Abstract:
A circuit board and a ball grid array (BGA) package having a solder joint with improved reliability are disclosed. The circuit board has a chip mounting surface in which wiring patterns are formed and a solder ball mounting surface in which a plurality of solder balls are mounted and electrically interconnected to the wiring patterns. The circuit board comprises a plurality of ball lands connected to the solder balls. The circuit board further includes solder ball opening area defined by a solder ball mask on the solder ball mounting surface and exposing the ball land from the solder ball mask, a plurality of pattern connecting portions each connected to corresponding one of the ball lands, and conductive wiring patterns linked together with the pattern connecting portions and electrically interconnected to the solder balls. The plurality of pattern connecting portions are aligned radially inwardly toward substantially a center point of the solder ball mounting surface.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to semiconductor packaging technology and, more particularly, to a BGA (Ball Grid Array) package. 
     2. Description of Related Art 
     As higher performance, more reliable, smaller and lighter IC devices are increasingly required, demands for smaller component packages and higher input/output (I/O) pin counts are increasing in the semiconductor packaging industry. The QFP (Quad Flat Package) and the BGA (Ball Grid Array) package offer a large number of I/O pins, as required by modem IC technology. In order to accommodate the increasing number of I/O pins, the QFP technology is forced to an ever finer lead pitch, which results in increasingly thinner, more fragile leads. Accordingly, the BGA package is more proper for a high I/O pin-count requirement while keeping the overall size of the package device smaller, using a far coarser pitch and more-freely-designed interconnections. The BGA package is an area array package that utilizes whole or part of the device footprint for interconnections made of balls composed of a conductive material such as a solder alloy. The BGA package is advantageous in that it can obtain the chip scale or chip size package (CSP) by reducing the package size by more than 30 percent of the normal lead frame plastic package and make the ball pitch less than 1.00 mm. 
     In a BGA package, reliability is important, in particulars the reliability of the solder joint, e.g., the joint between the solder ball and the ball land, is critical. When the solder joint is disconnected, electrical path is disconnected, resulting in undesirable device failure. Further, if cracks occur in the solder joint, electrical resistance in the joint increases and thus electrical characteristics of the device cannot be assured. The increase of the resistance in the joint produces an unwanted DC voltage drop in the signal path and may cause a charging delay in RC circuits and noise in system level. 
     Several attempts have been made to strengthen the joint between the solder balls and ball lands. One such example is disclosed in U.S. Pat. No. 5,796,163, in which a metal-to-metal annular bond is formed at the joint between the solder ball and the land around the plug of nonconductive material in the center of a via. Such a technology is also disclosed in U.S. Pat. No. 5,875,102, U.S. Pat. No. 5,936,848 and U.S. Pat. No. 5,706,178. 
     In U.S. Pat. No. 5,875,102, each via hole has a portion located within a solder pad to increase the routing space of the substrate, and additionally a portion located outside the solder pad to allow outgassing from the via hole. U.S. Pat. No. 5,936,848 discloses a technology using a plug via hole, while U.S. Pat. No. 5,706,178 describes a via hole structure formed within the solder ball land. Additionally, in U.S. Pat. No. 5,872,399, a dimple is formed in the solder ball land, and in U.S. Pat. No. 6,028,366, a groove is formed in the ball land. The purpose of both methods is to increase the joint strength between the solder ball and the ball land. 
     However, these conventional methods have not been fully successful in achieving a level of package reliability that is required by recent integrated circuit technology. 
     SUMMARY OF THE INVENTION 
     It is, therefore, an object of this invention to improve the package reliability, e.g. the reliability of the joints between solder balls and ball lands. 
     It is another object of this invention to prevent cracking from occurring in the joints between the solder balls and the ball lands. 
     For the purposes of the present invention, the inventors focus on the fact that the reliability of the BGA package largely depends on the package pad design. After reviewing and analyzing the causes of the cracks in the joints, the inventors discovered that the joint cracks occur in a direction to which a stress is applied. The inventors recognized that when a stress is applied to the joint in an arrow direction as shown in FIG. 2, the circle denoted as ‘A’ in the joint, i.e., the initial stressed portion of the joint is most susceptible to cracking. The stress may be applied, for example, because of a mismatch between the coefficient of thermal expansion (CTE) between the substrate  12  and the semiconductor chip  20  during thermal cycling in the reliability test of the package. In the reliability test, the package is subjected to heat and then cooled in room temperature. 
     According to the present invention, a circuit board has a chip mounting surface in which wiring patterns are formed and a solder ball mounting surface in which a plurality of solder balls are mounted and electrically interconnected to the wiring patterns. The circuit board comprises a plurality of ball lands each directly connected to the respective one of the solder balls; a older ball opening area defined by a solder ball mask generally deposited on the solder ball mounting surface and exposing the ball land from the solder ball mask; a plurality of pattern connecting portions each connected to corresponding one of the ball lands; and conductive wiring patterns linked together with the pattern connecting portions and electrically interconnected to the solder balls. The plurality of pattern connecting portions are arranged toward a center point of the solder ball mounting surface. 
     In an aspect of the present invention, a ball grid array package comprises the circuit board having centrally directional solder ball land types. The BGA package is a NSMD (non-solder mask defined) structure in that the size of the ball land is smaller than the ball land opening area. 
    
    
     These and other features, and advantages, will be more clearly understood from the following detailed description taken in conjunction with the accompanying drawings. It is important to point out that the illustrations may not necessarily be drawn to scale, and that there may be other embodiments of this invention which are not specifically illustrated. 
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a cross sectional view showing a BGA package and a circuit board mounting the BGA package according to one embodiment of the present invention; 
     FIG. 2 is a partial enlarged view showing the relationship between the defect in the solder ball joint and the direction of applied stress in the BGA package; 
     FIG. 3 is a bottom view of the BGA package showing the solder ball land pattern; 
     FIG. 4 is a partial enlarged view of the solder ball land pattern of the BGA package; 
     FIGS. 5 a  and  5   b  are a bottom view and a partial enlarged view respectively, of a conventional BGA package for evaluating the reliability of the solder ball joint; 
     FIGS. 6 a  and  6   b  are a bottom view and a partial enlarged view respectively, of another prior art BGA package for evaluating the reliability of the solder ball joint; 
     FIGS. 7 a  and  7   b  are a bottom view and a partial enlarged view respectively, of still another prior art BGA package for evaluating the reliability of the solder ball joint; and 
     FIGS. 8 a  and  8   b  are a bottom view and a partial enlarged view respectively, of the BGA package according to the present invention for evaluating the reliability of the solder ball joint. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     FIG. 1 shows a BGA package and a circuit board on which the BGA package is mounted in accordance with a preferred embodiment of the present invention. The package shown in FIG. 1 is a plastic package that employs over-molding and wire bonding technologies. However, it should be noted that the present invention is not limited to such a plastic package. Those skilled in the art should appreciate that the present invention can be applied to a TAB tape package using polyimide tape and Sn—Pb alloy (e.g., 10 percent of tin and 90 percent of lead) and a metal lid package as well as a ceramic package. 
     The BGA package device  10  includes a substrate  12 , a semiconductor chip  20  and solder balls  25 . The substrate  12  is formed of an organic material, e.g., BT (Bismaleimide-Triazine) resin or epoxy glass (also referred to as ‘FR-4’). On a chip mounting surface of the substrate  12  are disposed a die pad  14 , a conductive wiring, e.g. metal wiring  16  and a solder mask  19 , while the bottom surface or solder ball mounting surface  50  (FIG. 3) of the substrate  12  are provided with an interconnection ball land  18  and the solder mask  19 . The die pad  14  and the metal wiring  16  are conductive patterns, e.g., copper patterns, formed by a photolithography technology. When required input and output (I/O) pins are relatively few, the metal wiring (metal pattern)  16  is formed on both sides of the substrate  12 . In case of high I/O pin counts, the metal pattern  16  may be formed on the inner layer of the substrate as well as on both sides of the substrate. The semiconductor chip  20  is attached to the die pad  14  using a conductive adhesive  22 , e.g., a silver filled epoxy or a silver filled glass adhesive. The semiconductor chip and the metal wiring  16  are electrically interconnected by a conductive wire, e.g., metal wire  24 . The wiring pattern  16  on the chip mounting surface may extend to the bottom surface through via holes  28 . Heat generated from the chip  20  can be dissipated through thermal via holes  17 . The ball land  18  is surrounded by the solder mask  19 . By placing the solder balls  25  onto the ball land  18  and then performing reflow soldering, the solder balls  25  are soldered to the land  18 . At this time, a solder ball joint  27  is formed by the joint, e.g. metal-to-metal, between the solder ball  25  and the ball land  18 . The semiconductor chip  20  and metal wiring  16  are protected by an encapsulant such as a plastic resin  26  to form a package body. 
     The BGA package  10  is mounted onto a circuit board  30  (for example, a module board consisting of a memory module) by surface mounting the package so that the solder all  25  is soldered to the conductive pad  32 . The reliability of the solder ball joint  27  is affected by the design of the conductive pad  32 . However, the effect of the conductive pad design is less significant than the solder ball land type (i.e., design of ball land and conductive wiring) in view of the semiconductor package. 
     FIG. 3 is bottom view of a BGA package showing the solder ball mounting surface according to the present invention. The solder ball mounting surface is on the opposite side of the substrate  12  to the chip mounting surface. The surface of the solder ball mounting surface  50  is applied with a solder mask  52  except the ball land opening area  60 . The ball lands  62  are provided within the ball land opening area  60 . This structure is called Non-Solder Mask Defined (NSMD). The ball lands  62  are connected to pattern connecting portions  65 . 
     According to the present invention, the solder ball land type is designed so that the pattern connecting portions  65  are arranged substantially toward the center point  55  of the package. More specifically, the solder ball land type is designed so that the pattern connecting portions  65  lie within a centrally directional area B as shown in FIG.  4 . The central direction area B has a shape of an isosceles triangle having a base and two oblique sides. The base is diameter R crossing the center of the ball land  62  and substantially perpendicular to a line connecting the center  55  of the solder ball mounting surface  50  with approximately the center of the ball land  62 . The oblique sides are two straight lines C 1  and C 2  respectively connecting both ends of the base to the center  55 . The pattern connecting portion  65  is formed as one body, preferably integral, with the metal wiring pattern  66  that electrically interconnects the ball land  62  and the via holes  68 . It is preferable that the width W of the pattern connecting portion  65  is made smaller than the diameter R of the ball land  62 . 
     With the design of the solder ball land as shown in FIGS. 3 and 4, the resistance to stress applied in directions D 1 , D 2 , D 3  and D 4  is increased, and thus the reliability of the solder joint is significantly improved. 
     In order to evaluate the reliability of the various solder ball joints, FIGS. 5 to  8  show four types of the solder ball land type. 
     Type 1 has a pattern as shown in FIG.  5 . This type of solder ball land  70  is arranged in a random fashion with no overall alignment towards the center of the package. The diameter of the ball land opening area  76  is approximately 380 microns, which is smaller than that of the ball land  78 , i.e. approximately 450 microns, as shown in FIG. 5 b . This is called an SMD Solder Mask Defined) structure. The width of the pattern connecting portion  74  connected to a via hole  72  is 250 microns. 
     Type 2 is a NSMD structure having a non-directional solder ball land type  80  as shown in FIGS. 6 a  and  6   b . The size of the ball land  88  is 270 microns, the size of the ball land opening area  86  is 400 microns, and the width of the pattern connecting portion  84  connected to a via hole  82  is 75 microns. 
     Type 3 is an NSMD structure having a non-directional solder ball land type  90  as shown in FIGS. 7 a  and  7   b . The size of the ball land  96  is 270 microns, the size of the ball land opening area  98  is 400 microns, and the width of the pattern connecting portion  94  is 250 microns. 
     Type 4 is an NSMD structure having a solder ball land type  100  as shown in FIGS. 8 a  and  8   b . The size of the ball land  62  is 270 microns, the size of the ball land opening area  60  is 400 microns, and the width of the pattern connecting portion  65  is 75 microns. 
     Upon performing thermal cycling from −25° C. to 125° C. to the four types of BGA packages listed above, the cracks in the solder ball joints were found as shown in following table 1. The thermal cycling was performed in a period of 30 minutes, and the BGA package is a multi-chip package where a synchronous DRAM device and an NOR type flash memory is integrated into one chip. 
     
       
         
               
             
               
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
             
               
               
               
               
               
             
               
               
               
               
               
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 Cracks in the Solder Ball Joints 
               
             
          
           
               
                   
                 Number 
                   
                   
                   
                   
                   
                   
               
               
                   
                 of samples 
                 TC 300 
                 TC 700 
                 TC 1000 
                 TC 1200 
                 TC 1400 
                 TC 1600 
               
               
                   
                   
               
             
          
           
               
                 Type 1 
                 180 
                 0 
                 0 
                 0 
                 0 
                 2 
                 1 
               
             
          
           
               
                 Type 2 [180] 
                 [0] [Evaluation failed 
                 0 
                   
                 Evaluation failed 
               
               
                   
                 since all patterns 
                   
                   
                 since all patterns 
               
               
                   
                 failed] 180 
                   
                   
                 failed 
               
             
          
           
               
                 Type 3 
                 180 
                 0 
                 0 
                 0 
                 0 
                 3 
                 7 
               
               
                 Type 4 
                 180 
                 0 
                 0 
                 0 
                 0 
                 0 
                 0 
               
               
                   
               
             
          
         
       
     
     Comparing the results of types 1, 3 and 4, it is found that the reliability of the solder ball joint in the SMD structure is lower than in the NSMD structure. Further, the results from types 1 and 2 reveal that the width of the pattern connecting part has a great effect on the solder ball joint reliability. 
     In the SMD structure, the solder balls are attached onto the flat surface of the solder ball land, and crack occurs in the interfacial surface between the solder ball and the land when a predetermined amount of stress is applied. The crack progresses more speedy after the initial crack. On the other hand, since the joint is formed in a shape so that the solder ball surrounds the ball land, the stress is applied to the land of the NSMD structure on both the side and top surfaces, and therefore the resistance to the stress is greater than in the SMD structure. 
     Type 3 is generally an NSMD structure, however it is not different from the SMD structure when viewed from the direction in which the maximum stress is applied to the solder ball joint in that the width of the pattern connecting portion is 250 microns, little different from the size of the ball land-270 microns. Accordingly, by comparing types 1 and 3, it is found that the solder ball joint is more reliable when the width of the pattern connecting portion is smaller. 
     For type 2, cracks had occurred in all samples by the completion of TC 700 as shown in table 2 below, thus it is possible to evaluate the exact reliability of the solder ball joint. When performing thermal cycling from −25° C. to 125° C. with the packages of the above four types with attached solder balls having a diameter of 0.4 mm, pattern cracks occur as shown below in table 2. 
     
       
         
               
             
               
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
             
           
               
                 TABLE 2 
               
             
             
               
                   
               
               
                 Pattern Cracks 
               
             
          
           
               
                   
                 Number 
                   
                   
                   
                   
                   
                   
               
               
                   
                 of samples 
                 TC 300 
                 TC 700 
                 TC 1000 
                 TC 1200 
                 TC 1400 
                 TC 1600 
               
               
                   
                   
               
             
          
           
               
                 Type 1 
                 180 
                 0 
                 0 
                 0 
                 0 
                 0 
                 0 
               
               
                 Type 2 
                 180 
                 51 
                 129 
                 — 
                 — 
                 — 
                 — 
               
               
                 Type 3 
                 180 
                 0 
                 0 
                 0 
                 0 
                 0 
                 0 
               
               
                 Type 4 
                 180 
                 0 
                 15 
                 21 
                 7 
                 8 
                 8 
               
               
                   
               
             
          
         
       
     
     Here, it should be noted that the pattern cracks in type 4 of the present invention were found only in the solder ball land types denoted as circle  100 A in FIG. 8 a . The land pattern  100 A does not have pattern connecting portions  65  radially aligned toward substantially the center of the package in contrast to the remaining patterns  100 . 
     When the solder ball land types 1 and 4 are applied to packages other than the above multi-chip package, solder ball joint cracks occur as shown in below table 3. 
     
       
         
               
             
               
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
             
           
               
                 TABLE 3 
               
             
             
               
                   
               
               
                 Solder ball joint cracks 
               
             
          
           
               
                   
                 Number 
                   
                   
                   
                   
                   
                   
               
               
                   
                 of samples 
                 TC 300 
                 TC 700 
                 TC 1000 
                 TC 1200 
                 TC 1400 
                 TC 1600 
               
               
                   
                   
               
             
          
           
               
                 Type 1 
                 180 
                 0 
                 1 
                 0 
                 126 
                 — 
                 — 
               
               
                 Type 4 
                 180 
                 0 
                 0 
                 0 
                 0 
                 0 
                 0 
               
               
                   
               
             
          
         
       
     
     The relationship between the material of the solder ball and the reliability of the solder ball joint is shown in table 4 below. Here, material 1 represents eutectic solder ball having tin and lead in amounts of 63% and 37%, respectively. Material 2 has additional amounts of copper. 
     
       
         
               
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
             
           
               
                   
                 TABLE 4 
               
               
                   
                   
               
               
                   
                 Number 
                   
                   
                   
                   
                   
                   
               
               
                   
                 of samples 
                 TC 300 
                 TC 700 
                 TC 1000 
                 TC 1200 
                 TC 1400 
                 TC 1600 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                 Material 1 
                 180 
                 0 
                 1 
                 20 
                 — 
                 — 
                 — 
               
               
                 Material 2 
                 180 
                 0 
                 0 
                 0 
                 0 
                 2 
                 1 
               
               
                   
               
             
          
         
       
     
     As shown in table 4, when the copper is added, the solder ball joint is more reliable. 
     Although the preferred embodiments of the present invention have been disclosed for illustrative purposes, those skilled in the art will appreciate that various modifications, additions and substitutions are possible, without departing from the scope and spirit of the invention as described in the accompanying claims.