Abstract:
A broken trim die tool detection sensor. The lands of the tie bar die connect with the leads of the unit to form switches. The states of these switches indicate broken die lands or other malfunctions.

Description:
BACKGROUND AND SUMMARY OF THE INVENTION 
       [0001]    The present invention relates to integrated circuit structures and fabrication methods. In particular, the present application describes an improvement to back end processing. 
       BACKGROUND 
       [0002]    Part of the semiconductor device manufacturing process is packaging of the device. Packaging is necessary to protect a device from breakage, infiltration by dirt and debris, and to protect the device from atmospheric changes such as humidity which can affect device performance. Particulate contaminates can eventually penetrate the passivation layer of a chip and interfere with the functioning of a device. Particulates may also carry chemicals that can react with metallic interconnects. Penetration of the package by moisture is also an important concern, since moisture often causes subsequent degradation of the device. Packaging also gives some robustness to microelectronic devices, making them easier to work with and place on printed circuit boards. 
         [0003]    Plastics, ceramics, and glass are commonly used materials for packaging devices, though plastic is becoming the predominant material. In plastic encapsulation, the package material (usually a thermoset polymer with some inert material and other ingredients) is transferred to a mold cavity which holds the chips bonded to leadframes. Leadframes are metal frames that include the leads of a circuit package. The unit is placed in a central receiving area, and bond pads on the unit are electrically connected to bond fingers on the lead frame, which extend to become the external connections to the chip itself. These external connections are called leads. 
         [0004]    Packaging steps typically include mounting the semiconductor devices to a leadframe, forming wire bonds between the leadframe and the devices, followed by encapsulation of the devices within a packaging material. The devices are then trimmed of excess plastic or “flash” material that remains after encapsulation, and material from between the leads is removed. The material connecting the leads before separation is often called a tie bar or dambar. The leads are then cut to proper length, and bent to a desired configuration. 
         [0005]    Normal trimming and cutting tools are usually of a comb shape, with many fingers extending toward the chip carriage to remove the excess material from between leads. A typical trim die tool set includes a tie bar punch and a tie bar die. A unit is placed between a tie bar punch and a tie bar die within the trim die tool set. When the punch and the die are moved together, the fingers of the punch fit between the fingers or lands of the die to remove any material between them. This has the effect of removing tie bars from the metal leads on the unit. 
         [0006]    Since several chips are typically run in a lot, damage to one of the teeth or lands of the tie bar punch or the tie bar die can result in improper trimming or connections for all chips in a batch after the damage occurred. 
         [0007]    Typical solutions require manual visual inspection of processed units to detect breaks. This method cannot detect broken units until after the batch is run, so that if the lower trim die itself breaks, several improperly processed units will result, requiring a large part of that batch to be scrapped. 
         [0008]    Other solutions install systems that visually check each processed wafer as they exit the process, allowing stoppage at the first sign of a broken lower trim die, but this method cannot detect breakages if the leads are not yet detached from the package, but is already malformed. 
         [0009]    There is therefore a need in the art for a method of detecting broken trim die tools that detects errors in real time and reliably detects breaks in the trim die tool. 
       Broken Die Detect Sensor 
       [0010]    The present application discloses an innovative means to detect a broken trim die tool. In the preferred embodiment, the lands of the tie bar die are electrically connect with the leads of the unit in the tool. The connection between each lead and each land forms a switch. These switches are used to detect whether the connection is made properly on all lands in each unit. An improper connection (caused by misalignment, a broken trim die tool, or any other problem) causes the switch to be open. This switch is used in a detection circuit to monitor that all lands and leads are properly connected during the trimming process. If a switch is open, indicating a disconnect, packaging can be stopped and the tool repaired with a minimization of downtime or scrapped units. 
         [0011]    Advantages of the disclosed methods and structures, in various embodiments, can include one or more of the following: 
         [0012]    detects breakages of trim die tool in real time; 
         [0013]    reduction in scrappage; 
         [0014]    minimal loss of throughput. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0015]    The disclosed inventions will be described with reference to the accompanying drawings, which show important sample embodiments of the invention and which are incorporated in the specification hereof by reference, wherein: 
           [0016]      FIG. 1  shows a flow chart of the back end manufacturing process flow incorporating the preferred embodiment. 
           [0017]      FIG. 2   a  shows an untrimmed unit. 
           [0018]      FIG. 2   b  shows a trimmed unit. 
           [0019]      FIG. 3   a  shows a detail of an untrimmed unit. 
           [0020]      FIG. 3   b  shows a detail of a trimmed unit. 
           [0021]      FIG. 4  shows a trim die tool set that incorporates the preferred embodiment. 
           [0022]      FIG. 5  shows a close-up of the trim bar die with a unit therein. 
           [0023]      FIG. 6  shows a preferred embodiment of the detection switch. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0024]    The numerous innovative teachings of the present application will be described with particular reference to the presently preferred embodiment. However, it should be understood that this class of embodiments provides only a few examples of the many advantageous uses of the innovative teachings herein. In general, statements made in the specification of the present application do not necessarily delimit any of the various claimed inventions. Moreover, some statements may apply to some inventive features but not to others. 
         [0025]    The preferred embodiment is described with reference to the drawings.  FIG. 1  shows a flow chart of part of the back end process flow for integrated circuits. First, package molding is performed (Step  1 ) followed by tie bar trimming (Step  2 ) (removal of metal connections between the different leads extending from the chip). Next is deflash (Step  3 ), where the excess plastic mold material that remains after encapsulation is removed. These steps are followed by solder plating (Step  4 ), package marking (Step  5 ), lead forming (Step  6 ), and testing of the packaged chip (Step  7 ). 
         [0026]    The presently preferred embodiment relates to improvements to step  2  of  FIG. 1 , where the excess metal connecting the metal leads, also called tie bars or dambar, is removed.  FIG. 2   a  shows an untrimmed unit  200 . The unit  200  has leads  202  extending from the chip. Immediately after package molding, the leads  202  are still connected to both the chip itself and to one another by connecting pieces of metal  204 . These pieces of metal  204  (the tie bars) must be removed from the unit  200 , leaving a trimmed unit  206  as shown in  FIG. 2   b . Note that the metal leads  202  extending from the chip are now separated from one another. 
         [0027]      FIGS. 3   a  and  3   b  show details of an untrimmed unit ( FIG. 3   a ) and a trimmed unit ( FIG. 3   b ). As seen in  FIG. 3   a , the untrimmed unit still has the tie bar  204  connecting the leads  202  at a position very close to the chip itself. The trimmed unit in  FIG. 3   b  shows the tie bars removed, leaving the leads  202  clean and separated from one another. 
         [0028]    The removal of the tie bars is accomplished with a special tool called a trim die tool set.  FIG. 4  shows a trim die tool set, along with detailed individual parts of the trim die tool set. The set comprises an upper trim die  402  which includes a tie bar punch  404 , and a lower trim die  406  which includes a tie bar die  408 . The tie bar punch  404  refers to the part of the upper trim die  402  used to push the tie bar against the lower trim die  406 . The tie bar die  408  refers to the part of the lower trim die  406  used to remove the tie bar. 
         [0029]    The tie bar punch  404  comprises several teeth  410  spaced according to the dimensions of the leads, so that when the punch  404  is positioned above the unit, the teeth  410  of the punch  404  will be located above the tie bars that connect the leads. The tie bar die  408  has corresponding teeth or lands  412  that fit between the teeth  410  of the tie bar punch  404 , so that when the tie bar punch  404  and the tie bar die  408  are pressed together, the teeth  410  of the punch  404  interdigitate with the lands  412  of the die  408 . 
         [0030]    During trimming with the trim die tool set, the chip has leads attached, but the leads are untrimmed with tie bars still connecting each lead to its nearest neighbors. The chip is placed in the trim die tool set and the tie bar punch is lowered against the tie bar die. The chip leads are between the punch and the die, and the punches lower directly into contact with the tie bars but not the leads themselves. The leads make contact with the die lands. As the punch is lowered it trims the tie bars from the leads. 
         [0031]      FIG. 5  shows closeup of the trim bar die with a unit therein. The lands  502  make contact with the leads  504  of the unit. The present innovations take advantage of this contact to improve broken die detection in the lead trimming process. 
         [0032]    The present innovations use the principle of a latching circuit to detect broken die lands. The parts of the trim die tool itself along with the metal leads of the unit are used to form a switch in a latching circuit so that when the metal connection between the leads  504  and the lands  502  is broken a sensor is alerted. Thus, if an individual land  502  breaks, the switch (formed by the connection of the land  502  and the lead  504 ) will be open because of a loss of contact between the land  502  and the lead  504 . The loss of contact will cause the voltage at a detection node to be at a different voltage than if the switch were closed. 
         [0033]      FIG. 6  shows a preferred embodiment for the detection switch. The switch itself is created by contact between the die land  602  and the individual lead  604  that contacts it. A positive node  606  is connected to a negative node  608  through two parallel circuit connections. The first connection contains a resistor of value r 1   610 , the switch (formed by the contact between a die land  602  and a unit lead  604 ), and another resistor R 1   612 . The second parallel connection between the two nodes  606 ,  608  comprises another resistor  614  of the same value r 1  and a light  616  (or other load element) L 1 . In the preferred embodiment, L 1  and R 1  are designed to have the same resistance. When the die land  602  is in proper contact with the unit lead  604 , the switch remains closed. In the event that the die land  602  brakes, there will no longer be contact between the die land  602  and the lead  604 , causing the switch to be open. 
         [0034]    When the switch is closed, meaning that the land  602  and the lead  604  are making a proper connection and that the die land  602  is not broken, the current through the light L 1   616  is the same as the current through the resistor R 1   612 . When the switch is open, indicating a non-connection between the land  602  and the lead  604 , the current through L 1  is roughly doubled. This change in operation of the light L 1  is used as a broken die detection sensor, though it could be implemented in other ways. In the preferred embodiment, when the light is at a first brightness, this corresponds to a first current level through the light, which in turn corresponds to a closed switch position. When the light is at a second brightness, this indicates a second different current through the light, indicating a different switch state—open, meaning that the die land and the lead are not making proper contact. The detection circuitry can be bypassed by closing switch  618  in the circuit. 
         [0035]    The present innovations increase productivity in back end processing by detecting broken die lands (or other problems causing disconnection between the land and the lead). This detection is real-time, so that the process can be halted as soon as a broken land is detected. This prevents mass scrappage due to an undetected broken trim die, which would be used on succeeding units if not detected, causing these units to be improperly processed. Using the present innovative detection method, the system is able to stop at the instance of breakage, allowing an immediate fix and reducing downtime. 
       DEFINITIONS 
       [0036]    Following are short definitions of the usual meanings of some of the technical terms which are used in the present application. (However, those of ordinary skill will recognize whether the context requires a different meaning.) Additional definitions can be found in the standard technical dictionaries and journals. 
       Tie Bar: metal tie between leads. 
     Tie Bar Die: part of the lower trim die used to remove tie bar. 
     Tie Bar Punch: part of the upper trim die used to push the tie bar. 
     Tie Bar Trimming: removal of tie bars from a unit. 
     Trimmed Unit: the package after tie bar trimming. 
     Trim Die: metal tool set used in tie bar trimming. 
     Upper and Lower Trim Die: two parts of the trim die tool set. 
     Untrimmed Unit: the package after package molding. 
     Unit: refers to the package. 
     Modifications and Variations 
       [0037]    As will be recognized by those skilled in the art, the innovative concepts described in the present application can be modified and varied over a tremendous range of applications, and accordingly the scope of patented subject matter is not limited by any of the specific exemplary teachings given, but is only defined by the issued claims. 
         [0038]    The circuit used to detect the connection between the lands and the leads can obviously be of many varying configurations and still benefit from the innovations of the present application. For example, the switch could be part of a flip-flop or a latching circuit, or any other device where a voltage state that depends on the switch state can be monitored. 
         [0039]    Additional general background, which help to show the knowledge of those skilled in the art regarding variations and implementations of the disclosed inventions, may be found in the following documents, all of which are hereby incorporated by reference:
   C HIP  S CALE  P ACKAGING  by Lau and Lee, McGraw Hill (1999);   H ANDBOOK OF  E LETRONIC  P A CKAGING  by Harper, McGraw Hill (1969).