Patent Document

PRIORITY CLAIM 
     This application is a divisional of U.S. patent application Ser. No. 14/157,382 filed on Jan. 16, 2014, which claims priority to U.S. Provisional Application No. 61/755,889, filed Jan. 23, 2013, which is incorporated herein in its entirety. 
    
    
     TECHNICAL FIELD 
     The present disclosure relates to a method for manufacturing open cavity plastic packages for integrated circuits (ICs), in particular, using a cover over the IC die during the encapsulation process to provide a plastic package with an open cavity to allow the device to act as a sensing device. 
     BACKGROUND 
     Integrated circuits (ICs) may include sensors for a variety of reasons. For example, ICs may comprise moisture-sensitive sensors to detect liquid or humidity. Manufacturers may include a moisture-sensitive sensor to determine whether an IC has been damaged by immersion in water, so as to know whether a customer returning an IC is entitled to a replacement of the IC under warranty. Other ICs include sensors that are part of the functionality of the IC. For example, ICs have sensors to detect: radio frequency identification, temperature, ambient light, mechanical shock, liquid immersion, humidity, CO 2 , O 2 , pH, and ethylene. These IC chips may be used to monitor these ambient conditions. 
     Integrated circuit devices that operate as sensors may require a specific opening in the package to be able to act as an environmental sensing device. Conventional sensing devices use tape assist in the mold operation to create a cavity in the package. That technology is expensive and there are problems associated with it, in particular, damage to the sensor area (for example, die surface scratches, puncture marks in the sensor area or resin bleed from the molding operation). These problems continue to be the major issues with the tape assist art. Thus, a need exists for a plastic package for use as a housing for an integrated circuit device with an opening providing for a sensing area, wherein the sensing area must be free of resin bleed from the molding operation and must be free of damage. 
     SUMMARY 
     According to various embodiments, a spring-loaded solution is provided to resolve all those issues addressed above and make the process of manufacturing of an environmental sensing device even less expensive. According to an embodiment, a retractable spring loaded pin is provided in the mold to create an open cavity molded package. The open cavity concept is used to expose a certain sensor area to the environment (for example, a specific top area of an integrated circuit die). It is important that, during the manufacturing process, the sensor area is not scratched, punctured, nor covered with any resin residue from the mold operation. According to various embodiments, all those issues are resolved, and the sensor device can be economically produced. 
     According to various embodiments, a cavity or hole is created in a plastic package such that there will be no resin bleed from mold flash nor can there be any damage to the sensor area. To achieve these objects, a retractable spring-loaded pin or pins are used within the mold to create the cavity (for example, in a top half of a mold). The spring-loaded pins actually retracts back when the mold is closed and thus protect the sensor area on the silicon die from being covered by mold compound. 
     One aspect of the invention provides a method for manufacturing open cavity integrated circuit packages, the method comprising: placing a wire-bound integrated circuit in a mold; forcing a pin to contact a die of the wire-bound integrated circuit by applying a force between the pin and the mold; injecting plastic into the mold; allowing the plastic to set around the integrated circuit to form a package having an open cavity defined by the pin; and removing the open cavity integrated circuit package from the mold. 
     According to another aspect of the invention, there is provided a mold for forming a package for an integrated circuit sensor device, comprising: a bottom part for supporting an integrated circuit die; and a top part that is operable to be placed on top of said bottom part to form a cavity into which a plastic material can be injected to form the package, wherein the top part of the mold comprises a spring loaded pin arrangement comprising a cover that covers a sensor area on the integrated circuit die and provides for an opening when the plastic material is injected. 
     Still another aspect of the invention provides a process of manufacturing a sensor device comprising: placing an integrated circuit die on a support placed on a bottom part of a mold; wire-bonding the integrated circuit die; placing a top part of a mold on top of said bottom part to form a cavity for a package, wherein the top part of the mold comprises a spring-loaded pin arrangement comprising a cover that covers a sensor area on the integrated circuit die; and injecting plastic material into the mold formed by the top and bottom part, wherein the cover provides for an opening in the package formed by said injected plastic material. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIGS. 1A and 1B  illustrate a cross-sectional side view and a top view of an open cavity plastic IC package  10  made by the process of the present invention; 
         FIG. 1C  illustrates a cross-sectional side view of the IC package  10  of  FIGS. 1A and 1B  in a mold  105 ; 
         FIG. 2  illustrates a cross-sectional side view of an IC package in a mold of the present invention; 
         FIG. 3  shows a two-part mold according to various embodiments; 
         FIG. 4A  shows the manufacturing process in the form of a flow chart for an IC package of the present invention; 
         FIG. 4B  illustrates an IC at the end of the die attach sub-process; 
         FIG. 4C  illustrates an IC at the end of the wire bonder sub-process; 
         FIG. 4D  illustrates an IC at the end of the molding sub-process; 
         FIGS. 5A-5C  show top, side, and bottom views of an IC package  10  of an environmental sensor using a TQFN package; 
         FIGS. 6A and 6B  show a lead frame and a top mold cavity design for an array of sensor device; and 
         FIGS. 7A and 7B  show an exemplary embodiment of the spring-loaded pin arrangement according to various embodiments. 
     
    
    
     DETAILED DESCRIPTION OF INVENTION 
       FIGS. 1A and 1B  illustrate a cross-sectional side view and a top view of an open cavity plastic IC package  10  made by the process of the present invention. The open cavity plastic IC package  10  has a die  140  positioned on a lead frame  130 . Bond wires  135  extend from the die  140  to the lead frame  130 . The IC package  10  is covered with a plastic encapsulant  145 , which provides a sensor port or an open cavity  147  for a sensor on the die  140  to have access to the ambient conditions in which the IC package  10  may be placed. 
       FIG. 1C  illustrates a cross-sectional side view of the IC package  10  of  FIGS. 1A and 1B  in a mold  105 . During the manufacturing process, encapsulant in the form of a molding compound is pumped into the mold to cover the interconnects or bond wires. The mold  105  comprises two half-shell molds: a top part of the mold  110  and a bottom part of the mold  120 . The top part  110  of the mold has a pin  160  that contacts the upper surface of the die  140  when the mold is assembled on the lead frame  130 . The pin  160  prevents the plastic encapsulant from forming a cover in the area above the die  140  so that an open cavity  147  may be formed. 
       FIG. 2  illustrates a cross-sectional side view of an IC package in a mold of the present invention. The top part  110  of the mold  105  comprises a pin  160 . The pin  160  has a pin head  163 , which contacts the die  140 . The pin  160  also has a force element  161  and an anchor element  162 . The anchor element  162  holds the pin  160  in the top part  110  of the mold  105 . The force element  161  is connected at one end to the anchor element  162  and at the other end to the pin head  163 . The force element  162  pushes the pin head  163  away from the anchor element  162  toward the die  140 . The force element may take any form known to persons of skill in the art, such as a spring, a piston, an elastic rod, a magnetic rod, etc., wherein it has the capacity to force the position of the pin head  163  toward the die  140 . To provide more uniform contact between the pin head  163  and the die  140 , the pin head  163  may be flexibly or pivotably attached to the force element  161 . A flexible or pivotable attachment may allow the pin head  163  to adjust its contact face to align with the die  140 . This alignment may be particularly beneficial where the die is thicker on one side and relatively thinner on another side, or if the die is not perfectly bonded to the lead frame so as to be the same height at all points. 
     As IC dies may vary in thickness, the force element works beyond its means to protect the sensor area. Conventional manufacturing devices use a fixed pin and a high-temperature tape to protect the sensor area and form the open cavity. However, because fixed pins may apply different contact pressures to IC dies, depending on the IC die thicknesses, plastic encapsulate or resin may bleed or flow into the open cavity where the sensor is to be positioned or is positioned on the IC die. Further, use of a fixed pin and high-temperature tape may require additional process steps to remove the high-temperature tape. During the removal process, the sensor is further exposed and could be damaged. This conventional process uses a very expensive high-temperature tape to cover the sensor area during encapsulation and then an additional process to remove the high temperature tape from the open cavity either manually or using vacuum. These extra steps add to the unit cost. Also, additional process steps have their own process issues, i.e., they may scratch the sensor or cause resin to bleed into the open cavity over the sensor. 
     The force or spring-loaded concept according to various embodiments of the invention has been proven not to cause any damage to the sensor area. The technology according to various embodiments utilizes a spring-loaded pin similar to a spring-loaded pogo pin technology only present in equipment handlers for piece part testing (for example, to provide electrical connection of a test device with bond pads on a silicon die). 
     According to an embodiment, a spring load pogo or pin is used in the transfer molding process to create an opening in plastic packages such as in a Thin Quad Flat No Lead package. A similar concept can be applied to any other plastic package that is used in, for example, a gas or pressure sensor application that requires an opening to expose a sensor area of the device. 
     Using a spring-loaded bin a mold (for example in the top half of a mold), provides an economically sound process without causing any sort of damage to the sensor areas. The spring loaded cavity package furthermore will not cause any damage during the transfer molding process. 
       FIG. 3  shows a two-part mold according to various embodiments. The bottom part  120  of the mold provides support for a die  140 , which may be placed on a lead frame  130 . The top part  110  includes the spring-molded pin arrangement  190 , wherein a pin  160  supporting a cover  150  provides for a cover of the sensor area that may be arranged in the center of the die  140  on the lead frame  130 . The spring can be arranged inside a spring housing  170  and extends the cover  150  into or beyond the hollow space  100  when the top part  110  of the mold is not placed on the bottom part  120  as shown in  FIG. 3 . Once the top part  110  of the mold is placed onto the bottom part  120 , the spring-loaded pin  160  is pushed back by the die  140 . An additional opening  180  can be provided to retract the cover  150 . The extension length is designed such that the cover portion  150  of the spring loaded pin arrangement  190  will form an opening in the housing. The spring-loaded pin arrangement  190  further provides for automatic adjustment depending on the thickness of the die  140 . Thus, a single top mold part  110  can be used with various die thicknesses. Once the mold is closed by putting the top part  110  of the mold on top of the bottom part  120 , plastic can be injected into the hollow space  100  to encompass and seal the die  140  within the housing and at the same time form the opening in the top portion of the housing. Retracting the top part of the mold leaves the sensing device package completed without damage to the sensor area. 
       FIG. 4A  shows the manufacturing process in the form of a flow chart for an IC package of the present invention, wherein the process comprises three sub-processes: die attach, wire bonder, and molding. For the die attach sub-process, a supply of wafers  410  is brought into the process for inspection  411 . The wafers are then mounted  412  and sawed  413 . Dies are then attached  422  to the lead frames. A machine, such as an ASM AD898 may be used, wherein the machine may have an automatic wafer handling system with water cassette elevator for up to 8-inch wafers. Such a machine may be capable of handling die sizes from 0.25 mm×0.25 mm to 25.4 mm×25.4 mm. The machine may also apply a bond force of 30-2000 g and provide multi-grey levels PRS.  FIG. 4B  illustrates an IC at the end of the die attach sub-process. 
     Referring again to  FIG. 4A , for the wire bonder sub-process, a supply of lead frames  420  is brought into the process for inspection  421 . Bond wires are then made to bond  423  the dies to the lead frames. A step called 3 rd  Optical QA 424 is then performed. This sub-process creates wire-bound integrated circuits. A machine such as an ASM Eagle-60 or equivalent may be used. The wire size may be 15 um to 50.8 um Au. The maximum length of the wires may be 8 mm. The bonding speed may be 60+ ms for 2 mm wire. The bond placement accuracy may be ±3.0 um. The bonding area may be 54 mm×65 mm.  FIG. 4C  illustrates an IC at the end of the wire bonder sub-process. 
     Referring again to  FIG. 4A , for the molding sub-process, a supply of compound  430  is brought into the process for inspection  431 . The wafers are then molded  432  to form the IC packages. The wafers are then marked  433 . A saw singulation step  434  is then performed to separate the IC packages. A visual inspection  435  is performed, and the ICs are then packed and shipped  436 . An ASAHI Cosmo-T machine may be used for the molding sub-process. The mold temperature may be 180° C.±5° C. The transfer pressure may be 35 kgf. The clamp tonnage may be 45 ton. The in-mold cure time may be 90 seconds.  FIG. 4D  illustrates an IC at the end of the molding sub-process. 
     Materials known to persons of skill in the art may be used to manufacture the ICs. For example, the lead frame may be μPPF 0.2 mm thick, wherein the pad size may be 2.90 mm×2.90 mm and the exposed pad size may be 2.60 mm×2.60 mm. The die-attached epoxy may be Sumitomo CRM1076NS. Gold wire may be used having a diameter of MKE 0.8 mils. A mold compound may be Sumitomo G770HCD, wherein the pellet size may be 14×6.0 g. 
       FIGS. 5A-5C  show top, side, and bottom views of an IC package  10  of an environmental sensor using a TQFN package. The depth of the open cavity  147  may be about 0.35 mm±0.05, and the diameter of the open cavity  147  may be about 1 mm, for a package that is about 4 mm square. The lead frame  130  is visible from the side and bottom, as shown in  FIGS. 5B and 5C , respectively. 
       FIGS. 6A and 6B  show a lead frame and a top mold cavity design for an array of sensor devices, wherein  FIG. 6A  is a top view of the entire array  605 , and  FIG. 6B  is a cross-sectional side view of a portion of the array  605 . The exemplary top mold cavity design has four panels  610 , wherein each panel  610  is a 10×12 a for an array  610  having a total of 480 ICs. Each panel  610  has a lead frame  130  with an array of dies  140 , wherein a pin  160  is used relative to each die  140  to form an open cavity for a sensor on the die. According to one embodiment, the array  605  may be a TQFN top mold cavity design for TQFN 4×4 pressure sensor. 
       FIGS. 7A and 7B  show an exemplary embodiment of the spring-loaded pin arrangement according to various embodiments.  FIG. 7A  is a side view of a spring-molded pin arrangement  190 . The spring-molded pin arrangement  190  comprises a spring housing  170 , a pin  160 , and a cover  150 .  FIG. 7B  is an end view of the cover  150 . The cover  150  has an annular contact face  151  that allows the cover  150  to make more uniform contact with the die  140  (see  FIG. 3 ) to prevent flashing of the plastic encapsulant into the open cavity  147  (see  FIG. 1A ). The outside diameter of the cover  150  may be about 1.2 mm, and the inside diameter of the conical recess in the cover  150  may be about 1.0 mm, so that the annular contact face  151  may be about 0.1 mm wide. The spring-molded pin arrangement  190  may be constructed of material sufficient to endure exposure to plastic encapsulant heated to at least 300° C. The force of the spring inside the spring housing  170  may be about 80-120 g. 
     In alternative embodiments of the invention, a plurality of open cavities may be formed on a single IC. To form a plurality of open cavities, a plurality of covers  150  or pin heads  163  may be applied to a single IC during a molding sub-process. Where a plurality of pins are independently forced against the die, the independent application of force may ensure that the plastic encapsulant is unable to enter any of the open cavities because the plurality of pins are each able to make a firm contact with the die. Alternatively, a single cover  150  or pin head  163  may be applied to a single IC during a molding sub-process, but the single cover  150  or pin head  163  comprises a plurality of contact faces  151 . In the embodiment illustrated in  FIG. 7B , the contact face  151  is annular, but in alternatively embodiments, the contact face may be any shape or configuration and may comprise a plurality of contact faces. A plurality of open cavities  147  may be useful where a plurality of sensors are attached to a single IC.

Technology Category: b