Patent Document

BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates generally to semiconductor packaging, and more particularly, to a package of a semiconductor device having increased ability to limit moisture flow through the packaging material and into the area of the die and die pad 
     2. Discussion of the Related Art 
     Reference is made to FIGS. 1-3, wherein a typical packaged semiconductor structure  10  is shown. As is well known, a die pad  12  has a die  14  epoxied thereon and tie bars  16 ,  18  extending therefrom. Bonding wires  20  connect the die  14  to the inner ends of leads  22 . The die pad  12 , die  14 , wire bonding  20  and inner ends of the leads  22  are packaged in epoxy  24 , commonly referred to as plastic packaging, with the outer ends of the leads  22  extending from the epoxy  24 . These leads  22  may be plugged into a printed circuit board  26 , as shown in FIG.  3 . 
     Over a period of time, moisture  28  diffuses through the plastic package  24  and into the area of the die  14 , die pad  12 , and wire bonding  20 . Through the operation of the device  10 , heat buildup can cause moisture in those critical areas to vaporize, creating large amounts of steam pressure in pockets adjacent to these critical areas. In the case of moisture near the lower side of the die pad  12 , blisters may form of sufficiently large size to cause connection problems between the printed circuit board  26  and the device  10 . In the case of moisture near the upper side of the die  14 , wire bond failure, die face damage, and/or die fracture can occur. Obviously, situations where such delaminations can occur are highly undesirable and can result in severe reliability problems. 
     Current practice is to measure how well each packaged device performs under JEDEC stress testing (specified temperature and humidity environment for a specified time for each Level). Manufacturers of semiconductor devices strive to reach Level 1 as that is the only level for which neither device baking nor a hermetic bagging are required. This “bake and bag”process involves additional expense, and would not be necessary if the package were inherently resistant to moisture flow therethrough to the area of the die  14 , die pad  12  and wire bonding  20 . 
     FIG. 4 shows a packaged semiconductor device  29  which utilizes an anodized aluminum heat spreader  30  in contact with the die pad  32  for dissipation of heat buildup during the operation of the device  29 . In the fabrication of the device  29 , the heat spreader  30  is placed in the well of a transfer molding machine, legs  34  extending from the heat spreader  30  contacting the bottom of the well, and a lead frame  36  having a die  38  associated therewith and wire bonded thereto is placed over the heat spreader  30  with the die pad  32  in contact with the heat spreader  30 , so as to be an interference fit with the heat spreader  30 . As molding compound  40  is forced into the area of the die  38  and die pad  32 , air can be trapped above the heat spreader  30  and beneath the die pad  32  to form an air void therebetween. The heat spreader  30  includes apertures  42  therethrough to allow this air void to pass through the apertures  42  and away from the area of the die pad  32 , thereby relieving pressure from this area. While this configuration is effective for this purpose, the problem of limiting moisture ingress into the area of the die pad  32 , die  38 , and wire bonding is not addressed. Indeed, the apertures  42  in the heat spreader  30  will readily allow moisture therethrough directly into the area of the die pad  32  in contact with the heat spreader  30 . Furthermore, the problem of moisture ingress into the area of the die  38  and wire bonding through the packaging material thereabove is not addressed. 
     Therefore, it would be highly desirable to provide a system for limiting moisture diffusion through the plastic packaging material to the critical areas of the die, die pad, and wire bonding, so that the delamination problems described above are avoided. 
     SUMMARY OF THE INVENTION 
     In the present invention, in the environment of a semiconductor die epoxied to a die pad, with tie bars extending from the die pad, barrier structures are secured to the tie bars so as to have barrier bodies positioned on opposite sides of and overlying the die-die pad assembly. Each barrier body is spaced from the die-die pad assembly, and has a plurality of small apertures therethrough. As moisture diffuses through the plastic package material, the barrier bodies act as baffles to limit such diffusion in the direction of the die-die pad assembly to only that passing through the apertures. From these apertures the moisture diffuses in a spreading manner. Additional barrier structures may be provided which have barrier bodies overlying the above-described barrier bodies, the additional barrier bodies having apertures therethrough which are non-aligned with the apertures of the first-described barrier bodies to provide an even longer moisture flow path to the die-die pad area. 
     The present invention is better understood upon consideration of the detailed description below, in conjunction with the accompanying drawings. As will become readily apparent to those skilled in the art from the following description, there are shown and described embodiments of this invention simply by way of the illustration of the best mode to carry out the invention. As will be realized, the invention is capable of other embodiments and its several details are capable of modifications and various obvious aspects, all without departing from the scope of the invention. Accordingly, the drawings and detailed description will be regarded as illustrative in nature and not as restrictive. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The novel features believed characteristic of the invention are set forth in the appended claims. The invention itself, however, as well as said preferred mode of use, and further objects and advantages thereof, will best be understood by reference to the following detailed description of illustrative embodiments when read in conjunction with the accompanying drawings, wherein: 
     FIGS. 1-4 are sectional views of prior art semiconductor devices as described above; 
     FIG. 5 is a sectional view of a semiconductor device incorporating the present invention; 
     FIG. 6 is a sectional view of the device of FIG. 5, taken along the line  6 — 6  of FIG. 5; 
     FIG. 7 is a sectional view of the device of FIG. 5, taken along the line  7 — 7  of FIG.  5 . 
     FIG. 8 is a plan view of a preferred embodiment of barrier structure of the present invention; 
     FIG. 9 is a side view of the barrier structure of FIG. 8; 
     FIG. 10 is an enlarged view of a portion of the structure of FIG. 5, showing the barrier structures limiting moisture flow; 
     FIG. 11 it is a plan view of a typical lead frame for use in with the present invention; 
     FIG. 12 is a sectional view of a portion of the lead frame of FIG. 11, showing the barrier structure mounted thereto, and the step of wire bonding; 
     FIG. 13 is a plan view of a portion of an alternative embodiment of lead frame; 
     FIG. 14 is a plan view of an alternative embodiment of barrier structure; and 
     FIG. 15 is a sectional view showing the combination of the structures of FIGS. 13 and 14. 
    
    
     DETAILED DESCRIPTION 
     Reference is now made in detail to specific embodiments of the present invention which illustrate the best mode presently contemplated by the inventors for practicing the invention. 
     As shown in FIGS. 5,  6  and  7 , a die pad  50  has tie bars  52 ,  54  extending therefrom, and has epoxied thereon a silicon chip or die  56 . The embodiment of FIGS. 5-7 includes barrier structures  58 ,  60 ,  62 ,  64 , one of which is shown in detail in FIGS. 8 and 9. The barrier structure  58  is of very thin material, for example, Cu(Be) foil, and includes a rectangular barrier body  66  defining a plurality of apertures  67  therethrough and protrusions  68 ,  70  extending therefrom. The barrier structure  58  may be formed by a stamping process to define the rectangular overall configuration and to punch the plurality of apertures  67  through the barrier body  66 , and also to form the protrusions  68 ,  70  extending from the barrier body  66 . The barrier structure  58  may alternatively be fabricated of, for example, Alloy  42  (42 % Ni+58 % Fe), or copper alloy. 
     Again with reference to FIGS. 5-7, the barrier body  66  of the barrier structure  58  is positioned on the die side of the die  56 —die pad  50  assembly, with the protrusions  68 ,  70  of the barrier structure  58  being secured to the tie bars  52 ,  54  by welding or glue. In such state, the barrier body  66  generally overlies and is spaced from the die  56 —die pad  50  assembly, so as to be spaced from the die  56  and wire bonding  72 , which connects the die  56  to leads  73 . The barrier body  74  of the barrier structure  62  (similar in configuration to barrier structure  58 ) is positioned on the die pad side of the die  58 —die pad  50  assembly, with the protrusions  78 ,  80  of the barrier structure  62  likewise being secured to the tie bars  52 ,  54  by welding or glue. The barrier body  74  generally overlies and is spaced from the die  56 —die pad  50  assembly, so as to be spaced from the die pad  50 . Additional barrier structure  60  overlies the barrier structure  58 , and is spaced therefrom with the barrier body  66  of the barrier structure  58  positioned between the barrier body  82  of the barrier structure  60  and the die  58 —die pad  50  assembly. This barrier structure  60  includes protrusions  84 ,  86  which are longer than the protrusions  68 ,  70  of the barrier structure  58 , and is dimension so that the protrusions  84 ,  86  lie outward of the ends of the barrier structure  58 . These protrusions  84 ,  86  of the barrier structure  60  are also welded or glued to the tie bars  52 ,  54 . A similar structure  64  is provided on the die pad  50  side of the die  58 —die pad  50  assembly. That is, additional barrier structure  64  has its body  88  overlying and spaced from the body  74  of the barrier structure  62 , and has protrusions  90 ,  92  secured to the tie bars  52 ,  54 . 
     As will be seen from Figures, the apertures  67  in the barrier body  66  are non-aligned with the apertures  94  in the barrier body  82 , i.e., they are in staggered relationship. Similarly, the apertures  96  in the barrier body  74  are non-aligned with the apertures  98  in the barrier body  88 . As shown in FIG. 10, as moisture  99  flows through the packaging material  100  and toward the barrier body  82 , this moisture  99  is blocked to a substantial extent by the barrier body  82  from passing into the area between the barrier bodies  82 ,  66 . However, the moisture  99  which does pass through the apertures  94  of the barrier body  82 , it will be seen, diffuses downwardly and outwardly as shown. Thus, a very limited amount of moisture passes through the apertures  94  of the barrier body  82 , from which it is spread and diffused. It is only this very limited amount of moisture which may reach the barrier body  66 . Thus, a very minimal amount of moisture will pass through the aperture  67  of the barrier body  66  and toward the die  58 —die pad  50  assembly. 
     The barrier structures  62 ,  64  on the underside of the die  58 —die pad  50  assembly clearly limit moisture flow to that area in the same manner. 
     The spacing of the barrier bodies from the die  58 —die pad  50  assembly and from each other is significant in achieving the proper limitation of moisture flow into the critical areas. This spacing between elements allows the moisture to diffuse into relatively large areas so as to aid in the protection of critical structures. 
     While not essential, it is preferable that the barrier bodies not be continuous, but include such apertures shown and described Providing such apertures avoids the possible problem of molding compound intruding during transfer molding to trap bubbles of gas, creating voids. If such voids were present, local tensile stresses could increase and thermal impedance could be degraded. The apertures provided in the barrier bodies avoid such a potential problem by allowing such air to be vented away from the critical areas of the die, die and wire bonding. 
     The apertures in the barrier body may be spaced more widely in the least critical areas, those adjacent the junction of the die-die pad assembly, i.e., near the center of the barrier body, so that venting would move progressively inward from the periphery due to molding compound ingression. The effect is to provide the greatest limitation to moisture flow where the need is greatest, where the differential CTE (Coefficient of Thermal Expansion) is greatest, far from the die center. 
     The apertures in each barrier body may for example be 0.2 mm in diameter with a pitch of 1 mm. 
     In furtherance of describing the assembly of a semiconductor device incorporating the present invention, with reference to FIG. 11, a typical lead frame  110  is shown therein. As is well-known, a pair of rails  112 ,  114  are connected by dam bars  116 ,  118 , with tie bars  120 ,  122  extending outward from a die pad  124  between a pair of dam bars  116 ,  118 , the tie bars  120 ,  122  being connected to the rails  112 ,  114 . Leads  126  are connected to the dam bars  116 ,  118  as is also well-known. 
     A portion of this lead frame  10  is shown in sectional view of FIG. 12, with the barrier structure  62  and barrier structure  64  being mounted thereto as described above. The die pad  124  has a die  128  epoxied thereto, and the barrier structure  62 ,  64  are secured to the tie bars  120 ,  122  on the die pad side of the die  128 —die pad  114  assembly. The structure of FIG. 12 is now placed in a wire bonding machine  130  with a cavity  132  sufficiently deep to receive the barrier structures  62 ,  64 . Then, wire bonding is undertaken, after which the barrier structures  58 ,  60  are added, being secured to the tie bars  120 ,  122  as described above, on the die side of the die  128 —die pad  124  assembly. Then, transfer molding is undertaken to form the plastic package of the device, and the rails  112 ,  114  and portions of the dam bars  116 ,  118  are removed as is well known so that the leads  126  of the device are defined and extend from the package. 
     It is to be noted that spot welding of the protrusions to the tie bars  120 ,  122  takes place with the dam bars  116 ,  118 , rails  112 ,  114  and tie bars  120 ,  122  in place, that is, prior to the removal of the rails  112 ,  114  and portions on the dam bars  116 ,  118 . Thus, the entire structure is shorted out, and there will not be any voltage imposed on a pin of the device relative to any other pin due to the spot welding operation. 
     FIG. 13 shows an alternative embodiment of lead frame  150 , wherein each tie bar  152 ,  154  is bifurcated in the respective area  152 A,  154 A where it extends to connect to a rail  156 ,  158 . In this configuration, each barrier structure  160  has four protrusions  162 ,  164 ,  166 ,  168  stamped therein (FIG.  14 ), a protrusion being welded to each extended leg of a tie bar. The mounting of the barrier structures  160  to the bifurcated ends  152 A,  154 A by welding or glue as described above is shown in FIG.  15 . This embodiment can be used when increased mounting stability of the barrier structures is desired. 
     As yet another embodiment, the barrier structures may have protrusions which match up with the four diagonal tie bars of a quad flat pack lead frame configuration. 
     It will therefore be seen that the present invention provides for highly effective limitation of moisture flow through a package to the critical die-die pad area of a semiconductor device. This resistance to moisture in this critical areas avoids problems of delamination as described above, so that high device reliability is achieved. 
     The foregoing description of the embodiment of the invention has been presented for purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise form disclosed. Other modifications or variations are possible in light of the above teachings. The embodiments were chosen and described to provide the best illustration of the principles of the invention and its practical application to thereby enable one of ordinary skill of the art to utilize the invention in various embodiments and with various modifications as are suited to the particular use contemplated. All such modifications and variations are within the scope of the invention as determined by the appended claims when interpreted in accordance with the breadth to which they are fairly, legally and equitably entitled.

Technology Category: 5