Patent Publication Number: US-10334720-B1

Title: Printed circuit board test coupon for electrical testing during thermal exposure and method of using the same

Description:
INCORPORATION BY REFERENCE TO ANY PRIORITY APPLICATIONS 
     Any and all applications for which a foreign or domestic priority claim is identified in the Application Data Sheet as filed with the present application are hereby incorporated by reference under 37 CFR 1.57. 
     BACKGROUND 
     Field 
     The invention is directed to a printed circuit board (PCB) test coupon for use in electrical testing during thermal exposure, and more particularly to a PCB test coupon having a matrix of holes that can be electrically measured with a limited number of test channels 
     Description of the Related Art 
     Electrical testing of plated holes/vias (e.g., through, blind, buried, stacked) in double sided or multilayer printed circuit boards (PCB) to discover electrical reliability involves placing specifically designed test coupons into a thermal cycling environment that sequentially exposes them to hot and cold temperature extremes in cycles causing expansion and contraction of the plated hole/via with each thermal cycle. Sensing electrical failures due to fracturing or cracking of the plated holes/vias during thermal cycling historically involved making a test coupon consisting of an electrical test net containing more than one plated hole/via in a “daisy chain” pattern of plated holes/vias. The multiple plated holes/vias in this daisy chain test net were connected by copper trace patterns running between two plated holes/vias consecutively on alternating sides of the plated hole/via. Electrical resistance of the daisy chain test net of plated holes/vias was then measured using a 4-wire kelvin bridge measurement system to see if its resistance changed at a specified temperature during the thermal cycling. Resistance increases in the daisy chain test net at a specified temperature would indicate fracturing or cracking of the plated hole(s)/via(s) in the test net. Daisy chains are limited to detecting these fractures or cracks in an advanced stage, as the volume of copper in the plated hole is significantly greater that the volume of copper in the traces connecting them together in the daisy chain. Daisy chain test nets cannot electrically differentiate the degree of fractures or cracks in different/multiple plated holes/vias of the test net. Electrical resistance increases in the daisy chain detected by the 4-wire kelvin bridge measurement system at a given temperature during the thermal cycle would indicate that the volume of copper in one/some/all of plated hole(s)/via(s) had been reduced by fracture or cracking to a point where the difference between the volume of copper in the hole and volume of copper in the traces connecting them in the daisy chain was not significant enough to prevent detection. Detection from daisy chains would show evidence of fracturing or cracking of the copper in the plated hole(s)/via(s) at some indeterminate time after the initiation point of the fracture or crack. 
     In order to detect the initiation point of fracturing or cracking of a plated hole/via, the only solution is to forgo using daisy chain nets and measure a single plated hole/via in a test net for resistance change at a given temperature using a 4-wire kelvin measurement system. Testing of a single plated hole/via net in a PCB test coupon differs from testing a daisy chain net of plated holes/vias by being able to detect cracks or fractures in a plated hole/via as soon as they initiate. The disadvantage to this method is that it is difficult to get a statistically significant amount of plated holes/vias tested as each test net only provides information on one plated hole/via, whereas daisy chain test nets allow for resistance data to be collected from multiple plated holes/vias in one test net. 
     SUMMARY 
     Accordingly, there is a need for an improved PCB test coupon for use in electrical testing during thermal excursions that addresses one or more of the deficiencies in existing test coupons described above. 
     In accordance with one aspect of the disclosure, a PCB test coupon is provided having a matrix of plated holes/vias, where the resistance of the plated holes/vias in the matrix can be measured with a limited set of test channels to increase the amount of electrical resistance test data available from said limited set of test channels. 
     In accordance with another aspect of the disclosure, a PCB test coupon is provided having a matrix of sixteen test nets, each containing a single plated hole/via or daisy chain of plated holes/vias that can be measured with a limited set of test channels that would normally test 4 daisy chain test nets not in a matrix configuration. 
     In accordance with another aspect of the disclosure a printed circuit board test coupon for electrical testing during thermal exposure is provided. The test coupon comprises a substrate having one or more conductive layers and extending between a first side and a second side opposite the first side. The test coupon also comprises a plurality of plated holes/vias formed within the substrate of the test coupon or extending through the entire substrate of the test coupon from a first surface on the first side A of the plated hole/via to a second surface on the second side B of the plated hole/via. The test coupon also comprises a second plurality of trace patterns defining a second pattern, each of the second plurality of trace patterns interconnecting a different subset of the plurality of holes/vias on the second side of the plated holes/vias, the second pattern being different than the first pattern. The test coupon further comprises a connector pattern defined within or on the surface of the substrate, the first and second plurality of trace patterns connected to a plurality of connection points in the connector pattern. A measurement of resistance of each plated hole/via is provided by connecting 2 wires of a 4-wire kelvin bridge measurement system to the first side and connecting 2 wires of a 4-wire kelvin bridge measurement system to the second, side of the plated hole/via using connection points for one of the first plurality of trace patterns and one of the second plurality of trace patterns that connect to said plated hole/via. 
     In accordance with another aspect of the disclosure, a printed circuit board test coupon for electrical testing during thermal exposure is provided. The test coupon comprises a substrate having one or more conductive layers and extending between a first side and a second side opposite the first side. The test coupon also comprises a plurality of plated holes/vias comprising sixteen plated holes/vias formed within the substrate of the test coupon or extending through the entire substrate of the test coupon from a first surface on the first side of the plated hole/via to a second surface on the second side of the plated hole/via. The test coupon also comprises a first plurality of trace patterns defining a first pattern comprising four trace patterns on the first side, each of the four trace patterns interconnecting four of the plated holes/vias on the first side of the plated holes/vias. The test coupon also comprises a second plurality of trace patterns defining a second pattern comprising four trace patterns on the second side, each of the four trace patterns interconnecting plated holes/vias on the second side of the plated hole/via, the second pattern being different than the first pattern and each of the sixteen plated holes/vias connecting to one of the trace patterns on the first side and one of the trace patterns on the second side of the via. The test coupon further comprises a connector pattern defined in the substrate having twenty connection points, the first and second plurality of trace patterns connected to the twenty connection points in the connector pattern. A measurement of resistance of each of the sixteen plated holes/vias is provided by connecting 2 wires each of a 4-wire kelvin bridge measurement system to the first and second sides of the plated hole/via using connection points in the connector pattern for one of the first plurality of trace patterns and one of the second plurality of trace patterns that connect to said plated hole/via. 
     In accordance with another aspect of the disclosure, a method of making a printed circuit board test coupon for electrical testing during thermal exposure is provided. The method comprises forming a substrate having one or more conductive layers and extending between a first side and a second side opposite the first side, and forming a plurality of plated holes/vias within the substrate of the test coupon or such that they extend through the entire substrate of the test coupon from a first surface on the first side of the plated hole/via to a second surface on the second side of the plated hole/via. The method further comprises forming a first plurality of trace patterns in a first pattern, each of the first plurality of trace patterns interconnecting a subset of the plurality of plated holes/vias on the first side of the substrate, and forming a second plurality of trace patterns in a second pattern different than the first pattern, each of the second plurality of trace patterns interconnecting a different subset of the plurality of vias on the second side of the plated hole/via. The method further comprises providing a connector pattern in the substrate having a plurality of connection points, the first and second plurality of trace patterns connected to the plurality of connection points. A resistance measurement of each plated hole/via is provided by connecting 2 wires of a 4-wire kelvin bridge measurement system to the first side and connecting 2 wires of a 4-wire kelvin bridge measurement system to the second side of the plated hole/via using connection points for one of the first plurality of trace patterns and one of the second plurality of trace patterns that connect to said plated hole/via. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is perspective schematic view of a first side A of a test coupon  100  or the internal substrate layers of a multilayer PCB test coupon. 
         FIG. 2  is a perspective schematic view of a second side B of the test coupon  100  or the internal substrate layers of a multilayer PCB test coupon in  FIG. 1 . 
         FIG. 3  is a schematic view of an array of trace patterns T on both sides of the laminated PCB test coupon substrate S (e.g. external or internal layers of a multilayer PCB test coupon) of  FIGS. 1-2  superimposed on each other, with the trace patterns T on the second side of the laminated PCB test coupon substrate S shown in phantom. The integrated circular pads P ( 1 - 16 ) of the trace patterns T are attached to and terminate side A of the plated holes/vias H ( 1 - 16 ). The ends of each trace pattern T terminate in 2 pads C of connector pattern  30  which is comprised of pads C 1 -C 20 . Each of the pads C in connector pattern  30  has a plated through hole associated with it that facilitates connection of the 4-wire kelvin bridge measurement system to the test coupon  100 . 
         FIG. 4  is a schematic view of trace patterns T 1 -T 4  on the second side of the laminated PCB test coupon substrate S (e.g. external or internal layer of a multilayer PCB test coupon) of  FIGS. 1-2 . The integrated circular pads P ( 1 - 16 ) of these trace patterns T are attached to and terminate side B of the plated holes/vias H ( 1 - 16 ). The ends of each trace pattern T 1 -T 4  terminate in 2 pads C of connector pattern  30  which is comprised of pads C 1 -C 20 . Each of the pads C in connector pattern  30  has a plated through hole associated with it that facilitates connection of the 4-wire kelvin bridge measurement system to the test coupon  100 . 
         FIG. 5  is a schematic view of trace patterns T 5 -T 10  on the first side of the laminated PCB test coupon substrate S (e.g. external or internal layer of a multilayer PCB test coupon) of  FIGS. 1-2 . The integrated circular pads P ( 1 - 16 ) of these trace patterns are attached to and terminate side A of the plated holes/vias H ( 1 - 16 ). The ends of each trace pattern T 5 -T 8  terminate in 2 pads C of connector pattern  30  which is comprised of pads C 1 -C 20 . The ends of each trace pattern T 9 -T 10  terminate in 4 pads C of connector pattern  30  which is comprised of pads C 1 -C 20 . Each of the pads C in connector pattern  30  has a plated through hole associated with it that facilitates connection of the 4-wire kelvin bridge measurement system to the test coupon  100 . 
         FIG. 6  is a schematic view of trace patterns on both sides of a laminated PCB test coupon substrate S (e.g. external or internal layers of a multilayer PCB test coupon) superimposed on each other, with the trace patterns on the second side of the laminated PCB test coupon substrate S (e.g. external or internal layer of a multilayer PCB test coupon) shown in phantom. 
         FIG. 7  is a schematic view of trace patterns T 5 -T 10  and TDCA on a first side A of the laminated PCB test coupon substrate S (e.g. external or internal layer of a multilayer PCB test coupon) of  FIG. 6 . The integrated circular pads P ( 1 - 46 ) of these trace patterns T are attached to and terminate side A of the associated plated holes/vias. The ends of trace patterns T 5 -T 8  terminate in 2 pads C of connector pattern  30  which is comprised of pads C 1 -C 20 . The ends of trace patterns T 9 -T 10  terminate in 4 pads C of connector pattern  30  which is comprised of pads C 1 -C 20 . Each of the pads C in connector pattern  30  has a plated through hole associated with it that facilitates connection of the 4-wire kelvin bridge measurement system to the test coupon. The pads P ( 16 - 46 ) associated with TDCA trace pattern are attached to and terminate side A of the associated plated holes/vias. The trace pattern TDCA connects plated hole/via pairs together on side A (e.g.,  19  &amp;  20 ) which are then consecutively joined together to adjacent connected pairs (e.g.,  21  &amp;  22 ) with trace pattern TDCB ( 20  &amp;  21 ) on side B to form a daisy chain net. 
         FIG. 8  is a schematic view of trace patterns T 1 -T 4  and TDCB on a second side B of the laminated PCB test coupon substrate S (external or internal layer of a multilayer PCB test coupon) of  FIG. 6  that is opposite the first side A. The integrated circular pads P ( 1 - 46 ) of these trace patterns T are attached to and terminate side B of the associated plated holes/vias. The ends of trace patterns T 1 -T 4  terminate in 2 pads C of connector pattern  30  which is comprised of pads C 1 -C 20 . Each of the pads C in connector pattern  30  has a plated through hole associated with it that facilitates connection to the 4-wire kelvin bridge measurement system. The pads P ( 16 - 46 ) associated with TDCB trace pattern are attached to and terminate side B of the associated plated holes/vias. The trace pattern TDCB connects plated hole/via pairs together on side B (e.g.  22  &amp;  23 ) which are then consecutively joined together to adjacent connected pairs (e.g.  24  &amp;  25 ) with trace pattern TDCA (e.g.  23  &amp;  24 ) on side A to form a daisy chain test net. 
     
    
    
     DETAILED DESCRIPTION 
       FIGS. 1-2  show a printed circuit board (PCB) test coupon  100  or the internal layers of a multilayer PCB test coupon for use in electrical testing during thermal exposure. Thermal exposure includes cycling the test coupon  100  between extreme temperatures (e.g., extreme low temperature and extreme high temperature), for example by flowing a fluid (e.g., air) at different temperatures past the test coupon  100  and sensing the resistance of various components of the test coupon  100  through the connector pattern  30 . 
     The test coupon  100  can be made using the same process used to manufacture the final printed circuit boards which they represent and can have substrate S with plated holes/vias (blind, buried, stacked, through) H, multiple conductive layers, and trace patterns T (e.g., made of copper or other conductive material) that connect to either side of the plated holes/vias H using pads P. 
     With continued reference to  FIGS. 1-2 , the test coupon  100  has an array of sixteen plated holes/vias H ( 1 - 16 ) that extend either through the entire test coupon  100  or between internal layers of a multilayer test coupon from side A (a first side) to an opposite side B (a second side) of the test coupon or internal layers of a multilayer test coupon  100 . The test coupon  100  has a plurality of trace patterns T arranged to attach to multiple plated holes/vias H through pads P in trace patterns T from each side of the plated hole/via, as shown in  FIGS. 3-5 . The test coupon  100  has a connector pattern  30  with multiple plated through hole connection points C for electrical contacts, for example twenty connection points (e.g., C 1 , C 2 , C 3 , C 4 , C 5 , . . . C 18 , C 19 , C 20 ) to which the ends of the trace patterns T connect. 
     The trace patterns T viewed from side A of the substrate S differs from the trace patterns T viewed from the opposite side B of substrate S. Pads P in each trace pattern on side A connects to a different group of plated holes/vias H, and pads P on each trace pattern on side B connects to a different group of plated holes/vias H, each pad P with a plated hole/via H connecting to one of the trace patterns on side A and one of the trace patterns on side B of substrate S. 
     As shown in  FIG. 4 , on side B of the test coupon  100 , a trace pattern T 1  and its associated pads P ( 1 - 4 ) connect to one side of the associated plated holes/vias H ( 1 - 4 ) and terminates into connection points C 1  and C 2 , trace pattern T 2  and its pads P ( 5 - 8 ) connect to one side of the associated plated holes/vias H ( 5 - 8 ) and terminates into connection points C 5  and C 6 , trace pattern T 3  and its pads P ( 9 - 12 ) connect to one side of the associated plated holes/vias H ( 9 - 12 ) and terminates into connection points C 13  and C 14 , and trace pattern T 4  and its pads P ( 13 - 16 ) connects to one side of the associated plated holes/vias H ( 13 - 16 ) and terminates into connection points C 17  and C 18 . Trace pattern T 1 -T 4  and their pads P ( 1 - 16 ) connect one side of the plated holes/vias H ( 1 - 16 ) on side B via a substantially horizontal path (e.g., paths that extend generally parallel to an upper edge and/or a lower edge of the test coupon  100 ). 
     With reference to  FIG. 5 , on side A of the substrate S, a trace pattern T 5  and its pads P ( 1 ,  5 ,  9  and  13 ) connect to one side of the associated plated holes/vias H ( 1 ,  5 ,  9  and  13 ) and terminates into connection points C 3  and C 4 , trace pattern T 6  and its pads P ( 2 ,  6 ,  10  and  14 ) connect to one side of the associated plated holes/vias H ( 2 ,  6 ,  10  and  14 ) and terminates into connection points C 7  and C 8 , trace pattern T 7  and its pads P ( 3 ,  7 ,  11  and  15 ) connect to one side of the plated holes/vias H ( 3 ,  7 ,  11  and  15 ) and terminates into connection points C 15  and C 16 , and trace pattern T 8  and its pads P ( 4 ,  8 ,  12  and  16 ) connect to one side of the plated holes/vias H ( 4 ,  8 ,  12  and  16 ) and terminates into connection points C 19  and C 20 . Trace patterns T 5 -T 8  and their pads P ( 1 - 16 ) connect one side of the associated plated holes/vias H ( 1 - 16 ) on side A via a substantially angled path (e.g., paths that extend at a non-parallel and non-perpendicular angle to an upper edge and/or a lower edge of the test coupon  100 ). 
     Trace pattern T 9  does not connect to any tested plated holes/vias and terminates into connection points C 9 , C 10 , C 11  and C 12  and can be used for calculating the temperature of the side A surface of the substrate S. Copper conductors have a predictable resistance change with temperature, so trace T 9  allows the generation of a resistance to temperature correlation profile for surface A of substrate S by measuring the resistance of that circuit (e.g., trace T 9 ), at different temperatures during calibration or exposure of the test coupon  100  to thermal cycles to calculate the side A surface temperature of the substrate S associated with that circuit. 
     Trace pattern T 10  extends between trace pattern T 6  and trace pattern T 9 . The resistance of trace pattern T 10  can be measured by connecting the two wires (electrodes) of the kelvin bridge measurement system to connection points C 7  and C 8  (endpoints of trace pattern T 6 ) of the test coupon  100  and connecting the other two wires (electrodes) of the kelvin bridge measurement system to connection points C 9  and C 10  (endpoints of trace pattern T 9 ) of the test coupon  100 . The measurement of the trace pattern T 10  generally should always be the same at any specified temperature and can provide a reference resistance (e.g., for calibration) in a range similar to the resistance range of the plated hole/via to evaluate the consistency and drift of the resistance measurements made of the plated holes/vias using the kelvin bridge measurement system (e.g., drift of components in the meter/switching/interconnection systems). Such a reference resistance can be used to monitor and compensate for electrical drift in the kelvin bridge measurement system during electrical testing of the nets in test coupon  100  during thermal exposure. During thermal exposure of the test coupon  100 , as the holes or vias H in pads P may fracture or crack causing the measured resistance of the test net to increase at a specified temperature. Though the reference resistance of the trace pattern T 10  should remain the same at a specified temperature, one or more of the components of the kelvin bridge measurement system may be sensitive to temperature or other environmental changes, which may result in drift in the repeated resistance measurement of the trace pattern T 10  at a specified temperature. Advantageously, the measurement system can compensate for drift by using changes in the resistance measurement of the trace pattern T 10  at a specified temperature (e.g., as it applies to the resistance measurements of one of the plated holes/vias). 
     Optionally, one or more of the plated holes/vias H ( 1 - 16 ) placed in pads P ( 1 - 16 ) on the associated trace patterns can vary in one or more characteristics, such as via diameter, pad diameter, trace pattern width, grid size and connect sequence that mimics the PCB structure evaluated using test coupon  100 . In other embodiments, the plated holes/vias H ( 1 - 16 ) can vary by via type (e.g., through, blind, buried, stacked) and be internal or external to the substrate S. 
     The test coupon  100  provides a matrix for testing made up of  20  individual connection points C 1 -C 20  in the connector pattern  30  and allows for the individual resistance measurement of each of the plated holes/vias H ( 1 - 16 ) associated with pads P ( 1 - 16 ). In order to make a resistance measurement of one of the plated holes/vias H, 2-wires of the 4-wire kelvin bridge measurement system are attached to the endpoints associated with the trace pattern of side A connected to the hole H under test and the other 2-wires of the 4-wire kelvin bridge measurement system are attached to the endpoints associated with the trace pattern of side B connected to the hole H under test of the test net. For example, to measure the resistance of plated hole/via H 1  associated with pad P 1 , two electrodes of the kelvin bridge measurement system are connected to connection points C 1  and C 2  of the test coupon  100  to form one side of the kelvin bridge test circuit (e.g., via trace pattern T 1  on side B), and the other two electrodes of the kelvin bridge measurement system are connected to connection points C 3  and C 4  of the test coupon  100  to form the second side of the kelvin bridge test circuit (e.g., via trace pattern T 5  on side A). The connections of the electrodes of the kelvin bridge measurement system to these connection points C 1 , C 2 , C 3  and C 4  isolate the plated hole/via H 1  associated with pad P 1  from the rest of the plated holes/vias so that only the resistance of plated hole/via H 1  is measured. Since the plated hole/via H 1  associated with pad P 1  is the common hole between the pads of the two measurement patterns (e.g., defined in part by trace patterns T 1  and T 5 ) on either side of the plated hole/via H 1 , the resistance of plated hole/via H 1  is what is measured when the electrodes of the kelvin bridge measurement system are connected to connection points C 1 , C 2  and the other two electrodes of the kelvin bridge measurement system are connected to connection points C 3 , C 4  of the test coupon  100 . Advantageously, each plated hole/via net connects to two connection points C on the connector pattern  30  via a single trace pattern T on side A, and connects to two different connection points C on the connector pattern  30  via a single trace pattern T on side B of the test coupon  100 , as further described in below. 
     Table 1 below identifies the measurement scheme for the test coupon  100  shown in  FIGS. 1-5 . Specifically, Table 1 lists the connection points C in the connector pattern  30  of the test coupon  100  that connects two of the four wires (e.g., electrodes) of the 4-wire kelvin bridge measurement system to the side A trace patterns, and the connection points C in the connector pattern  30  that connects the other two wires (e.g., electrodes) of the 4-wire kelvin bridge measurement system to side B trace patterns, in order to test (e.g., measure the resistance of) each net consisting of a plated hole/via H, temperature net and the calibration/drift net. 
     
       
         
           
               
             
               
                 TABLE 1 
               
             
            
               
                   
               
               
                 Measurement Scheme for PCB Test Coupon 
               
            
           
           
               
               
               
               
            
               
                   
                   
                 Kelvin Bridge 
                 Kelvin Bridge 
               
               
                   
                   
                 Connection  
                 Connection  
               
               
                   
                   
                 Points to Trace 
                 Points to Trace 
               
               
                   
                 Test Net 
                 Pattern Side A 
                 Pattern Side B 
               
               
                   
                   
               
               
                   
                 Plated Hole/Via H1 
                 C1 &amp; C2 
                 C3 &amp; C4 
               
               
                   
                 Plated Hole/Via H2 
                 C1 &amp; C2 
                 C7 &amp; C8 
               
               
                   
                 Plated Hole/Via H3 
                 C1 &amp; C2 
                 C15 &amp; C16 
               
               
                   
                 Plated Hole/Via H4 
                 C1 &amp; C2 
                 C19 &amp; C20 
               
               
                   
                 Plated Hole/Via H5 
                 C5 &amp; C6 
                 C3 &amp; C4 
               
               
                   
                 Plated Hole/Via H6 
                 C5 &amp; C6 
                 C7 &amp; C8 
               
               
                   
                 Plated Hole/Via H7 
                 C5 &amp; C6 
                 C15 &amp; C16 
               
               
                   
                 Plated Hole/Via H8 
                 C5 &amp; C6 
                 C19 &amp; C20 
               
               
                   
                 Plated Hole/Via H9 
                 C13 &amp; C14 
                 C3 &amp; C4 
               
               
                   
                 Plated Hole/Via H10 
                 C13 &amp; C14 
                 C7 &amp; C8 
               
               
                   
                 Plated Hole/Via H11 
                 C13 &amp; C14 
                 C15 &amp; C16 
               
               
                   
                 Plated Hole/Via H12 
                 C13 &amp; C14 
                 C19 &amp; C20 
               
               
                   
                 Plated Hole/Via H13 
                 C17 &amp; C18 
                 C3 &amp; C4 
               
               
                   
                 Plated Hole/Via H14 
                 C17 &amp; C18 
                 C7 &amp; C8 
               
               
                   
                 Plated Hole/Via H15 
                 C17 &amp; C18 
                 C15 &amp; C16 
               
               
                   
                 Plated Hole/Via H16 
                 C17 &amp; C18 
                 C19 &amp; C20 
               
               
                   
                 Temperature 
                  C9 &amp; C10 
                 C11 &amp; C12 
               
               
                   
                 Calibration/Drift 
                  C9 &amp; C10 
                 C7 &amp; C8 
               
               
                   
                   
               
            
           
         
       
     
     In the test coupon  100  of  FIGS. 1-5 , each of the sixteen plated holes/vias connects to a separate test net. Two additional test nets are provided to measure surface temperature and calibration/drift (via trace patterns T 9  and T 10 ), as discussed above. Advantageously, the routing of the trace patterns T on the substrate S (e.g., trace patterns T 1 -T 4  on side A and trace patterns T 5 -T 8  on side B) allow each of the sixteen plated holes/vias and the two reference conductors (for surface temperature and calibration/drift) to be measured via only 20 connections points (i.e., C 1 -C 20 ). Therefore, the test coupon  100  increases the amount of electrical test net data available from thermal exposure of test coupon  100 , using a matrix of  20  nets that can be measured with only 20 connections that would normally test four daisy chain test nets if a matrix is not used. 
     In use, the connector pattern  30  of test coupon  100  is attached to a connection system of a thermal exposure and electrical testing machine (e.g., a Highly Accelerated Thermal Shock, or HATS, machine). The connection system in the machine interfaces with the connector pattern  30  of the test coupon  100 . The machine can have an matrix switch to facilitate the connection of four test pattern inputs associated with each test net matrixing connection points C from test coupon  100  to the four inputs of the kelvin bridge measurement system (e.g., connect two wires of the 4-wire kelvin bridge measurement system to two connection points C of test coupon  100  connected to a trace pattern on side A of the substrate S and connect two wires of the 4-wire kelvin bridge measurement system to two connection points C of test coupon  100  connected to a trace pattern on side B of the substrate S). Therefore, operation of the array switch will connect the four inputs of the 4-wire kelvin bridge measurement system to four connection points C (two connected with a trace pattern on side A and two connected to a trace pattern on side B of substrate S) to measure the resistance of one net containing a plated hole/via (e.g., one of the sixteen plated holes/vias). The test coupon  100  is subjected to thermal exposure in the machine to test the electrical reliability of the test coupon  100  (e.g., of each of the holes/vias). During thermal exposure and electrical testing, stresses on the holes or vias due to the thermal exposure, can cause one or more plated holes/vias to fracture or crack and resistance measurements to increase at a specified temperature. Therefore, by measuring resistance of each of the plated holes/vias at a specified temperature during thermal exposure allows for collection of data on the reliability of each of the plated holes/vias. 
       FIGS. 6-8  schematically illustrates a test coupon  100 ′. Some of the features of the test coupon  100 ′ are similar to features of the test coupon  100  in  FIGS. 1-5 . Thus, references numerals used to designate the various components of the test coupon  100 ′ are identical to those used for identifying the corresponding components of the test coupon  100  in  FIGS. 1-5 , except that a “′” has been added to the numerical identifier. Therefore, the structure and description for the various features of the test coupon  100  in  FIGS. 1-5  are understood to also apply to the corresponding features of the test coupon  100 ′ in  FIGS. 6-8 , except as described below. 
     The test coupon  100 ′ differs from the test coupon  100  in the shape of the trace pattern T 4 ′, and in that the test coupon  100 ′ includes a daisy chain DC of plated holes/vias interconnected by trace pattern TDCA and TDCB. Trace pattern TDCA is defined by trace segments T 12 , T 14 , T 16 , T 18 , T 20 , T 22 , T 24 , T 26 , T 28 , T 30 , T 32 , T 34 , T 36 , T 38  and T 40 , each trace segment connecting two plated holes on side A. Trace pattern TDCB is defined by trace segments T 11 , T 13 , T 15 , T 17 , T 19 , T 21 , T 23 , T 25 , T 27 , T 29 , T 31 , T 33 , T 35 , T 37  and T 39 , each trace segment connecting two plated holes on side B. The trace segments of trace pattern TDCA on side A alternate with trace segments of trace pattern TDCB on side B of test coupon  100 ′, creating a daisy chain of interconnected plated holes/vias. The end points of the daisy chain DC (pads P 16  and P 17 ) terminate into connection points C 17 , C 18 , C 19  and C 20  via trace patterns T 8  and T 4 . During thermal exposure (e.g., using a HATS machine), the electrical resistance of the daisy chain DC can be measured at a specified temperature to evaluate the combined electrical reliability of the thirty one holes in the daisy chain DC. 
     Table 2 below identifies the measurement scheme for the test coupon  100 ′ shown in  FIGS. 6-8 . Specifically, Table 2 lists the connection points C in the connector pattern  30  of test coupon  100 ′ that connect to two of the four wires (e.g., electrodes) of the 4-wire kelvin bridge measurement system to side A trace patterns, and the connection points C in the connector pattern  30  that connect the other two wires (e.g., electrodes) of the 4-wire kelvin bridge measurement system to side B trace patterns, in order to test (e.g., measure the resistance of) each net consisting of a plated holes/vias, daisy chain DC, temperature net and the calibration/drift net. 
     
       
         
           
               
             
               
                 TABLE 2 
               
             
            
               
                   
               
               
                 Measurement Scheme for PCB Test Coupon 
               
            
           
           
               
               
               
               
            
               
                   
                   
                 Kelvin Bridge 
                 Kelvin Bridge 
               
               
                   
                   
                 Connector 
                 Connector  
               
               
                   
                   
                 Pattern to Trace 
                 Pattern to Trace 
               
               
                   
                 Test Net 
                 Side A 
                 Side B 
               
               
                   
                   
               
               
                   
                 Plated Hole/Via H1 
                 C1 &amp; C2 
                 C3 &amp; C4 
               
               
                   
                 Plated Hole/Via H2 
                 C1 &amp; C2 
                 C7 &amp; C8 
               
               
                   
                 Plated Hole/Via H3 
                 C1 &amp; C2 
                 C15 &amp; C16 
               
               
                   
                 Plated Hole/Via H4 
                 C1 &amp; C2 
                 C19 &amp; C20 
               
               
                   
                 Plated Hole/Via H5 
                 C5 &amp; C6 
                 C3 &amp; C4 
               
               
                   
                 Plated Hole/Via H6 
                 C5 &amp; C6 
                 C7 &amp; C8 
               
               
                   
                 Plated Hole/Via H7 
                 C5 &amp; C6 
                 C15 &amp; C16 
               
               
                   
                 Plated Hole/Via H8 
                 C5 &amp; C6 
                 C19 &amp; C20 
               
               
                   
                 Plated Hole/Via H9 
                 C13 &amp; C14 
                 C3 &amp; C4 
               
               
                   
                 Plated Hole/Via H10 
                 C13 &amp; C14 
                 C7 &amp; C8 
               
               
                   
                 Plated Hole/Via H11 
                 C13 &amp; C14 
                 C15 &amp; C16 
               
               
                   
                 Plated Hole/Via H12 
                 C13 &amp; C14 
                 C19 &amp; C20 
               
               
                   
                 Plated Hole/Via H13 
                 C17 &amp; C18 
                 C3 &amp; C4 
               
               
                   
                 Plated Hole/Via H14 
                 C17 &amp; C18 
                 C7 &amp; C8 
               
               
                   
                 Plated Hole/Via H15 
                 C17 &amp; C18 
                 C15 &amp; C16 
               
               
                   
                 Daisy Chain (DC) 
                 C17 &amp; C18 
                 C19 &amp; C20 
               
               
                   
                 Temperature 
                  C9 &amp; C10 
                 C11 &amp; C12 
               
               
                   
                 Calibration/Drift 
                  C9 &amp; C10 
                 C7 &amp; C8 
               
               
                   
                   
               
            
           
         
       
     
     Therefore, the daisy chain DC of plated holes/vias in the test coupon  100 ′ provides a test net that replaces the test net associated with pad P 16  on substrate S, as shown in Table 1 above. Though the test coupon  100 ′ replaces a single plated hole/via test net (e.g., of hole/via H 16 ) with a daisy chain DC test net of thirty one plated holes/vias, one of skill in the art will recognize that the substrate S of test coupon  100 ′ can include additional daisy chain test nets of varying quantity and size of plated holes/vias, each of which can replace other single plated hole/via test nets in the trace pattern on the substrate S of test coupon  100 ′. Therefore, the test coupon  100 ′ can include any combination of daisy chain and single plated hole/via test nets. 
     Additional Embodiments 
     In embodiments of the present invention, a printed circuit board test coupon for electrical testing during thermal exposure, and method of making the same, may be in accordance with any of the following clauses:
         Clause 1. A printed circuit board test coupon for electrical testing during thermal exposure, comprising:
           a substrate having one or more conductive layers and extending between a first side and a second side opposite the first side;   a plurality of plated holes/vias formed within the substrate of the test coupon or extending through the entire substrate of the test coupon from a first surface on the first side A of the plated hole/via to a second surface on the second side B of the plated hole/via;   a first plurality of trace patterns defining a first pattern, each of the first plurality of trace patterns interconnecting a subset of the plurality of plated holes/vias on the first side of the plated holes/vias;   a second plurality of trace patterns defining a second pattern, each of the second plurality of trace patterns interconnecting a different subset of the plurality of holes/vias on the second side of the plated holes/vias, the second pattern being different than the first pattern; and   a connector pattern defined within or on the surface of the substrate, the first and second plurality of trace patterns connected to a plurality of connection points in the connector pattern,   wherein a measurement of resistance of each plated hole/via is provided by connecting 2 wires of a 4-wire kelvin bridge measurement system to the first side and connecting 2 wires of a 4-wire kelvin bridge measurement system to the second, side of the plated hole/via using connection points for one of the first plurality of trace patterns and one of the second plurality of trace patterns that connect to said plated hole/via.   
           Clause 2. The test coupon of clause 1, wherein the plurality of plated holes/vias consist of sixteen plated holes/vias formed within or completely through the substrate. Clause 3. The test coupon of any preceding clause, wherein the first plurality of trace patterns comprises four trace patterns, each connecting to the first side of four plated holes/vias.   Clause 4. The test coupon of clause 3, wherein each of the four trace patterns connects to the first side of four plated holes/vias along a row that extends generally parallel with an edge of the substrate, the four trace patterns defining a horizontal row trace pattern.   Clause 5. The test coupon of any preceding clause, wherein the second plurality of trace patterns comprises four trace patterns, each connecting to the second side of four plated holes/vias.   Clause 6. The test coupon of clause 5, wherein each of the four trace patterns connects to the second side of four plated holes/vias along a row that extends at a non-parallel angle relative to an edge of the substrate, the four trace patterns defining an angled row trace pattern.   Clause 7. The test coupon of any preceding clause, wherein one of the second plurality of trace patterns are configured to measure temperature and two of the second plurality of trace patterns are configured to measure calibration/drift by resistance measurements.   Clause 8. The test coupon of any preceding clause, further comprising a daisy chain of plated holes/vias in the substrate consecutively connected to connection points in the connector via one of the first plurality of trace patterns on the first side and one of the second plurality of trace patterns on the second side.   Clause 9. The test coupon of any preceding clause, wherein each of the plurality of plated holes/vias comprises one of a blind via, a buried via, a stacked via and a through hole.   Clause 10. A printed circuit board test coupon for electrical testing during thermal exposure, comprising:
           a substrate having one or more conductive layers and extending between a first side and a second side opposite the first side;   a plurality of plated holes/vias comprising sixteen plated holes/vias formed within the substrate of the test coupon or extending through the entire substrate of the test coupon from a first surface on the first side of the plated hole/via to a second surface on the second side of the plated hole/via;   a first plurality of trace patterns defining a first pattern comprising four trace patterns on the first side, each of the four trace patterns interconnecting four of the plated holes/vias on the first side of the plated holes/vias;   a second plurality of trace patterns defining a second pattern comprising four trace patterns on the second side, each of the four trace patterns interconnecting plated holes/vias on the second side of the plated hole/via, the second pattern being different than the first pattern and each of the sixteen plated holes/vias connecting to one of the trace patterns on the first side and one of the trace patterns on the second side of the via; and   a connector pattern defined in the substrate having twenty connection points, the first and second plurality of trace patterns connected to the twenty connection points in the connector pattern,   wherein a measurement of resistance of each of the sixteen plated holes/vias is provided by connecting 2 wires each of a 4-wire kelvin bridge measurement system to the first and second sides of the plated hole/via using connection points in the connector pattern for one of the first plurality of trace patterns and one of the second plurality of trace patterns that connect to said plated hole/via.   
           Clause 11. The test coupon of clause 10, wherein each of the four trace patterns on the first side connects four plated holes/vias along a row that extends generally parallel with an edge of the substrate, the four trace patterns defining a horizontal row pattern.   Clause 12. The test coupon of clause 10 or 11, wherein each of the four trace patterns on the second side connects four plated holes/vias along a row that extends at a non-parallel angle relative to an edge of the substrate, the four trace patterns defining an angled row pattern.   Clause 13. The test coupon of any of clauses 10-12, wherein one of the second plurality of trace patterns are configured to measure temperature and two of the second plurality of trace patterns are configured to measure calibration/drift by resistance measurements.   Clause 14. The test coupon of any of clauses 10-13, wherein each of the plurality of plated holes/vias comprises one of a blind via, a buried via, a stacked via and a through hole.   Clause 15. A method of making a printed circuit board test coupon for electrical testing during thermal exposure, comprising:
           forming a substrate having one or more conductive layers and extending between a first side and a second side opposite the first side;   forming a plurality of plated holes/vias within the substrate of the test coupon or such that they extend through the entire substrate of the test coupon from a first surface on the first side of the plated hole/via to a second surface on the second side of the plated hole/via;   forming a first plurality of trace patterns in a first pattern, each of the first plurality of trace patterns interconnecting a subset of the plurality of plated holes/vias on the first side of the substrate;   forming a second plurality of trace patterns in a second pattern different than the first pattern, each of the second plurality of trace patterns interconnecting a different subset of the plurality of vias on the second side of the plated hole/via; and   providing a connector pattern in the substrate having a plurality of connection points, the first and second plurality of trace patterns connected to the plurality of connection points,   wherein a resistance measurement of each plated hole/via is provided by connecting 2 wires of a 4-wire kelvin bridge measurement system to the first side and connecting 2 wires of a 4-wire kelvin bridge measurement system to the second side of the plated hole/via using connection points for one of the first plurality of trace patterns and one of the second plurality of trace patterns that connect to said plated hole/via.   
           Clause 16. The method of clause 15, wherein the plurality of plated holes/vias consist of sixteen plated holes/vias formed within the substrate of the coupon or extending through the entire substrate of the test coupon.   Clause 17. The method of clause 15 or 16, wherein the first plurality of trace patterns comprises four trace patterns, each connecting four plated holes/vias.   Clause 18. The method of any of clauses 15-17, wherein each of the four trace patterns connects four plated holes/vias along a row that extends generally parallel with an edge of the substrate, the first pattern defined by the four trace patterns being a horizontal row pattern.   Clause 19. The method of any of clauses 15-18, wherein the second plurality of trace patterns comprises four trace patterns, each connecting four plated holes/vias.   Clause 20. The method of any of clauses 15-19, wherein each of the four trace patterns connects four plated holes/vias along a row that extends at a non-parallel angle relative to an edge of the substrate, the second pattern defined by the four trace patterns being an angled row pattern.   Clause 21. The method of any of clauses 15-20, wherein one of the second plurality of trace patterns is configured to measure temperature and two of the second plurality of trace patterns are configured to measure calibration/drift by resistance measurement.   Clause 22. The method of any of clauses 15-21, further comprising forming a daisy chain of plated holes/vias in the substrate consecutively connected to connection points in the connector pattern by one of the first plurality of trace patterns and one of the second plurality of trace patterns.   Clause 23. The method of any of clauses 15-22, wherein forming each of the plurality of plated holes/vias comprises forming one of a blind via, a buried via, a stacked via and a through hole.       

     While certain embodiments of the inventions have been described, these embodiments have been presented by way of example only and are not intended to limit the scope of the disclosure. Indeed, the novel methods and systems described herein may be embodied in a variety of other forms. Furthermore, various omissions, substitutions and changes in the systems and methods described herein may be made without departing from the spirit of the disclosure. The accompanying claims and their equivalents are intended to cover such forms or modifications as would fall within the scope and spirit of the disclosure. Accordingly, the scope of the present inventions is defined only by reference to the appended claims. 
     Features, materials, characteristics, or groups described in conjunction with a particular aspect, embodiment, or example are to be understood to be applicable to any other aspect, embodiment or example described in this section or elsewhere in this specification unless incompatible therewith. All of the features disclosed in this specification (including any accompanying claims, abstract and drawings), and/or all of the steps of any method or process so disclosed, may be combined in any combination, except combinations where at least some of such features and/or steps are mutually exclusive. The protection is not restricted to the details of any foregoing embodiments. The protection extends to any novel one, or any novel combination, of the features disclosed in this specification (including any accompanying claims, abstract and drawings), or to any novel one, or any novel combination, of the steps of any method or process so disclosed. 
     Furthermore, certain features that are described in this disclosure in the context of separate implementations can also be implemented in combination in a single implementation. Conversely, various features that are described in the context of a single implementation can also be implemented in multiple implementations separately or in any suitable subcombination. Moreover, although features may be described above as acting in certain combinations, one or more features from a claimed combination can, in some cases, be excised from the combination, and the combination may be claimed as a subcombination or variation of a subcombination. 
     Moreover, while operations may be depicted in the drawings or described in the specification in a particular order, such operations need not be performed in the particular order shown or in sequential order, or that all operations be performed, to achieve desirable results. Other operations that are not depicted or described can be incorporated in the example methods and processes. For example, one or more additional operations can be performed before, after, simultaneously, or between any of the described operations. Further, the operations may be rearranged or reordered in other implementations. Those skilled in the art will appreciate that in some embodiments, the actual steps taken in the processes illustrated and/or disclosed may differ from those shown in the figures. Depending on the embodiment, certain of the steps described above may be removed, others may be added. Furthermore, the features and attributes of the specific embodiments disclosed above may be combined in different ways to form additional embodiments, all of which fall within the scope of the present disclosure. Also, the separation of various system components in the implementations described above should not be understood as requiring such separation in all implementations, and it should be understood that the described components and systems can generally be integrated together in a single product or packaged into multiple products. 
     For purposes of this disclosure, certain aspects, advantages, and novel features are described herein. Not necessarily all such advantages may be achieved in accordance with any particular embodiment. Thus, for example, those skilled in the art will recognize that the disclosure may be embodied or carried out in a manner that achieves one advantage or a group of advantages as taught herein without necessarily achieving other advantages as may be taught or suggested herein. 
     Conditional language, such as “can,” “could,” “might,” or “may,” unless specifically stated otherwise, or otherwise understood within the context as used, is generally intended to convey that certain embodiments include, while other embodiments do not include, certain features, elements, and/or steps. Thus, such conditional language is not generally intended to imply that features, elements, and/or steps are in any way required for one or more embodiments or that one or more embodiments necessarily include logic for deciding, with or without user input or prompting, whether these features, elements, and/or steps are included or are to be performed in any particular embodiment. 
     Conjunctive language such as the phrase “at least one of X, Y, and Z,” unless specifically stated otherwise, is otherwise understood with the context as used in general to convey that an item, term, etc. may be either X, Y, or Z. Thus, such conjunctive language is not generally intended to imply that certain embodiments require the presence of at least one of X, at least one of Y, and at least one of Z. 
     Language of degree used herein, such as the terms “approximately,” “about,” “generally,” and “substantially” as used herein represent a value, amount, or characteristic close to the stated value, amount, or characteristic that still performs a desired function or achieves a desired result. For example, the terms “approximately”, “about”, “generally,” and “substantially” may refer to an amount that is within less than 10% of the stated amount. As another example, in certain embodiments, the terms “generally parallel” and “substantially parallel” refer to a value, amount, or characteristic that departs from exactly parallel by less than or equal to 15 degrees. 
     The scope of the present disclosure is not intended to be limited by the specific disclosures of preferred embodiments in this section or elsewhere in this specification, and may be defined by claims as presented in this section or elsewhere in this specification or as presented in the future. The language of the claims is to be interpreted broadly based on the language employed in the claims and not limited to the examples described in the present specification or during the prosecution of the application, which examples are to be construed as non-exclusive. 
     Of course, the foregoing description is that of certain features, aspects and advantages of the present invention, to which various changes and modifications can be made without departing from the spirit and scope of the present invention. For example, though embodiments above describe a test coupon having a particular number of plated holes/vias, test nets, or particular number of connection points in the connector pattern, the invention is not limited to any particular number of plated holes/vias, test nets, any particular number of connection points in the connector pattern, nor any particular number of trace patterns or connector patterns. Additionally, the PCB test coupon described above is not limited to any particular size (e.g., vary from ½ inch by 1 inch to 1 inch by 2 inches, but may be smaller or larger). Moreover, the test coupon need not feature all of the objects, advantages, features and aspects discussed above. Thus, for example, those of skill in the art will recognize that the invention can be embodied or carried out in a manner that achieves or optimizes one advantage or a group of advantages as taught herein without necessarily achieving other objects or advantages as may be taught or suggested herein. In addition, while a number of variations of the invention have been shown and described in detail, other modifications and methods of use, which are within the scope of this invention, will be readily apparent to those of skill in the art based upon this disclosure. It is contemplated that various combinations or subcombinations of these specific features and aspects of embodiments may be made and still fall within the scope of the invention. Accordingly, it should be understood that various features and aspects of the disclosed embodiments can be combined with or substituted for one another in order to form varying modes of the discussed test coupon.