BRIEF DESCRIPTION OF THE DRAWING
The present invention is best understood from the following detailed description when read in conjunction with the accompanying drawing. It is emphasized that, according to common practice, the various features of the drawing are not necessarily to scale. On the contrary, the dimensions of the various features are arbitrarily expanded or reduced for clarity. Like numerals denote like features throughout the specification and drawing.
FIG. 1 is a cross-sectional view of a conventional semiconductor package exhibiting warpage according to the PRIOR ART;
FIG. 2 is a bottom view of a conventional semiconductor package with solder ball pads of the same dimension according to the PRIOR ART;
FIG. 3 is a bottom view of an exemplary BGA semiconductor package according to the invention; and
FIG. 4 is a cross-sectional view of an exemplary BGA semiconductor package according to the invention.
DETAILED DESCRIPTION
The invention provides for measuring surface topology of the bonding or coupling surface of a package substrate in a semiconductor package, in particular measuring the relative elevation of locations on the surface, i.e. the degree of non-planarity, and, responsive to the measurements, forming solder ball pads of different sizes to produce corresponding solder balls of different heights to compensate for the non-planarity of the package substrate and provide an array of solder balls having different heights but such that the tops of all the solder balls are essentially coplanar. The invention is applicable to various ball grid array package types such as PBGA (plastic ball grid array) packages, LFBGA (low profile ball grid array) packages, flip-chip packages, SBGA/VBGA (super/viper BGA) packages and may be used for packages of various dimensions and using solder balls of different dimensions and formed of different materials. For example, the invention may be used for CSP (chip scale packages) applications.
Referring to FIGS. 3 and 4, an exemplary BGA package according to the invention is illustrated. Semiconductor package 3 includes package substrate 5, semiconductor chip 7, heat spreader 9, stiffeners 10, adhesives 12 and underfill material 14. The components are joined using thermal processes that produce a non-planarity in coupling surface 11 of package substrate 5 indicated by distance 21 showing the degree of warpage. An aspect of the invention, however, is that the highest points, i.e. apices 115 of the respective solder balls 113, are coplanar 117 and at the same height and do not include the non-planarity indicated by distance 21 between the highest and lowest points of coupling surface 11, as shown in the cross-sectional view of FIG. 4. The components and relative positions of the components in semiconductor package 3 are intended to be exemplary only. Heat spreader 9 and stiffener 10 are exemplary and in other exemplary embodiments, other components including shields and the like, may be used in conjunction with one or more semiconductor chips 7 that may be formed on package substrate 5.
FIG. 3 shows solder ball pads 119 formed on coupling surface 11 of package substrate 5. According to the method of the invention, after the various components are joined to form semiconductor package 3, but prior to the formation of solder ball pads 19 on bonding surface 11 of package substrate 5, conventional techniques may be used to measure the surface topography of coupling surface 11. Various tools for mapping or otherwise measuring the relative height of coupling surface 11 are available and can be used to determine the elevation at the various locations of coupling surface 11 including distance 21 between high and low points of coupling surface 11. In some exemplary embodiments, the warpage may produce a non-planar surface whereby bonding surface 11 is essentially concave and in other exemplary embodiments, bonding surface 11 may be essentially convex. In still other exemplary embodiments, undulations or ridges may appear throughout coupling surface 11.
According to the method of the invention, solder ball pads 119 are then formed responsive to the surface topology data generated. In particular, at locations of relatively low elevation on coupling surface 11, solder ball pads 119 are formed to have a relatively smaller dimension and at locations of relatively high elevation on coupling surface 11, solder ball pads 119 are formed to have a relatively greater dimension. In the exemplary embodiment illustrated in FIGS. 3 and 4, coupling surface 11 is concave, i.e., the height at the edges of coupling surface 11 is greater than the height at the central portion of coupling surface 11. According to the illustrated exemplary embodiment, two distinct regions of different elevation and different solder ball pad sizes are provided responsive to surface topographical measurements. FIG. 3 illustrates peripheral portion 123 having a relatively higher elevation than central portion 125. Boundary 121 separates peripheral portion 123 and central portion 125. Accordingly, solder ball pads 119A formed within central portion 125 are of smaller dimension than solder ball pads 119B formed in peripheral portion 123. Each of solder ball pads 119A and 119B is essentially circular according to the illustrated exemplary embodiment and solder ball pads 119A formed in central portion 125 include a diameter 135 that is less than diameter 133 of solder ball pads 119B formed in peripheral portion 123 that includes a relative high elevation compared to central portion 125. Diameter 133 may be 10 to 20 percent greater than diameter 135, but other size differences may be used in other exemplary embodiments, depending upon the difference in elevation of the various regions of coupling surface 11 and further depending upon the amount of solder material used. Each of solder ball pads 119A include the same dimensions in the illustrated embodiment as do each of solder ball pads 119B but the two distinct portions, each with identically sized solder balls, is exemplary only. In other exemplary embodiments, more than two different regions, i.e., central portion 125 and peripheral portion 123, may be used. There may be a peripheral portion, a central portion and an intermediate portion therebetween. In yet other exemplary embodiments, the solder ball pad sizes may vary gradually or irregularly throughout coupling surface responsive to the contours mapped by the topography tool.
The dimensions of semiconductor package 3 and coupling surface 11 may vary in exemplary embodiments. Similarly, the sizes of solder ball pads 119 and pitch 141 may also vary in exemplary embodiments. Pitch 141 may vary from about 0.4 mm to about 1.27 mm in one exemplary embodiment, but other suitable pitches may be use din other exemplary embodiments. According to one exemplary embodiment in which pitch 141 is about 1 mm, at least one of the diameters 133, 135 may be about 0.5±0.05 mm, but this is exemplary only and various other pitches and diameters may be used in other exemplary embodiments. Diameters 133, 135 may each lie within the range of 0.2 to 0.8 mm in one exemplary embodiment.
An equal amount of solder material is then deposited on each of solder ball pads 119 using conventional methods. Conventional solder materials such as SnAg, other lead-free or lead-containing solder materials may be used. After the solder material is deposited, conventional reflowing processes are used to form solder balls 113 shown in FIG. 4. Applicants have discovered that, when the same amount of solder material is used on solder ball pads having different dimensions, the formed solder balls have different heights after reflow. The solder balls formed on solder ball pads having greater dimensions are formed to include a lower height than solder balls formed using the same amount of solder material on solder ball pads having smaller dimensions. Applicants attribute this difference to surface tension phenomenon, as the solder material does not laterally encroach the initial peripheral boundaries of solder ball pads 119. Solder balls 113 will be spherical or ovoid in shape depending on the amount of solder used and the size of solder ball pad 119 upon which the solder ball is formed. The heights of solder balls 113 may vary and depend upon the amount of solder material used and dimensions of solder ball pads 119. An advantage of the invention is that the apices 115 of solder balls 113A and 113B form plane 117. Coupling surface 11, now with the BGA of solder balls 113 formed thereon, can then be electrically and physically coupled to further electronic components using various conventional techniques.
The preceding merely illustrates the principles of the invention. It will thus be appreciated that those skilled in the art will be able to devise various arrangements which, although not explicitly described or shown herein, embody the principles of the invention and are included within its spirit and scope. Furthermore, all examples and conditional language recited herein are principally intended expressly to be only for pedagogical purposes and to aid the reader in understanding the principles of the invention and the concepts contributed by the inventors to furthering the art, and are to be construed as being without limitation to such specifically recited examples and conditions. Moreover, all statements herein reciting principles, aspects, and embodiments of the invention, as well as specific examples thereof, are intended to encompass both structural and functional equivalents thereof. Additionally, it is intended that such equivalents include both currently known equivalents and equivalents developed in the future, i.e., any elements developed that perform the same function, regardless of structure.
This description of the exemplary embodiments is intended to be read in connection with the figures of the accompanying drawing, which are to be considered part of the entire written description. In the description, relative terms such as “lower,” “upper,” “horizontal,” “vertical,” “above,” “below,” “up,” “down,” “top” and “bottom” as well as derivatives thereof (e.g., “horizontally,” “downwardly,” “upwardly,” etc.) should be construed to refer to the orientation as then described or as shown in the drawing under discussion. These relative terms are for convenience of description and do not require that the apparatus be constructed or operated in a particular orientation. Terms concerning attachments, coupling and the like, such as “connected” and “interconnected,” refer to a relationship wherein structures are secured or attached to one another either directly or indirectly through intervening structures, as well as both movable or rigid attachments or relationships, unless expressly described otherwise.
Although the invention has been described in terms of exemplary embodiments, it is not limited thereto. Rather, the appended claims should be construed broadly, to include other variants and embodiments of the invention, which may be made by those skilled in the art without departing from the scope and range of equivalents of the invention.
Patent Application
20080054455
