Abstract:
An approach for through-hole component soldering for a PCB, and a resulting PCB assembly, that eliminates protruding solder joints, is provided. The approach comprises back-drilling, from a bottom surface of a PCB, one or more through-holes, wherein each back-drilled through-hole is back-drilled to a depth partially through the PCB and at a diameter that is larger than the diameter of the through hole. Solder paste is applied to the PCB. The components are placed on the PCB, inserting each pin into a corresponding through-hole. The PCB is passed through a solder process, whereby, within each through-hole having a component pin inserted therein, the solder paste is wicked into the through-hole, and forms a solder joint with the respective pin. Each solder joint of a back-drilled through-hole is situated within the through-hole in a manner whereby the solder joint does not protrude beyond the bottom surface of the PCB.

Description:
BACKGROUND 
       [0001]    Since the second generation of computers in the 1950&#39;s, until the rise of surface-mount technology, through-hole soldering has been the predominant process for mounting integrated circuit (IC) and other electrical components on printed circuit boards. Even though surface-mount technology has largely replaced through-hole circuit board assembly, however, through-hole technology remains a significant part of electronic device manufacturing in various technology areas and for various components that are still manufactured in through-hole packaging. Through-hole technology refers to an electronic component mounting process, whereby electrical conductor leads of a component are inserted through holes drilled in the printed circuit board (PCB), and are soldered to conductor pads on the board either by manual assembly or automated insertion mount machines. 
         [0002]      FIGS. 1A, 1B and 1C  show various views of a typical through-hole PCB assembly/solder process.  FIG. 1A  illustrates a top view of a section of a printed circuit board (PCB)  101  with the components (an IC  110 , a coil  112  and a capacitor  114 ) inserted into the PCB for a typical through-hole PCB component assembly/solder process.  FIG. 1B  illustrates a cross-sectional view of the PCB section of  FIG. 1A  (along the line X-X′), showing a cross-section along the through-holes of the PCB and the component pins inserted through the through-holes  FIG. 1C  illustrates the cross-sectional view of the PCB section of  FIG. 1B , after completion of the typical through-hole PCB component solder process, along with an expanded view of a single solder joint and wiring layers of the PCB. With reference to  FIG. 1B , in a typical through-hole process, the pins or electrical conductor leads of the component (e.g., the pins  116  of the IC  110 , the pins  118  of the coil  112 , the pins  120  of the capacitor  114 ) are inserted through plated holes  122  in the PCB, such that pin protrusions  124  from the component leads extend through the back side of the PCB  101 . When soldered, for example, through a wave solder process, the component leads (including the pin protrusions through the back of the board) and the plated through-hole form a solder joint. During the normal process, the boards are first run through a surface-mount stencil printer that applies a solder paste over the top side of the board, forcing the solder paste into the plated through-holes of the PCB  101 . After the solder paste is applied, the board is run through a component placement machine that places the surface mount (SMT) components, and then operators place the pin-in-hole (PIH) components by inserting the component leads into the respective PCB through-holes  122 . The PCB  101  is then put through a solder reflow oven (or in other applications/processes, a solder wave bath), which heats the board and melts the solder paste to form a solder joint between each component lead and the metalized portion of the respective through-hole. This process leaves protruding component pins on the bottom of the printed circuit card assembly (PCBA) with a conical shaped solder joint attached to the annular ring of the metal through hole. 
         [0003]    With reference to  FIG. 1C , and the expanded view of a single solder joint as a result of the solder reflow process, ideally, the solder flows into the through-hole and fills the space  126  between the component leads and the metalized surface of the through-hole  122 . Additionally, the solder paste forms a protruding conical-shaped solder joint  128  around the pin protrusion  124 . Accordingly, this protruding solder joint around the pins of the electrical component creates a point for a potential electrical short and for potential damage during further assembly of the overall product and during operation and repair thereof. For example, in the event that the circuit board comprises a daughter card that is to be mounted or soldered on a further system card or motherboard. Additionally, for example, the protruding solder joints produce points for potential electrical shorts with respect to jumper wires or other components that may be added to the board at a subsequent point. Further, in order to avoid future electrical shorts within the overall product, the protruding solder joints create constraints on the minimum distance between two adjacent boards in the overall assembly—thus creating constraints on such things as product size reduction. Alternatively, in the use of such traditional solder processes that create protruding solder joints, a separate substrate would be attached to the primary assembly to serve as an insulating layer to protect and insulate the protruding solder joints. 
         [0004]    There is, therefore, a need for an electrical circuit board component solder/assembly process, and resulting circuit board product, that eliminates any solder protrusion points in a through-hole electrical component solder/assembly process. 
       SOME EXAMPLE EMBODIMENTS 
       [0005]    Embodiments of the present invention advantageously address the needs above, as well as other needs, by providing an approach for through-hole component soldering/assembly for an electrical circuit card or printed circuit board (PCB), and a resulting circuit card or PCB assembly, that eliminates solder protrusion points or protruding solder joints resulting from the through-hole solder process. 
         [0006]    In accordance with example embodiments of the present invention, an approach is provided, for through-hole component soldering/assembly for an electrical circuit card, wherein the component through-holes in the circuit board are partially back-drilled with a wider aperture. Then, when heated, the solder or solder paste wicks or draws back from the surface of the oversized back-drilled aperture. Consequently, at the at the back of the board, the solder joints are recessed below the surface of the back side of the PCB. Accordingly, this through-hole solder process eliminates any solder protrusions from the back of the assembled circuit card. 
         [0007]    In accordance with example embodiments, a method comprises back-drilling, from a bottom surface of a printed circuit board (PCB), one or more component through-holes in the PCB, wherein each back-drilled through-hole is back-drilled to a depth partially through the PCB and at a diameter that is larger than the diameter of the through hole. The method further comprises applying solder paste to at least one side of the PCB. One or more components are placed on a top surface of the PCB, inserting each pin of each component into a corresponding through-hole. The PCB is passed through a solder process, whereby, within each through-hole of the PCB having a component pin inserted therein, the solder paste is wicked into the through-hole, and forms a solder joint with the respective pin. Each solder joint of a back-drilled through-hole is situated within the through-hole in a manner whereby the solder joint does not protrude beyond the bottom surface of the PCB. By way of example, each back-drilled through-hole is back-drilled to a depth of approximately 20% of the overall depth of the through-hole. By way of further example, each back-drilled through-hole is back-drilled at a diameter that is approximately 200% of the diameter of the through hole. By way of further example, the through-holes of the PCB are plated either before performing the back-drilling step or after performing the back-drilling step. By way of further example, the solder paste is applied at least to the bottom surface of the PCB, whereby the solder paste is forced into at least the back-drilled portion of each back-drilled through-hole. 
         [0008]    In accordance with further example embodiments, an apparatus comprises a printed circuit board (PCB), and one or more components mounted on a top surface of the PCB, wherein each pin of the one or more components is secured within a respective through-hole in the PCB by a solder joint. One or more of the through-holes of the PCB that contains a component pin solder joint is back-drilled, from a bottom surface of the PCB, to a depth partially through the PCB and at a diameter that is larger than the diameter of the through hole. Further, each solder joint of a back-drilled through-hole is situated within the through-hole in a manner whereby the solder joint does not protrude beyond the bottom surface of the PCB. By way of example, the back-drilled portion of each back-drilled through-hole is of a depth of approximately 20% of the overall depth of the through-hole. By way of further example, the back-drilled portion of each back-drilled through-hole is of a diameter that is approximately 200% of the diameter of the through hole. By way of further example, plating within each back-drilled through-hole either does not extend into the back-drilled portion of the through-hole or extends into the back-drilled portion of the through-hole. 
         [0009]    Still other aspects, features, and advantages of the present invention are readily apparent from the following detailed description, simply by illustrating a number of particular embodiments and implementations, including the best mode contemplated for carrying out the present invention. The present invention is also capable of other and different embodiments, and its several details can be modified in various obvious respects, all without departing from the spirit and scope of the present invention. Accordingly, the drawings and description are to be regarded as illustrative in nature, and not as restrictive. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0010]    Embodiments of the present invention are illustrated by way of example, and not by way of limitation, in the figures of the accompanying drawings, wherein like reference numerals refer to similar elements and wherein: 
           [0011]      FIG. 1A  illustrates a top view of a section of a printed circuit board (PCB) with the components inserted into the PCB for a typical through-hole PCB component assembly/solder process; 
           [0012]      FIG. 1B  illustrates a cross-sectional view of the PCB section of  FIG. 1A  (along the line X-X′), showing a cross-section along the through-holes of the PCB and the component pins inserted through the through-holes; 
           [0013]      FIG. 1C  illustrates the cross-sectional view of the PCB section of  FIG. 1B , after completion of the typical through-hole PCB component solder process, along with an expanded view of a single solder joint and wiring layers of the PCB; 
           [0014]      FIG. 2A  illustrates a bottom view of a section of a printed circuit board (PCB) with back-drilled through-holes, in accordance with example embodiments of the present invention; 
           [0015]      FIG. 2B  illustrates a cross-sectional view of the PCB section of  FIG. 2A  (along the line Y-Y′), showing a cross-section along the through-holes of the PCB, in accordance with example embodiments of the present invention; 
           [0016]      FIG. 2C  illustrates the cross-sectional view of the PCB section of  FIG. 2B , after completion of a through-hole PCB component solder process, along with two expanded views (i) and (ii) of a single solder joint, in accordance with example embodiments of the present invention; 
           [0017]      FIG. 3A  illustrates a flow chart depicting a process for a through-hole PCB soldering/assembly process, employing back-drilled through-holes, in accordance with an example embodiment of the present invention; and 
           [0018]      FIG. 3B  illustrates a flow chart depicting a process for a through-hole PCB soldering/assembly process, employing back-drilled through-holes, in accordance with another example embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0019]    An approach for through-hole component soldering/assembly for an electrical circuit card or printed circuit board (PCB), and a resulting circuit card or PCB assembly, that eliminates solder protrusion points or protruding solder joints resulting from the through-hole solder process, is described. In the following description, for the purposes of explanation, numerous specific details are set forth in order to provide a thorough understanding of the present invention. The present invention is not intended to be limited based on the described embodiments, and various modifications will be readily apparent. It will be apparent that the invention may be practiced without the specific details of the following description and/or with equivalent arrangements. Additionally, well-known structures and devices may be shown in block diagram form in order to avoid unnecessarily obscuring the invention. Further, the specific applications discussed herein are provided only as representative examples, and the principles described herein may be applied to other embodiments and applications without departing from the general scope of the present invention. 
         [0020]    A typical multilayer printed circuit board (PCB)  101  includes a plurality of wiring layers within the layers of the PCB. With reference to the expanded view of the one solder joint illustrated in  FIG. 1C , example wiring layers of the PCB are also depicted. For example, wiring layers  132  and  134  run on the top and bottom of the PCB, respectively, and wiring layers  136  and  138  run within layers sandwiched inside the PCB. In the example of  FIG. 1C , the depicted wires,  142  within the top layer  132 ,  144  within the bottom layer  134  and  146  and  148  respectively within the middle layers  136  and  138 , each connects to the pin  116  of the IC  110  via the plating  126  within the through-hole and the solder joint  122 . 
         [0021]      FIG. 2A  illustrates a bottom view of a section of a printed circuit board (PCB)  201  with back-drilled through-holes  221 / 223 , in accordance with example embodiments of the present invention. As depicted in  FIG. 2A , the back of each through-hole  221  is partially drilled at a larger diameter than the diameter of the through-hole, resulting in a partially back-drilled portion  223  of the rough-hole. The back-drilled hole  223  is drilled only a portion of the way through the PCB. By way of example, the back drilled aperture  223  is drilled to a depth that is approximately 20% of the total depth of the through-hole (20% of the thickness of the PCB), and at a diameter that is approximately 200% of the overall diameter, of the through-hole  221 . For an alternate view,  FIG. 2B  illustrates a cross-sectional view of the PCB section  201  of  FIG. 2A  (along the line Y-Y′), showing a cross-section along the through-holes of the PCB, in accordance with example embodiments of the present invention. As depicted, the cross-sectional view of  FIG. 2B  illustrates a side view of the through-hole  221 , and the back-drilled hole  223 , drilled at a relatively larger diameter (e.g., 2×) than the diameter of the through-hole  221 , and drilled only partially through the PCB  201  (e.g., approximately ⅕ th  of the total depth). In accordance with one embodiment, the back-drilled portion  223  of the through-hole is drilled after the plating process, whereby the back-drilling removes the plating from the back-drilled portion  223  of the through-hole (as depicted in the expanded view (i) of  FIG. 2C ). In accordance with an alternative embodiment, the back-drilled portion  223  of the through-hole is drilled prior to the plating process, whereby the plating is added to both the through-hole  221  and the back-drilled portion  223  thereof (as depicted in the expanded view (ii) of  FIG. 2C ). 
         [0022]      FIG. 2C  illustrates the cross-sectional view of the PCB section  201  of  FIG. 2B , after completion of a through-hole PCB component solder process, along with two expanded views (i) and (ii) of a single solder joint, in accordance with example embodiments of the present invention. With reference to the expanded views (i) and (ii) of the single solder joint of  FIG. 2C , (as in the expanded view of  FIG. 1C ) the example wiring layers of the PCB are also depicted. For example, wiring layers  232  and  234  run on the top and bottom of the PCB  201 , respectively, and wiring layers  236  and  238  run within layers sandwiched inside the PCB. In the expanded view (i) of  FIG. 2C , the depicted wires  242  within the top layer  232 , and  246  within the middle layer  236 , each connects to the pin  216  of the IC  210  via the plating  226  within the through-hole  221 / 223  and the solder joint  222 . In the expanded view (ii) of  FIG. 2C , the depicted wires  242  within the top layer  232 ,  244  within the bottom layer  234 , and  246  and  248  respectively within the middle layers  236  and  238 , each connects to the pin  216  of the IC  210  via the plating  226  within the through-hole  221 / 223  and the solder joint  222 . As further depicted in  FIG. 2C , the PCB section  201  includes an IC  210 , a coil  212  and a capacitor  214  inserted into the PCB. Further, the pins or electrical conductor leads of each component (e.g., the pins  216  of the IC  210 , the pins  218  of the coil  212 , the pins  220  of the capacitor  214 ) are inserted through the plated holes  221 / 223  in the PCB, such that there are no pin protrusions extending through the back side of the PCB. When soldered (e.g., through a wave solder process), each of the component leads ( 216 / 218 / 220 ) and the respective plated through-hole ( 221 / 223 ) form a solder joint  222 . 
         [0023]    By way of example, the PCB  201  is first manufactured with the appropriate back-drilled through-holes  221 / 223 . Then, using a pin-in-paste process, solder paste is applied to the PCB  201  via a solder paste printing process, and is thereby squeegeed into the through-holes  221 / 223 . According to one example embodiment, the solder paste is applied to the bottom or back side of the PCB. After the solder paste is applied to the PCB, the pin-in-hole (PIH) components are placed through the top or front side of the board, with the component pins or leads being inserting into the respective PCB through-holes  221 / 223 . By way of example, the component leads are pre-cut to a length that protrudes through to the end of the through-holes (just flush with the back side of the PCB), or, alternatively, to a length that is just short of the back side of the PCB. Accordingly, having been run through the solder paste application, with the solder paste being squeegeed or forced into the through-holes, the back-drilled through-holes are filled with solder paste around the component pins. The PCB  201  is then put through a solder reflow oven, which heats the board and melts the solder paste to form a solder joint  222  between each component lead and the metalized portion of the respective through-hole  221 / 223 . In an alternative process, the PCB is put though a solder wave bath process, which heats the board and melts the solder paste to form a solder joint  222  between each component lead and the metalized portion of the respective through-hole  221 / 223 . When heated, the solder or solder paste wicks or draws back from the oversized back-drilled aperture, and flows into the through-hole  221  and fills the space  226  between the component leads and the metalized surface of the through-hole. 
         [0024]    Consequently, at the at the back of the PCBA, the component pins are recessed below the back of the PCB plane, and each solder joint is also below the plane, forming a recessed bell shaped solder joint, with a relatively convex bottom, around the component pins. In other words, the resulting solder joint exhibits an “L” shaped cross-section or anchor flange around the inside of the back-drilled portion  223  of the through-hole. Accordingly, this through-hole solder process eliminates any solder protrusions from the back of the assembled circuit card, keeping both the solder and component pins of the solder joint below the back surface of the PCBA. Moreover, such a bell shaped solder joint within the plated through-hole portion  221  and the back-drilled portion  223  of the through-hole (which is non-plated according to the PCBA of the expanded view (i) of  FIG. 2C , and plated according to the PCBA of the expanded view (ii) of  FIG. 2C ), provides a physical anchor at the base of the pin, resulting in a solder joint of significantly improved strength over that of the traditional process. In other words For example, the strength of this solder joint makes it suitable not only for low stress environments, but also for high stress applications that require PCB assemblies that can withstand high G forces. 
         [0025]    As such, in accordance with the solder process of example embodiments, the elimination of the resulting pin protrusions and protruding solder joints of prior through-hole solder processes also eliminates the points for potential electrical shorts and for potential damage associated with such protrusions. Additionally, because the resulting solder joints of such example embodiments are below the back surface of the PCBA, the solder joints are protected during handling of the boards throughout the remainder of the product assembly process. For example, the present solder process according to example embodiments eliminates the aforementioned potential shorts and/or damage in assemblies with daughter cards mounted on a further system card or motherboard, and with respect to jumper wires or other components that may be added to the board at a subsequent point. Additionally, the elimination of the protruding solder joints facilitate a reduction of the minimum distance required between two adjacent boards in the overall assembly—thus facilitating reductions in overall product size and other associated benefits. Further, the back-drilled through-hole solder process facilitates the manufacture of a component assembly that eliminates any potential requirement for a second insulating substrate, eliminating the associated labor and cost of cutting the pin protrusions after the solder process as well as that of adding the extra insulating PCB. 
         [0026]      FIG. 3A  illustrates a flow chart depicting a process for a through-hole PCB  201  soldering/assembly process, employing back-drilled through-holes  221 / 223 , in accordance with an example embodiment of the present invention. The PCB  201  proceeds through the normal process of fabricating the wiring layers of the PCB (Step  301 ). The through-holes are drilled at a first diameter (Step  303 ), and plated (Step  305 ). Then, once the through-holes are plated, the PCB  201  is put through a further process step, whereby each through-hole is partially back-drilled from the bottom or back side of the PCB at a diameter relatively larger than the diameter of the original through-hole (Step  307 ). By way of example, the back-drilled portion  223  of the through-hole is drilled approximately 20% of the way though the through-hole (approximately ⅕ th  of the thickness of the PCB) from the bottom or back side of the PCB. By way of further example, the diameter of the back-drilled portion  223  of the through-hole is approximately 200% of the diameter of the through-hole  221  (two times the diameter of the through-hole  221 ). The back-drilling removes the metal plating from the back-drilled portion  223  of the through-hole, and thereby creates a reservoir for the solder paste. 
         [0027]    The solder paste is then screened onto the board, and during that process step the solder paste is squeegeed and forced into the back-drilled portion  223  of the through-hole, and may be at least partially forced into the narrower portion of the through-hole (Step  309 ). By way of example, the solder paste is screened onto the bottom or back side of the PCB. Further, once the board is removed from the solder paste screening step, the components are inserted into the solder paste filled through-holes from the top or front side of the board (Step  311 ), and the component pins extend to a point flush with or slightly above (within) the end of the back-drilled portions  223  of the through-holes (slightly above the surface of the back-side of the PCB). For example, during a component preparation process, the component pins are cut to an appropriate length prior to insertion into the PCB. The boards are then passed through the reflow oven and heated in stages to heat the solder paste and melt the solder within the paste (Step  313 ). As a result of the heating process, the melted solder is wicked up into the narrower portion of the through-hole and adheres to the metal plating in that portion of the hole, and the excess solder remains below, in the wider portion of the through-hole, and acts as an anchor anchoring the solder joint from the bottom of the PCB (without protruding through the PCB). 
         [0028]      FIG. 3B  illustrates a flow chart depicting a process for a through-hole PCB soldering/assembly process, employing back-drilled through-holes, in accordance with a further example embodiment of the present invention. With reference to  FIG. 3B , the PCB  201  proceeds through the normal fabrication process of fabricating the wiring layers of the PCB (Step  321 ). The through-holes are drilled through the PCB at a first diameter (Step  323 ). In this embodiment, the through-holes are then partially back-drilled (from the bottom or back side of the PCB) at the second diameter relatively larger than the first, prior to the plating of the holes (Step  325 ). Then, once back-drilled, the back-drilled through-holes  221 / 223  are plated (Step  327 ). The solder paste is then screened onto the board, and during that process step the solder paste is forced into the back-drilled portion  223  of the through-hole, and may be at least partially forced into the narrower portion of the through-hole (Step  329 ). By way of example, the solder paste is screened onto the bottom or back side of the PCB. The components are inserted into the solder paste filled through-holes from the top or front side of the board (Step  331 ). Here also, the component pins extend to a point flush with or slightly above the end of the back-drilled portions  223  of the through-holes (slightly above the surface of the back-side of the PCB). The boards are then passed through the reflow oven and heated in stages to heat the solder paste and melt the solder within the paste (Step  333 ). As a result of the heating process, the melted solder is wicked up into the narrower portion of the through-hole and adheres to the metal plating in that portion of the hole, and the excess solder remains below in the wider portion of the through-hole and adheres to the metal plating in that portion of the hole. Again, the portion of the solder joint  222  within the wider portion of the back-drilled through-hole  221 / 223  acts as an anchor anchoring the solder joint from the bottom of the PCB (without protruding through the PCB). The expanded view of  FIG. 2C  illustrates the result, where the plating and wire layer connections are through the entire back-drilled through-hole  221 / 223 . 
         [0029]    Moreover, in accordance with a further embodiment, surface-mount components may also be placed on surface-mount pads on the top side of the PCB  201  and solder paste can be further screened on the top side for the surface-mount components. Accordingly, the present process according to example embodiments facilitates the mounting of both through-hole components and surface-mount components, through the same manufacturing/assembly process, while eliminating a step for the mounting of the surface-mount components. 
         [0030]    The foregoing description of the present invention provides illustration and description, but is not intended to be exhaustive or to limit the invention to the precise one disclosed. Modifications and variations are possible consistent with the above teachings or may be acquired from practice of the invention. Thus, it is noted that the scope of the invention is defined by the claims and their equivalents.