Patent Document

BACKGROUND OF THE INVENTION  
         [0001]    (1) Field of the Invention  
           [0002]    The invention relates to the fabrication of integrated circuit devices, and more particularly, to a molded semiconductor device package and method for the creation thereof.  
           [0003]    (2) Description of the Prior Art  
           [0004]    For the packaging of semiconductor devices frequent use is made of methods of encapsulation of the devices in packages that are aimed at further usage. These packages have to meet requirements of high speed processing environments and are therefore heavily influenced by such considerations as cost, usability, quality, ease and repeatability of manufacturing, throughput and others.  
           [0005]    One of the more commonly used molding materials that is used for the purpose of creating an encapsulated semiconductor device package is resin. Resins occur freely in a natural environment, industrially applied resins are synthetically prepared and can be created with many properties that are of value for a given application. Synthetic resins (such as alkyd resins or phenolic resins) usually have high molecular weight and may have some of the properties of natural resins. Synthetic resins however are typically very different from natural resins. Synthetic resins may be thermoplastic or thermosetting, they can be made by polymerization or by condensation, and they are used mostly as plastics or the essential ingredients of plastic, in varnishes or other coatings, in adhesives and in ion exchange.  
           [0006]    In the semiconductor industry, resins are frequently molded into particular forms or shapes that are used to house or package semiconductor chips. These completed molds then serve as chip carriers and may contain parts within the mold that facilitate or enable this function such as a die pad (to position the chip onto), metal extensions (lead fingers) that serve to interconnect the packaged chip with its surrounding electrical environment and means (such as wire bonding) for connecting the chip to metal extensions.  
           [0007]    It is thereby also common practice to adapt plastic or resin chip carriers to a high speed semiconductor manufacturing environment, for the main reason that this is the predominant environment that is being used to produce high volumes of semiconductor chips at a competitive price. The chip carriers must thereby also be adaptable to a variety of chip sizes, again to make the chip carrier acceptable from a cost point of view. To adapt the chip carrier to a high-speed manufacturing environment, the design must be such that no parts of the carrier can interfere with the manufacturing process due to protruding parts of the carrier. This could cause deformation of the protruding parts in addition to slowing down the manufacturing process due to the required intervention to remove the offending carrier.  
           [0008]    A mold cavity frequently consists of two sections, an upper section and a lower section. The lower section forms, after molding, the support for mounting the chip and for supporting lead fingers. These supporting components are inserted in the lower mold prior to the formation of the pre-molded plastic chip carrier.  
           [0009]    In addition to the above considerations relating to the creation of a mold, considerations of adhesion between the epoxy mold compound and the substrate of the package play an important role in the creation of a mold-packaged semiconductor device. This concern applies to the four corners and the edges of the mold cap where stress concentrations are most likely to occur, a stress that is highly temperature dependent. Present practices to alleviate the impact of corner stress focus on substrate cleanliness, achieved by for instance surface plasma treatment, and by matching the stress related properties of physically interfacing elements of the package. Stress related adverse impact on the overall package is typically and most likely concentrated at the most exposed or weakest points of the mold cap. The invention addresses these concerns of mold cap creation and reliability.  
         SUMMARY OF THE INVENTION  
         [0010]    A principle objective of the invention is to provide a mold cap that is free of problems of adhesion to surrounding surfaces.  
           [0011]    Another objective of the invention is to provide an anchor for the mold cap such that the mold cap is more firmly secured and kept in place.  
           [0012]    In accordance with the objectives of the invention a new method is provided for the creation of a mold cap. The mold cap-anchoring feature of the invention is designed and incorporated from the start of the design and fabrication of the substrate. Various design options of the mold anchor of the invention can be implemented. The mold anchor of the invention allows the mold compound to flow underneath the substrate where the mold compound will remain in place until the process of mold formation is completed. The mold compound of the package will penetrate all available cavities surrounding and being accessible from the mold anchor of the invention where the mold compound will remain in place and harden. After hardening, the mold compound surrounding the mold anchor will support the anchored area. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0013]    [0013]FIGS. 1 a  through  1   c  show conventional methods of creating a mold cap over the surface of a supporting substrate.  
         [0014]    [0014]FIGS. 2 a  through  2   c  show methods of the invention of creating a mold cap over the surface of a supporting substrate.  
         [0015]    [0015]FIGS. 3 a  and  3   b  show details of the mold cap to substrate interface and the anchoring that is achieved between these two elements.  
         [0016]    [0016]FIGS. 3 c  and  3   d  show an additional implementation that closely resembles the implementation shown in FIGS. 3 a  and  3   b.    
         [0017]    [0017]FIG. 4 a  and  4   b  shows implementation details of the substrate that are provided for purposes of anchoring the mold cap to the substrate.  
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0018]    The invention provides a method for anchoring the mold cap of a mold compound to the underlying and supporting substrate of the package. The mold anchor of the invention is preferably applied in the creation of relatively thin semiconductor device packages. The mold anchor of the invention secures the mold cap to the substrate. The mold cap of the invention may be provided at the edges or corners of the mold cap. The active area of the mold cap may in this manner by increased.  
         [0019]    The conventional method of providing a mold cap over the surface of a supporting substrate in a semiconductor device package will first be highlighted, using FIGS. 1 a  through  1   c  for this purpose. Shown in FIG. 1 a  is a top view  10  of a substrate  12  over the surface of which a mold cap  14  has been deposited using conventional methods of mold cap formation. A film  13  of polyimide has for protective purposes been applied over the surface of substrate  12  prior to the creation of the mold cap  14 .  
         [0020]    Further detail of one of the corners  16  of the substrate  12  with the thereover provided mold cap  14  has been shown in the cross section of FIG. 1 b , the cross section being taken along the line  1   b - 1   b ′ of FIG. 1 a  and bounded by the highlighted circle  16  shown in FIG. 1 a . The cross section shown in FIG. 1 b  represents a typical substrate  12  design with a mold cap  14  applied over the surface of substrate  12 . Not shown in FIG. 1 b  is the effect of temperature that is experienced by the substrate/mold cap combination as a result of mold reflow. This effect is shown in the cross section of FIG. 1 c , which shows in this instance a cross section along the line  1   b - 1   b ′ of FIG. 1 a  without however being bounded by the circle  16  of FIG. 1 a  but being extended between the extremities of the cross section as highlighted with circles  16  and  16 ′ in the cross section of FIG. 1 c . One of the reasons for mold reflow is to assure that the mold  14  properly adheres to the surface of the substrate  12 . This adhesion however tends to warp the substrate  12  due to a combination of contraction of the mold compound  14  and the relative good adhesion that exists between the mold compound  14  and the substrate  12 . This warpage of the substrate  12  is shown in the cross section of FIG. 1 c . It is clear from the cross section of FIG. 1 c  that extreme tension of separation will be created between the mold cap  14  at the extremities of the mold cap, that is in cross section areas  16  and  16 ′. This tension or force of separation results in the delamination  18  shown in the cross section of FIG. 1 c , where the mold cap  14  separates from the underlying substrate  12 . This delamination is highly undesirable since it exposes the underlying substrate  12  over the surface areas of the delamination  18 , thereby introducing the possibility of creating deposits over these exposed surface areas which have a negative impact on package performance and reliability.  
         [0021]    To prevent the delamination  18  that is shown in the cross section of FIG. 1 c , the invention provides anchor points in the four corners of the substrate as has been highlighted in and will be described using FIGS. 2 a  through  2   d . Anchoring can also be provided at any other surface area around the perimeter of the substrate that is sued to create the device package.  
         [0022]    [0022]FIG. 2 a  shows a top view  20  of the supporting substrate  22  over the surface of which has been applied a mold cap  24 . A polyimide tape  23  has been applied over the surface of substrate  22  prior to the formation of the mold cap  24 , copper interconnect traces  25  created over the surface of substrate  22  have been highlighted in FIGS. 2 c  and  2   d.    
         [0023]    Of special interest to the invention are the surface areas in the four corners of the substrate  22 , of which one illustrative example has been highlighted by surface area  26  in FIG. 2 a . A cross section, taken along the line  2   b - 2   b ′ of FIG. 2 a , of this surface area  26  is shown in FIGS. 2 b  and  2   c.    
         [0024]    Specifically notable in the cross section of FIG. 2 b  is the opening  28  that has been created through the substrate  22 , an opening that is provided for each of the corners of substrate  22  of which the cross section shown in FIG. 2 b  is a representative example. It is clear that the mold compound  24  will, at the time of filling of the mold cavity with mold  24 , penetrate opening  28  and in so doing will, after hardening of the mold, firmly anchor the mold compound  24  in each of the corners of substrate  22 .  
         [0025]    To further emphasize this anchoring effect, it is beneficial to enable the mold to penetrate underneath the substrate. An example of this is shown in the cross section of FIG. 2 c  in which an additional relief or opening  27  is provided for this purpose in the lower part of the mold cavity (the cavity bar). This opening  27  is filled with mold compound at the time that the mold compound enters into the mold cavity. This additional relief  27  has been shown in the cross section of FIG. 2 c  as being of rounded cross section, resembling a segment of a circle.  
         [0026]    There is no reason for this additional relief to be limited to such a cross section, any shape or form that further enhances the anchoring of the mold compound to the underlying substrate can be applied for this purpose of anchoring. For instance, a finned cross section, resembling for instance cooling fins of a heatsink, wherein parts of the additional relief fan-out as separate sub-elements from a central part can be envisioned as providing extreme anchoring capabilities.  
         [0027]    [0027]FIG. 2 d  shows a top view of surface are  26  of FIG. 2 a , more clearly highlighting the location of the anchoring point  26  with respect to both the substrate  22  and the applied mold cap  24 .  
         [0028]    With the basic concept of the invention in mind, that is providing an anchor that forms a solid interconnection between the supporting substrate and the overlying mold cap, it is clear that a number of variations of this concept can be used. Some of these variations are highlighted using FIGS. 3 a  through  3   d  for this purpose.  
         [0029]    Referring first specifically to the cross section that is shown in FIG. 3 a , there is shown a cross section  32  of a top cavity bar, a cross section  30  of a bottom cavity bar, a substrate  34  with copper interconnect traces  35  provided over the surface thereof. Anchor opening  37  has been provided through (each of the four) corners of substrate  34 , by modifying the contours of the top and bottom cavity bars  32 / 30  where these cavity bars are aligned with the anchor opening  37 , the contours of the applied mold compound can be controlled.  
         [0030]    In the example that is shown in the cross section of FIG. 3 a , a top cavity relief  36  has been indicated that extends over a distance of substrate  34 , allowing additional mold compound to collect over the surface of the substrate  34  and surrounding the anchor opening  37 . A bottom cavity relief  38  has the same effects as this effect is now introduced for mold collection underneath the substrate  34 . The combined effect of these relief  36  and relief  38  is shown in the completed mold compound  38  as shown in the cross section of FIG. 3 b , where the anchor area  33  is now provided with mold compound  38  that extends above the upper and below the lower surface of substrate  34 . Copper traces  35  are also highlighted in the cross sections of FIGS. 3 a  and  3   b.    
         [0031]    An additional implementation that closely resembles the implementation shown in FIGS. 3 a  and  3   b  is highlighted in FIGS. 3 c  and  3   d . In the latter implementation the relief  38 , FIG. 3 a , in the bottom cavity bar  30 ′ is omitted, resulting in a completed mold compound  38 ′ shown in cross section in FIG. 3 d . The anchor area  33 ′ is now provided with mold compound  38 ′ that extends above the upper surface of substrate  34 .  
         [0032]    Additional details relating to the design of the substrate of the invention are shown in FIG. 4 a  and  4   b , both FIGS. 4 a  and  4   b  showing a top view of one (of the four) corner of substrates  40  and  42 . The difference between substrates  40  and  42  is created by the difference in the creation of the anchor holes  45  (FIG. 4 a ) and  43  (FIG. 4 b ).  
         [0033]    The anchor hole  45 , FIG. 4 a , has been created using a drilling or punch-through process, which as shown in the cross section of FIG. 4 b  as not differentiating between the presence or absence or copper  41 .  
         [0034]    The anchor hole  43 , FIG. 4 b , has been created applying an etch process, which creates the anchor through hole  43  while not affecting copper  46 , creating overhang copper  46 .  
         [0035]    Elements  44 , FIG. 4 a , are copper pads created over the surface of substrate  40 ,  48  is polyimide tape applied over the surface of substrate  40 .  
         [0036]    Although the invention has been described and illustrated with reference to specific illustrative embodiments thereof, it is not intended that the invention be limited to those illustrative embodiments. Those skilled in the art will recognize that variations and modifications can be made without departing from the spirit of the invention. It is therefore intended to include within the invention all such variations and modifications which fall within the scope of the appended claims and equivalents thereof.

Technology Category: 5