Patent Publication Number: US-8533657-B2

Title: Printed circuit boards having pads for solder balls and methods for the implementation thereof

Description:
PRIOR APPLICATION DATA 
     The present application is a continuation application of prior U.S. application Ser. No. 12/000,366, filed on Dec. 12, 2007, entitled “PRINTED CIRCUIT BOARDS HAVING PADS FOR SOLDER BALLS AND METHODS FOR THE IMPLEMENTATION THEREOF”. And this application is a divisional application of Ser. No. 10/735,638, filed Dec. 16, 2003, and entitled “PRINTED CIRCUIT BOARDS HAVING PADS FOR SOLDER BALLS AND METHODS FOR THE IMPLEMENTATION THEREOF” incorporated by reference herein in its entirety. 
    
    
     BACKGROUND OF THE INVENTION 
     A package suitable to contain one or more semiconductor dies may comprise solder-balls in a ball grid array (BGA). Any or all of the solder-balls may serve as external electrical terminations of the package. A package with one or more semiconductor dies installed therein is known as a device. 
     Once a device is soldered onto a printed circuit board (PCB), mechanical and/or thermal and/or thermomechanical strains may occur between the package and the PCB. In packages comprising BGA solder-balls, such strains may result in damage to the soldered solder-balls over time. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Embodiments of the invention are illustrated by way of example and not limitation in the figures of the accompanying drawings, in which like reference numerals indicate corresponding, analogous or similar elements, and in which: 
         FIGS. 1-3  show a top view of an exemplary printed circuit boards comprising a pad suitable to be soldered to a respective solder-ball of a device and merging with a trace according to some alternative embodiments of the invention; 
         FIGS. 4-7  shows a top view of an exemplary printed circuit boards comprising a pad suitable to be soldered to a respective solder-ball of a device and having a microvia located therein according to some alternative embodiments of the invention; 
         FIG. 8  shows a top view of an exemplary printed circuit boards comprising pad suitable to be soldered to a respective solder-balls of a device and having microvias located therein according to some embodiments of the invention; 
         FIG. 9  is a top view of an exemplary printed circuit board in accordance with some embodiments of the invention; 
         FIG. 10  is a top view of an exemplary printed circuit board having devices installed thereon, in accordance with some embodiments of the invention; and 
         FIG. 11  is a top view of an exemplary apparatus including an exemplary printed circuit board, the exemplary printed circuit board having devices installed thereon, in accordance with some embodiments of the invention. 
     
    
    
     It will be appreciated that for simplicity and clarity of illustration, elements shown in the figures have not necessarily been drawn to scale. For example, the dimensions of some of the elements may be exaggerated relative to other elements for clarity. 
     DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION 
     In the following detailed description, numerous specific details are set forth in order to provide a thorough understanding of embodiments of the invention. However it will be understood by those of ordinary skill in the art that the embodiments of the invention may be practiced without these specific details. In other instances, well-known methods, procedures, components and circuits have not been described in detail so as not to obscure the embodiments of the invention. 
     A printed circuit board (PCB) may comprise metal pads on its external conductive layer that are soldered to respective solder-balls of a device. Such solder balls may suffer from mechanical fatigue due to, for example, cyclic stress, or thermo-mechanical fatigue due to, for example, coefficient of thermal expansion (CTE) mismatch. As a result of that mechanical fatigue, any or all of these solder balls may develop cracks (fractures), which may gradually degrade the reliability of the electrical coupling these balls provide between the PCB and a semiconductor die contained in the device, and may result in a total failure of the solder joint. Consequently, the reliability of the system in which that PCB is installed may be degraded over time, possibly resulting in a total system failure. 
     Such cracks may start developing in a solder ball at a point (e.g. “crack initiation point”) that is close to the perimeter of the pad, and may propagate through the solder ball in such a way that a projection of the crack onto the pad forms a substantially straight line crossing the center of the pad to a point on the perimeter of the pad that is opposite to the crack initiation point. 
     Moreover, and although the scope of the invention is not limited in this respect, a projection onto a PCB of the crack propagation directions in solder balls of a device soldered to pads of the PCB may point toward a common point, and the crack initiation point of each solder ball may be near the point of the pad that is the farthest from that common point. 
     Using currently known and future methods, it may be possible to anticipate the location of the crack initiation point on the perimeter of the pad, and to further anticipate the crack propagation direction. 
     A solder-ball soldered to a solder-mask defined pad (i.e. a pad that is partially covered with solder mask along its perimeter) may be more prone to crack initiation than a solder-ball soldered to a metal defined pad (i.e. a pad that is not covered with solder mask). For example, this may be due to a protruded solder mask that leads to stress concentration and shape formation on solder-mask defined pads, possibly resulting in “necking”. In another example, this may be due to thermo-mechanical tensions between the solder-ball and the solder-mask. Moreover, a solder-ball soldered to a pad that has a trace merging with it (i.e. the pad and the trace are part of the same metal layer of the PCB) along a portion of the pad&#39;s perimeter that includes a vicinity of the crack initiation point, may be more prone to crack initiation than a solder-ball soldered to a pad that has a trace merging with it along a portion of the pad&#39;s perimeter that does not include a vicinity of the crack initiation point. 
     A metal pad, suitable to be soldered to a respective solder ball of a device, may have a microvia located therein. A microvia may be a metal-plated through-hole in at least the metal pad and the isolation layer of the PCB underneath the metal pad, providing electrical coupling between the metal pad and one or more internal conductive layers of the PCB and/or another external conductive layer of the PCB. 
     During the process of soldering a pad having a microvia to a solder-ball of a device, the solder-ball may change its shape. The external shape of the solder-ball may change by, for example, having a part of the solder-ball material filling, at least in part, the through-hole of the microvia. The internal shape of the solder-ball may change, for example, by a formation of a void in the solder-ball material above the microvia. Such a void inside the soldered solder-ball may weaken the solder joint. 
     The inventors have come to the conclusion that the length of the path between the crack initiation point and the surface of the void may affect the solder fatigue. A longer path may prolong the fatigue life of a solder joint. 
     Although the size of a void in a solder ball may vary, a void may be substantially centered above the center of the microvia, and consequently, the longer the path between the crack initiation point and the center of the microvia, the longer the path between the crack initiation point and the surface of the void. 
     According to some embodiments of the invention, instructions may be applied to the implementation of microvias inside pads suitable to be soldered to respective solder-balls of devices, and to the implementation of merging such pads with traces. Instructions according to embodiments of the invention may be used in generating one or more files for manufacturing of a printed circuit board using. Such files may be generated, for example, by a computer aided design (CAD) software in an automatic process or in a human-assisted process. Different priorities may be assigned to different instructions, and it may happen in the implementation of a particular printed circuit board that not all instructions can be implemented for all the pads and all the traces to which the instructions apply. 
       FIGS. 1-3  present three exemplary alternative implementations, using instructions according to some embodiments of the invention, of traces merged with pads that are suitable to be soldered to respective solder-balls of a device. 
       FIGS. 4-7  present four exemplary alternative implementations, using instructions according to some embodiments of the invention, of microvias in pads that are suitable to be soldered to respective solder-balls of a device 
       FIG. 1  shows an exemplary PCB  1  comprising a pad  2  suitable to be soldered to a solder-ball of a device. Pad  2 , which for the clarity of the explanation is substantially round, merges with a trace  3  along a portion  4  of a perimeter  5  of pad  2  according to some embodiments of the invention. A point  6  is the anticipated location of the crack initiation point on the perimeter of pad  2 . 
     A first instruction according to some embodiments of the invention used in the implementation of pad  2  and trace  3  is to merge pad  2  and trace  3  along a portion of perimeter  5  that does not include an arc  7  that is in the vicinity of point  6 . 
     A second instruction according to some embodiments of the invention used in the implementation of pad  2  and trace  3  is to merge pad  2  and trace  3  along a portion of perimeter  5  a length of which is no longer than one quarter of the length of perimeter  5 . 
       FIG. 2  shows an exemplary PCB  11  comprising a pad  12  suitable to be soldered to a respective solder-ball of a device. Pad  12 , which for the clarity of the explanation is substantially round, merges with a trace  13  along a portion  14  of a perimeter  15  of pad  12 . A point  16  is the anticipated location of the crack initiation point on the perimeter of pad  12 . A vector  19  beginning at point  16  is the projection onto pad  12  of the anticipated crack propagation direction. A line  18  is a tangent to perimeter  15  at the middle point of portion  14 . Line  18  is substantially parallel to vector  19 . 
     A first instruction, according to some embodiments of the invention, used in the implementation of pad  12  and trace  13  is to merge pad  12  and trace  13  along a portion of perimeter  15  that does not include an arc  17  that is in the vicinity of point  16 . 
     A second instruction, according to some embodiments of the invention, used in the implementation of pad  12  and trace  13  is to merge pad  12  and trace  13  along a portion of perimeter  15  so that a tangent to perimeter  15  at a middle point of the merging portion will be substantially parallel to the projection onto pad  12  of the anticipated crack propagation direction. 
     A third instruction, according to some embodiments of the invention, used in the implementation of pad  12  and trace  13  is to merge pad  12  and trace  13  along a portion of perimeter  15  a length of which is no longer than one quarter of the length of perimeter  15 . 
       FIG. 3  shows an exemplary PCB  21  comprising a pad  22  suitable to be soldered to a respective solder-ball of a device. Pad  22 , which for the clarity of the explanation is substantially round, merged with a trace  23  along a portion  24  of a perimeter  25  of pad  22  according to some embodiments of the invention. A point  26  is the anticipate location of the crack initiation point on the perimeter of pad  22 . A vector  29  initiated at point  26  is the projection onto pad  22  of the anticipated crack propagation direction. 
     A first instruction, according to some embodiments of the invention, used in the implementation of pad  22  and trace  23  is to merge pad  22  and trace  23  along a portion of perimeter  25  that does not include an arc  27  that is in the vicinity of point  26 . 
     A second instruction, according to some embodiments of the invention, used in the implementation of pad  22  and trace  23  is to merge pad  22  and trace  23  along a portion of perimeter  25  so that a straight line joining point  26  and the middle point of the merging portion will be parallel to vector  29 . 
     A third instruction, according to some embodiments of the invention, used in the implementation of pad  22  and trace  23  is to merge pad  22  and trace  23  along a portion of perimeter  25  a length of which is no longer than one quarter of the length of perimeter  25 . 
       FIG. 4  shows an exemplary PCB  31  comprising a pad  32  suitable to be soldered to a respective solder-ball of a device. Pad  32 , which for the clarity of the explanation is substantially round, comprises a microvia  33  according to some embodiments of the invention. A point  36  is the anticipated location of the crack initiation point on perimeter  35  of pad  32 . The center  34  of microvia  33  is farther than the center  37  of pad  32  from point  36 . For the clarity of the drawing, a dashed curve  39  is shown, which is an equidistance curve from point  36  containing the center  37  of pad  32 . 
     An instruction according to some embodiments of the invention, used in the implementation of pad  32  and microvia  33  is to locate microvia  33  inside pad  32  such that the center of microvia  33  is farther than the center of pad  32  from point  36 . 
       FIG. 5  shows an exemplary PCB  41  comprising a pad  42  suitable to be soldered to a respective solder-ball of a device. Pad  42 , which for the clarity of the explanation is substantially round, comprises a microvia  43  according to some embodiments of the invention. A point  46  is the anticipated location of the crack initiation point on a perimeter  45  of pad  42 . The center  44  of microvia  43  is farther than the center  47  of pad  42  from point  46 . In addition, at least one point of perimeter  48  of microvia  43  is located on perimeter  45  of pad  42 . For the clarity of the drawing, a dashed curve  49  is shown, which is an equidistance curve from point  46  containing the center  47  of pad  42 . 
     A first instruction according to some embodiments of the invention, used in the implementation of pad  42  and microvia  43  is to locate microvia  43  inside pad  42  such that the center of microvia  43  is farther than the center of pad  42  from point  46 . 
     A second instruction according to the invention, used in the implementation of pad  42  and microvia  43  is to locate microvia  43  inside pad  42  such that at least one point of perimeter  48  of microvia  43  is located on perimeter  45  of pad  42 . 
       FIG. 6  shows an exemplary PCB  51  comprising a pad  52  suitable to be soldered to a respective solder-ball of a device. Pad  52 , which for the clarity of the explanation is substantially round, comprises a microvia  53  according to some embodiments of the invention. A point  56  is the anticipated location of the crack initiation point on a perimeter  55  of pad  52 . A vector  58  beginning at point  56  is the projection onto pad  52  of the anticipated crack propagation direction. The center  54  of microvia  53  is located on vector  58  and is farther than the center  57  of pad  52  from point  56 . For the clarity of the drawing, a dashed curve  59  is shown, which is an equidistance curve from point  56  containing the center  57  of pad  52 . 
     A first instruction according to some embodiments of the invention, used in the implementation of pad  52  and microvia  53  is to locate microvia  53  inside pad  52  such that the center of microvia  53  is farther than the center of pad  52  from point  56 . 
     A second instruction according to some embodiments of the invention, used in the implementation of pad  52  and microvia  53  is to locate center  54  of microvia  43  on vector  58 . 
       FIG. 7  shows an exemplary PCB  61  comprising a pad  62  suitable to be soldered to a respective solder-ball of a device. Pad  62 , which for the clarity of the explanation is substantially round, comprises a microvia  63  according to some embodiments of the invention. A point  66  is the anticipated location of the crack initiation point on a perimeter  65  of pad  62 . A vector  60  initiated at point  66  is the projection onto pad  62  of the anticipated crack propagation direction. The center  64  of microvia  63  is located on vector  60  and is farther than the center  67  of pad  62  from point  66 . In addition, at least one point of perimeter  68  of microvia  63  is located on perimeter  65  of pad  62 . For the clarity of the drawing, a dashed curve  69  is shown, which is an equidistance curve from point  66  containing the center  67  of pad  62 . 
     A first instruction according to some embodiments of the invention, used in the implementation of pad  62  and microvia  63  is to locate microvia  63  inside pad  62  such that the center of microvia  63  is farther than the center of pad  62  from point  66 . 
     A second instruction according to some embodiments of the invention, used in the implementation of pad  62  and microvia  63  is to locate center  64  of microvia  63  on vector  60 . 
     A third instruction according to some embodiments of the invention, used in the implementation of pad  62  and microvia  63  is to locate microvia  63  inside pad  62  such that at least one point of perimeter  68  of microvia  63  is located on perimeter  65  of pad  62 . 
       FIG. 8  shows an exemplary PCB  71  comprising a “footprint”  72 . Footprint  72  may be suitable to have installed thereon a device having a ball grid array (BGA) package of sixteen solder balls arranged in an array of four columns and four rows. Footprint  72  may therefore comprise sixteen pads  73  suitable to be soldered to respective ones of the sixteen solder-balls of the device. Pads  73  have microvias  74  located therein. 
     Moreover, and although the scope of the invention is not limited in this respect, a projection  75  onto PCB  71  of the anticipated crack propagation directions in solder balls of a device soldered to pads  73  may point toward a common point  76 , and an anticipated crack initiation point  77  of each solder ball may be located near the point of the pad that is the farthest from common point  76 . Microvias  74  are located such that the center of microvias  74  are farther than the center of the respective pad  73  from common point  76 . 
       FIG. 9  shows an exemplary PCB  80  in accordance with some embodiments of the invention. PCB  80  may comprise, for example, “footprints”  81 ,  82 ,  83 ,  84 , and  85  for respective devices. Mechanical features and additional footprints of PCB  80  are not shown for clarity. Footprint  81  may be suitable to have installed thereon a device having a ball grid array (BGA) package of thirty solder balls arranged in an array of six columns and five rows. Footprint  82  may therefore comprise thirty pads  86  suitable to be soldered to respective ones of the thirty solder-balls of the device. 
     PCB  80  may comprise traces (not shown) merging with some or all of pads  86  according to embodiments of the present invention. In addition, PCB  80  may comprise microvias located in some or all of pads  86  according to embodiments of the invention. 
       FIG. 10  shows an exemplary PCB  90  in accordance with some embodiments of the invention. PCB  90  may have installed thereon some devices, for example, devices  93 ,  94 , and  95 , and may optionally have installed thereon a voltage monitor  92 . PCB  90  may have installed on footprint  81  a device  91  having a BGA package of thirty solder balls arranged in an array of six columns and five rows. Mechanical features and additional footprints of PCB and additional devices installed on PCB  90  are not shown for clarity. As a non-limiting example, PCB  90  may be a motherboard. 
       FIG. 11  shows an exemplary apparatus  100  in accordance to some embodiments of the invention. Apparatus  100  may comprise PCB  90  of  FIG. 10 , and may optionally comprise an audio input device  101 . A non-exhaustive list of examples for apparatus  100  includes a personal computer (PC), a notepad computer, a notebook computer, a laptop computer, a server computer, a pocket PC, a personal digital assistant (PDA), a personal information manager (PIM), a cellphone, a pager, a mobile or non-mobile memory storage device, a hard disk drive (HDD), a floppy disk drive (FDD), a monitor, a projector, a digital video disc (DVD) player, a video compact disc (VCD) player, an MP3 player, a mobile media player, a calculator, a wireless local area network (LAN) access point, a LAN router, and the like. 
     As a result of using instructions according to embodiments of the invention during the implementation of a PCB, solder balls of a device that are soldered to pads of PCB implemented using these instructions may be more immune to crack initiation, and cracks may propagate more slowly and/or may cause damage to the solder balls more slowly than if that PCB was implemented without these instructions. It will be appreciated that the instructions need not be satisfied for all the pads and all the traces to which the guidelines apply for the solder balls to be more immune to crack initiation and for cracks to propagate more slowly and/or to cause damage to the solder balls more slowly. 
     Embodiments of the invention may include an article comprising a storage medium having stored thereon instructions that, when executed by a computing platform, result in generating one or more files for manufacturing a printed circuit board, where generating the one or more files comprises ensuring that a pad of the printed circuit board that is suitable to be soldered to a respective solder-ball of a device merges with a respective trace along a portion of a perimeter of the pad, wherein the portion does not include a vicinity of a crack initiation point, and the crack initiation point is located on the perimeter at a location where cracks in the solder-ball are anticipated to start after the solder-ball is soldered to the pad. 
     Embodiments of the invention may include an article comprising a storage medium having stored thereon instructions that, when executed by a computing platform, result in generating one or more files for manufacturing a printed circuit board, wherein generating the one or more files comprises ensuring that for a pad of the printed circuit board that is suitable to be soldered to a respective solder-ball of a device and has a microvia located therein, a center of the microvia is farther than a center of the pad from a crack initiation point located on a perimeter of the pad at a location where cracks in the solder-ball are anticipated to start after the solder-ball is soldered to the pad. 
     While certain features of the invention have been illustrated and described herein, many modifications, substitutions, changes, and equivalents will now occur to those of ordinary skill in the art. It is, therefore, to be understood that the appended claims are intended to cover all such modifications and changes as fall within the true spirit of the invention.