Abstract:
One embodiment sets forth an integrated circuit package that includes a substrate, one or more devices mounted on the substrate, a layer of under-fill configured to secure the one or more devices on the substrate, and a solder trench formed in the substrate, where the aggregate volume of the solder trench is large enough to capture a flow of excess under-fill during fabrication. One advantage of the disclosed integrated circuit package is that the solder trench is used in lieu of solder dam structures, thereby allowing a stencil to be lowered closer to the substrate surface during fabrication, which facilitates depositing solder paste during fabrication.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention generally relates to integrated circuit chip packaging, and, more specifically, to absorbing excess under-fill flow with a solder trench. 
         [0003]    2. Description of the Related Art 
         [0004]    Consumer demand for increasingly smaller electronic devices has grown dramatically, primarily in the form of thinner mobile phones and lighter laptops. Continued miniaturization of consumer electronic devices has generated a demand for thinner integrated circuit (IC) packages that incorporate the processing devices and memory devices that run consumer electronic devices. However, limits exist on what methods may be employed by IC package fabricators to further reduce the height of IC packages and yet continue producing IC packages precisely to the required specifications. 
         [0005]    One common problem in the art is attempting to control the height of a “solder dam” when producing thin IC packages. A solder dam is formed around an IC that is mounted on a packaging substrate, and may be analogized to the walls of an above-ground pool, wherein the walls of the pool contain the water in the pool and prevent people outside the pool from getting wet. In the instant case, a solder dam contains “under-fill” and protects chip capacitors or other structures mounted on the packaging substrate from the flow of excess under-fill within the package. As is well-known, under-fill is used to support and secure various parts of the IC package. 
         [0006]    During fabrication, a stencil is placed on a surface of a packaging substrate, and the holes of the stencil are used to precisely deposit solder paste onto targeted areas of the surface of the substrate. Because the solder dam protrudes above the surface of the packaging substrate, if the solder dam has too much height, the stencil cannot be placed in direct contact with the substrate surface, thereby reducing the precision of solder paste deposition. Furthermore, if the solder dam is misaligned, an outcome that is more likely as the height of the solder dam increases, the solder dam may cause the stencil to be placed askew, resulting in inaccurate deposition of solder paste. As a general matter, imprecise deposition of solder paste is undesirable because improper deposition may result in incomplete electrical connections in the IC package, which could cause the IC package to malfunction or become inoperable. 
         [0007]    As the foregoing illustrates, there is a need in the art for a more effective way to guard against the flow of under-fill within an IC package. 
       SUMMARY OF THE INVENTION 
       [0008]    One embodiment of the present invention sets forth an integrated circuit package that includes a substrate, one or more devices mounted on the substrate, a layer of under-fill configured to secure the one or more devices on the substrate, and a solder trench formed in the substrate, where the aggregate volume of the solder trench is large enough to capture a flow of excess under-fill during fabrication. 
         [0009]    One advantage of the disclosed integrated circuit package is that the solder trench is used in lieu of solder dam structures, thereby allowing a stencil to be lowered closer to the substrate surface during fabrication, which facilitates depositing solder paste during fabrication. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0010]    So that the manner in which the above recited features of the invention can be understood in detail, a more particular description of the invention, briefly summarized above, may be had by reference to embodiments, some of which are illustrated in the appended drawings. It is to be noted, however, that the appended drawings illustrate only typical embodiments of this invention and are therefore not to be considered limiting of its scope, for the invention may admit to other equally effective embodiments. 
           [0011]      FIG. 1  illustrates a schematic, cross-sectional view of a conventional integrated circuit (IC) package; 
           [0012]      FIG. 2  illustrates an implementation of a conventional solder dam design as well as a solder trench design, according to one embodiment of the invention; 
           [0013]      FIG. 3  illustrates a cross-sectional view of a solder trench, according to one embodiment of the invention; 
           [0014]      FIG. 4  illustrates one configuration of the solder trench of  FIG. 3 , according to one embodiment of the invention; and 
           [0015]      FIG. 5  illustrates another configuration of the solder trench of  FIG. 3 , according to another embodiment of the invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0016]    In the following description, numerous specific details are set forth to provide a more thorough understanding of the present invention. However, it will be apparent to one of skill in the art that the present invention may be practiced without one or more of these specific details. In other instances, well-known features have not been described in order to avoid obscuring the present invention. 
         [0017]      FIG. 1  illustrates a schematic cross-sectional view of a conventional integrated circuit (IC) package  100 . As shown, the conventional IC package  100  includes, without limitation, a lid  102 , a die  104 , microbumps  106 , a substrate  108 , solder balls  110 , a solder dam  116  surrounding the die  104 , and chip capacitor pads  118  located outside the solder dam  116 . The microbumps  106  are immersed in under-fill  114 , which serves to support and secure the placement of microbumps  106  and die  104  within the IC package  100 . By way of example and not limitation, die  104  may include one or more devices such as a central processing unit, a graphics processing unit, a memory device or any combination thereof to form a system-on-chip device. Lid  102  is coupled to substrate  108  with a lid adhesive (not shown). Lid  102  is configured to shield and protect the die  104 . As also shown, IC package  100  is coupled to a printed circuit board (PCB)  112  with solder balls  110 . 
         [0018]    As discussed above, consumer demand has trended towards thinner electronic devices, which, in turn, has increased demand for thinner IC packages. Because of advances in reducing the height of IC packages, solder dams in IC package designs have begun causing fabrication issues. These issues are described below in conjunction with  FIG. 2 . 
         [0019]      FIG. 2  illustrates an implementation of a conventional solder dam design as well as a solder trench design, according to embodiment of the invention. As shown, a conventional IC package  200  includes, without limitation, a substrate  204 , solder balls  110 , a solder dam  210  surrounding the C4 pads  208 , and chip capacitor pads  118  surrounding the solder dam  210 . Solder dam  210  is substantially similar to solder dam  116  of  FIG. 1  and has a height  214 . As is well-known, the fabrication process includes the use of stencil  202  in order to direct the deposition of solder paste (not shown) onto the substrate  204 . In practice, stencil  202  is typically lowered towards substrate  204  before the solder paste (not shown) is deposited onto the substrate  204  to form C4 pads  208 . 
         [0020]    A common problem occurs when a solder dam is too tall. For example if solder dam  210  is too tall, then solder dam  210  may obstruct stencil  202  as stencil  202  is lowered towards substrate  204 , thereby preventing stencil  202  from being lowered to an appropriate distance  216  from the substrate  204 . If stencil  202  is located too far from the substrate  204 , then properly depositing the solder paste (not shown) onto the substrate  204  becomes quite difficult. Again, imprecise deposition of solder paste is undesirable because improper deposition may result in incomplete electrical connections in the IC package, which could cause the IC package to malfunction or become inoperable. Also, if the solder dam is too tall, the solder dam could interfere with attempts to reduce the overall height of the IC package. 
         [0021]      FIG. 2  also illustrates the use of a solder trench  212  within an IC package  250 . Here, solder dam  210  has been replaced with solder trench  212 . Because IC package  250  no longer includes any solder dams, the problems described above resulting from the solder dams interfering with stencil  202  and interfering with attempts to reduce the overall height of the IC package  250  are addressed. More specifically, as is evident with respect to IC package  250 , solder trench  212  allows the stencil  202  to be placed quite close to substrate  204  to allow the solder paste (not shown) to be deposited more accurately for the formation of C4 pads  208 . Accurately depositing solder paste on substrate  204 , among other things, reduces the number of improperly built or malfunctioning IC packages, thereby increasing yield and lowering overall production cost. Further, because the solder dams are no longer present, the solder dams no longer need to be accommodated when trying to reduce the overall height of the IC package  250 . 
         [0022]    Also as shown in  250  in  FIG. 2 , an additional benefit of solder trench  212  is that solder trench  212  may have a depth  218  that is greater than height  216 , thereby allowing for more under-fill  114  to be captured and held. Further, because solder trench  212  does not impact the positioning of stencil  202  relative to substrate  204 , solder trench  212  may be formed with less precision than solder dam  210 . Thus, the process of forming a more precise solder dam structure is replaced with a less precise process of forming a solder trench, which results in reduced complexity in IC package fabrication and reduced overall cost. 
         [0023]      FIG. 3  illustrates an implementation of an embodiment of the invention in the form of a solder trench  212 . As shown, and similar to IC package  250 , IC package  300  is substantially similar to IC package  100  of  FIG. 1  except that solder dam  116  of  FIG. 1  is replaced with a solder trench  312  having a depth  318 . By implementing solder trench  312 , the problem of under-fill  114  flow onto chip capacitor pads  118  is avoided because under-fill flow  114  flows into solder trench  312 . In other words, excess under-fill  114  is captured in solder trench  312  instead of being contained by solder dam  116 , as in conventional designs. 
         [0024]      FIG. 4  illustrates one configuration of the solder trench  312  of  FIG. 3 , according to one embodiment of the invention. As shown, the solder trench  312  includes numerous individual apertures  406  arranged in a predetermined pattern. Those skilled in the art will appreciate that the aggregate volume of the different apertures  406  should be great enough to capture and hold excess under-fill  114  so that the under-fill  114  does not reach chip capacitors  118  during fabrication. 
         [0025]    As shown, the apertures  406  are disposed in a staggered arrangement such that there is no direct flow path through the solder trench  312  to the chip capacitors  118  for the under-fill  114  to follow, thereby decreasing the likelihood that the under-fill  114  would reach the chip capacitors  118  during fabrication. In other embodiments, however, the apertures  406  may be disposed in any arrangement, and all technically feasible arrangement fall within the scope of the present invention. 
         [0026]    Apertures  406  are advantageously cylindrically shaped to reduce the amount of stress placed on substrate  108 . For example, as persons skilled in the art will appreciate, designing a solder trench with squared or cornered features, such as a corner, would increase the local stresses placed on substrate  108 , thereby increasing the likelihood of fracture or other failure. 
         [0027]      FIG. 5  illustrates another configuration of a solder trench  312  that may be implemented in the IC package  300  of  FIG. 3 , according to another embodiment of the invention. As shown, solder trench  312  comprises a continuous trench disposed around the periphery of substrate  108  between die  104  and chip capacitors  118 . Again, trench  312  should be designed to have an internal volume great enough to capture and hold excess under-fill  114  so that under-fill  114  does not reach chip capacitors  118  during fabrication. Again, solder trench  312  is configured with rounded edges  514  and  516  to reduce the amount of stress placed on substrate  108 , thereby reducing the likelihood of fracture or other failure. 
         [0028]    Persons skilled in the art will understand that IC packages designed according to the disclosed techniques may be included and implemented in any type of computing device such as, for example, a cellular telephone, a tablet computer, a handheld computing device, a personal digital assistant, a laptop computer, a desktop computer, or the like. 
         [0029]    The invention has been described above with reference to specific embodiments. Persons of ordinary skill in the art, however, will understand that various modifications and changes may be made thereto without departing from the broader spirit and scope of the invention as set forth in the appended claims. The foregoing description and drawings are, accordingly, to be regarded in an illustrative rather than a restrictive sense. 
         [0030]    Therefore, the scope of embodiments of the present invention is set forth in the claims that follow.