Abstract:
A semiconductor device package is assembled using a jig that alters the shape of gel material disposed in a cavity in the package. In one embodiment, a jig having a concave bottom surface is inserted onto uncured gel material disposed within a cavity in a housing of the package to change a top surface of the gel from having a concave shape to a convex shape. The gel is then cured with the jig in place. When the jig is subsequently removed, the cured gel retains the convex shape, which helps to avoid any bond wires from being exposed. The re-shaped gel material reduces internal stresses during thermal cycling and can therefore reduce permanent damage to the package otherwise resulting from such thermal cycling.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates generally to semiconductor device packages, and more particularly to packages with a gel filled cavity. 
     For certain semiconductor device packages, such as those that include pressure-sensing dies, it is known to apply a pressure-sensitive gel material over the pressure-sensing die to protect the die while still allowing the die to sense the atmospheric pressure outside of the package. 
     In some package designs having one or more dies, the gel fills the entire bottom portion of the package housing and is intended to cover all of the dies as well as any bond wires used to connect the dies to one another and/or to package leads. Unfortunately, due to the mechanical properties of some gels and some housing materials, when the gel is dispensed into the cavity, the gel&#39;s meniscus behavior results in the top surface of the gel having a concave shape. The concavity of the gel can increase during staging and curing, where staging refers to the time period from the dispensing of the gel until the beginning of the curing process. 
       FIGS. 1(A) and 1(B)  show simplified cross-sectional side views of a conventional partially assembled semiconductor device package  100  having two dies  102  and  104  mounted within a package housing  106  that is partially filled with gel  108  and having at least one interconnecting bond wire  110 . Other bond wires, if any, are not shown.  FIG. 1(A)  shows the package  100  just after the gel  108  has been dispensed, while  FIG. 1(B)  shows the package  100  after the gel  108  has been cured. 
     As shown in  FIG. 1(A) , the uncured gel  108  has a slightly concave top surface, while the top surface of the cured gel  108  in  FIG. 1(B)  has greater concavity. This greater concavity can result from the uncured gel  108  creeping up the walls of the housing  106  due to capillary action and/or shrinkage of the gel  108  during the curing process. 
     Unfortunately, as represented in  FIG. 1(B) , the increase in the concavity of the gel  108  can result in the exposure of portions of one or more of the bond wires  110  outside of the cured gel. 
     Furthermore, the concavity of the cured gel  108  in  FIG. 1(B)  corresponds to a relatively large variation in the thickness of the gel  108  across the width of the package  100 , with the gel  108  being thicker at the edges of the package and thinner at the middle of the package. 
     Conventional package qualification processes involve thermal cycling during which the fully assembled packages are repeatedly heated up and cooled down over the range of expected operating temperatures for the package. The varying gel thickness across the width of the package can result in relatively large internal stresses during thermal cycling that can cause permanent damage to the package, such as broken and/or disconnected bond wires. Accordingly, it would be advantageous to have an assembly process that ensures the bond wires are covered with gel. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention is illustrated by way of example and is not limited by the accompanying figures, in which like references indicate similar elements. Elements in the figures are illustrated for simplicity and clarity and have not necessarily been drawn to scale. For example, the thicknesses of layers and regions may be exaggerated for clarity. 
         FIGS. 1(A) and 1(B)  show simplified cross-sectional side views of a conventional partially assembled semiconductor device package having, respectively, uncured and cured gel material; 
       FIGS.  2 (A)-(C) show simplified cross-sectional side views of three different steps in the assembly of a semiconductor device package according to an exemplary embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Detailed illustrative embodiments of the present invention are disclosed herein. However, specific structural and functional details disclosed herein are merely representative for purposes of describing example embodiments of the present invention. The present invention may be embodied in many alternate forms and should not be construed as limited to only the embodiments set forth herein. Further, the terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of example embodiments of the invention. 
     As used herein, the singular forms “a,” “an,” and “the,” are intended to include the plural forms as well, unless the context clearly indicates otherwise. It further will be understood that the terms “comprises,” “comprising,” “includes,” and/or “including,” specify the presence of stated features, steps, or components, but do not preclude the presence or addition of one or more other features, steps, or components. It also should be noted that in some alternative implementations, the functions/acts noted may occur out of the order noted in the figures. For example, two figures shown in succession may in fact be executed substantially concurrently or may sometimes be executed in the reverse order, depending upon the functionality/acts involved. 
     In one embodiment, the present invention provides a method of assembling a semiconductor device package. The method includes (a) dispensing uncured gel material into a cavity of a partially assembled semiconductor device package, where the package includes at least one die mounted within a package housing, and where the gel material has a top surface with a first shape; (b) applying a jig over the uncured gel material to change the top surface of the uncured gel material to have a second shape different from the first shape; (c) curing the uncured gel material with the applied jig; and (d) removing the jig such that the top surface of the cured gel material substantially retains the second shape. 
     In another embodiment, the present invention is a semiconductor device package assembled in accordance with the above-described method. 
     In yet another embodiment, the present invention provides a semiconductor device package comprising a package housing, at least one die mounted within the package housing, one or more bond wires connected to the at least one die, and cured gel material covering the at least one die and filling a bottom portion of the package housing, wherein a top surface of the cured gel material has a convex shape. 
     Referring now to FIGS.  2 (A)-(C), simplified cross-sectional side views of three different steps in the assembly of a semiconductor device package  200  are shown. The package  200  has two dies  202  and  204  mounted within a package housing  206 . In a preferred embodiment, at least one of the dies is a pressure sensor die. The housing  206  may comprise a multi-layer substrate or a pre-molded lead frame, having molded sidewalls formed thereon. The housing  206  is at least partially filled with a gel  208  that covers the dies  202  and  204 , or at least the pressure sensing die. The two dies  202 ,  204  may be connected to each other and/or to a substrate bond pad with bond wires  210 , one of which is shown. It is preferred that the gel  208  also covers the bond wires  210 .  FIG. 2(A) , which is analogous to  FIG. 1(A) , shows the partially assembled package  200  just after the gel  208  has been dispensed into a cavity formed by the housing  206 , with a top surface of the gel  208  automatically assuming a concave shape. 
     As shown in  FIG. 2(B) , before the gel  208  is cured, a jig (i.e., tool)  212 , having a bottom surface with a concave shape and lateral dimensions corresponding to the lateral dimensions of the housing cavity, is inserted into the package housing  206  and pressed into the gel  208 , such that the top surface of the gel assumes the shape of the bottom surface of the jig  212 . The gel  208  is then cured with the jig  212  in place. The jig  208  subsequently is removed after the curing process is complete (or at least after the gel  208  has solidified enough to retain the shape imposed by the jig  212 ). 
     Although not represented in the cross-sectional views of  FIG. 2 , if the housing  206  has a cylindrical cavity, then the jig  212  will have a circular lateral shape that substantially matches the cylindrical cavity. Alternatively, if the housing  206  has a rectilinear cavity, then the jig  212  will have a rectangular lateral shape that substantially matches the rectilinear cavity. 
       FIG. 2(C)  shows the package  200  after the gel  208  has been cured and the jig  212  has been removed. As shown in  FIG. 2(C) , the top surface of the cured gel  208  has a convex shape that substantially matches the concave shape of the bottom surface of the jig  212 . 
     As a result of that convex shape, all of the bond wires  210  are completely covered by the cured gel  208 . Furthermore, as also a result of the convex shape and taking into account (e.g., subtracting) the thicknesses of the one or more dies  202 ,  204 , the variation in the thickness of the gel  208  in  FIG. 2(C)  is less than the variation in the gel thickness in  FIG. 1(B) . As such, the internal stresses during thermal cycling of the package  200  will be less than the internal stresses during thermal cycling on the corresponding, conventional package  100 , which stress reduction can reduce the incidence of permanent damage to the package  200  as compared to the package  100 . 
     Although  FIG. 2  represents the assembly of a single semiconductor device package, in practice, one- or two-dimensional arrays of multiple packages are assembled simultaneously, typically as part of a single multi-package structure before they are separated into individual packages. In such cases, a multi-jig structure can be used having an array of jigs, each similar to jig  212  and coinciding with a corresponding package housing in the multi-package structure. 
     By now it should be appreciated that there has been provided an improved semiconductor device package and a method of forming the semiconductor device package. Circuit details are not disclosed because knowledge thereof is not required for a complete understanding of the invention. 
     Although the invention has been described using relative terms such as “upper,” “lower,” “front,” “back,” “top,” “bottom,” “over,” “under” and the like in the description and in the claims, such terms are used for descriptive purposes and not necessarily for describing permanent relative positions. It is understood that the terms so used are interchangeable under appropriate circumstances such that the embodiments of the invention described herein are, for example, capable of operation in other orientations than those illustrated or otherwise described herein. 
     Unless stated otherwise, terms such as “first” and “second” are used to arbitrarily distinguish between the elements such terms describe. Thus, these terms are not necessarily intended to indicate temporal or other prioritization of such elements. Further, the use of introductory phrases such as “at least one” and “one or more” in the claims should not be construed to imply that the introduction of another claim element by the indefinite articles “a” or “an” limits any particular claim containing such introduced claim element to inventions containing only one such element, even when the same claim includes the introductory phrases “one or more” or “at least one” and indefinite articles such as “a” or “an.” The same holds true for the use of definite articles. 
     Although the invention is described herein with reference to specific embodiments, various modifications and changes can be made without departing from the scope of the present invention as set forth in the claims below. Accordingly, the specification and figures are to be regarded in an illustrative rather than a restrictive sense, and all such modifications are intended to be included within the scope of the present invention. Any benefits, advantages, or solutions to problems that are described herein with regard to specific embodiments are not intended to be construed as a critical, required, or essential feature or element of any or all the claims.