation step during package assembly, thereby leaving the package with a plurality of encapsulated leads  104  that are electrically isolated from each other.  
     [0022] Leads  104  extend inward from dam bar  102  toward all four sides of die pad  106 , as in a quad package. Each lead  104  has an inner end segment, denoted herein as bond finger  104   a , that is proximate to die pad  106 , and a longer, outer second portion  104   b  that is between bond finger  104   a  and dam bar  102 . In FIG. 1, bond fingers  104   a  are shown within the dashed line. Ultimately, the bond finger  104   a  of each of the leads  104  is electrically connected by a bond wire, tab, or some other electrical conductor to the semiconductor chip that is to be mounted on die pad  106  (see, e.g., FIG. 4). Typically, bond fingers  104   a  of leads  104  are plated with silver or some other common metal to facilitate connection to the bond wire or other conductor that extends to the chip. A nonconductive adhesive strip  112 , which may be formed of polyimide, may be applied in a ring onto second portion  104   b  of leads  104  for stability during processing and to maintain leads  104  at proper positions relative to one another. This can help to prevent two adjacent leads  104  from bending.  
     [0023] As mentioned, leadframe  100  is normally formed from a solid rectangular metal sheet that is patterned to create the configuration shown in FIG. 1. Conventionally, the patterning process involves either a chemical etching process or a mechanical stamping process.  
     [0024] A typical chemical etching process uses photolithography, a photoresist mask, and a metal-dissolving liquid chemical to etch a pattern into the metal sheet that is being used to make leadframe  100 . The liquid chemical etches away all portions of the metal sheet not masked by the photoresist mask, leaving behind the desired pattern that forms leadframe  100 . The stamping process, on the other hand, uses a series of progressive dies to cut out portions of the metal sheet to create leadframe  100 .  
     [0025] As mentioned above, there is a trend in the industry toward reducing bond finger pitch. There are, however, several constraints that limit how much bond finger pitch can be reduced when they are made using the conventional methods described above. Some of these constraints generally stem from the fabrication techniques used to form leads. For instance, the width of the spaces between bond fingers  104   a  can only be minimized so far using conventional etching and stamping techniques. Another constraint is the width of each bond finger  104   a . There is a limit to how much the width of bond fingers  104   a  can be minimized because the surface area of bond fingers  104   a  cannot go below a standard limit for the attachment of bond wires or other chip coupling means. Bond fingers  104   a  must allow for the space taken up by the bond wire or other chip coupling means, as well as allowing for tolerances in the bonding system. Although the width of bond fingers  104   a  can decrease as wire diameters decrease, it is still desirable to decrease the spaces between bond fingers  104   a  as well.  
     [0026] The present invention provides for reducing the spacing between adjacent bond fingers  104   a , and thereby achieves tighter packing of leads  104  and extends leads  104  further into the package, while maintaining the width of the bond fingers  104   a  at a width appropriate for whatever types of conductor (e.g., bond wires) and conductor attaching equipment that are used to electrically connect the bond fingers  104   a  to the semiconductor chip to be mounted on the leadframe. To achieve this objective, the conventional methods of forming bond fingers  104   a  (i.e., wet chemical etching and/or mechanical stamping) must be discarded, since these methods are relatively crude and leave considerable unused space between bond fingers  104   a.    
     [0027] In accordance with embodiments of the present invention, bond fingers  104   a  are formed using a fine laser beam. The use of such a laser beam to form bond fingers  104   a  allows for a substantial decrease in the width of the spaces between the bond fingers, which in turn allows for tighter packing of leads  104 . A method of making a leadframe in accordance with one embodiment of the present invention uses two steps for forming the leadframe. A first step employs chemical etching, mechanical stamping, or some other metal removal method to pattern a metal sheet to create the above-described portions of leadframe  100 , except for the bond finger  104   a  of the inner end portion of the leads  104 . A second step uses a fine laser beam to form the bond fingers  104   a  of the leads  104 .  
     [0028]FIG. 2 is a plan view of a portion of an incomplete leadframe  100  after the first step of the above-described two-step process. In particular, the portion of leadframe  100  shown here consists of leads  104  and dam bar  102 . The inner end portions of the leads  104  are not separate, but rather are integrally joined in a block, called end block  200  herein. This may be done, for example, by modifying the photoresist mask used in an etching process, or by modifying the dies in a stamping operation, that initially patterns the metal sheet. Second portions  104   b  of leads  104  are joined to dam bar  102  at one end and to end block  200  at the other. Dashed lines  202  in FIG. 2 represent the boundaries of the individual bond fingers that will be formed after the second step of the above-mentioned two-step process.  
     [0029]FIG. 3 is a plan view of the same portion of leadframe  100  as shown in FIG. 2 after end block  200  has been separated into individual bond fingers  104   a  in accordance with the above-mentioned two-step process. Here, rather than using etching or stamping techniques, the formation of bond fingers  104   a  is carried out using a laser, and in particular, a narrow beam laser. For example, a diode pumped YAG laser from the Rofin Basil/Sinar company of Germany may be used. The laser beam is directed at end block  200  and cuts through end block  200  to form individual bond fingers  104   a , as shown in FIG. 3. The laser beam forms ultra-narrow spaces  300  between adjacent bond fingers  104   a . Each space  300  is sufficient to electrically isolate the bond fingers  104   a  that are on either side of the space  300  from one another. Unlike in previous chemical etching or stamping techniques, however, this spacing between adjacent bond fingers is substantially minimized, thereby allowing leads  104  and bond fingers  104   a  to be packed more tightly within leadframe  100 . This tighter packing of leads  104  and bond fingers  104   a  also provides room for additional leads  104  and bond fingers  104   a  if so desired. The width of bond fingers  104   a  shown in FIG. 3 corresponds to at least the minimum width necessary for the attachment of bond wires or other chip coupling means.  
     [0030] In another embodiment of a method of forming a leadframe in accordance with the present invention, one can half-etch away or otherwise remove a portion of the thickness of end block  200  prior to the laser-cutting step. The portion may include a top, bottom, and/or side surface of end block  200 . The amount removed may be, for example, 50% or 33% to 75% of the thickness of end block  200 . This process of half-etching end block  200  is generally done as part of an initial step that forms leadframe  100  of FIG. 2, but can alternatively be done in a second etching step or removal step that takes place after the incomplete leadframe of FIG. 2 is formed. In the half-etch step, the etchant proceeds to etch through the exposed portion of end block  200 , and when the etchant has etched a selected distance through the thickness of end block  200 , the etching process is halted. The reduction in thickness of end block  200  can make the subsequent laser cutting easier and cleaner, and can increase the cutting speed of the laser.  
     [0031]FIG. 4 is a cross-sectional side view of a semiconductor package  400  made using the laser method described above with respect to FIGS. 2 and 3. Semiconductor package  400  includes a semiconductor chip  402  mounted on die pad  106  using an adhesive layer  403 , which may be any conventional adhesive, adhesive film, or adhesive tape, among other possibilities. Die pad  106  is downset from leads  104 . Chip  402  has a plurality of bond pads  404  that are each electrically coupled to an upper side  405  of a respective one of the bond fingers  104   a  of leads  104  by a metal (e.g. gold) bond wire  406 . In other embodiments, this electrical coupling can be facilitated by means other than bond wires  406 , such as tabs.  
     [0032] In semiconductor package  400  of FIG. 4, bond fingers  104   a  are formed by a laser cutting process as described above. Therefore, each bond finger  104   a  of lead  104  is separated from its neighboring bond fingers  104   a  by a pair of narrow spaces  300  (see FIG. 3) created using a laser beam. Bond fingers  104   a  are also shown as having a lesser thickness (approximately half the thickness) as the remaining second portion  104   b  of lead  104 , in accordance with the above-described optional step of reducing the thickness of bond fingers  104   a  prior to laser cutting. In particular, a recessed horizontal surface  408  is formed in lower side  410  of the leads  104  at bond finger  104   a.    
     [0033] Semiconductor package  400  also includes an encapsulant  412  that covers die pad  106 , chip  402 , bond wires  406 , bond fingers  104   a , and second portion  104   b  of leads  104 . Encapsulant  412  is typically a nonconductive polymer that is molded and cured to harden. The outer, unencapsulated portions of leads  104  may be bent into a variety of configurations, such as gull wing or J-lead configurations.  
     [0034] In an alternative embodiment of package  400 , die pad  106  may be omitted, such as in the case of a leadframe for a package where the chip is electrically coupled to the laser-formed bond fingers  104   a  using a flip-chip technique.  
     [0035]FIG. 5 is a plan view of an alternative embodiment of a leadframe  500  where the laser cutting method forms two or more leads that are integrally formed or tied together. Laser cuts  502  are made according to the methods disclosed herein, and these cuts form several leads  504 . Laser cuts  502  also form two leads  506  that are joined or tied together by an integral bar  508  that is laser-formed from end block  200 . Bar  508  extends transversely to the longitudinal direction of leads  506  around bond fingers  510  of leads  504  that are laterally between the two joined leads  506 . The integral connection of leads  506  allows for a single bond wire or other electrical connection to a semiconductor chip, and also allows a common signal or potential to be communicated to or from a semiconductor chip on joined leads  506 . The single bond wire or other connector may be connected to one of bond fingers  512  of joined leads  506  or to bar  508 . The thickness of leads  504 , including joined leads  506 , and bar  508  may be reduced (e.g. by halfetching) to facilitate the laser forming step, as is discussed above with respect to FIG. 4.  
     [0036]FIG. 6 is a plan view of an alternative embodiment of a leadframe  600  where laser cuts  602  form bond fingers  604  with a shape designed to optimize the bonding area of each bond finger  604 , while allowing for an even tighter packing. For instance, when bond wires are attached to bond fingers, the actual bonds tend to be crescent-shaped due to the bonder capillary shape of thermosonic ball bonders. So in leadframe  600 , bond fingers  604  have a wine-glass shape to accommodate the crescent-shaped bonds, where the wine-glass shape consists of a relatively wide body  606 , and a narrower stem  608 . The body  606  of each wine-glass shaped bond finger  604  has a semi-circular form that the crescent-shaped bond fits on. The lateral width of body  606  is wide enough to allow reliable bonding of a bond wire or another electrical conductor (e.g. a TAB bond) using conventional bonding equipment and methods.  
     [0037] To bring bond fingers  604  closer together, bond fingers  604  are laser-formed from an end block  200  in an alternating fashion such that adjacent wine-glass shapes are oriented in opposite directions, i.e., each wine-glass shape is rotated 180 degrees relative to adjacent wineglass shapes. So when one bond finger  604  has its body  606  proximate to the die pad, the flanking adjacent bond fingers  604  will have their stems  608  proximate to the die pad. This allows the body  606  of each bond finger  604  to squeeze between the stems  608  of adjacent bond fingers  604 . Prior to laser-forming the wine-glass shaped bond fingers  604  from an end block  200 , the thickness of end block  200  may be reduced so that the laser cutting step may be facilitated. Accordingly, bond fingers  604  would have a side profile similar to the leads  104  of FIG. 4.  
     [0038] Of course, the wine-glass shaped bond finger leads  604  of FIG. 6 are merely exemplary. The shape of the laser-formed bond fingers can be varied in a way that provides for a wider bond finger area where the actual connection will be made between the bond finger and the bond wire or other electrical conductor, and a narrow bond finger area where no connection is made. For instance, the bond fingers may have an alternating oppositely oriented T-shapes, as shown in FIG. 7.  
     [0039] Accordingly, the present invention uses a laser cutting technique to form finelypitched bond fingers of a leadframe. Unlike previously developed techniques for forming the bond fingers, in which a substantial amount of unused space was left between bond fingers of the leadframe, the laser beam forms ultra-narrow spaces between the bond fingers. This results in less wasted space and allows leads  104  and bond fingers  104   a  to be packed more tightly within leadframe  100 .  
     [0040] While exemplary embodiments of the invention have been shown and described, it will be apparent to those skilled in the art that numerous alterations may be made without departing from the inventive concepts presented herein. For example, the entire portion of the lead  104  inward of dam bar  102  may be formed by etching or stamping into a block, and the block may then be cut with a laser to singulate the entire inner lead, including the bond finger  104   a  (see FIG. 1) and the second portion  104   b  of the lead  104 . Thus, the invention is not to be limited except in accordance with the following claims and their equivalents.