Robust high density substrate design for thermal cycling reliability

The present invention relates to packaging for semiconductors. Recent silicon technology advances have placed increased demand for high density signal routing on organic BGA substrates. Packages with a full area ball array are also being used to reduce the package body size and provide a high input/output (IO) signal density.

Increased signal routing density in the substrate is obtained by using fine pitch vias through the core so that routing layers below the core can be efficiently utilized. The via pitch reduction requires the use of thin core substrates which are susceptible to warpage during thermal excursions. Typically, the regions are under the die corner are regions of stress concentration. Under cycled thermal excursions, cracks can initiate from the ball pad edges and spread into the layers above the ball pad layer. Depending on the design, these cracks may pose a reliability hazard.

Problems exist with the packages which are currently in wide use. Currently, in full array packages, signals are typically routed directly over the ball pads under the die corner on the bottom routing layer. As an example, FIG. 1 shows a typical multi-layer organic BGA flip chip substrate that uses a 9-layer stackup 20 with a thin core. The 9-layer stackup 20 includes a layer L 1 , a layer L 2 , a layer L 3 , a layer L 4 , a layer L 5 , a layer L 6 , a layer L 7 , a layer L 8 , and a layer L 9 . Each of the layers is formed of a conductive material such as copper. Insulative material 21 is provided between each of the conductive layers. An upper protective mask 22 is provided proximate layer L 1 and a lower protective layer 24 is provided proximate layer L 9 . Layer L 1 provides a plurality of bump pads 26 (one of which is shown) on which a plurality of solder masses 28 (one of which is shown) are provided. The upper surfaces 28 a of the solder masses 28 provide a die site on which the die (not shown) will be placed. A plurality of ball pads 30 (one of which is shown) are provided on Layer L 9 .

A portion of Layer L 9 of the 9-layer stackup 20 is shown in FIG. 2 . Circularly shaped ball pads 34 of layer L 9 are dispersed throughout layer L 9 . The ball pads 34 may be dispersed in a partial or full-array format. Placement of a quadrant of the die over the solder masses 28 is represented by line 38 . Placement of one of the corners of the die is represented by the point 38 a.

A portion of Layer L 8 of the 9-layer stackup 20 is shown in FIG. 3 . Layer L 8 provides the bottom routing layer. Placement of a quadrant of the die over the solder masses 28 is represented by line 38 . Signal traces 36 are dispersed throughout layer L 8 . Numerous signal traces 36 and other metal structures extend across the layer L 8 proximate the die corner 38 a.

The signal traces 36 of layer L 8 are routed over the ball pads 34 of layer L 9 . Areas of high stress are associated with the die corner 38 a and in particular the ball pads which are positioned under the area surrounding the die corner 38 a . The edges of the ball pads 34 which are associated with the die corner 38 a act as stress concentration points and under temperature cycling conditions, cracks are initiated from the edges of the ball pads 34 and extend into the dielectric layer above layer L 9 . If traces 36 are routed or other metal structures are provided on layer L 8 over the ball pads 34 associated with the die corner 38 a , the cracks can extend through the traces 36 and cause failures due to trace cracks under cycled stress conditions.

One existing solution to overcoming this problem is to place metal planes on the layer L 9 under the die corner 28 . A disadvantage of placing metal planes on the layer L 9 , however, is that it reduces the signal density. Another existing solution to overcoming this problem is to define a circular shaped area with a one (1) millimeter radius using placement of the die corner 38 a as the center of the circle. When routing traces on layer L 8 , traces are not routed within this circular region. A disadvantage of having a one (1) millimeter radius region under the die corner 38 a is that it is not sufficient to avoid trace cracks in the layer L 8 under temperature cycling conditions for all packaging technolgies.

Therefore, an improved package is needed which will reduce cracks in the signal traces and therefore avoids functional failures caused by stresses under the die corner. The present invention provides such a package. Features and advantages of the present invention will become apparent upon a reading of the attached specification, in combination with a study of the drawings.

A primary object of an embodiment of the present invention is to provide an improved package with no failures.

Another object of an embodiment of the present invention is to provide an improved package which eliminates the occurrences of cracks in the signal traces.

Briefly, and in accordance with the foregoing, an embodiment of the present invention provides an improved package in which a high stress area associated with the die corner is defined and in which the signal traces are routed away from the high stress areas at the die corner. The invention describes a methodology for signal trace routing in the high stress area under the die corder that incorporates a range of die sizes.

The features of the present invention which are believed to be novel, are described in detail herein below. The organization and manner of the structure and operation of the invention, together with further objects and advantages thereof, may best be understood by reference to the following description taken in connection with the accompanying drawings wherein like reference numerals identify like elements in which:

FIG. 1 is diagram illustrating a 9-layer stackup;

FIG. 2 is a diagram representing layer L 9 of the 9-layer stackup of FIG. 1 ;

FIG. 3 is a diagram representing layer L 8 of the 9-layer stackup of FIG. 1 ;

FIG. 4 is a diagram of a portion of layer L 9 of a substrate package which is in accordance with an embodiment of the present invention, illustrating high stress zones; and

FIG. 5 is a diagram representing layer L 8 of a 9-layer stackup of a substrate package which is in accordance with an embodiment of the present invention.

While this invention may be susceptible to embodiment in different forms, there is shown in the drawings and will be described herein in detail, a specific embodiment with the understanding that the present disclosure is to be considered an exemplification of the principles of the invention, and is not intended to limit the invention to that as illustrated and described herein.

An embodiment of the present invention provides a substrate package in which signal traces are routed away from the high stress area associated with the ball pads and the corner of the die.

A quadrant of the L 9 layer 50 of a 9-layer stackup is shown in FIG. 4 . The remaining quadrants of the layer L 9 are similar to the quadrant shown. Ball pads 52 are dispersed through the L 9 layer 50 . It is to be understood that although the invention is described in connection with a 9-layer stackup, the invention applies to any multi-layer stack up that uses a thin core. Lines 54 , 56 represent placement of die quadrants. Varying sized dies can be used. Line 54 represents placement of a die and the associated corner 54 a of one size and line 56 represents placement of a die of a larger size and the associated corner 56 a . A high stress zone 58 is associated with the die corner 54 a and a high stress zone 60 is associated with the die corner 56 a . Each high stress zone 58 , 60 is similarly shaped. A portion of high stress zone 58 over laps a portion of high stress zone 60 . The high stress zone 58 extends approximately two ball pitches away from the die corner 54 a in each direction. The high stress zone 60 extends approximately two ball pitches away from the die corner 56 a in each direction. Several ball pads 52 are provided within the high stress zone 58 and several ball pads are provided within the high stress zone 60 .

A portion of layer L 8 is shown in FIG. 5 . Layer L 8 is the bottom routing layer of the 9-layer stackup. Layer L 8 includes signal traces and other metal structure, such as, for example, voltage bus bars, dispersed throughout the layer.

Line 54 represents placement of a relatively small sized die relative to layer L 8 . Point 54 a represents placement of the die corner.

The high stress area 58 defined in connection with layer L 9 , also is represented on layer L 8 . Signal traces 64 and other metal structures are provided throughout layer L 8 , however, within the high stress area 58 , the signal traces 64 and other metal structures, for example, voltage bus bars, are routed away from the die corner 54 a . Thus, the high stress area 58 on layer L 8 includes a very small number of signal traces 64 and other metal structures. Preferably, all of the signal traces 64 are routed outside the high stress area 58 . If, however, the traces 64 cannot be moved outside the high stress zone 58 , the traces 64 are routed so as to avoid routing the traces 64 over the ball pads 52 of layer L 9 within the high stress area 58 . Traces on L 8 may be routed over sections of continuous solid plane on L 9 within the high stress region. In the event a larger size die is used, the signal traces 64 will be routed outside the high stress zone 60 defined in connection with layer L 9 .

Because the signal traces 64 and other metal structures are not provided over the ball pads 52 , cracks initiated from the ball pad edges during thermal cycling do not affect the traces 64 and other metal structures and therefore thermal cycling reliability of the package is improved.

Although the invention has been described with respect to a nine layer stack-up, the present invention and method of routing signal traces away from the die corner can be used in connection with PTFE or thin core substrate technology in multiple layer stack ups such as, five layer or seven layer stack ups. The present invention can also be used in connection with other multi-layer build-up substrate designs with a thin core and signals routed below the die corner, such as, for example, six layer, eight layer, and ten layer, etc. The present invention can also be applied to other substrate and packaging technologies such as wirebond BGAs.

While a preferred embodiment of the present invention is shown and described, it is envisioned that those skilled in the art may devise various modifications of the present invention without departing from the spirit and scope of the appended claims.