Patent Publication Number: US-10333055-B2

Title: Methods for magnetic sensor having non-conductive die paddle

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is continuation of U.S. patent application Ser. No. 15/049,732, filed on Feb. 22, 2016, which is a divisional application of U.S. patent application Ser. No. 14/090,037 filed on Nov. 26, 2013, now U.S. Pat. No. 9,299,915 entitled: M ETHODS  A ND  A PPARATUS  F OR  M AGNETIC  S ENSOR  H AVING  N ON -C ONDUCTIVE  D IE  P ADDLE , which is a continuation of U.S. patent application Ser. No. 13/350,970 filed on Jan. 16, 2012, now U.S. Pat. No. 8,629,539 entitled: M ETHODS  A ND  A PPARATUS  F OR  M AGNETIC  S ENSOR  H AVING  N ON -C ONDUCTIVE  D IE  P ADDLE , which is incorporated herein by reference in its entirety. 
    
    
     BACKGROUND 
     As is known in the art, eddy currents can degrade the performance of integrated circuits having magnetic sensors. Magnetic sensors typically include a magnetic transducer, such as a Hall cell element, on the surface of an integrated circuit, which is mounted on a metal leadframe. The sensor is connected to the leadframe with wires and overmolded with thermoset plastic. While such magnetic sensors may be suitable for sensing static magnetic fields, at higher frequencies increasing eddy currents are generated in the conductive leadframe in response to the changing magnetic field. Eddy currents flow in circular loops perpendicular to the direction of the magnetic flux vectors. The eddy currents create an opposing magnetic field underneath the Hall cell, which can cause unacceptably large errors in the magnetic field strength detected by the sensor. 
     While prior art attempts have been made to provide slots in conductive leadframes to reduce eddy current flow, such slots provide only limited reductions in eddy current levels. U.S. Pat. No. 6,853,178 to Hayat-Dawoodi, for example, shows various slots across the leadframe and crossed slots. 
     SUMMARY 
     In one aspect of the invention, a method comprises: employing a conductive leadframe; forming a non-conductive die paddle in relation to the leadframe; placing a die on the non-conductive die paddle to form an assembly; forming at least one electrical connection between the die and the leadframe; and overmolding the assembly to form an integrated circuit package. 
     The method can further include one or more of the following features: providing a non-conductive die paddle on which the die is disposed, the non-conductive die paddle comprises a plastic material, a magnetic layer aligned with the die to affect magnetic fields proximate the die, a back-bias magnet as part of the IC package, the conductive leadfinger material is at least a given distance more than a height from the leadfingers to the magnetic sensing element, the conductive leadfinger material is at least two times a vertical height from the leadfingers to the magnetic sensing element, the magnetic sensor element is formed in the die, the magnetic sensor element includes a Hall element, the magnetic sensor element includes a magnetoresitive element, the leadfinger material extends from only one side of the magnetic field sensor device, and/or applying an underfill material proximate the wafer bumps. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The foregoing features of this invention, as well as the invention itself, may be more fully understood from the following description of the drawings in which: 
         FIG. 1  is a graphical representation of propagation time for a prior art magnetic integrated circuit; 
         FIG. 2  is a graphical representation of response time for a prior art magnetic integrated circuit; 
         FIG. 3  is a graphical representation of rise time for a prior art magnetic integrated circuit; 
         FIG. 4  is a prior art magnetic sensor IC package; 
         FIG. 5  is a prior art magnetic sensor IC package with a slot in a conductive leadframe; 
         FIG. 6  is a schematic depiction of a conductive leadframe that can form a part of an IC package having a non-conductive die paddle; 
         FIG. 7  is a schematic representation of partially fabricated IC package in accordance with exemplary embodiments of the invention; 
         FIG. 7A  is a side sectional view of the IC package of  FIG. 7  without a magnetic layer; 
         FIG. 7B  is a side sectional view of the IC package of  FIG. 7  with a magnetic layer; 
         FIG. 8  is a schematic representation of a partially fabricated IC package in accordance with exemplary embodiments of the invention; 
         FIG. 8A  is side sectional view of the assembly of the IC package of  FIG. 8  without a magnetic layer; 
         FIG. 8B  is side sectional view of the assembly of the IC package of  FIG. 8  with a magnetic layer; 
         FIG. 8C  is a side view of an assembly with a magnetic layer secured to a back of the non-conductive die paddle; 
         FIG. 8D  is a side view of an assembly with a hard ferromagnetic material layer secured to the magnetic layer of  FIG. 9C ; 
         FIG. 9  is a schematic representation of an IC package in accordance with exemplary embodiments of the invention; 
         FIG. 9A  is side sectional view of the assembly of the IC package of  FIG. 9  without a magnetic layer; 
         FIG. 9B  is side sectional view of the assembly of the IC package of  FIG. 9  with a magnetic layer; 
         FIG. 10  is a flow diagram showing an exemplary sequence of steps for fabrication an IC package in accordance with exemplary embodiments of the invention; and 
         FIG. 11  is a schematic representation of an exemplary flip chip embodiment of an IC package in accordance with exemplary embodiments of the invention. 
     
    
    
     DETAILED DESCRIPTION 
     The present invention provides methods and apparatus for an integrated circuit (IC) package including a die on a non-conductive die paddle to reduce eddy current effects on a magnetic sensor. In one embodiment, a Single In-line Package (SIP) with a non-conductive or high resistivity plastic die paddle allows design flexibility and improved magnetic sensor performance when encapsulating magnetic semiconductor Integrated Circuits (ICs). The non-conductive or high resistivity is large enough such that an eddy current that results in an unacceptably large magnetic field error is not induced in the application. The non-conductive die paddle improves the response time and bandwidth of magnetic sensors for high frequency applications, such as DC-DC converters and inverters in switch mode power supplies. In an exemplary embodiment, a layer of ferromagnetic or magnet material is placed inside the package. The ferromagnetic or magnetic material may be either a soft ferromagnetic or a hard ferromagnetic material, or in some cases both a soft and hard ferromagnetic material layer and multilayer. It is understood that the term “die paddle” refers to the area of the leadframe or package that a die or multiple die may locate in the final package construction. 
     Before describing exemplary embodiments of the invention, some information is provided. Magnetic sensor integrated circuits, which contain transducers, including but not limited to, Hall Effect, MR (magnetoresistive), GMR (giant magnetoresistive, AMR (anisotrpic magnetoresistive) and TMR (tunneling magnetoresistive) type devices have inherent bandwidth limitations due to the physical and electrical design of the Integrated Circuit (IC). Magnetic sensor circuits have inherent capacitance, inductance, and resistance that form some type of tuned circuit determining the overall frequency response/bandwidth of the transducer circuit on the magnetic IC. This bandwidth is typically relatively high, e.g., from about 50 Hz to hundreds of kHz for sensor output. This bandwidth is often filtered on the IC in amplification and filtering stages to optimize device performance and lower output noise. It is understood that filtering can be minimized, usually at the expense of accuracy. With a high bandwidth design, the physical packaging should be considered because it will limit the response time for high frequency magnetic events, as discussed below. 
       FIG. 1  shows the propagation delay (t PROP ) of a conventional magnetic integrated circuit. The propagation delay is the time required for the magnetic sensor output to reflect a change in the applied magnetic field. Propagation delay is attributed to magnetic transducer signal conditioning in the IC and to inductive loading within the linear IC magnetic sensor package, as well as the inductive loop formed by the primary conductor geometry creating the magnetic field. 
       FIG. 2  shows the device response time (t RESPONSE ), which is defined as the time interval between when the applied magnetic field reaches 90% of its final value and when the magnetic sensor IC output reaches 90% of its output value corresponding to the applied magnetic field. 
       FIG. 3  shows the device rise time (t r ), which is the time interval between the magnetic sensor output reaching 10% of its full scale value and reaching 90% of its full scale value. The rise time to a step response is used to derive the approximate bandwidth of the magnetic sensor, and is calculated as f(−3 dB)=0.35/t r . It should be noted that the rise time t r  and response time t RESPONSE  are detrimentally affected by eddy current losses observed in the conductive IC die paddle, which is often also the ground plane. Therefore, the bandwidth and overall response time for a high frequency magnetic sensor is determined by the IC design, as well as the packaging. 
     In a conventional SIP configuration shown in  FIG. 4 , the die IC is mounted on the die paddle DP of the leadframe LF, which is often connected to the GND lead of the package, shown as pin  4 . The die IC is attached to the leadframe die paddle DP with a conductive adhesive and contact from the die active areas to the leads is made with a gold wire bond WB. The assembly is then over-molded, for example with a mold compound, to protect the die IC and wire bonds WB. Typically, many devices are over-molded at the same time and singulated from the matrix leadframe after molding into individual units. 
     In conventional ICs, the leadframe material, e.g., plated copper, is conductive. The conductive leadframe LF allows eddy currents to form during high frequency magnetic events. As is known in the art, eddy currents are currents induced in conductors that oppose the change in magnetic flux that generated the eddy currents. Eddy currents are generated when a conductor is exposed to a changing magnetic field due to relative motion of the field source and conductor and/or field variations over time. The resultant eddy currents create induced magnetic fields that oppose the change of the original magnetic field change in accordance with Lenz&#39;s Law. The opposing field delays the response time of the magnetic sensor IC to reach the value of the measured magnetic field. As seen in  FIG. 4 , the magnetic transducer element MT is subject to both the incident and opposing magnetic fields. 
       FIG. 5  shows a prior art device having a portion of a copper leadframe behind the magnetic transducer removed to form a slot SL to reduce eddy current levels. While forming slots in a conductive leadframe may reduce eddy currents to acceptable levels, higher frequency operation may still be limited. 
     In one aspect of the invention, a magnetic sensor IC includes a non-conductive die paddle to minimize the amount of electrically conductive material proximate the IC in order to reduce, if not eliminate, eddy currents. The die is attached to a non-conductive material, such as plastic, for example a non-conductive mold compound, instead of copper leadframe material. With this arrangement, eddy currents near the integrated circuit are minimized, which concomitantly minimizes the strength of the opposing field generated by the eddy currents, and therefore, lowers the instantaneous error and reduces the response time. 
       FIG. 6  shows a leadframe  100  that can form the basis for an IC package with a non-conductive die paddle in accordance with exemplary embodiments of the invention. Prior to formation of the non-conductive die paddle, the leadframe  100  has only conductive portions  102 . The conductive portions  102  can be formed from copper or other metal to provide lead fingers. In one embodiment, a Fe—Ni alloy, such as KOVAR (trademark of Carpenter Technology Corporation), is used. In general, the conductive leadframe material is outside a perimeter of the die. The nonconductive die paddle to eliminate conductive material, e.g., copper, behind the sensor IC for reducing eddy currents can be formed as described below. 
       FIG. 7  shows the assembly of  FIG. 6  overmolded in a first mold step to create a non-conductive die paddle  200  with an optional magnetic layer  202  in relation to a conductive leadframe  204 . In other embodiments, the magnetic layer  202  can be provided as a ferromagnetic material that can be used as a magnet or concentrator for magnetic fields behind a die. With a back biased ferromagnetic magnetic layer  202  in the plastic package, the bandwidth may be limited. However, in some applications a ferromagnetic material, or a back biased magnet, may be more desirable than high frequency operation. It should also be noted that the thickness of the magnetic layer is typically be less than that of the leadframe material. In the case of a back biased magnetic material the conductivity is lower, therefore resulting in lower eddy currents in the magnetic layer  202 . 
     It is understood that the geometry and dimensions of the components in exemplary embodiments of the invention can vary to meet the needs of a particular application. For example, die paddle materials can have different lead thicknesses, which can vary depending on the package design. Exemplary thicknesses include 8 mils, 10 mils, and 15 mils. However, packages such as MLP (micro leadframe) or QFN (quad flat no leads) may use less material, e.g., 5 mils. It is contemplated that thickness will continue to decrease as technology improves, e.g., as package sizes and volumes continue to decrease. 
     In the illustrated embodiment, the conductive leadframe material  204  does not overlap at all with the die. That is, where the die  206  is located in a horizontal plane and the leadframe is located in the same or different horizontal plane, no vertical line intersects both the die and the leadframe. It should be noted that as long as any leadframe overlap does not come near the magnetic field transducer the spirit of the invention is maintained. 
     The magnetic layer  202  can be provided in a wide range of geometries, dimensions and materials, to meet the needs of a particular application. In one embodiment, the magnetic layer is provided as a back biased magnet comprising, but not limited to: NeFeB, a hard ferrite, and/or SmCo. In other applications, the magnetic layer  202  is provided as a soft magnetic material when used to direct flux and a magnet is provided as a relatively hard magnetic material that applies flux. In the case of a desire to isolate electrical influences, the magnetic layer may be a conductive layer, e.g., a ground plane. 
       FIG. 7A  shows a side view of the assembly without a magnetic layer and  FIG. 7B  shows a side view of the assembly with the magnetic layer  202 . It should be noted that while  FIG. 7B  shows the magnetic layer  204  flush with the material  200 , the material  202  may extend beyond edge or be short of the edge of the material  200  for certain applications. 
     As shown in  FIG. 8 , once the first mold step to provide the non-conductive die paddle  200  is complete, with or without the magnetic layer  202 , a die  206  can be mounted on the plastic die paddle  200  and wire-bonded to create connections  208  from the die to the lead fingers. A magnetic transducer  210 , such as a Hall element or magnetoresistor (giant magneotresistance (GMR), anisotropic magnetoresitive element (AMR), magnetic tunnel junction (MTJ), or tunneling magnetoresistor (TMR)), can be provided in the die in a manner well known in the art. In general, there is no overlap between the die  206  and the conductive leadframe  204 . It is understood that the spacing from the edge of the die to any leadframe material would be considered by the designer for a given application. 
       FIG. 8A  shows a side view of the assembly without a magnetic layer and  FIG. 8B  shows a side view of the assembly with the magnetic layer  202 .  FIG. 8C  shows a side view of an assembly with a magnetic layer  202  secured to a back of the non-conductive die paddle  200 . In another embodiment shown in  FIG. 8D , a hard ferromagnetic material layer  205  can provide a back-bias magnet instead of or in addition to the magnetic layer  202  provided by the soft ferromagnetic material. 
     As shown in  FIG. 9 , to complete the packaging a final overmold step with mold material  212  yields the final IC package.  FIG. 9A  shows a side view of the assembly without a magnetic layer and  FIG. 9B  shows a side view of the assembly with the magnetic layer  202 .  FIGS. 9A and 9B  show an optional dimple or reduced thickness of the package behind the die. 
       FIG. 10  shows an exemplary sequence of steps for fabricating an IC having a non-conductive die paddle in accordance with exemplary embodiments of the invention. In step  300 , a leadframe is formed. In one embodiment, the leadframe is fabricated from a conductive material, such as copper, and is configured to provide leadfingers for the IC package. In step  302 , a die paddle is fabricated from a non-conductive material, such as an electrically insulating, or non-conductive plastic. In one embodiment, the die paddle is formed using a mold process. The die paddle is oriented with respect to the leadframe. 
     An optional magnetic layer can be provided in step  304 . In one embodiment, a magnetic concentrator or a permanent magnet is positioned in the die paddle as part of the die paddle molding process. The magnetic material can be formed from a soft ferromagnetic material to protect the die from magnetic fields behind the IC package. In another embodiment, a hard ferromagnetic material may be utilized to provide a back-bias magnet instead of or in addition to the magnetic layer provided by the soft ferromagnetic material, as shown in  FIG. 8D . 
     In step  306 , a die is placed on the leadframe/die paddle assembly. In general, the die paddle is configured such that there is no conductive material overlapping or directly adjacent the die so as to reduce, if not eliminate, eddy currents proximate the die. In one embodiment, an adhesive, preferably, but not limited to, a non-conductive adhesive, secures the die to the die paddle. The die can include one or more magnetic transducer elements. It is understood that eddy currents in an adhesive would be lower due the reduced thickness. 
     In step  308 , wirebonds are formed between active areas of the die and lead fingers of the leadframe to provide the desired connections. In step  310 , the assembly can be overmolded to provide an IC package. Any suitable overmolding material can be used. 
       FIG. 11  shows an exemplary flip chip configuration for an IC package having a die  400  positioned on an optional non-conductive die paddle  402 . A magnetic transducer  403 , such as a Hall element or a magnetoresistive element, can be provided in the die. Conductive lead fingers  404  have a bump area  406  to provide a connection to active areas of the die  400 , which may have solder balls or stud bumps (for example copper pillars). The connection of the die to the leadframe is typically achieved via a reflow step. In an alternative embodiment, an epoxy adhesive is used at designated locations. An overmold material  408  is overmolded about the assembly to provide the IC package. 
     It is understood that a magnetic layer may also be used in conjunction with flip-chip embodiment. It is further understood that other methods, such as chip on lead technology, can also be used without departing from the scope of the invention. 
     In an exemplary flip chip embodiment, the die paddle step  306  and wirebond step  308  of  FIG. 10 , are modified to reflow bumps onto the lead fingers and apply an optional underfill material. In one embodiment, after place and reflow of the bumped die, the assembly is overmolded in a single molding step. 
     In one flip chip embodiment, conductive leadframe material is kept away from the magnetic transducer, e.g., the Hall plate. A boundary region  405  can define an area that contains no conductive material. In general, the boundary region  405  should minimize eddy current influences. In one particular embodiment, conductive leadframe material is at least 0.25 mils away from a boundary of the Hall element. In another embodiment, the conductive leadframe material is at least two times the vertical height from the leadframe to the transducers. In flip chip configurations, if the after reflow bump height is 50 to 75 microns, for example, a distance of 100 to 200 um may be required. For wirebonded parts, this distance may need to be larger. 
     It is understood that the boundary region can comprise any suitable geometry to meet the needs of a particular application. Exemplary geometries include rectangular, circular, ovular, and other shapes that enclose an area. 
     Exemplary embodiments of the invention provide a magnetic sensor IC capable of increased frequency as compared to conventional sensors. Overmolding without an electrical or magnetic layer of conductive, soft ferromagnetic, or hard magnetic material in the first mold process produces a package with minimal nearby copper leadframe material to conduct eddy currents. The packaged device is physically optimized for increased frequency applications. 
     Using a layer of ferromagnetic material in the first overmold process lowers the bandwidth, but provides shielding from nearby interfering fields coming from the back side of the package for applications where a sensor is looking for a field coming from one side of the package. This layer, in this case a magnetic concentrator layer, also concentrates or focuses incident desired fields on the front of the package in cases where the field to be sensed is weak and allows for improved sensor performance under weak field conditions. 
     Using a layer of hard or permanent magnetic material allows for an integrated back biased magnetic solution to sense the motion of soft ferromagnetic material in front of the magnetic sensor IC. This back-biased magnet can be relatively thin, so that the generated field is relatively small. This configuration may be preferable for magneto-resistive solutions like GMR, AMR and TMR. This configuration can be used in IC packages for gear tooth sensors, such as ABS (anti-lock braking systems) or transmission gear tooth sensors with relatively small form factors. A thicker magnet allows for significant improvement in the generated back biased magnetic field for Hall back biased sensors which may result in increased working air gaps depending on a particular magnetic design. 
     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.