Abstract:
According to the present invention a pad arrangement is provided for applying reworks or engineering changes to an electronic circuits to be formed on the circuit board, the pad arrangement comprising a first signal pad for being electrically connected to a first signal line, a second signal pad for being electrically connected to a second signal line, a ground pad being connected to a ground line provided on the circuit board, and a voltage pad being connected to a supply voltage line provided on the circuit board, whereby the pads are arranged in proximity to each other for facilitating a placement of electronic devices between at least two of the pads.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention generally relates to a printed circuit board having at least one pad on which a surface mounted device can be mounted. Particularly, the present invention relates to a pad arrangement on a printed circuit board for applying reworks or engineering changes to an electronic circuits to be formed on said circuit board. 
     2. Description of the Related Art 
     The majority of electronic devices are now packaged with their electronic components mounted on printed circuit boards (PCB). PCBs have electrically conductive paths that are often connected to electrically conductive pads upon which surface mounted devices (SMDs) can be mounted. Typically, these SMDs are resistors or capacitors. However, SMDs can also be inductors, jumpers, i.e., a removable wire or small plug whose presence or absence is used to determine some aspect of hardware configuration, or other electrical components known to those of ordinary skill in the art. 
     Generally, the electrical element of an SMD is carried within a ceramic or plastic package having two conductive end portions that are connected internally to the electrical element and a nonconductive center portion. To permanently connect the SMD electrically to the PCB, its conductive end portions are soldered to appropriately spaced conductive pads provided on the surface of the PCB. 
     Problems can arise, whenever changes to the layout of the electronic circuit get necessary. May it because of reworks or engineering changes, e.g., due to design requirements or the desire to substitute one SMD for another, the size of an SMD might change. In order to avoid a potentially costly redesign of the PCB layout some arrangements could be made. 
     U.S. Pat. No. 5,303,122 by Clifford G. Adams, Jr. et. al., assigned to Ford Motor Company, filed May 19, 1993, issued Apr. 12, 1994, “Printed circuit board having a commonized mounting pad which different sized surface mounted devices can be mounted”, discloses a printed circuit board having a unique commonized pad upon which different sized surface mounted devices can be mounted. The preferred form of the commonized pad comprises two electrically conductive pads, each having a tapered portion that electrically connects a narrow portion to a wide portion. The present invention is advantageous over known pads in that it (1) allows for using different sizes of surface mounted devices without redesign and (2) it does not require wasting what might otherwise be antiquated inventory or continuation of an old process to use up what would be antiquated inventory. 
     In some cases it is more than a question of the size of a SMD. Additional, wires might be required to be connected to the PCB or an additional electrical connection from one surface of the PCB to the other surface may be needed. For this case simple pad arrangements were introduced that would only be used in case of a rework or an engineering change. 
     Such a pad arrangement  100  is depicted in FIG.  1 . As it can be seen from FIG. 1, there is a first conductive pad  102  and a second conductive pad  104 . The first conductive pad  102  is formed by a basically rectangularly shaped central portion  106  having truncated corners and a rectangularly shaped extension  108  being provided for receiving a via  110  connecting the first conductive pad  102  to a respective pad (not shown) on the opposite surface of a PCB (not shown). The second conductive pad  104  comprises tree basically rectangularly shaped conductive portions  120 ,  122  and  124  being connected to each other by two trapezoidal conductive portions  126  and  128 , all together forming basically an annulus running approximately half around the first conductive pad  102 . The second conductive pad  104  further comprises in one of the trapezoidal conductive portions  128  a via  130  connecting the second conductive pad to a ground line (not shown) provided in the PCB. 
     OBJECT OF THE INVENTION 
     Starting from this, the object of the present invention is to provide a pad arrangement on a printed circuit board that allows flexibly applying reworks or engineering changes to an electronic circuits to be formed on said circuit board. 
     BRIEF SUMMARY OF THE INVENTION 
     The foregoing object is achieved by a pad arrangement as laid out in the independent claims. Further advantageous embodiments of the present invention are described in the sub claims and are taught in the following description. 
     According to the present invention a pad arrangement is provided for applying reworks or engineering changes to an electronic circuits to be formed on the circuit board, the pad arrangement comprising a first signal pad for being electrically connected to a first signal line, a second signal pad for being electrically connected to a second signal line, a ground pad being connected to a ground line provided on the circuit board, and a voltage pad being connected to a supply voltage line provided on the circuit board, whereby the pads are arranged in proximity to each other for facilitating a placement of electronic devices between at least two of the pads. 
     In a preferred embodiment according to the present invention the group of two signal pads, a ground pad and a voltage pad is substantially being arranged in the edges of a square, whereby in a refinement the ground pad and the voltage pad are arranged next to each other. 
     In order to reduce the inductance on the PCB the connection between the ground pad to the ground line and between the voltage pad to the voltage line are formed basically on the edge of the respective pad which is closest to the respective other of both pads. 
     According to another embodiment of the present invention the ground pad extends on the respective surface of the circuit board basically around the adjacent signal pad forming further areas for soldering electrical elements onto. In a refinement the ground pad even extends further to the opposite side of the second signal pad, providing an additional shielding from electrical interferences and more possibilities for connecting surface mounted devices (SMD) and electrical wires, such as coaxial cable, to the pads and in particular to the ground pad. A coaxial cable is a kind of cable having a solid central conductor surrounded by an insulator, in turn surrounded by a cylindrical shield woven from fine wires. It is used to carry high frequency signals, in the area of radio frequencies. The shield is usually connected to electrical ground to reduce electrical interference. In the present case the shield may be connected to a portion of the ground pad. 
     In order to provide further flexibility and to allow a connection from one surface of the PCB to the other, the pad arrangement further comprises in another embodiment a third signal pad being electrically connected to the first signal pad, a fourth signal pad being electrically connected to the second signal pad, another ground pad being connected to a ground line provided on the circuit board, and another voltage pad being connected to a supply voltage line provided on the circuit board, whereby the pads are placed on the opposite surface of the PCB and are again arranged in proximity to each other for facilitating a placement of electronic devices between at least two of the pads. To improve crosstalk behavior, the connection between the first signal pad to the third signal pad and between the second signal pad to the fourth signal pad are formed basically on the edge of the respective pad which is furthest away from the respective other of both pads. 
     The pad arrangement in accordance with the present invention is placed as prevention of costly and time consuming modifications of cards or PCBs because of potential reworks and engineering changes. It is advantageously applied in order to get connectivity between the front and back side of double side mounted cards and PCBs. In addition, with embedded wires placed in an intermediate layer of the PCB, the pad arrangement according to the present invention may be used, to tunnel blockages or get in to closed areas. Hence, with the design as taught by the present invention one has an tremendous improvement and advantages compared to the known designs. 
     In addition to the traditional pads, the novel pad arrangement according to the present invention offers particularly the following advantages. The combination of the four pads to a quadrant, with one supply voltage (VCC), one ground (GND) and two signal connections provides the capability and variations to place 0603-SMT (Surface Mounted Technology) components manually or 0805-SMT components automatically on such pads. The variation of placing SMT components within these quadrants can be done in X-direction or Y-direction, to get the functional engineering change (EC) and rework (REA) capability of a serial resistor EC/REA (+2 EC-wire), serial capacitor EC/REA (+2 EC-wire), tie up resistor (+EC-wire), tie down resistor (+EC-wire), capacitor+2 EC-Wires (Highpass), capacitor+1 EC-Wire (Lowpass) or a bypass caps (+up to 2 EC-Wires). The usage of these different EC/REA capabilities can even be combined, except the one with the Serials and the Highpass functions. 
     The design with all these capabilities is anyhow very small (210 mil×300 mil), so that with one or more of pad arrangements according to the present invention the hazard of card redesigns or scrapping defect cards during manufacturing is much lower. Smaller ground pads adjacent to the signal pads may be used for coaxial cable repair to solder down the respective shield. 
     The supply voltage and the ground pad via is moved very close to each other and nearby the SMT component contact area, to reduce the overall lead inductance. This will make the pad arrangement according to the present invention also very robust for high frequency design ECs (Engineering Changes) and REAs (Reworks). 
     Since two signal pads are coming along the novel design (and this very close to each other and very homogenous), one now has the capability to EC/REA also differential pairs without any problems. In short, particularly the following repair scenarios may be possible, Single Net EC/Repair (Front to Rear), Differential Pair EC/Repair (Front to Rear), Resistor EC/Repair, tie-up Resistor+EC-wire, tie-down Resistor+EC-wire, Serial Resistor+2 EC-Wire, Capacitor EC/Repair, Bypass, Capacitor+2 EC-Wires (Highpass), Capacitor+1 EC-Wire (Lowpass) Further a combined usage may be possible within very dense dimensions only 210 mil times 300 mil, furthermore, overlay capability during multiple usage combined usage of the different advantages and test point access capability. 
    
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
     The above, as well as additional objectives, features and advantages of the present invention, will be apparent in the following detailed written description. 
     The novel features of the invention are set forth in the appended claims. The invention itself, however, as well as a preferred mode of use, further objectives, and advantages thereof, will best be understood by reference to the following detailed description of an illustrative embodiment when read in conjunction with the accompanying drawings, wherein: 
     FIG. 1 shows a pad arrangement according to the prior art; 
     FIG. 2 shows a low inductance multiple resistor EC capacitor pad arrangement in accordance with the present invention; 
     FIG. 3 shows a usecase for employing the pad arrangement of FIG. 2 on a PCB; 
     FIG. 4 shows the pad arrangement of FIG. 2 with an engineering change/rework with a coaxial cable and a low inductance bypass capacitor in accordance with the present invention; 
     FIG. 5 shows the pad arrangement of FIG. 2 with a tie up/tie down engineering change/rework circuitry with two coaxial cables in accordance with the present invention; 
     FIG. 6 shows a first example of placement overlapping of two pad arrangements of FIG. 2 in accordance with the present invention; and 
     FIG. 7 shows a second example of placement overlapping of two pad arrangements of FIG. 2 in accordance with the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     With reference now to FIG. 2, there is depicted a low inductance multiple resistor engineering change/rework capacitor pad arrangement  200  in accordance with the present invention. The drawings is not in scale and the written dimensions are all in mils ({fraction (1/1000)} inch). However, it is acknowledged that these dimensions are only by way of example and may be varied in accordance to design constraints, such as the size of SMT components. 
     The pad arrangement  200  comprises a first signal pad  202 , a second signal pad  204 , a ground pad  206  and a voltage pad  208 . All pads  202 ,  204 ,  206  and  208  are made from conductive material. The area between the pads  202 ,  204 ,  206  and  208  is non conductive, so that the pads  202 ,  204 ,  206  and  208  are electrically separated from each other. 
     The first signal pad  202  and the second signal pad  204  are both formed by a basically rectangularly shaped central portion  212  and  214 . Both show a basically trapezoid extension  216  and  218 . In each of the basically trapezoid extensions  216  and  218  a via  220  and  222  is arranged connecting the first signal pad  202  to a respective pad (not shown) on the opposite surface of a PCB (not shown) and the second signal pad  204  to a respective pad (not shown) on the opposite surface of the PCB (not shown), whereby the vias  220  and  222  are formed basically on the edge of the respective pad  202  and  204  which is furthest away from the respective other of both pads  202  and  204 . 
     The voltage pad  208  is formed by a basically square shaped main portion  230  with a basically square shaped extension  232  in one of its corners. In the basically square shaped extension  232  a via  234  is arranged connecting the voltage pad  208  to a supply voltage line (not shown) provided on the PCB. 
     Like the voltage pad  208 , the ground pad  206  includes as well a basically square shaped main portion  240  with a basically square shaped extension  242  in one of its corners. In the basically square shaped extension  242  a via  244  is arranged connecting the ground pad  206  to a ground line (not shown) provided on the PCB. However the ground voltage pad  208  further comprises four basically rectangularly shaped conductive pads  246 ,  248 ,  250  and  252  which are electrically connected to one another by intermediate conductive portions  254 ,  256 ,  258  and  260 . Both the conductive pads  246 ,  248 ,  250  and  252  together with the intermediate conductive portions  254 ,  256 ,  258  and  260  form a integral area of conductive material running from close to the voltage pad  206  around the neighboring second signal pad  204  and basically half way around the first signal pad  202 . One more via  262  is provided in the intermediate portion  258 . 
     Now with reference to FIG. 3, there is shown a usecase for employing the pad arrangement of FIG. 2 on a printed circuit board (PCB). A PCB  300  comprises a first surface  302  and a second surface  304 . On the first surface  302  there is mounted an integrated circuit  306  having a plurality of pins  308  extending through respective openings  310  provided in the PCB  300 . The integrated circuit  306  is abuts on a support device  312  sitting on to of the first surface  302  of the PCB  300 . On the second surface  304  the tips of the pins  308  extending through the openings  310  provided in the PCB are covered by a housing  314 . Further along the PCB on its second surface  304  another electronic device  316  is provided having four pins  318  extending through respective holes  320  formed in the printed circuit board  300 . 
     A first pad arrangement  330  in accordance to the present invention is placed on the first surface  302  of the PCB  300  basically in the middle of the integrated circuit  306  and the pins  318  of the electronic device  316  extending through the PCB. A second pad arrangement  332  is placed inside the housing  314  on the second surface  304  of the PCB  300 . Since the pad arrangements  330  and  332  are shown in a cross section according to the line III—III of FIG. 2, only two portions of the ground pad  334  and  336  and one signal pad  338  and  349  can be seen from each pad arrangement  330  and  332 . The signal pads  338  and  340  from both pad arrangements  330  and  332  are electrically connected to each other by an embedded wire  342 . The ground pad, however, is connected to a respective ground line (not shown) provided on the PCB. 
     As an example of an engineering change or rework a first coaxial cable  344  and a second coaxial cable  346  are employed. Both coaxial cables  344  and  346  have a solid central conductor  348  and  350  surrounded by an insulator, in turn surrounded by a cylindrical shield  352  and  354  woven from fine wires. The cylindrical shield  352  of the first coaxial cable  344  is on one side electrically connected to one of the pins  318  of the electronic device  316 . The other side of the cylindrical shield  352  of the first coaxial cable  344  is electrically connected to a portion of the ground pad  334  of the first pad arrangement  330 . The central conductor  348  of the first coaxial cable  344  is on one side electrically connected to another of the pins  318  of the electronic device  316 . The other side of the central conductor  348  of the first coaxial cable  344  is electrically connected to a portion of the signal pad  338  of the first pad arrangement  330 . 
     Correspondingly, the cylindrical shield  354  of the second coaxial cable  346  is on one side electrically connected to one of the pins  308  of the integrated circuit  306 . The other side of the cylindrical shield  354  of the second coaxial cable  346  is electrically connected to a portion of the ground pad  336  of the second pad arrangement  332 . The central conductor  350  of the second coaxial cable  346  is on one side electrically connected to another of the pins  308  of the integrated circuit  306 . The other side of the central conductor  350  of the second coaxial cable  346  is electrically connected to a portion of the signal pad  340  of the second pad arrangement  332 . By means of the embedded wire  342  the central conductor  348  of the first coaxial cable  344  is electrically connected to the central conductor  350  of the second coaxial cable  346 , thus, providing a electrical connection from the first surface  302  of the PCB  300  to its second surface  304 . In addition to the first coaxial cable  344  a SMT component  356  is electrically connected to the ground pad  334  and the signal pad  338  of the first pad arrangement  330 . 
     FIG. 4 shows a pad arrangement  400 . the pad arrangement  400  comprises a first signal pad  402 , a second signal pad  404 , a ground pad  406  and a voltage pad  408  as explained in greater detail with reference to FIG.  2 . To the pad arrangement  400  an engineering change/rework has been applied comprising a first coaxial cable  410  and a second coaxial cable  412  as well as a low inductance bypass capacitor  414 . The low inductance bypass capacitor  414  is electrically connected between the voltage pad  408  and the ground pad  406 . 
     Both coaxial cables  410  and  412  have a solid central conductor  416  and  418  surrounded by an insulator, in turn surrounded by a cylindrical shield  420  and  422  woven from fine wires. The cylindrical shield  420  of the first coaxial cable  410  is electrically connected to a portion of the ground pad  406 . The central conductor  416  of the first coaxial cable  410  is electrically connected to the first signal pad  402 . The cylindrical shield  422  of the second coaxial cable  412  is as well electrically connected to a portion of the ground pad  406 . The central conductor  418  of the second coaxial cable  412 , however, is electrically connected to the second signal pad  404 . 
     With reference to FIG. 5, there is depicted the pad arrangement  500  comprises a first signal pad  502 , a second signal pad  504 , a ground pad  506  and a voltage pad  508  as explained in greater detail with reference to FIG.  2 . To the pad arrangement  500  an engineering change/rework has been applied comprising a first coaxial cable  510  and a second coaxial cable  512  as well as a a tie up/tie down engineering change/rework circuitry consisting of a first resistor  514  and a second resistor  515 . The first resistor  514  is electrically connected between the voltage pad  508  and the first signal pad  502 , whereas the second resistor  515  is electrically connected between the ground pad  506  and the second signal pad  504 . 
     Both coaxial cables  510  and  512  have a solid central conductor  516  and  518  surrounded by an insulator, in turn surrounded by a cylindrical shield  520  and  522  woven from fine wires. The cylindrical shield  520  of the first coaxial cable  510  is electrically connected to a portion of the ground pad  506 . The central conductor  516  of the first coaxial cable  510  is electrically connected to the first signal pad  502 . The cylindrical shield  522  of the second coaxial cable  512  is as well electrically connected to a portion of the ground pad  506 . The central conductor  518  of the second coaxial cable  512 , however, is electrically connected to the second signal pad  504 . 
     In FIG. 6 a first example of placement overlapping of two pad arrangements as explained in further detail with reference to FIG. 2 is shown in accordance with the present invention. There is depicted a complex pad arrangement  600  basically consisting of a first pad arrangement and a second pad arrangement that is basically a mirrored first pad arrangement. The complex pad arrangement  600  comprises a first signal pad  602 , a second signal pad  604 , a third signal pad  606  and a fourth signal pad  608 . Furthermore it comprises a first voltage pad  610  and a second voltage pad  612  and a ground pad  614 . According to the embodiment shown in FIG. 6 the portion of the ground pad running next to the first and second signal pad  602  and  604  is shared. 
     Finally with reference to FIG. 7, there is depicted a second example of placement overlapping of two pad arrangements as explained in further detail with reference to FIG. 2 is shown in accordance with the present invention. There is depicted a complex pad arrangement  700  basically consisting of a first pad arrangement and a second pad arrangement that is basically a mirrored first pad arrangement. The complex pad arrangement  700  comprises a first signal pad  702 , a second signal pad  704 , a third signal pad  706  and a fourth signal pad  708 . Furthermore it comprises a first voltage pad  710  and a second voltage pad  712  and a ground pad  714 . According to the embodiment shown in FIG. 7 the portion of the ground pad running between the second and the third signal pad  704  and  706  is shared.