Abstract:
A method for product assurance for semiconductor devices having wire bonded die and carrier assemblies comprises “puffing” the arc of each bonded wire individually with a stream of gas to force the wire arc into a substantially vertical plane which is positioned laterally away from adjacent wire arcs. The “puff” may be accomplished by a stream of compressed gas from a nozzle positioned beneath the wire or alternatively, by a vacuum drawn from a nozzle positioned above the wire arc, or by both techniques. The force exerted is sufficient to debond defective bonds, break structurally deficient wires and move debonded wires or broken wire ends away from the die, lead frame and other wires. The method is preferably applied just following wire bonding andlor just prior to encapsulation of the die.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates generally to the fabrication of semiconductor devices. More specifically, the invention pertains to the testing of wire bonds and lead wires, also called bond wires, for integrity. 
     2. State of the Art 
     The secure bonding and maintenance of bond wires extending between the bond pads of a semiconductor die and the leads of a corresponding lead frame are critical in the manufacture of semiconductor devices employing this technique. Typically, the method of interconnecting the bond pads of an integrated circuit (IC) device to a lead frame or other carrier having conductive traces comprises individual wire bonding techniques such as thermocompression, thermosonic, or ultrasonic bonding. The wires are typically formed of gold, aluminum, or alloys thereof, and have wire diameters of e.g. 0.001 to 0.003 inch. As wire sizes have become increasingly miniaturized, the inherent strength of the wires and of the wire bonds to the bond pads and leads has necessarily been reduced. 
     The now-common use of “leads over chip” (LOC) semiconductor die assemblies replaces a traditional lead frame having a central, integral support (commonly called a die-attach tab, paddle, or island) to which the back surface of a semiconductor die is secured, with a lead frame arrangement wherein the dedicated die-attach support is eliminated and at least some of the leads extend over, and are secured to, the active surface of the die. LOC die assemblies may have centrally located bond pads, thus increasing the length of the leads, which may flex during a wire bonding operation. 
     Because of the high cost of IC devices and the difficulty of correcting defective wire interconnects once the devices are encapsulated, it is vital to achieve a very high degree of reliability during wire bonding. More sophisticated methods and apparatus are required to limit the frequency of wire and wire bond failure. 
     As well-known in the art, the inadequacy or failure of bond wires and wire bonds may be caused by a wide range of factors, and may result from events occurring (a) at the time of bonding, (b) during assembly steps following bonding but before encapsulation, and (c) during encapsulation. 
     Inadequately bonded wires may occur because of many reasons, including bond pad surfaces which are not adequately cleaned, incomplete metallization of bond pads, wire impurity, inadequate bonding temperature, stress-strain mismatches, excessive flexing, corrosion, intermetallic grain growth followed by stress-induced creep, as well as by other causes. 
     The wires themselves may occasionally break because of impurity-induced weakness, corrosion, and mishandling such as an accidental excessive force applied during wire-pull testing. 
     Wires may also be weakened or fail during die processing steps subsequent to bonding but before encapsulation. Handling of the die during intermediate processing steps such as testing and inspection may result in weakening or separation of inner (to a bond pad) or outer (to a lead or trace) wire bonds as well as occasional breakage of a wire itself. 
     The step of encapsulating the die with a plastic or ceramic material may also result in wire or bond failure. In transfer molding of a lead frame-mounted semiconductor die, the die is suspended from its active surface from the underside of inner lead extensions of a lead frame (typically Cu or Alloy 42) by a tape, screen print or spin-on dielectric adhesive layer. The bond pads of the die and the inner lead ends of the frame are then conductively connected by wire bonds (typically Au, although Al and other metal alloy wires have also been employed) by means known in the art. When any intermediate steps are completed after wire bonding, the resulting die assembly is placed in a mold cavity and encapsulated in a heated, thermosetting particulate-filled polymer which, upon curing, forms a highly cross-linked matrix no longer capable of being melted. Typically, a post-cure step completes the curing of the polymer. The die and lead frame assembly may comprise the framework of a dual-in-line package (DIP), zig-zag in-line package (ZIP), small outline j-lead package (SOJ), thin small outline package (TSOP), quad flat pack (QFP), plastic leaded chip carrier (PLCC), surface mount device (SMD) or other plastic configuration. 
     During transfer molding, a defect known as “wire sweep” may become a troublesome problem. In this type of defect, the advancing flow front of liquid thermoset molding compound sweeps the wires against each other, causing short-circuiting. Factors that tend to exacerbate wire sweep include high wire loop heights, long wire bond lengths, bond orientation perpendicular to the advancing polymer flow front, rapid mold compound transfer times, high transfer pressures, rapid viscosity rise of the polymer melt, relatively low wire modulus, and insufficiently bonded wires. The difficulty in precisely controlling all of the above factors results in packaged, yet defective, semiconductor devices which must be discarded at considerable loss. 
     As is well known in the art, the wires and wire bonds may also fail during or following post-curing because of the stresses formed in the polymer package. 
     Of course, once the die has been encapsulated by transfer molding, it is very difficult, if even possible, to correct a wire which is broken, shorted, or which has become unbonded from a bond pad or lead finger. Even though the cost of manufacturing a semiconductor device through the encapsulation step is substantial, it is rarely economical to attempt repair of a defective wire or wire bond after transfer molding. Removal of the encapsulant without destroying the interconnecting wires is extremely difficult, particularly when the encapsulant is a transfer-molded filled polymer. 
     Although the state of the art in wire bonding is continually improving, ever-increasing demands for further miniaturization, increased circuit complexity, enhanced production speed, reduced cost, product uniformity and reliability require further improvements in quality control. 
     It is apparent that to avoid failure of wires or wire bonds, both the wire modulus and the bond strength should be as high as reasonably possible, given known process parameters. It is thus desirable to provide a method for confirming that prior to encapsulation, all of the wires and/or bonds meet a predetermined minimum value of strength. 
     One test which is used to determine the wire bond strength comprises the use of a hook to pull a lead wire loop upward with an increasing measured force until a wire bond breaks. This is a destructive test and is not used routinely on production dies. 
     In a related test, a lead wire is pulled upward by a hook at a minimum threshold force value indicative of satisfactory bonds. Only inadequately bonded leads fail the test, so the test is at least arguably “non-destructive”. Wire pull testing is typically conducted under 25×-50× magnification and is a tedious and time consuming task. Further, damage is occasionally incurred by the wire under test, or by adjacent wires. The bond pulling test is not generally appropriate for testing wire bonds of very closely spaced lead wires used in many recently developed semiconductor devices. 
     U.S. Pat. No. 3,581,557 of Drees et al. briefly indicates the common problem of inadequate bonding of wires to semiconductor die bond pads and package leads. Drees et al. discloses an apparatus for exerting a “puff” of gas substantially transversely across and at a selected angle to each wire following its bonding to break away weakly bonded wires. The test is conducted on each wire immediately after it has been bonded at both ends, so that any failed wires will not be displaced by the relatively horizontal puff of gas into adjacent wires. Thus, the test must be conducted on a lead wire before the next adjacent wire is bonded to the die and lead frame. If the final wire fails in this test, it may be blown into the adjacent first wire and may displace or break that wire as well. Thus, the test has distinct problems if used following completion of multiple wire bonds. In addition, the “puff” duration is severely limited in order to avoid wire vibration induced by the gas stream. 
     U.S. Pat. No. 5,085,084 of Salatino discloses a method for testing the bond integrity of lead frame leads to bond pads by applying a fluid to the underside of a plurality of leads simultaneously. A conductive sensor is positioned above the leads, and if a lead-to-bond pad bond fails due to the applied fluid force, its broken end is blown upward to contact the sensor and close an electrical circuit. The Salatino method is also disclosed as being applicable to testing of wire bonds. 
     Several problems are inherent in the Salatino test. First, the actual force exerted on each lead will depend upon the free area for gas flow in the vicinity of the lead. Thus, the width and proximity of adjacent leads will affect the applied force. Both of these factors may be highly variable, not only from die to die, but in respect to the leads on a single die as well. Further, failure of a single lead-to-bond pad bond results in movement of that lead out of the lead pattern, decreasing the resistance to air flow and reducing the force exerted on adjacent leads. Thus, the force is not evenly applied to the leads. 
     SUMMARY OF THE INVENTION 
     The present invention comprises a method for aligning and testing integrity of bond wire interconnections extending between the bond pads of a semiconductor die and leads of a lead frame or conductive traces on a carrier substrate such as a circuit board. Broken wires and inadequate bonding of the wire ends are detected at times in the manufacturing process where the wires may be readily replaced, i.e., following bonding and before encapsulation. 
     In the method of the invention, the upwardly extending “arch” or arc (also commonly termed a “loop”) formed by each acceptable bonded wire is motivated upwardly into a generally vertical plane passing through the inner and outer bonds of the wire. Thus, the lateral distance between adjacent bond wires is generally maximized. The upward motivation of defective, i.e. broken or inadequately bonded, bond wires results in their upward movement away from the die, lead frame and other interconnection wires. A broken or debonded wire will not contact an active surface, bond or other wire, thus avoiding short circuiting or damage to other wires. 
     The method of the invention comprises a quality control procedure which is preferably performed at least two times during the manufacture of a semiconductor device, i.e., 1) following wire bonding to ensure that all bond wires have been securely bonded at both ends by the wire bonding capillary and that no bond wires have broken, and 2) following any intermediate manufacturing steps and handling and prior to die encapsulation, to ensure that the wire bonds and wires have not been damaged between wire bonding and the intended encapsulation of the die in a filled-polymer package by transfer molding, or by other techniques such as so-called “glob topping” with a silicone gel or other viscous encapsulant. 
     Thus, broken bond wires and/or inadequately bonded wires will be detected and replaced before a badly-bonded die is encapsulated. 
     Each interconnecting bond wire bonded between the die and lead frame is subjected to an upward fluid force on the underside of the wire arc or arch. The force may be produced by a brief stream of compressed gas from a nozzle positioned beneath the wire arc. Alternatively, a vacuum nozzle closely positioned to the upper side of a ad wire arc may be utilized to draw a wire upwardly. In some cases, a lower compressed gas nozzle and an upper vacuum nozzle may be used in combination to provide a desired upwardly directed force. 
     The force exerted on the underside of an arc of an interconnection wire is selected to debond an inadequately bonded wire. The method is performed at a time when the defective bond may be reworked at minimum time and expense, or the device discarded before the additional manufacturing expense of encapsulation is expended thereon. The detection and removal of badly wire-bonded dice prior to insertion in the transfer-molding apparatus ensures that only adequately bonded dice are encapsulated. 
     The method of the invention may also reduce the frequency of incidence of wire sweep because bent and leaning wires are substantially vertically reoriented at, or close to, a maximum separation distance immediately prior to transfer-molding. 
     The correction of weak bonds prior to encapsulation also reduces failure due to expansion/contraction of the encapsulant during and following encapsulation. 
     The use of a stream of gas to orient a bond wire upwardly to a maximal arc position overcomes the tendency to bond wire vibration which may occur in lateral “puffing” of the wire, as described in U.S. Pat. No. 3,581,557. Thus, the “puffing” duration is not limited to extremely short periods to avoid wire destruction. 
    
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
     The invention is illustrated in the following figures, wherein the elements are not necessarily shown to scale. 
     FIG. 1 is a perspective view of an integrated circuit die connected to a lead frame by wire bonding and apparatus of the invention for conducting an alignment-and-test procedure on the wires; 
     FIG. 2 is a side view of a die and lead frame with apparatus of the invention for conducting an alignment-and-test procedure on the interconnecting wires; 
     FIG. 3 is a side view of a die and lead frame with apparatus of another embodiment of the invention for conducting an alignment-and-test procedure on the interconnecting wires; and 
     FIG. 4 is a side view of a die and lead frame with apparatus of a further embodiment of the invention for conducting an alignment-and-test procedure on the interconnecting wires. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     A method and apparatus are provided by the invention for testing the wire bonds of a semiconductor die, wherein (a) a weak or broken bond or wire is identified by completing breakage thereof, (b) strongly bonded wires are aligned in positions at a maximum distance from each other, and (c) wires extending to a failed bond are moved to a vertical orientation away from the active surface of the die, the lead frame, and other wires. 
     With reference to the drawings, and particularly to FIGS. 1 and 2, a semiconductor die  10  supported on a die paddle  13  of a conventional lead frame  14  is shown on a chuck or other substrate  12  and having leads  30  of the lead frame  14  supported by support members  16 , e.g. clamps in a position surrounding the die  10  but separated from it by peripheral space  18 . The substrate  12  and lead frame  14  are mounted to be movable in unison, whereby wires  22  may be sequentially bonded between the die  10  and the lead frame  14 . A bonding tool  20 , shown as a wire feeding capillary, is used to bond the wire  22  to a bond or die pad  26  of the die  10  and a lead  30  of the lead frame  14 . The bonding tool  20  comprises a capillary for thermocompression or thermosonic bonding, or a wedge for ultrasonic bonding. Conventional interconnecting conductive wires  22  have inner ends  24  shown as being ball-bonded at  45  to peripheral die pads  26  on the die  10 , and outer ends  28  wedge-bonded by bonds  44  to leads  30  of the lead frame  14 . 
     In a first embodiment of the invention shown in FIGS. 1 and 2, a compressed gas nozzle  32  is selectively positioned beneath a bond wire  22 . An upwardly directed stream  36  of gas is provided through a nozzle orifice  34  of nozzle  32  and in an orientation along line  38  to exert an upward force on the underside  42  of the bond wire  22  (e.g. the inside of the wire “arc” or “arch”). The nozzle  32  and wire  22  are aligned so that (a) the wire will generally bisect the gas stream  36  and (b) the distance  40  from the nozzle orifice  34  to the wire  22  is within a relatively narrow range for each wire undergoing the operation. Given these constraints, the fluid force exerted on each tested wire  22  meets the test criteria. 
     The nozzle  32  is configured to move in a horizontal (x-y) plane for alignment with each wire  22  in turn. In addition, the nozzle  32  is moveable in a vertical direction  46  to provide the desired proximity  40  to the underside  42  of the wire  22  undergoing the test. Furthermore, the nozzle  32  is moveable to a limited degree from the vertical position. Thus, the nozzle  32  may be positioned at an angle  48  of up to about 30 degrees from the vertical in any direction, so that a wire  22  on any side of the die  10  may be subjected to a selectively-angled fluid force aligning it in a generally vertical plane, or in a somewhat non-vertical plane to place the wire  22  at a maximum distance from adjoining wires  22 . Such selectivity may be useful, for example, with interconnecting wires  22  adjacent the die comers  47 . 
     The stream  36  of gas is directed at the underside  42  of the wire  22  at a sonic or subsonic velocity at which a laminar flow regime is established, as is well-known in the field of fluid flow dynamics. The velocity may be varied (alone or in conjunction with proximity to the wire) to achieve a desired minimum test force, depending upon the wire diameter, wire modulus, required bond strength, wire length, wire-to-orifice distance  40 , and other factors. The optimum velocity may be calculated from the above factors, or determined by testing. 
     The method of the test comprises mounting the die  10  and lead frame  14 , supported respectively by chuck  12  and clamps  16 , on a bonding machine pedestal  17  and conductively bonding the inner end  24  and outer end  28  of a bond wire  22  to a die pad  26  and lead  30  of the lead frame  14 , respectively, forming an arc, arch or loop  29  therebetween. A central portion of the arc  29  of the wire  22  is subjected to a generally brief upward burst of fluid force to align the arc  29  of a firmly bonded wire  22  into a vertical plane passing through said inner and outer ends of the wire. Should either or both of the wire bonds  44 ,  45  be too weak to withstand the fluid force, i.e. separate from the die pad  26  or lead  30 , the defectively connected wire  22  will be forced by the gas stream  36  upwardly and away from the die  10 , lead frame  14  and other interconnecting wires  22  bonded to the die and lead frame. Likewise, ends of a broken wire  22  will similarly be blown upwardly. 
     The “central portion” of the wire  22  is defined as comprising approximately the middle 80 percent of the wire between the respective inner and outer ends  24 ,  28 . 
     Another form of apparatus for performing the invention is illustrated in FIG. 3. A die  10  is shown attached to a carrier substrate  12  which may be e.g. a circuit board with lead traces  52  of a lead frame. Such a configuration, termed “chip on board” or “COB,” is conventionally used when a plurality of dice is back-bonded to a substrate and then wire bonded, as in fabrication of memory modules. In this instance, there is insufficient access space to place a compressed gas nozzle beneath the wires  22 . Interconnection wires  22  are shown bonded to die pads  26  by bonds  45  and to the lead traces  52  with bonds  44 . A substantially vertically oriented vacuum nozzle  50  is positioned immediately above a bonded wire  22  for drawing a vacuum for a short time period. The vacuum nozzle  50  draws an upward stream  54  of gas past a central portion  60  of the wire  22 , drawing it upward. As a result, (a) adequately bonded wires  22  are aligned in a generally vertical plane separated from adjacent wires  22 , and (b) inadequately bonded wires  22  are separated at a bond site and moved upward away from the die  10 , lead traces  52  and other wires  22 . The defective bonds may thus be readily identified, and the wires removed and replaced. Again, broken wires  22  will also be clearly identified by their elongated, free ends. 
     The vacuum nozzle is moveable vertically in direction  56 , in a horizontal plane, and at an angle  58  from the vertical of up to about 20 degrees. 
     The typical force useful for aligning and testing a 1-3 mil. diameter gold wire may, for example, be about 1-4 grams and more preferably, about 1.5-2.5 grams force. Such a force will align an adequately bonded wire in the desired vertical plane, and debond and position a defectively bonded wire, or a broken wire, upwardly away from potential short-circuiting or damage caused by inappropriate contact with other bond wires  22 , bond pads  26 , or traces  52 . 
     The time during which a gas stream  36  is blown against the wire  22 , or a vacuum is drawn above a wire, is desirably as brief as possible to maintain a high production rate. Thus, the wire  22  may be “puffed” for, e.g., about 0.1 second to no more than several seconds. However, there is no set time limit imposed by induced detrimental wire vibration, as in a prior art test method using substantially laterally oriented puffs. Thus, for example, the force may be continuously applied while a broken wire or one having a defective bond is being removed, to facilitate projection thereof above the other wires  22 . 
     It should be recognized that the embodiment of FIG. 3 is also especially applicable to testing bond wires and wire bonds in an LOC die and lead frame configuration, since such a configuration necessarily places the arcs, arches or loops  29  of the bond wires  22  over the active surface of the die (since the leads extend onto the die), preventing placement of a nozzle  32  under the wires  22 . 
     In another version of the apparatus useful in this invention and as shown in FIG. 4, a test apparatus may include both a compressed gas nozzle  32  positioned below a wire  22 , and a vacuum nozzle  50  positioned above the wire. The wire  22  may be subjected to an upward gas stream from the compressed gas nozzle  32  as in FIG. 2, to an upward gas stream from a vacuum nozzle  50  as in FIG. 3, or to an upward gas stream  62  effected by use of both nozzles concurrently or sequentially. In the latter mode and when concurrent flow is employed, the two nozzles  32 ,  50  are aligned along centerline  64  to provide a coherent laminar stream  62  of gas. The force exerted on the wire  22  may be more accurately controlled through the use of both nozzles. 
     It is significant that the operation of this invention may be conducted at two separate times during manufacture of the semiconductor device. First, use of the method immediately following the wire bonding step (of each individual wire or of all wires collectively) ensures that all wires are adequately bonded prior to further handling and, possibly expensive intermediate manufacturing steps such as testing. Second, use of the method following any intermediate steps but prior to encapsulation, e.g. in a filled-polymer package by transfer molding, ensures that any bond/wire damage is corrected and the wires are optimally aligned before the encapsulation step. 
     As described herein, the invention provides a non-destructive quality assurance method which reveals defects and inadequate bonding of wires to a semiconductor die and to a lead frame or traces on a carrier substrate such as a circuit board. The method may be applied to various die and lead frame configurations as described herein, without limitations including the aforementioned LOC configurations and chip-on-board (COB) assemblies. In the latter, wire bonds are formed between the bond pads of the dice and conductive traces on the board. Thus, the method may be applied before the die-and-board assemblies are “glob-topped” e.g. with a silicone gel or an epoxy, or otherwise encapsulated, as by potting. 
     It is apparent to those skilled in the art that various changes and modifications may be made in the methods of testing wires and wire bonds between any type of semiconductor die and a lead frame or carrier substrate as disclosed herein without departing from the spirit and scope of the invention as defined in the following claims.