Patent Document

This application is a division of application Ser. No. 11/253,940 filed Oct. 19, 2005, now U.S. Pat. No. 7,244,638. 
    
    
     FIELD OF THE INVENTION 
     The present invention is related in general to the field of semiconductor devices and processes and more specifically to the structure and method of semiconductor chip attach in the device assembly process. 
     DESCRIPTION OF THE RELATED ART 
     Any user of semiconductor components, who has the task of assembling components on electronic boards, can describe the aggravation encountered when just a single component lacks sufficient coplanarity for the board assembly. To avoid the difficulties, stringent requirements have often been established. As an example, for semiconductor devices with surface mount leads, the requirements included a coplanarity of about 0.14 mm for a lead pitch of 1 mm, and a coplanarity of about 0.05 mm for a lead pitch of 0.3 mm. The requirements increased rapidly with shrinking lead pitch. 
     For devices with solder balls, such as ball grid arrays (BGAs), insufficient coplanarity is manifested by a minority of solder balls not touching the substrate concurrently with the majority of balls. Consequently, efforts have been undertaken in literature to correct the coplanarity problems with solder balls of unequal size or delayed-reflow solder pastes. These efforts brought only limited success. 
     SUMMARY OF THE INVENTION 
     Applicants recognize the fact that, in BGAs, coplanarity problems are in most cases caused by device warpage and, consequently, an improvement of coplanarity should most readily be based on a reduction or elimination of device warpage. This approach is an inherently low-cost and robust methodology, since it eliminates coplanarity problems at the root cause. In addition, the approach is applicable to many device types and product families, and can be fine-tuned to a wide variety of materials characteristics of the substrates. 
     One embodiment of the invention is a semiconductor device with a chip, its position defining a plane, and an insulating substrate with first and second surfaces; the substrate is substantially coplanar with the chip, without warpage. One of the chip sides is attached to the first substrate surface using adhesive material, which has a thickness. The thickness of the adhesive material is distributed so that the thickness under the central chip area is equal to or smaller than the material thickness under the peripheral chip areas. Encapsulation compound is embedding all remaining chip sides and the portions of the first substrate surface, which are not involved in the chip attachment. 
     When a plurality of conductive contact pads are distributed over at least portions of the second substrate surface and a reflow element attached to each contact pad, this plurality of reflow elements is substantially coplanar with the chip. 
     Another embodiment of the invention is a method for fabricating a semiconductor device. An insulating substrate with first and second surfaces is provided. The second surface is placed on a chuck with openings for vacuum suction; the surface of the chuck is effectively convex, often practically achieved by a raised middle portion. The vacuum suction is activated so that the substrate is bent practically convex over the chuck surface. Adhesive material is then placed on the first substrate surface; the material is viscous and has a thickness. Next, a semiconductor chip is provided, which has sides, wherein one of the sides is intended for mechanical attachment. This attachment chip side is placed on the adhesive material; the thickness of the material becomes thereby distributed so that the thickness under the central area of the chip side is smaller than or equal to the material thickness under the peripheral areas of the chip side; the position of the chip defines a plane. 
     The vacuum suction is then de-activated and the substrate is removed from the chuck together with the assembled chip. All remaining chip surfaces and the portions of the first substrate surface not involved in the chip attachment are embedded in a thermoset encapsulation compound. The compound is polymerized, causing volumetric compound shrinkage, which pulls the substrate into a position substantially coplanar with the chip. The device does not exhibit warpage. 
     When the substrate is provided with a plurality of conductive contact pads distributed over at least portions of the second substrate surface, a reflow element can be attached to each contact pad; the plurality of reflow elements are then substantially coplanar with the substrate and the chip. 
     The technical advantages represented by certain embodiments of the invention will become apparent from the following description of the preferred embodiments of the invention, when considered in conjunction with the accompanying drawings and the novel features set forth in the appended claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic cross section of a substrate with an assembled semiconductor chip resting on a chuck with planar surface, in known technology. 
         FIG. 2  is a schematic cross section of an assembled device in known technology. 
         FIG. 3  shows a schematic cross section of a finished device with polymerized molding compound and a plurality of solder balls, in known technology. 
         FIG. 4  is a microphotograph of a cross section of an actual large-chip semiconductor device after polymerization of the molding compound in known technology. 
         FIG. 5A  is a schematic cross section of an example of a substrate with an assembled semiconductor chip resting on a chuck with a surface contour according to the invention. 
         FIG. 5B  is a schematic cross section of another example of a substrate with an assembled semiconductor chip resting on a chuck with a surface contour according to the invention. 
         FIG. 6  is a schematic cross section of an assembled device, after the device has been assembled on a vacuum chuck according to the invention. 
         FIG. 7  shows a schematic cross section of a finished device with polymerized molding compound and a plurality of solder balls, after the device has been assembled on a vacuum chuck according to the invention. 
         FIG. 8  is a microphotograph of a cross section of an actual large-chip semiconductor device after assembly on a vacuum chuck according to the invention and after polymerization of the molding compound. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       FIG. 1  illustrates schematically a heatable chuck  101 , which has a planar, flat surface  101   a . In surface  101   a  are a plurality of openings (not shown in  FIG. 1 ), through which a vacuum can be applied so that it sucks an object on surface  101   a  to hold it tight against the surface. 
     Before the vacuum is turned on, a substrate  102  is placed on surface  101   a;  the substrate is also flat. Then the vacuum is turned on and substrate  102  is held tight against surface  101   a . A pre-determined amount of viscous adhesive material  103  (usually polyimide or epoxy) is deposited on substrate  102  (usually by a syringe). A semiconductor chip  104  is slightly pressed against attach material  103  and the material is partially polymerized by the thermal energy of the chuck. Chip  104  and substrate  102  are coplanar. 
     When the assembled chip and substrate are lifted from the chuck (see  FIG. 2 ), the substrate  102  still has its planar configuration; it is still coplanar to chip  104 . 
     The assembly is then encapsulated in thermoset polymer compound  301 , usually molding compound. After the encapsulation, compound  301  is polymerized by storing the device at elevated temperatures for several hours. During the polymerization process, compound  301  undergoes a volumetric shrinkage. As a result, the finished device acquires a shape as schematically indicated in  FIG. 3 : Substrate  102  is pulled towards the polymer compound and becomes curved outward (concave) under the chip  104 . Substrate  102  is no longer coplanar with chip  104 . Attach material  103  ends up thicker ( 301   b ) in the center of chip  104  compared to its thickness under the chip edges ( 301   a ). And the polymerized encapsulation material may have an outer surface  301   a , which not planar, but exhibits a slight dip  301   b  in the center. 
     When solder balls  320  are attached to the outer surface  102   a  of substrate  102  and the balls are of equal size, the plane of the balls cannot be coplanar with chip  104 . When a device as depicted in  FIG. 3  is to be attached to a circuit board, the solder is molten. Whenever the lack of coplanarity is so pronounced that it cannot be compensated by the reflowing solder, failures in board attach will occur. 
     The microphotograph in  FIG. 4  of a cross section of an actual device encapsulated by molding material  401  and assembled on a chuck with planar surface illustrates the unequal thickness of the attach material. Chip  404  has a thickness  404   a  of 275 μm and a length  404   b  of 5.75 mm. The chip attach material  403  has a thickness  403   b  of about 108 μm in the center, but only a thickness  403   a  of approximately 87 μm at the chip edges; the difference between center and periphery is about 21 μm. 
     As the microphotograph shows, the substrate  402  in  FIG. 4  follows the curved outline of the chip attach material  403 . The following parts of the substrate are visible: The intermittent white-and-black layer  402   a  consists of the copper traces separated by the solder mask; the grey layer  402   b  is the polyimide tape. The whitish spheres are the solder balls  420 . The plurality of solder balls is not coplanar with the chip. (Finer detail of the substrate such as solder mask, nickel layer, etc. is not shown in  FIG. 4 ). 
     The embodiment of  FIG. 5A  depicts an assembly apparatus modified according to the invention. The heatable chuck  501  has a surface suitable for placing sheet-like substrates. The surface has openings for vacuum suction (not shown in  FIG. 5A ). The chuck surface has area portions  502   a  in a first plane and at least one area portion  502   b  in a second plane elevated relative to the first plane so that the summary contour of the chuck surface becomes convex. An originally flat sheet-like substrate  503 , when placed on the chuck surface and pulled towards the surface by the activated vacuum, will follow the summary convex contour and adopt this contour, as illustrated in  FIG. 5A . 
     As an example, the height difference  510  between surface  502   a  and  502   b  in  FIG. 5A  may be 25 μm for devices such as the μStar™ BGA. In other embodiments, the height difference may be larger of smaller. In yet other, more expensive embodiments, the whole surface of chuck  501  is smoothly micro-machined to obtain a continuously convex contour. The degree of convexity of the chuck surface is dependent on the material of the substrate, the area of the substrate, and the amount, composition, and polymerization of the encapsulation compound. 
     In the next process step, a pre-determined amount of viscous adhesive material  504  (usually polyimide or epoxy) is deposited on substrate  503  (usually by a syringe). A semiconductor chip  505  is slightly pressed against attach material  504  and the material is partially polymerized by the thermal energy of the chuck. 
     As a result of the convex substrate shape, the thickness of the attach material  504  is distributed so that the thickness  504   b  in the central area (under the chip) is smaller than the material thickness  504   a  in the peripheral areas of the chip. When a different height  510  of the central chuck surface is selected, the central material thickness  504   b  may be left up to (but will not be larger than) the peripheral thickness  504   a.    
     The considerations described above are preferably realized for relatively hard substrates, which operate in the plastic regime of the stress-strain relationship. For softer substrates operating in the plastic regime, the substrate behavior on the chuck  520  with the raised surface center looks schematically as shown in  FIG. 5B . The height difference  530  between surface  522   a  and  522   b  is 25 μm. While the summary contour of the chuck surface is again convex, the originally flat sheet-like substrate  523 , when placed on the chuck surface and pulled towards the surface by the activated vacuum, will follow the actual surface contour more closely and adopt a more wavy profile as illustrated in  FIG. 5B . 
     In the next process step, a pre-determined amount of viscous adhesive material  524  (usually polyimide or epoxy) is deposited on substrate  523  (usually by a syringe). A semiconductor chip  525  is slightly pressed against attach material  524  and the material is partially polymerized by the thermal energy of the chuck. 
     As a result of the wavy convex substrate shape, the thickness of the attach material  524  is distributed so that the thickness  524   b  in the central area (under the chip) is smaller than the material thickness  524   a  in the peripheral areas of the chip. When a different height  530  of the central chuck surface is selected, the central material thickness  524   b  may be left up to (but will not be larger than) the peripheral thickness  524   a.    
     As  FIG. 6  illustrates, when the assembled chip and substrate are lifted from the chuck, the substrate  503  still retains its convex configuration. The thickness of the attach material  504  is distributed so that the thickness  504   b  under the central area of the chip  505  is smaller than (or up to equal to) the material thickness  504   a  under the peripheral areas of the chip. The next process steps are summarized in  FIG. 7 . 
       FIG. 7  describes the effect of the encapsulation of the assembly in thermoset polymer compound  701 , preferably a molding compound. Because of the thermoset nature, compound  701  has to be polymerized after the encapsulation, preferably by storing the device at elevated temperatures for several hours. During the polymerization process, compound  701  undergoes a volumetric shrinkage. As a result, the finished device acquires a shape as schematically indicated in  FIG. 7 : Substrate  503  is pulled towards the polymer compound and becomes planar under the chip  505 . Attach material  504  ends up with a thickness distribution so that the material thickness  504   b  under the central chip area is equal to (or still slightly smaller than) the material thickness  504   a  under the peripheral chip areas. Consequently, substrate  503  is now coplanar with chip  505 . The polymerized encapsulation material  701  has an outer surface  701   a , which is also coplanar with chip  505 . 
     When solder balls  720  are attached to the outer surface of substrate  503  and the balls are of equal size, the plane of the balls is coplanar with chip  505 . When a device as depicted in  FIG. 7  is to be attached to a circuit board, the solder is molten and there will be no failures in board attach. 
     The microphotograph in  FIG. 8  of a cross section of an actual device, assembled on a chuck with effectively convex surface and encapsulated by molding material  801 , illustrates the equal thickness of the attach material  803 . Chip  804  has a thickness  804   a  of 275 μm and a length  804   b  of 5.75 mm. The chip attach material  803  has an approximately uniform thickness  803   a  of about 95 μm throughout its length. More precisely, the difference between the thickness in the center of 94 μm and the thickness at the periphery between 91 and 99 μm has reduced the difference to 10 μm. 
     As the microphotograph shows, the substrate  802  in  FIG. 8  follows the planar outline of the chip attach material  803  (for an explanation of the photograph detail visible in  FIG. 8  see description under  FIG. 4 ). The whitish spheres are the solder balls  820 . The plurality of solder balls is coplanar with the chip  804 . 
     Statistical data collected from many manufacturing lots about coplanarity and board assembly have confirmed the reduced device warpage, improved solder ball coplanarity (at least 26%) and improved board attach reliability based on the effectively convex assembly chuck according to the invention. The only significant reliability factor left was related to solder ball variability. 
     While this invention has been described in reference to illustrative embodiments, this description is not intended to be construed in a limiting sense. Various modifications and combinations of the illustrative embodiments, as well as other embodiments of the invention, will be apparent to persons skilled in the art upon reference to the description. As an example, more than one raised center of the assembly chuck may be advisable for large area chips. As another example, for production with high throughput the cost of a micro-machined concave chuck surface may be justified; if would enhance the precision of the attach material thickness uniformity. It is therefore intended that the appended claims encompass any such modifications.

Technology Category: 5