Patent Publication Number: US-2023156928-A1

Title: Designing a printed circuit board (pcb) to detect slivers of conductive material included within vias of the pcb

Description:
RELATED APPLICATION 
     This application is a continuation of U.S. Pat. Application No. 17/248,569, filed Jan. 29, 2021 (now U.S. Pat. No. 11,570,908), which is a divisional of U.S. Pat. Application No. 15/647,984, filed Jul. 12, 2017 (now U.S. Pat. No. 10,917,976), the contents of which are incorporated herein by reference in their entireties. 
    
    
     BACKGROUND 
     A printed circuit board (PCB) may include one or more material layers that mechanically support and electrically connect electronic components using conductive pathways. The conductive pathways may be etched from copper sheets laminated onto a non-conductive substrate. 
     SUMMARY 
     According to some possible implementations, a method may include obtaining a printed circuit board (PCB) that includes a set of pads associated with facilitating a connection to a component, a set of vias electrically connected to the set of pads, and a set of layers perpendicular to the set of vias. The set of vias include a set of stub regions. The set of layers include a signal layer and a ground layer. The ground layer may be located between the set of stub regions and the signal layer. The method may include drilling to remove at least a portion of a stub region of a via of the set of vias. The drilling may create a ring of conductive material around a portion of the ground layer that connects to the via. The method may include performing an electrical test to determine whether a sliver of conductive material is included within the via after drilling to remove the at least a portion of the stub region of the via. 
     According to some possible implementations, a printed circuit board (PCB) may include a set of pads associated with facilitating a connection to a component, a set of vias electrically connected to the set of pads, and a set of layers perpendicular to the set of vias. The set of vias may be back drilled to remove at least a portion of a set of stub regions associated with the set of vias. The set of layers may include a signal layer and a ground layer. The ground layer may be located between the set of stub regions and the signal layer. The ground layer may include a ring of conductive material around portions of the ground layer that connect to the set of vias. 
     According to some possible implementations, a method may include obtaining a printed circuit board (PCB) that includes a set of vias and a set of layers perpendicular to the set of vias. The set of vias may include a set of stub regions. The set of layers may include a signal layer and a ground layer. The ground layer may be located between the set of stub regions and the signal layer. The method may include drilling to remove at least a portion of a stub region of a via of the set of vias. The method may include performing an electrical test to determine whether a sliver of conductive material is included within the via after drilling to remove the at least a portion of the stub region of the via. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIGS.  1 A- 1 C  are diagrams of an overview of an example implementation described herein; 
         FIG.  2    is a diagram of an example PCB; and 
         FIG.  3    is a flow chart of an example process for detecting slivers of conductive material included within vias of a PCB. 
     
    
    
     DETAILED DESCRIPTION 
     The following detailed description of example implementations refers to the accompanying drawings. The same reference numbers in different drawings may identify the same or similar elements. 
     A PCB mechanically supports and electrically connects electronic components using conductive pathways etched from copper sheets that are laminated onto a non-conductive substrate. The conductive pathways may be organized as a number of layers on the PCB to increase the signal transmission density of the PCB. 
     PCBs may be used in high frequency applications. For example, a PCB may be populated with an integrated circuit used to enable high speed serial links to and from the PCB, as may be utilized for an Ethernet switch, a serializer/deserializer (SerDes), or the like. In a high frequency application, the PCB can experience significant signal integrity disturbances as a result of the unused portions of through-holes and vias, such as a stub portion that extends past the last connected layer of the PCB. The stub may present resonances, present impedance discontinuities, and increase the loss of the channel, thereby limiting performance. 
     To remove the stub, PCB manufacturers implement a technique known as back drilling. Back drilling involves using a drill bit slightly larger in diameter than the drill bit used to create the original via hole to remove the undesired conductive plating in the stub region of the via. However, if a drill is not properly registered, the drill bit may not be properly aligned in the center of the hole of the via (e.g., a center point of the drill bit needs to be aligned to a center point of the hole of the via). Back drilling without a properly centered drill bit may cause a sliver of conductive material (e.g., a copper sliver) to remain within the via (i.e., a remnant of the conductive plating), which may cause signal integrity disturbances. Furthermore, manual detection of conductive slivers may be difficult when a PCB has hundreds, or even thousands, of vias. 
     Some implementations described herein provide a PCB design process to detect slivers of conductive material using an electrical test. For example, the process may include designing a PCB to exclude an antipad between a ground layer and a via. The ground layer may be between a stub region of the via and a signal layer that is nearest to the stub region. In this case, the stub region of the via may be back drilled to create a ring of conductive material around the ground layer of the via. An electrical test may be performed on the via. If the back drilling is not positioned properly and creates a sliver of conductive material, the sliver will come into contact with the ring of conductive material around the ground layer. This may create a short circuit that causes the via to fail the electrical test. If the back drilling is positioned properly and does not create a sliver of conductive material, an electrical charge will not come into contact with the ring of conductive material, resulting in the via passing the electrical test. 
     By reducing stub length via back drilling, a PCB design process reduces signal distortion (e.g., deterministic jitter). Furthermore, by using an electrical test to detect slivers of conductive material within vias of a PCB, the PCB design process reduces signal attenuation and crosstalk relative to a PCB design process that is unable to detect the slivers of conductive material. 
       FIGS.  1 A- 1 C  are diagrams of an overview of an example implementation  100  described herein.  FIGS.  1 A- 1 C  show a PCB design process with respect to a single via. In practice, the PCB design process shown may apply to a set of vias (e.g., hundreds, thousands, tens of thousands, or even hundreds of thousands of vias) within a PCB. 
     As shown in  FIG.  1 A , by via A, a PCB design engineer may design a PCB that includes an antipad between a ground layer and via A. In contrast, and as shown by via B, a PCB design engineer may design a PCB so that a ground layer of via B comes into physical contact with via B (e.g., by designing the PCB without an antipad between the ground layer and via B). The ground layer may be located between a stub region of via B and a signal layer that is nearest to the stub region (e.g., in relation to other signal layers). In some cases, the PCB may be designed using a computer-aided design (CAD) technique, a computer-aided manufacturing (CAM) technique, or the like. 
     By designing the PCB to exclude an antipad between the ground layer and the via, the stub region of the via may be back drilled to leave a ring of copper that may be used to detect copper slivers, as described further herein. 
     As shown in  FIG.  1 B , the PCB design process may include back drilling the via. For example, the PCB design process may modify the via by using a back drilling technique to remove the stub region of the via. As shown by the “optimum condition,” a drill bit that is properly centered between sides of a primary drill hole may remove the copper within the via (and not leave a copper sliver). In this case, the drill bit may leave a ring of copper around the ground layer as a result of back drilling without an antipad around the ground layer. 
     As shown by the “offset condition,” a drill bit that is not properly centered between sides of the primary drill hole may remove a portion of the copper, but may leave a copper sliver. A drill bit may not be properly centered if the drill is not properly registered (e.g., a drilling location identified by software may not be centered), if the drill slips out of position upon making initial contact with the PCB, or the like. In this case, the back drilling may create a ring of copper around the ground layer as a result of back drilling without an antipad around the ground layer. Additionally, the copper sliver may come into contact with the copper ring around the ground layer. 
     By removing the antipad of the ground layer and back drilling, the PCB design process may create a ring of copper around the ground layer. 
     As shown in  FIG.  1 C , the PCB design process may include an electrical test. For example, the PCB design process may perform an electrical test (e.g., an in-circuit test (ICT)) using an electrical probe. In some cases, a test machine (i.e., a device capable of automatically performing electrical tests) may include a set of probes that may perform a set of electrical tests concurrently. In this case, the test machine may move a probe of the set of probes to a first via, conduct an electrical test, store a test result (e.g., pass, fail, etc.), and may move the probe to a second via to perform an additional electrical test. An electrical test may detect a copper sliver if the electrical test detects a short circuit. For example, if a copper sliver is present within a via, electrical current may travel through the copper sliver to the ring of copper associated with the ground layer, creating a short circuit at the ground layer. 
     As shown by the optimum condition, an electrical probe may provide electrical current to the via. In this case, the electrical current may not reach the ground layer because the back drilling did not create a copper sliver. As shown by the offset condition, an electrical probe may provide current to the via, and the current may travel from the copper sliver to the copper ring around the ground layer, electrifying the ground layer to cause a short circuit. 
     In some implementations, the electrical test may be performed on tens of vias, hundreds of vias, thousands of vias, tens of thousands of vias, hundreds of thousands of vias, or more, concurrently. Additionally, or alternatively, the electrical test may be performed using a test structure (e.g., a test coupon). 
     In this way, the PCB design process may use an electrical test to detect copper slivers. By detecting copper slivers, vias associated with the copper slivers may be re-drilled and re-tested to determine that the copper sliver has been removed, thereby reducing signal attenuation and crosstalk relative to a PCB that includes copper slivers. 
     As indicated above,  FIGS.  1 A- 1 C  are provided merely as an example. Other examples are possible and may differ from what was described with regard to  FIGS.  1 A- 1 C . For example, while implementations described here provide for designing a PCB that excludes an antipad associated with a ground layer, other implementations may exist that provide for removing the antipad associated with the ground layer from a PCB that has already been designed to include an antipad. 
       FIG.  2    is a diagram of an example implementation  200  of a PCB. Specifically,  FIG.  2    is a diagram illustrating a cross-sectional view of a PCB. The cross-sectional view shown in  FIG.  2    conceptually illustrates a layout of a PCB  210  as the layout of the PCB relates to the connection and routing of signals for a component  220 . As shown, component  220  is illustrated as connected to PCB  210  via a ball grid array (BGA)  230 . BGA  230  may, for example, be soldered to pads  235  of PCB  210 . 
     PCB  210  is shown as including a number of vias  240 . A via  240  may generally be a vertical electrical connection between different layers of PCB  210 . A via  240  may include, at each layer, a pad that provides electrical connections between conductive traces on the layer or an antipad that defines a non-conductive “void” around the via  240  to insulate the via  240  from that layer. A number of example layers  250 ,  252 ,  254 , and  256  are shown in PCB  210 . Each layer  250 - 256  may include conductive traces (e.g., copper traces) that route power, signals, and/or ground through PCB  210 . Each layer  250 - 256  may be generally electrically isolated from one another, but may be potentially connected through vias  240 . 
     In some designs, it may be desirable to dedicate certain layers  250 - 256  as power layers, signal layers, or ground layers. A ground layer (also known as a ground plane) is an electrically conductive surface (e.g., often made of copper foil) connected to a power supply ground terminal. The ground layer may serve as a return path for current from different components of the PCB. As shown, a ground layer may be located between a signal layer  254  and a stub region  245 . In some cases, the ground layer may be included in stub region  245 . As shown, signal layer  254  may be nearest to stub region  245  relative to other signal layers (e.g., signal layer  252 , signal layer  250 , etc.). 
     The portion of the vias  240  that extend below the last layer to which the vias connect may be referred to as stub region  245 . Stub region  245  may create undesirable distortions in the signals that traverse the vias and/or signals being routed at a layer adjacent to the vias. The distortions may become more acute as the operating frequency increases. To decrease the deleterious effect of stub region  245 , a back drill may be used to physically remove stub region  245 . 
     The rectangles shown in  FIG.  2   , such as rectangle  260 , may represent conductive traces for the layers. The breaks in the rectangles around vias  240  may define the antipads (i.e., the non-conductive areas around a via  240 ). An antipad may function to isolate a via  240  from the PCB layer traversed by the via  240 . 
     As shown by reference number  265 , a first via  240  may be back drilled to physically remove the stub. The first via  240  may extend from BGA  230  to signal layer  254 . Back drilling to remove the stub may create a circular ring of conductive material around a portion of ground layer  256  that connects to via  240  (the circular ring being visible from a top view). 
     The number and arrangement of layers and vias shown in  FIG.  2    are provided as an example. In practice, there may be additional layers/vias, fewer layers/vias, different layers/vias, or differently arranged layers/vias than those shown in  FIG.  2   . Furthermore, two or more layers/vias shown in  FIG.  2    may be implemented as a single layer/via, or a single layer/via shown in  FIG.  2    may be implemented as multiple, different layers/vias. 
       FIG.  3    is a flow chart of an example process  300  for detecting slivers of conductive material included within vias of a PCB. In some implementations, example process  300  may be applied to the design of a PCB used to route a set of high speed signals. 
     As shown in  FIG.  3   , process  300  may include designing a printed circuit board (PCB) that excludes an antipad associated with a ground layer (block  310 ). For example, a PCB design process may design a PCB that excludes an antipad associated with a ground layer that is located between a stub region and a signal layer that is nearest to the stub region (e.g., in relation to other signal layers). In this case, a user may interact with a device to utilize a computer-aided design (CAD) technique or a computer-aided manufacturing (CAM) technique to design the PCB in a manner that excludes the antipad associated with the ground layer. The antipad may define a non-conductive “void” around the via to insulate the via from the ground layer. 
     In some implementations, a user may interact with a device to utilize a CAD technique or a CAM technique to design a PCB without an antipad associated with a ground layer. For example, assume the ground layer is located between a stub region of the via and a signal layer that is nearest to the stub region. In this case, the user may design the PCB to include antipads for one or more layers of the PCB (e.g. power layers, signal layers, ground layers, etc.), but may not design the PCB to include an antipad for the ground layer. In this way, the ground layer may come into contact with conductive material (e.g., copper plating) associated with the via. 
     Additionally, or alternatively, a user may interact with a device to remove an antipad associated with a ground layer of a PCB. For example, assume a PCB is designed to include an antipad associated with a ground layer. In this case, a user may interact with a device to utilize a CAD technique or a CAM technique to “fill” the antipad to allow the ground layer to come into contact with conductive material associated with the via. 
     By designing a PCB to exclude an antipad associated with a ground layer, the stub region of the via may be back drilled to create a ring of conductive material that may be used in conjunction with an electrical test to detect whether slivers of conductive material are present after the back drilling, as described further herein. 
     As further shown in  FIG.  3   , process  300  may include back drilling a stub region of a via to create a ring of conductive material around a portion of the ground layer (block  320 ). For example, a PCB design process may use a back drill to remove a stub region of a via to create a ring of conductive material around a portion of the ground layer that connects to the via. 
     In some implementations, a back drill may be used to remove at least a portion of the stub region of the via. For example, assume a primary drill is used to create a first hole with a diameter D. Further assume that the hole is plated with conductive material. In this case, a back drill may be used to drill a second hole with diameter D + x (e.g., the second hole may be larger than the first hole). As an example, the back drill may remove all but approximately 2 thousandths of an inch (mils) to approximately 12 mils of the stub. 
     In some implementations, back drilling may be used to remove conductive material associated with the stub region. For example, back drilling may remove conductive material by drilling from a first area to a second area. The first area may be a side of the via (e.g., a “stub” side that is opposite from a side of the via that connects to component  220 ). The second area may be an area within the via that is between the ground layer and a signal layer. The signal layer may be nearest to the stub region of the via relative to other signal layers. In this case, a back drill may drill from the first area to the second area (e.g., past the ground layer which does not have an antipad). This may create a ring of conductive material around a portion of the ground layer that connects to the via. 
     In some implementations, a back drill may remove all conductive material associated with a stub region of the via. For example, if a drill bit of a back drill is properly centered between sides of a primary drill hole, the back drill may remove all conductive material within the via (and will not leave a sliver of conductive material). In this case, the drill bit may leave a ring of conductive material around the ground layer due to back drilling without an antipad between the ground layer and the via. 
     In some implementations, a back drill may remove a portion of the conductive material associated with a stub region of the via but may leave a sliver. For example, if a drill bit of a back drill is not properly centered between sides of a primary drill hole, the back drill may leave a sliver of conductive material within the via. A drill bit may not be properly centered if the drill is not registered properly (e.g., a drilling location identified by software may not be centered), if the drill slips out of position upon making initial contact with the PCB, or the like. In this case, the drill bit may leave a ring of conductive material around the ground layer due to back drilling without an antipad between the ground layer and the via. Additionally, the sliver of conductive material may connect to the ring of conductive material. 
     In this way, a back drill that is not properly centered may create a sliver of conductive material that connects to a ring of conductive material around the ground layer, thereby allowing an electrical test to detect the sliver. 
     As further shown in  FIG.  3   , process  300  may include performing an electrical test to determine whether a sliver of conductive material is included within the via (block  330 ). For example, the PCB design process may include an electrical test (e.g., an in-circuit test (ICT)) that may be performed to determine whether a sliver of conductive material is included within the via. 
     In some implementations, the PCB design process may perform an electrical test on the via. For example, an electrical probe may perform an ICT on a PCB to check for shorts, opens, resistance, capacitance, or the like. In some cases, the electrical probe may provide electrical current to the via to determine whether a sliver of conductive material connected to the ground layer creates a short circuit. 
     As an example, assume a drill bit used for back drilling is properly centered and removes a portion of the stub region. In this case, an electrical probe may provide electrical current to the via, and the electrical current may remain within the via (and not come into contact with the ground layer, as shown by the first image in  FIG.  1 C ). This may cause the via to pass the electrical test, indicating that there are no slivers of conductive material within the via. 
     As another example, assume a drill bit used for back drilling is not properly centered and removes a portion of the stub region (and in doing so, creates a sliver of conductive material). In this case, an electrical probe may provide current to the via, and the current may travel through the sliver of conductive material and through the ground layer to create a short circuit (shown by the second image in  FIG.  1 C ). This may cause the via to fail the electrical test, indicating that there is a sliver of conductive material within the via. 
     In some implementations, a test machine (i.e., a device capable of automatically performing electrical tests) may include a set of probes that may perform a set of electrical tests concurrently. For example, the test machine may move a probe of the set of probes to a first via, conduct an electrical test, store a test result (e.g., pass, fail, etc.), and may move the probe to a second via to perform an additional electrical test. In some cases, probes may be configured to stagger the electrical tests (e.g., perform the electrical tests at different time intervals), and may record a time stamp indicating a time that the electrical test begins and a time stamp indicating a time that the electrical test ends. Additionally, the probes may record a time stamp in the event of a short circuit. This may allow multiple probes to test vias associated with the same ground layer. For example, assume one copper sliver causes the entire ground layer to short circuit (and causes all concurrently performing electrical tests to fail). In this case, the test machine may check time stamps to identify which concurrently tested via includes the copper sliver. 
     In some implementations, an electrical test may be performed on hundreds, thousands, tens of thousands, even hundreds of thousands of vias concurrently. In this way, an electrical test may be used to detect all (or some) copper slivers included within a PCB in a time efficient manner (relative to testing one via at a time). 
     Additionally, or alternatively, an electrical test may be implemented using a test coupon. For example, assume a PCB is designed to include a test coupon (e.g., a test structure). In this case, an electrical probe may perform an electrical test on the test coupon, which may be located at an edge of the PCB. In this way, the test coupon may be used to determine whether a sliver of conductive material is included within vias of the PCB. In some implementations, an electrical test may be performed on a test coupon and a subset of layers within the PCB (e.g., the critical layers). 
     In some implementations, vias associated with identified slivers may be re-drilled and re-tested. For example, assume an electrical test detects a sliver of conductive material included in a via. In this case, the via may be re-drilled to remove the copper sliver. Additionally, the test machine may use a probe to run an additional electrical test on the via to ensure that the sliver of conductive material has been removed. 
     By performing an electrical test to detect the slivers of conductive material, vias associated with the slivers of conductive material may be re-drilled and re-tested to determine that the sliver has been removed, thereby reducing signal attenuation and cross talk relative to a PCB that does not detect and/or remove slivers of conductive material. 
     Although  FIG.  3    shows example blocks of process  300 , in some implementations, process  300  may include additional blocks, fewer blocks, different blocks, or differently arranged blocks than those depicted in  FIG.  3   . Additionally, or alternatively, two or more of the blocks of process  300  may be performed in parallel. 
     By reducing stub length via back drilling, a PCB design process reduces signal distortion (e.g., deterministic jitter). Furthermore, by using an electrical test to detect slivers of conductive material within vias of a PCB, the PCB design process reduces signal attenuation and crosstalk relative to a PCB design process that is unable to detect the slivers of conductive material. 
     The foregoing disclosure provides illustration and description, but is not intended to be exhaustive or to limit the implementations to the precise form disclosed. Modifications and variations are possible in light of the above disclosure or may be acquired from practice of the implementations. 
     As used herein, the term component is intended to be broadly construed as hardware, firmware, and/or a combination of hardware and software. 
     It will be apparent that systems and/or methods, described herein, may be implemented in different forms of hardware, firmware, or a combination of hardware and software. The actual specialized control hardware or software code used to implement these systems and/or methods is not limiting of the implementations. Thus, the operation and behavior of the systems and/or methods were described herein without reference to specific software code—it being understood that software and hardware can be designed to implement the systems and/or methods based on the description herein. 
     Even though particular combinations of features are recited in the claims and/or disclosed in the specification, these combinations are not intended to limit the disclosure of possible implementations. In fact, many of these features may be combined in ways not specifically recited in the claims and/or disclosed in the specification. Although each dependent claim listed below may directly depend on only one claim, the disclosure of possible implementations includes each dependent claim in combination with every other claim in the claim set. 
     No element, act, or instruction used herein should be construed as critical or essential unless explicitly described as such. Also, as used herein, the articles “a” and “an” are intended to include one or more items, and may be used interchangeably with “one or more.” Furthermore, as used herein, the term “set” is intended to include one or more items (e.g., related items, unrelated items, a combination of related and unrelated items, etc.), and may be used interchangeably with “one or more.” Where only one item is intended, the term “one” or similar language is used. Also, as used herein, the terms “has,” “have,” “having,” or the like are intended to be open-ended terms. Further, the phrase “based on” is intended to mean “based, at least in part, on” unless explicitly stated otherwise.