Lead frame design for reduced wire sweep having a defined gap between tie bars and lead pins

Wire sweep/crossing during resin molding is significantly reduced or prevented by reducing the gap spacing between corner lead pins and the tie bars of a die-attach pad. Embodiments of the present invention include spacing the tie bars from the corner lead pins by a distance no greater than about 18 mils, e.g., about 4 to 12 mils. Embodiments of the present invention also comprise a lead frame wherein the inner ends of the lead pins are arranged in a substantially planar array to define a substantially circular region surrounding the die-attach pad.

FIELD OF THE INVENTION
 The present invention relates to plastic packaging of integrated circuits,
 particularly to the design of a lead frame. The present invention is
 particularly applicable to plastic quad flat pack (PQFP) devices.
 BACKGROUND ART
 Conventional semiconductor chip or die packaging techniques comprise
 bonding a semiconductor chip to a die-attach pad secured by tie bars and
 then sealing by synthetic resin molding, whereby the resulting
 semiconductor device or package is formed. As the geometry of transistors
 continues to plunge into the deep sub-micron regime, plastic packaging of
 integrated circuits presents greater challenges to manufacturers. The
 reduction in the transistor size results in a smaller die with a reduced
 perimeter around its edges for securing bond pads which provide
 input/output connections to external circuitry. Each bond pad on the die
 is connected to an internal package lead for transferring electrical
 signals between the integrated circuit and external circuitry.
 Conventional lead frame designs are attendant with various problems. As the
 geometric size of a die shrinks, at least three problems are exacerbated.
 Firstly, as the die shrinks in size, there is less perimeter around the
 die, resulting in tighter bond pad pitch. To accommodate this tighter bond
 pad pitch, finer diameter bond wires are conventionally used, e.g., down
 to about 0.9 mil. However, finer diameter wires tend to sweep to a greater
 degree than thickner wires during molding, thereby generating greater
 shorts. Secondly, there are manufacturing limits to the minimum lead frame
 pitch at a given lead width. Accordingly, as the die is reduced in size,
 the lead fingers cannot be moved proportionately, thereby resulting in
 longer wires. Longer wires are more vulnerable to sweep than shorter
 wires, thereby increasing shorts. The phenomenon known as wire sweep
 occurs in packaging integrated circuit devices and is particularly acute
 at the corner of a die because the distance between each of the corners of
 a polygonal lead frame and an adjacent bonding pad on the circuit chip
 tends to be the longest.
 Conventional approaches to minimizing wire sweep and crossing involve
 increasing the bond pad pitch proximate the corners of the die-attach pad.
 However, greater bond pad pitch proximate the corners increase the
 dimensions of the chip, thereby increasing manufacturing costs. Another
 conventional approach is to shorten the bond wires by reducing the width
 of a metal lead. This approach, however, compromises the overall yield of
 wire bonding semiconductor chips and increases lead frame manufacturing
 costs.
 Accordingly, there exists a need for a lead frame design that minimizes or
 eliminates electrical shorting, particularly due to mold sweep. There
 exists a particular need for such a lead frame design suitable for use
 with a submicron integrated circuit die.
 DISCLOSURE OF THE INVENTION
 An advantage of the present invention is a lead frame design that
 eliminates or substantially reduces wire sweep and wire crossing, i.e.,
 short circuiting.
 Additional advantages and other features of the present invention will be
 set forth in part in the description that follows and in part will become
 apparent to those having ordinary skill in the art upon examination of the
 following disclosure or may be learned from the practice of the present
 invention. The advantages of the present invention may be realized and
 obtained as particularly pointed out in the appended claims.
 According to the present invention, the foregoing and other advantages are
 achieved in part by a lead frame for an integrated circuit, the lead frame
 comprising: a die-attach pad for supporting a semiconductor die thereon; a
 plurality of lead pins with inner ends arranged in an array to define a
 central region surrounding the die-attach pad; and a plurality of tie bars
 for supporting the die-attach pad, the tie bars extending from the
 die-attach pad between lead pins to form a gap between a tie bar and a
 lead pin; wherein the gap is no greater than about 18 mils.
 Embodiments of the present invention comprise a substantially polygonal
 shaped die-attach pad having four corners and a tie bar extending from
 each corner. Embodiments of the present invention also comprise
 controlling the gap between each tie bar and a lead pin to no greater than
 about 15 mils, such as less than about 10 mils. Embodiments of the present
 invention also comprise a lead frame wherein the inner ends of the lead
 pins are arranged in a substantially planar array to define a
 substantially circular central region surrounding the dieattach pad.
 Additional advantages of the present invention will become readily apparent
 to those having ordinary skill in the art from the following detailed
 description, wherein the embodiments of the present invention are
 described, simply by way of illustration of the best mode contemplated for
 carrying out the present invention. As will be realized, the present
 invention is capable of other and different embodiments, and its several
 details are capable of modifications in various obvious respects, all
 without departing from the present invention. Accordingly, the drawings
 and description are to be regarded as illustrative in nature, and not as
 restrictive.

DESCRIPTION OF THE INVENTION
 The present invention addresses and solves the shorting problem stemming
 from wire crossing, particularly during resin molding semiconductor
 packages, most particularly in semiconductor packages involving
 miniaturized semiconductor dies. During extensive experimentation and
 investigation, it was found that such wire crossing and, hence, shorting,
 is primarily due to wire sweep during resin molding. Upon further
 experimentation and investigation it was found that a significantly large
 gap is typically formed between the corner tie bars and lead pins of a
 conventional lead frame, e.g., greater than about 40 mils. It is believed
 that the wire sweep problem is caused by turbulence of the mold compound
 flow during molding. Experimentation was then conducted on techniques to
 reduce the turbulence of the mold compound flow. As a result of such
 experimentation, it was found that a reduction in the gap spacing between
 the tie bars and lead pins resolved this issue.
 A conventional lead frame design is depicted in FIG. 1 and comprises a
 die-attach pad 10, generally of a square shape, with tie bars 11 extending
 from each of the four corners of die-attach pad 10. The lead frame
 comprises a plurality of lead pins 12, numbered 1 through 160, with
 separations at each corner to accommodate the tie bars 11 generating a gap
 G between a tie bar and a lead pin of about 40 mils or greater. It was
 found that a reduction in the gap between the tie bars and lead pins
 significantly reduced and in many cases eliminated wire sweep, thereby
 significantly reducing or eliminating shorting due to crossed wires during
 resin molding.
 Embodiments of the present invention, therefore, comprise reducing the gap
 between a tie bar and a lead pin to no greater than about 18 mils, such as
 no greater than about 15 mils, e.g., no greater than about 10 mils. A
 suitable range for the gap between the tie bar and lead pin is about 4
 mils to about 12 mils. Embodiments of the present invention are applicable
 to any or various shaped die-attach pads, such as various polygonal shaped
 pads, e.g., rectangular and square shaped die-attach pads having four
 corners and a tie bar extending from each of the corners.
 An embodiment of a lead frame according to the present invention is
 schematically illustrated in FIG. 2 and comprises a substantially square
 die-attach pad 20 with tie bars 21 extending from the four corners thereof
 which tie bars 21 are relatively wide to occupy more space at the corners
 between the lead pins, thereby reducing the gap R between the tie bars and
 lead pins. In accordance with embodiments of the present invention, R is
 no greater than about 18 mils, such as no greater than about 15 mils,
 e.g., no greater than about 10 mils.
 It was also found that wire sweep can be virtually eliminated by providing
 a circular lead frame design, wherein the inner ends of the lead pins are
 arranged in a substantially planar array to define a substantially
 circular central region surrounding the die-attach pad. The lead pins
 typically have outer ends disposed in non circular array with transitions
 to radial orientation proximate the inner ends thereof occurring
 intermediate the inner and outer ends.
 Another embodiment of a lead frame according to the present invention is
 schematically illustrated in FIG. 3 and comprises a substantially square
 shaped die-attach pad 30 with tie bars 31 extending from the corners
 thereof. The lead frame design depicted in FIG. 3 comprises a plurality of
 lead pins 32 arranged to define a substantially central circular region
 about die-attach pad 30.
 In the circular lead frame design, the inner lead pins of the circumference
 of a circle result in a reduced distance from the lead pins to the bond
 pads at the corners of the die. Each of the lead pins connects to a
 corresponding bond pad on a semiconductor die through a bond wire. Since
 the lead pins are arranged in a substantially geometric circular shape,
 the width of the metal leads required along the circumference of the
 circular lead frame do not differ as much as with other geometric shapes
 for lead frames.
 Circular lead frames, in effect, permit shorter wires to wire bond to the
 four corners of a semiconductor die and to the corresponding lead tips of
 the circular lead frame. Wire sweep is, therefore, minimized by a circular
 lead frame design. Unlike a polygonal lead frame, the short wires from the
 corner of the die to the lead frame are less likely to touch adjacent
 wires, thereby reducing electrical shorts in the packaged integrated
 circuit. The use of a circular lead frame design minimizing wire sweep
 enables a further reduction in the pitch between the bond pads on an
 integrated circuit die and, consequently, pad dimensions. Accordingly, a
 circular lead frame design is conducive to further miniaturization. In
 U.S. patent application Ser. No. 08/590,392 (now abandoned) a circular
 lead frame design is disclosed.
 Adverting to FIG. 3, the inner ends of the lead pins 32 form a
 substantially geometric circular cavity in the center of the lead frame
 and are aligned substantially along radii from the center of the circular
 region. Thus, the metal lead pins 32 are oriented along radial courses
 near the inner ends and near non-radial courses near the outer ends, with
 transitions in courses occurring through transition regions intermediate
 the inner end and outer ends of the lead pins. The circular cavity
 provides a framework to wire bond the inner ends of the lead pins to bond
 pads 32 on semiconductor die 31. The die-attach pad 30 can be adhesively
 attached beneath the lead pins in a conventional manner, e.g., as a
 heatspreader or heatsink, via an insulating layer to support a
 semiconducted die thereon and to aid in retaining the inner ends of the
 lead pins in fixed positions.
 When a heatspreader is attached to a lead frame then the critical gap is
 not the "tiebar gap" but rather slots in the heatsink and tape that allow
 molding compound to flow unimpeded by the heatsink. The most critical area
 of this gap is in the location of the heatsink that is closer to the die
 than the lead tips.
 A circular lead frame in accordance with an embodiment of the present
 invention is schematically illustrated in FIG. 4. The lead frame 40
 comprises of a plurality of metal leads 42 arranged adjacent to one and
 another in a planar array. The outer ends 43 of the metal leads from a
 rectangular shape for conventional plastic packaging. The inner ends 44 of
 the plurality of metal leads 42 form a geometric circular cavity 46 in the
 center of the lead frame 40, and are aligned substantially along radii
 from the center of the circular region. Thus, the metal leads 42 are
 oriented along radial courses near the inner ends 44 and along non-radial
 courses near the outer ends 43, with transitions in courses occurring
 through transition regions 46 intermediate the inner ends 44 and outer
 ends 43 of the metal leads 42. The circular cavity 46 provides a framework
 to connect the inner ends 44 to bonding pads (not shown) on a die (not
 shown).
 An alternate manifestation contained in the concept of FIG. 4 would be the
 attachment of a heatspreader to a lead frame that contains a separate die
 attach pad. The central base of that die attach pad (not shown) would be
 supported relative to the lead frame by metal support bars (not shown)
 that are substantially evenly spaced about the cavity. That manifestation
 is alternate to that shown in FIG. 4 where the heatspreader serves as the
 die attach pad and is adhesively attached to a padless frame. In both
 cases the insulating adhesive layer may be disposed outwardly from the
 circular lead frame.
 The central base or die-attach pad 41 may be supported relative to the lead
 frame by metal support bars (not shown) that are substantially evenly
 spaced about the circular cavity. Alternatively, a die-attach pad 41 may
 be adhesively attached beneath the leads 44 in conventional manner via an
 insulating layer to support a die thereon and to aid in retaining the
 inner lead ends 44 in fixed positions. The insulating adhesive layer may
 be disposed outwardly from the circular inner lead tips 44 and inwardly
 from the outer perimeter 47 of the die-attach pad 41. Also, a die-attach
 pad supported in this manner with respect to the lead frame may be formed
 of thick metal of substantial thermal mass to serve as a heat sink for an
 attached die.
 The present invention provides a lead frame design that substantially
 reduces wire sweep and, hence, shorting of bond wires. The present
 invention is applicable to the manufacture of any or various type of
 semiconducted packaging, particular highly miniaturized semiconductor
 packages.
 Only the preferred embodiment of the present invention and but a few
 examples of its versatility are shown and described in the present
 disclosure. It is to be understood that the present invention is capable
 of use and various other combinations and environments and is capable of
 changes or modifications within the scope of the inventive concept as
 expressed herein.