Abstract:
An apparatus and method includes an integrated circuit disposed in a ball grid array (“BGA”) package having interconnects on at least one corner without signal assignments.

Description:
BACKGROUND 
   Ball grid array (herein “BGA”) packages are known and commonly used in many electronic applications. During manufacture and test of printed circuit boards (herein “PCB”), failures stemming from the BGA occur. Under the prior art, identification and diagnosis of the failures requires a costly and time consuming destructive test of the PCB or a marginally reliable optical inspection test. While the failure is identified and diagnosed, more of the same product is manufactured potentially subject to the same failure pattern. 
   There is a need, therefore, for improved test and diagnostic tools to identify certain BGA failures. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     An understanding of the present teachings can be gained from the following detailed description, taken in conjunction with the accompanying drawings of which like reference numerals in different drawings refer to the same or similar elements. 
       FIG. 1  is a plan view illustration of a BGA package according to the present teachings. 
       FIG. 2  is a side view illustration of a BGA package attached to a printed circuit board (“PCB”) according to the present teachings. 
       FIG. 3  is a side view illustration of the BGA package and PCB of  FIG. 2  with the PCB exhibiting some amount of flex. 
       FIGS. 4 through 6  illustrate different corner definitions appropriate according to the present teachings. 
       FIGS. 7 and 8  illustrate embodiments of corner series circuits for purposes of solder joint failure testability. 
       FIG. 9  is a flow chart of a process according to the present teachings. 
   

   DETAILED DESCRIPTION 
   In the following detailed description, for purposes of explanation and not limitation, example embodiments disclosing specific details are set forth in order to provide a thorough understanding of the present teachings. It is apparent to one having ordinary skill in the art with benefit of the present disclosure that other embodiments according to the present teachings that depart from the specific details disclosed herein remain within the scope of the appended claims. Moreover, descriptions of well-known apparatus and methods may be omitted so as to not obscure the description of the example embodiments. Such methods and apparatus are considered within the scope of the present teachings. 
   With specific reference to  FIGS. 1 and 2  of the drawings, based upon destructive tests of PCBs populated with one or more BGAs that did not pass functional test, it was determined that a common failure mode of the PCB  101  is failure of a solder joint  102  between the BGA package  100  and the PCB  101  to which the BGA package  100  is attached. Additionally, it was determined that the failures of the solder joints  102  are significantly more common with thick BGA packages than with thin BGA packages. 
   Upon further inspection, the identified solder joint failure mode was further characterized as occurring primarily at one or more of the four corners  103  of the BGA packages  100 . The solder joint  102  between interconnects  104  of the BGA  101  and the PCB  101  provide both an electrical and mechanical connection. As one of ordinary skill in the art appreciates, the interconnects  104  of a typical BGA package  100  are small spherical electrically conductive metal. Based upon the characterization of the failure mode, it is proposed that a reason for the failure of the solder joint  102  is flexing of the PCB  101  relative to the BGA package  100  disposed on it. 
   Thin BGA packages are more flexible than thick BGA packages. Thin BGA packages, therefore, can flex more readily with the PCB  101  and do not place a strain on the solder joint  102 . Thick BGAs are stiffer that thin BGA packages. Flexing a PCB with the thick BGA package, therefore, places a higher separation force on the connection between the BGA package  100  and the PCB  101  than similar flexing with the thin BGA package. 
   With specific reference to  FIG. 3  of the drawings, one of ordinary skill in the art can appreciate that as the PCB  101  flexes in the z-direction relative to the BGA package  100 , the relatively stiffer BGA package  100  resists the flexing. The resistance to flex generates a tensile stress  105  to the solder joint  102 . The tensile stress  105  is represented as a vector in  FIG. 3  of the drawings. 
   In most cases, the weakest attachment between the BGA package  100  and the PCB  101  is the solder joint  102 . As can be appreciated by one of ordinary skill in the art, therefore, when the tensile stress  105  exceeds the strength of the solder joint  102 , there is a failure of the solder joint  102 . Because the solder joint  102  provides both a mechanical and electrical connection, the mechanical failure of the solder joint  102  also results in a failure of the electrical connection. 
   The solder joints  102  at the outer edges  106  of the BGA package  100  receive the greatest amount of tensile stress  105 . The solder joints  102  closer to the center of the BGA package  100  receive relatively less tensile stress. As one of ordinary skill in the art appreciates, flexure of the PCB  101  can occur in both the x-z direction as well as the y-z direction. Flexure may be the result of physical bending of the PCB  101  or may occur as a result of different thermal expansion between the PCB  101  and the BGA package  100 . If the tensile stress as a result of flexure in both directions add, the tensile stress at the corners  103  of the BGA package  100  is greatest. Accordingly, it is reasonable to expect that failures of one or more solder joints  102  as a result of the identified failure mode will occur at the corners  103  of the BGA package  100 . 
   Empirical evidence of solder joint failures shows this to be true, which provides confidence that the identified failure mode is the cause of the failures. 
   When the solder joint  102  fails, the tensile stress  105  on the failed solder joint  102  goes to zero and there is no resistance to displacement between the BGA package  100  and the failed solder joint  102 . As a result, the BGA package  100  is permitted to separate from the PCB  101  at the site of the failed solder joint  102 . Tensile stress remains on the adjacent solder joint  102  and it also fails if the tensile stress exceeds the strength of the solder joint  102 . In some cases, failure of the corner solder joint may indicate that stress was applied to the PCB and the failure absorbed enough of the stress to prevent failure of adjacent interconnects. 
   Solder joints  102  that fail under the failure mode identified as part of the present teachings, the failure is most likely to occur at one or more of the corners of the BGA package  100 . Reasons proposed for the increased likelihood are that the solder joints  102  at the corners  103  of the BGA package  100  do not have benefit of adjacent solder joints  102  that help to distribute resistance to the tensile stress  105  applied. Therefore, the displacement of the flexure of the PCB  101 , the direction of the flexure, and the stiffness of the BGA package  100  determine which corner  103  experiences one or more failed solder joints  102  and how many of the solder joints  102  actually fail. 
   If it is accepted that flexing forces are applied to all PCBs  101  at various stages of the manufacturing process, then failure of the solder joint  102  may be accepted as a likelihood. It is further proposed to beneficially use acceptance of this likelihood to improve a manufacturing process of a PCB  101  populated with one or more BGA packaged integrated circuits. 
   Specifically, according to one embodiment of the present teachings, the BGA package  100  may be designed without signal assignments at one or more of the corners  103  of the BGA package  100 . In another embodiment, each of the four corners  103  of the BGA package  100  have interconnects  104  without signal assignments. With specific reference to  FIG. 4  of the drawings, each corner  103  of the BGA package  100  may be defined as the single interconnect  104  closest to the respective corners  103  of the BGA package. As one of ordinary skill in the art can appreciate, many definitions of “corner  103 ” are applicable to the present teachings. With specific reference to  FIG. 5  of the drawings, there is shown a configuration where the term “corner  103 ” is defined as three interconnects total, one that delineates each extreme corner  103  of the BGA package and the two interconnects just adjacent to it along respective perpendicular sides of the package  100 . With specific reference to  FIG. 6  of the drawings, there is shown another embodiment where five interconnects  104  define each corner  103  of the BGA package  100 . In the embodiment of  FIG. 6  of the drawings, the five interconnects  104  are the extreme corner interconnect, the two adjacent to it, and the three next adjacent to the two. Many other corner definitions are will occur to one of ordinary skill in the art without departing from the intentions of the present teachings. 
   The interconnects  104  at the corners  103  of the BGA package  100  may be left completely unassigned thereby acting as a stress relieving and sacrificial buffer for interconnects  104  adjacent the corners  103  of the BGA package  100 . This will serve to reduce the likelihood of PCB failure as it travels through the manufacturing process. If the solder joint  102  fails, the interconnects  104  at the corners  103  of the BGA package  100  provide a nondestructive test for presence of mechanical stress during the manufacturing process. 
   The interconnects  104  at the corners  103  of the BGA package  100  may be assigned only to power or ground or a combination of both. Because power and ground are typically redundant connections, failure of a small subset of the power and ground connections does not result in failure of the PCB  101 . Therefore, the interconnects  104  at the corners of the BGA package  100  provide connections that remain valuable to the PCB  101  in the event of no failure of the solder joint  102  but are still dispensable in the event that a failure does occur. 
   In some cases, it may be possible to repair the identified failed solder joints  102  by reflowing the solder to reconnect the interconnects to the PCB  101 . Beneficially, therefore, identification of the failure mode and isolation of the location of the failure can prevent loss of one or more fully manufactured by repairing a PCB  101  that might have otherwise been scrapped. 
   In another embodiment and with specific reference to  FIG. 7  of the drawings, the interconnects  104  at the corners  103  of the BGA package  100  may be interconnected in a series circuit. Beneficially, the series circuit provides visibility into whether one or more solder joints  102  failed for each BGA package  100  that populates the PCB  101 . 
   For purposes of illustration,  FIG. 7  shows a single corner  103  of the BGA package  100  as defined in the embodiment of  FIG. 5  of the drawings. It is apparent, however, that the present teachings may be applied to any of the other illustrated embodiments, or embodiments not shown, but within the scope of the present teachings. The series circuit shown in  FIG. 7  of the drawings, places package continuity traces  106  between first  108 , second  109  and third  110  interconnects  104  at the corner  103  of the BGA package  100 . The package continuity traces  106  are disposed on the BGA package  100  and may be generated as part of the overall BGA package design for purposes of testability. The package continuity traces  106  may include small resistors or may be short circuits comprising printed traces. As part of this testability in the embodiment of  FIG. 7 , therefore, first, second, and third visibility ports  111 ,  112 ,  113  are supplied on the PCB  101  connected to the solder joints  102  of the interconnects  108 ,  109 ,  110  respectively. A continuity test or resistance measurement is made between the visibility ports  111 ,  112 ,  113  for purposes of PCB testing. For purposes of this discussion, continuity and measurement of a resistance below a certain threshold is used synonymously. For example, in  FIG. 7 , a detected discontinuity or high impedance between the first and second visibility ports  111 ,  112  indicates that one or both of the solder joints to the first and second interconnects  108 ,  109  failed. A detected discontinuity between the second and third visibility ports  112 ,  113  indicates that one or both of the solder joints  102  to the second and third interconnects  109 ,  110  failed. If continuity is found between the first and second visibility ports  111 ,  112  and between the second and third visibility ports  112 ,  113 , it may be inferred that all three of the solder joints  102  for the interconnects  108 ,  109 ,  110  are in tact. If a discontinuity is found, it may be inferred that one or more of the solder joints  102  at the corner  103  of the BGA package  100  has failed. If it is desirable to identify which solder joint failed, a third continuity test may be made. For example, if continuity is found between ports  111  and  112  and a discontinuity is found between ports  112  and  113 , it may be inferred that the solder joint  102  to interconnect  110  failed and the other two are in tact. It is typically most convenient that the visibility ports  111 ,  112 ,  113  be disposed on the PCB  101 . In another embodiment, in order to distinguish with the continuity check whether two of the three solder joints failed or all three solder joints failed, a visibility port is also disposed on the BGA package  101 . 
   There may be one discrete series circuit for each corner  103  or two or more of the corners  103  may be interconnected into a larger series circuit. Discrete circuits at each corner  103  of the BGA package provide visibility into a corner location of the detected failure and takes additional time to make the measurement to isolate the location of the failure. A single interconnected circuit that tests all corners  103  at the same time takes less test time to identify a failure and provides only BGA package level visibility into the location of the failure. Depending upon how repair is performed or if repair is attempted, the unique advantages of each method will make one method more appropriate than another in different applications. 
   With specific reference to  FIG. 8  of the drawings, there is shown another embodiment of a series circuit suitable for testing solder joint failure at corners  103  of the BGA package  100 . As one of ordinary skill in the art appreciates,  FIG. 8  illustrates an embodiment where the corners  103  are defined as the three interconnects  104  that are closest to each of the respective four corners of the BGA package  100 . In the embodiment illustrated in  FIG. 8 , each corner  103  is interconnected into a series circuit and the series circuits of all four corners  103  are also interconnected. In the embodiment of  FIG. 8 , the continuity traces between the interconnects  108 ,  109 ,  110  alternate between disposition on the BGA package  100  and disposition on the PCB  101 . Specifically, the series circuit for continuity testing comprises alternating package continuity traces  106  and PCB continuity traces  107 . The visibility ports  111 ,  112  are disposed on opposite ends of the larger series circuit and on the PCB  101  in this embodiment. A continuity test or resistance measurement, therefore, indicates whether one or more solder joints  102  at any of the interconnects failed, but does not indicate which one or more of the interconnects has a failed solder joint. In an alternate embodiment, additional visibility ports  111 ,  112  may be disposed along the series circuit for purposes of improved isolation of an identified discontinuity. 
   After the continuity test for the corner solder joints  102  or after repair is attempted, a test may be made of the PCB  101  to determine if functions of the PCB work as expected. If the PCB  101  passes the functional test even when it is known that the solder joints  102  at the corner  103  failed, it is further known that failure was limited to one or more of the corner interconnects  108 ,  109 ,  110  or that repair was successful if it were attempted. If the PCB  101  does not pass the functional test, there is the possibility that solder joints  102  to interconnects  104  that are adjacent to the corner  103  may also have failed and were not repaired or repair was not attempted. For purposes of reliability, therefore, a PCB designer may choose to improve the manufacturing process by defining a larger BGA package corner  103  or may make adjustments to the PCB manufacturing process to reduce flexure of the PCB  101  or both. 
   With specific reference to  FIG. 9  of the drawings, there is shown a flow chart of an embodiment of a method according to the present teachings in which at least one corner of a BGA package  100  is defined  120 . A BGA designer constrains  121  signal assignments of an IC that is to be packaged into the BGA to those interconnects  104  that do not populated the defined corners  103 . Signals of the IC are assigned  122  according to the constraints and the IC is packaged  123  according to the constrained signal assignments. Optionally, and not shown in the flow chart of  FIG. 9 , a continuity test or resistance measurement may be made after a PCB  101  is manufactured to identify and isolate failures of solder joints  102  of the BGA package  100 . 
   Embodiments of the teachings are described herein by way of example with reference to the accompanying drawings describing various embodiments of an apparatus and method to improve visibility into PCB manufacturing failures. Other variations, adaptations, and embodiments of the present teachings not specifically disclosed will occur to those of ordinary skill in the art given benefit of the present teachings. For example, other embodiments with variants and combinations of alternating package and PCB continuity traces not specifically disclosed are also contemplated. A single corners defined by 4 or more interconnects may comprise a series circuit with alternating package and PCB continuity traces. Two or more corners may be interconnected in a series circuit with BGA continuity traces and any number of visibility ports disposed on the PCB. Visibility ports may also be disposed on the BGA package and a combination of both the BGA package and the PCB.