Abstract:
An injection casting system for encapsulating semiconductor products and method of use includes a mold unit having a cavity, a substrate material placed against the cavity, the cavity being filled by a liquid dispenser in contact with the bottom of the cavity and a running channel at the bottom of the cavity to receive the liquid dispenser for even dispersion of epoxy in the cavity from the bottom of the cavity upward to the top of the cavity.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The present invention generally concerns encapsulating semiconductor devices and specifically an injection casting system for encapsulating semiconductor devices.  
         [0003]     2. Related Information  
         [0004]     Encapsulating semiconductors is an important step for protecting a semiconductor surface such as a printed circuit board (PCB) from degrading. In addition, encapsulating permits the forming of lens for use with Infrared Data Transceivers (IrDT) while still protecting the PCB.  
         [0005]     The use of IrDT as a device to transmit and receive data wirelessly has been gaining popularity. Devices are equipped with infrared modules such as personal digital assistants (PDA), notebook/laptop computers, digital cameras, mobile phones, and many more.  
         [0006]     Traditionally, most semiconductor devices use leadframes as substrate materials, but recently PCBs have been widely used to replace leadframes. Some advantages of using a PCB over leadframes are as follows: 1) better coplanarity, 2) easier to scale down, 3) better electromagnetic interference immunity and improved thermal dissipation, and 4) flexible singulation process (sawing over trim/form). These advantages translate into lower investment for equipment. Therefore, an objective of the present invention is the ability to incorporate PCBs into the injection casting system.  
         [0007]     Because of its simplicity, low cost, and fast turn-around time, a cast process is extensively used in the encapsulation process. The cast process is especially economical for low volume production and research and development (R&amp;D) work when compared to a transfer mold. Transfer molding onto PCB substrates with an unfilled epoxy also presents more challenges than transfer molding onto leadframes substrates. Accordingly, a further objective of the present invention is to integrate the advantages of the cast process including simplicity, low cost, and fast turn around time.  
         [0008]     The current cast process uses room temperature vulcanizing (RTV) silicone as the mold material. RTV silicone comes in two parts which are then mixed and cured in a master mold to form the epoxy casting RTV silicone molds. The resulting RTV silicone molds are then used to encapsulate the semiconductor substrates. The RTV silicone molds are problematic because they wear out fast and normally last for 10 to 20 cast cycles after which new molds will have to be made again. Also, the RTV silicone mold&#39;s properties depend on mix ratio and cure temperature and are therefore susceptible to unintended variations. Furthermore, the use of molds using silicone are problematic because silicone absorbs moisture which can create problematic air bubbles. Silicone is not the best processing agent either because of long preheat and cure time because silicone has poor thermal conductivity. Therefore, a further objective of the present invention is to avoid the problems associated with RTV silicone molds, minimize mold wear, minimize water and air absorption, and provide for shorter preheat and cure times.  
         [0009]     In the prior art, once the RTV silicone molds were formed an operator was required to fill each individual cavity with epoxy, each cavity may have lens aperture which would also have to be individually filled. This process is manual and requires a long cycle time. In addition, the yield varies depending on the operator&#39;s skill. An automatic solution for this process is difficult to achieve because of the individual manipulation of epoxy for each cavity and lens aperture. Therefore, a still further objective of the present invention is the provision of a fast, simple and easy process that shortens cycle time and improves yield.  
         [0010]     Traditionally, after applying the epoxy, the PCB is placed onto the mold. This process results in a high probability of air entrapment that can not be removed. This step is especially problematic because neither a vacuum chamber nor a pressurized chamber may be used to prevent air entrapment. Accordingly, a still further objective of the present invention is the use of a vacuum or pressurized chamber to eliminate air bubbles.  
         [0011]     In addition, several other problems are identifiable with RTV silicone molds. For example, the substrate or PCB size is limited due to a large thermal expansion mismatch between silicone RTV and PCB substrates and the narrow processing (i.e. curing) temperature for RTV silicone molds. Therefore, a still further objective of the present invention is to minimize the expansion mismatch and the narrow processing temperature associated with RTV silicone molds.  
         [0012]     A still further objective of the present invention is the provision of a injection casting system for encapsulating semiconductor devices and method of use that is economical to manufacture, durable in use, and efficient in operation.  
       BRIEF SUMMARY OF THE INVENTION  
       [0013]     An injection casting system for encapsulating semiconductor devices and method of use has been developed to improve yield, to reduce encapsulation time, and to allow encapsulating deep lens cavity for optoelectronic devices. This invention utilizes a combination of a controllable liquid dispensing system and a runner system to achieve a balanced flow of material in a cast process. To prevent air entrapment, the cast mold has a vertical cavity with an opening on the top. Instead of pouring or dispensing material from the top (as in the standard cast process), encapsulating material is injected into a desired location (e.g. bottom of cavity), which is then distributed by the runner system throughout the mold cavity. This way, the mold cavity is filled from the bottom up, and as the material flows up, it pushes air upward. The vertical mold position also allows the air to easily rise to the surface, thereby minimizing defect due to air entrapment. The process is economical, reduces cycle time, provides better process control, reduces defects, permits larger allowed substrate size, and produces a yield that is less operator-dependent.  
         [0014]     The injection casting system for encapsulating semiconductor devices and method of use has several features absent from the prior techniques of using leadframes, transfer molding, and RTV silicone molds. These features are summarized as follows: 
        Fast, simple and easy process.     Flow can be easily controlled.     Minimum air entrapment.     Vacuum or pressurized chamber can be utilized to eliminate air bubbles if necessary.     Very little expansion mismatch between stainless steel and PCB, allowing better tolerance over larger PCB.     Fast preheat and cure due to higher thermal conductivity of stainless steel.     No moisture sensitivity issue.     Mold wear is negligible.        
 
         [0023]     These, as well as other features and advantages of the present invention will become apparent from the following specification and claims. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0024]      FIG. 1  is a perspective view of the injection casting system in an exploded perspective view.  
         [0025]      FIG. 2  is a perspective view of the injection casting system with the plate, printed circuit board, and mold unit fastened together.  
         [0026]      FIG. 3  is a cross-sectional view taken along line  3 - 3  of  FIG. 1 .  
         [0027]     FIGS.  4 A-C are cross-sectional views taken along line  44  of  FIG. 2  showing the filling of the mold unit with epoxy.  
         [0028]      FIG. 5  is a perspective view of the finished product from the injection casting system, specifically an IR data transceiver. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0029]     Referring to  FIG. 1  an injection casting system is generally designated by the numeral  10 . The injection casting system  10  includes a mold unit  12  which comprises top side  14 , bottom side  16 , longitudinal sides  18 , an inner face  22 , and an outer face  24 . The mold unit is typically stainless steel.  
         [0030]     A cavity  26  extends into the inner face  22 . The cavity  26  is defined by inset bottom side  28 , inset longitudinal sides  30 , and an inset back face  32 . As most clearly seen in  FIG. 3 , the cavity  26  has an open face  34  and an open top side  36 . The inset back face  32  may have aperture  38  further inset into the cavity  26 .  
         [0031]     As seen in  FIG. 1 , the cavity has a dispensing channel  40  extending from the top side  14  into the cavity  26 . The dispensing channel  40  may parallel the longitudinal sides  30 . In addition, the cavity  26  has a running channel  42  adjacent to the dispensing channel  40 . The running channel  42  may parallel the bottom side  28 . The running channel  42  may be used without a dispensing channel  40 .  
         [0032]     A liquid dispenser  44  is provided having a reservoir  46  and a needle  48 . The liquid dispenser  44  is positioned over the dispensing channel  40  and inserted such that the needle  48  comes in contact with the running channel  42 . The reservoir  46  stores the encapsulation material or epoxy  54  and the dispensing needle  48  provides a throughway for dispensing the epoxy  54  in a controlled manner to the running channel  42 .  
         [0033]     A plate  50  is provided for securing to the inner face  22  of the mold unit  12  over the open face  34  of the cavity  26 . The plate  50  is typically stainless steel.  
         [0034]     Positioned intermediate the mold unit  12  and the plate  50  is the substrate material  52 . As shown in  FIG. 1 , the substrate material  52  is a printed circuit board. The substrate material may be other devices.  
         [0035]     As seen in  FIG. 1 , holes in the printed circuit board mate with pegs on the mold unit  12 . The plate  50  mates with knobs on the mold unit  12  to clamp the printed circuit board  52  firmly in position.  FIG. 2  shows the plate  50  securely clamping the printed circuit board  52  to the mold unit  12 .  
         [0036]     As further seen in  FIG. 1 , the mold unit  12  has four cavity  26  segments.  
         [0037]     Furthermore, each cavity  26  may have an arrangement of repeating apertures. In other words, each cavity  26  consists of many potential devices. The apertures may be arranged in a longitudinal series, a lateral series, or a combination longitudinal and lateral series.  
         [0038]     The size of each cavity  26  is designed to maximize the number of parts while minimizing defects from warpage. When using a clear epoxy  54 , defects from warpage are especially troublesome because clear molding epoxy typically has an expansion coefficient of around 60×10 −6 /° C. This warpage is minimized by pressing the PCB firmly between the plate  50  and mold unit  12 . The seal may also be assisted by tenting material (not shown) a kind of dry film solder mask on PCB substrate to create a better seal. The tenting material is typically around 0.002 inches thick.  
         [0039]     As seen in  FIG. 3 , the method of using the injection casting system begins with the liquid dispenser  44  in alignment with the dispensing channel  40 .  
         [0040]     Before closing the plate  50  and PCB  52  upon the mold unit, a thin mold release agent is applied uniformly across the mold surface and specifically upon the cavity  26  and aperture  38  areas. The PCB  52  and plate  50  are then aligned and placed upon the mold unit  12 . Once the mold unit  12 , PCB  52 , and plate  50  are aligned, they may be placed together  30  securely by screws, a press vice, or other system. Typically, the molds are preheated depending on the viscosity of the encapsulation material or epoxy  54 . However, preheating the mold unit  12  may not be necessary if the viscosity of the encapsulation material  54  is thin enough to flow readily into the cavity  26 .  
         [0041]     As seen in  FIG. 4A , the dispensing needle  48  is inserted into the dispensing channel  40 . The dispensing needle  48  is typically positioned at the bottom of the dispensing channel  40  so that it contacts the running channel  42 .  
         [0042]     As seen in  FIG. 4B , the epoxy  54  is then distributed from the reservoir  46  through the needle  48  to begin filling the running channel  42  and the cavity  26 . As epoxy  54  fills the cavity  26  it flows upward against gravity and presses air upward from the cavity  26 . The epoxy  54  is applied gradually so that few if any air bubbles and moisture are left within the cavity  26  and the apertures  38 . Once the encapsulation material  54  fills the cavity it will enter into an overflow area  58 . A divider  56  prevents any encapsulation material  54  from traveling into a neighboring cavity  26 .  
         [0043]     The user may then remove excess material if present in the overflow area  58 . The encapsulating material  54  can then be cured a short time in the mold and then removed for a longer cure.  
         [0044]     Final preparation of the encapsulated material takes the encapsulated PCB and saws or dices it up into individual devices. An illustration of this is the infrared data transceiver as shown in  FIG. 5 . After sawing the finished product  60 , it is completed by inserting solder pads  62 . Although an infrared data transceiver is shown in  FIG. 5 , the method can be applied to many other applications including a simple flat-surface cavity and a cavity with deep-lens apertures.  
         [0045]     The invention has been shown and described above with the preferred embodiments, and it is understood that many modifications, substitutions, and additions may be made which are within the intended spirit and scope of the invention. From the foregoing, it can be seen that the present invention accomplishes at least all of its stated objectives.