Abstract:
Methods and apparatus for providing an integrated circuit using a multi-stage molding process to protect wirebonds. In one embodiment, a method includes attaching a die to a leadframe having a lead finger, attaching a wirebond between the die and the leadfinger, applying a first mold material over at least a portion of the wirebond and the die and the leadfinger to form an assembly, waiting for the first mold material to at least partially cure, and applying a second mold material over the assembly.

Description:
BACKGROUND 
     As is known in the art, integrated circuits (ICs) can include a leadframe to support a die and to provide external connections for the IC package. Connections between the die and the leadframe fingers can be made with wire bonds, for example. For plastic packages, a leadframe/die assembly is overmolded with a plastic material. 
     In one conventional package, a sensor includes a so-called K-type assembly having a backside to which a magnet is adhered. The assembly with the attached magnet is molded or encased into a sensor housing. In one known process, a die is attached to a leadframe, a flux concentrator and/or magnet is attached, and the assembly is overmolded. 
     During the molding process, the relatively delicate wirebonds may be broken by the pressurized flow of plastic material. In addition, the fragility of the wirebonds may limit the efficiency of the molding process. 
     SUMMARY 
     The present invention provides methods and apparatus for an integrated circuit having a first mold material applied to protect wirebond connections between a die and a leadfinger and a second mold material applied over the first mold material. By protecting the wirebonds with the first mold material, the second mold material can be applied at relatively high pressures. With this arrangement, yields are improved due to less wirebond damage. While the invention is primarily shown and described in conjunction with sensor integrated circuits, it is understood that the invention is applicable to integrated circuits in general in which it is desirable to protect delicate connections from a die. 
     In one aspect of the invention, a method comprises attaching a die to a leadframe having a lead finger, attaching a wirebond between the die and the leadfinger, applying a first mold material over at least a portion of the wirebond and the die and the leadfinger to form an assembly, waiting for the first mold material to at least partially cure, and applying a second mold material over the assembly. 
     The method can further comprise one or more of the following features: applying the mold material to only a die-side of the leadframe, applying the first mold material to encapsulate the wirebond, applying the first mold material to encapsulate the die on the leadframe, selecting a material for the first mold material based at least in part upon similarity of coefficient of expansion with a magnet, the first and second mold materials are different, selecting a material for the first and/or second mold material based at least in part upon filler size, applying the second mold material at a substantially higher pressure than a pressure at which the first mold material was applied, the die includes one or more Hall elements, attaching a concentrator to the assembly, and the wirebond includes a gold wire. 
     In another aspect of the invention, an integrated circuit device comprises a die attached to a leadframe having a lead finger, a wirebond to make a connection between the die and the leadfinger, a first mold material covering at least a portion of the wirebond and the die and the leadfinger to form an assembly, and a second mold material overmolding the assembly. 
     The device can further include one or more of the following features: the first mold material is only on a die-side of the leadframe, the first mold material encapsulates the wirebond, the first mold material encapsulates the die on the leadframe, the first mold material has a coefficient of expansion compatible with a coefficient of expansion for a magnet, the first and second mold material are different, the die includes one or more Hall elements, the wirebond includes a gold wire. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The exemplary embodiments contained herein will be more fully understood from the following detailed description taken in conjunction with the accompanying drawings, in which: 
         FIG. 1  is a schematic representation of an exemplary integrated circuit having a multi-stage molding process in accordance with exemplary embodiments of the invention; 
         FIG. 1A  is a pictorial representation of the integrated circuit of  FIG. 1 ; 
         FIG. 2A  is a transparent top view line drawing of an exemplary integrated circuit having a multi-stage molding process in accordance with exemplary embodiments of the invention; 
         FIG. 2B  is a transparent side line drawing of the integrated circuit of  FIG. 2A ; 
         FIG. 3  is a partially transparent perspective view of an exemplary integrated circuit having a multi-stage molding process in accordance with exemplary embodiments of the invention; 
         FIG. 4  is a flow diagram showing an exemplary sequence of steps to implement a multi-stage molding process for an integrated circuit in accordance with exemplary embodiments of the invention; and 
         FIG. 5  is a pictorial representation of an integrated circuit sensor having wirebonds that can be protected by a first mold material to form an assembly that can be overmolded with a second mold material. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  shows an exemplary integrated circuit (IC)  100  provided as a sensor fabricated with a multi-step molding process in accordance with exemplary embodiments of the invention. In the illustrative embodiment, the sensor  100  is a gear tooth sensor (GTS) to detect motion of teeth  10  on a gear  12 . In general, a first molding step is performed to protect wirebonds. After the first molding process is complete, a second molding step is performed to provide the final package configuration. 
     In an exemplary embodiment, the sensor  100  includes a Hall IC  102  having first and second Hall elements  104 ,  106  disposed on a leadframe  108 . A concentrator  110 , shown as a pole piece concentrator, is disposed on a backside of the assembly with a magnet  112  secured to the concentrator. As described more fully below, in an exemplary embodiment the assembly is molded in a first step to protect wirebonds from the IC  102  to the leadframe  108  prior to overmolding the assembly having the concentrator  110  and magnet  112 . 
       FIG. 1A  shows an exemplary IC package  100  for the Hall integrated circuit of  FIG. 1  in a 4-pin  120   a - d  SIP (single inline package) configuration. The IC  100  is a two-wire differential peak-detecting gear tooth sensor (GTS). 
       FIGS. 2A and 2B  show cross-sectional views illustrating an exemplary integrated circuit  200  having a first mold material  202  and a second mold material  204  fabricated in a multi-stage molding process in accordance with exemplary embodiments of the invention. The IC  200 , shown as a sensor IC, includes a die  206  disposed on a leadframe  208 . A series of wirebonds  210  make electrical connections between the die  206  and leadfingers  212  of the leadframe  208 . The die/wirebond assembly is overmolded with the first mold material  202 . The IC  200  further includes a concentrator  214  and magnet  216  overmolded by the second material  204 . 
     In the illustrated embodiment, the wirebonds  210  extend from the die  206  to only one side of the leadframe  208 . In other embodiments, wirebonds can extend from both sides and/or ends of the die  206 . 
     The first mold material  202  is applied in a manner to protect the integrity of the wirebonds  210 . In one embodiment, the first mold material  202  encapsulates only the die  206  and wires  210  on the die side of the leadframe  208 . No molding occurs on the backside of the package until additional process steps are performed before completion. 
     It is understood that wirebonds  210  can be formed from materials and/or in dimensions that render the wirebonds susceptible to damage. While some materials are desirable for electrical conductivity, ease of connection, and/or ease of fabrication, e.g., ductility, such materials may be easily deformed. Gold is an exemplary material commonly used for wirebonds. Gold wire diameter for sensor applications generally ranges from about 0.9 mil to about 1.3 mils in diameter. Such dimensions provide suitable electrical connections, however, mold material applied under excessive pressure can easily move, bend, break or otherwise degrade the wirebond connection. 
     A variety of suitable epoxy molding compounds well known to one of ordinary skill in the art can be used to encapsulate the pre-mold assembly, as well as the overmold of the second mold material  204 . One such compound is MG52F by Hysol Corporation for the first and second mold material  202 ,  204 . This material has process characteristics that have some similarity to the magnet material. The coefficient of expansion between the encapsulate assembly and the magnet  216  should be similar so that cracking does not occur when parts are subjected to extreme temperature cycles, such as the conditions seen by a part employed in an automobile. 
     An alternative compound for the first and/or second mold material  202 ,  204  is SUMIKON EME-6600R by Sumitomo Bakelite Co. Ltd. A factor in selecting a suitable compound is obtaining one in which the filler distribution is towards the high side of the range. For example, a filler size of 5-10 um will produce significant amounts of flash and bleed, whereas 30 um or more will almost eliminate it. As is well known in the art, semiconductor molds have vents that allow air to escape while containing the molding compound. Minute resin bleed will be seen in these areas as resin rich material will seep through. Molds are designed with a particular compound in mind with vents regulated to what the compound of choice will allow to pass. 
     Factors to consider when selecting the first mold material  202  are similarity of coefficient of expansion with the magnet  216  and the amount of flash and resin bleed, which can interfere with the magnet  216  and concentrator  214  attachment and overmolding of the second mold material  204 . 
     In one embodiment, the same material is used for the first and second mold materials  202 ,  204 . In other embodiments, the first and second mold materials are different to meet the needs of a particular application. In addition, while first and second molding steps are shown and described, it is understood that further molding steps can be used to apply the same or different materials, to protect other assembly features, to achieve particular geometries and the like. 
     In general, mold setup and cure times in the premold (first mold material  202 ) and overmold (second mold material  204 ) processes can be selected based upon the mold materials and mold geometry. In an exemplary embodiment, processing begins by using 0 seconds preheat time, 5 second transfer time, and 120 seconds cure time. Suitable transfer pressure and transfer times are used to encapsulate the die and wires so no resultant wire sway became evident. Significant increases in transfer pressure and transfer time can be applied during the overmold process without the risk of damaging or moving the bonded wire, which otherwise cannot be done without pre-molding. The significantly higher transfer pressure and time during the overmold process are helpful to properly compact the second mold material due to the relatively large depth of the mold cavity created by the size of the magnet. 
     It is understood that the premold and overmold processes have different parameters. In general, the premold process (first mold material  202 ) uses a relatively slow injection speed and initial packing pressure to keep the wires from sweeping. The first mold material  202  protects the wires and die while the subsequent overmold operations for the second mold material  204  take place. The overmold process can use relatively high transfer speeds and pressures to allow the second mold material  204  to get around the deep cavity around the magnet. Without the higher pressures for the second mold material  204  enabled by the presence of the first mold material  202  protecting the wirebonds  210  and die  202 , it would be difficult, if not impossible, to eliminate voids in or around the magnet area. By protecting the wirebonds  210  from damage during subsequent processing steps, yields increase for more efficient IC fabrication. 
     Once application of the first mold material  202  is complete, the pre-mold assembly is manipulated to have the concentrator  214  and magnet  216  attached to the backside of the leadframe  208  in a manner well known to one of ordinary skill in the art. In an exemplary embodiment, the first mold material  202  molds only the front of the package to protect the die  206  and wires  210  leaving the backside free for further processing. In an exemplary embodiment, the concentrator  214  and magnet  216  are attached to the backside using an adhesive and then oven cured prior to final overmolding. 
     For example, application of the first mold material  202  is limited in time and pressure due to the presence of fine wirebonds, e.g., 1 mil gold wires. The package needs to be completely molded with no voids with relatively low pressure so as not to move or otherwise impact the gold wires. Once the first mold material  202  is set, the second mold material  204  can applied without limitations of the wirebonds since they are encapsulated in the first mold material  202 . Various post processing steps can be performed without regard to the wirebonds. 
       FIG. 3  is a perspective view of the finished assembly  200  of  FIGS. 2A and 2B  in which the second mold material  204  is partially transparent. As can be seen, the first mold material  202  encapsulates the die/leadframe/wirebond assembly. In an exemplary embodiment, the second mold material  204  partially encapsulates the first mold material  202 . Leads  280  extend from the device for external connection. 
       FIG. 4  shows an exemplary sequence of steps to fabricate an IC, such as a sensor, in a multi-step molding process in accordance with exemplary embodiments of the invention. In step  300 , a die is attached to a leadframe in a manner well known to one of ordinary skill in the art. Wirebonds are then attached between selected locations on the die and the leadfingers to provide desired electrical connections in step  302 . A first mold material is then used to at least partially cover the wirebonds in step  304 . In one embodiment, the first mold material encapsulates the wirebonds. 
     In step  306 , the first mold material cures until at least a threshold level is achieved. In optional step  308  a flux concentrator is secured to the leadframe. In optional step  310 , a magnet is secured to the concentrator. This assembly is then overmolded with a second mold material in step  312  to provide IC in a finished package. It is understood that the finished package may undergo additional processing to meet certain desired physical and/or electrical specifications. 
     While exemplary embodiments shown and described herein are directed to sensors, and particularly sensors having concentrators and/or magnets, it is understood that the invention is applicable to IC fabrication processes in general in which an assembly includes wire-bonds, or other delicate connections, between a leadframe and a die. In addition, it is understood that embodiments having more than two molding steps using the same or different materials are contemplated. 
       FIG. 5 , for example, shows a miniaturized, integrated current sensor  410  including a magnetic field transducer, here in the form of Hall Effect sensor  412 , a current-carrying conductor  416  and a magnetic core  424 . The conductor  416  includes features for receiving portions of the Hall Effect sensor  412  and the magnetic core  424  such that the elements are maintained in a fixed position relative to each other. In the illustrated embodiment, the conductor  416  has a first notch  418   a  and a second notch  418   b  substantially aligned with the first notch. In assembly, at least a portion of the Hall Effect sensor  412  is disposed in the first notch  418   a . The magnetic core  424  is substantially C-shaped and has a central region  424   a  and a pair of substantially parallel legs  424   b ,  424   c  extending from the central region. In assembly, at least a portion of the central region  424   a  is disposed in the second notch  418   b  of the conductor such that each leg  424   b ,  424   c  covers at least a portion of a respective surface of the Hall Effect sensor  412 . 
     The Hall Effect sensor  412  is provided in the form of an integrated circuit containing a sensor die  414  with wirebonds  417  fixed in position with a first mold material to form an assembly that is overmolded with a second mold material, as described above. The integrated Hall Effect sensor  412  can be provided in different package types, such as the “K” single in line (SIP) package. Further structural details are set forth in U.S. Pat. No. 6,781,359, which is incorporated herein by reference. 
     While exemplary embodiments of the invention are primarily shown and described in conjunction with a Hall sensor, it is understood that the invention is applicable to integrated circuits in general, and sensors, in particular magnetic field sensors, provided for example as Hall sensors, GMR (giant magnetoresistance), AMR (anisotropic magnetoresistance), TMR (tunnel magnetoresistance), and MTJs (Magnetic tunnel junctions). 
     Having described exemplary embodiments of the invention, it will now become apparent to one of ordinary skill in the art that other embodiments incorporating their concepts may also be used. The embodiments contained herein should not be limited to disclosed embodiments but rather should be limited only by the spirit and scope of the appended claims. All publications and references cited herein are expressly incorporated herein by reference in their entirety.