Abstract:
A removable, permanent or reconfigurable debug probing device for use in debug probing of a printed circuit assembly, the printed circuit assembly having at least one through via, the debug probing device comprising at least one leader thread configured to be threaded through the at least one through via. Using the probing device comprises inserting a selected one of the at least one leader threads through a selected one of the at least one through via to thereby probe a surface of the printed circuit assembly; and responsive to detecting a defect in the selected through via, using a flexible circuit connected to the selected leader thread to repair the detected defect.

Description:
FIELD OF INVENTION 
     The present disclosure relates to a debug probing device for probing a printed circuit assembly. More particularly the present invention relates to a debug probing device and technique for repair or rework of a defective printed circuit assembly. 
     BACKGROUND 
     Printed circuit assemblies are used for numerous electronic applications these days. The printed circuit assemblies contain a number of interconnections, wirings, through vias and plated through holes extending from external connections on one surface of the printed circuit assembly to wirings and connections on the opposite surface. 
     Often the printed circuit assemblies—because of incorrect design or manufacturing errors—end up carrying defects. The defects in the printed circuit assemblies may include undesirable short circuits, open circuits or misrouted traces. Even a single defect in one of a plurality of connections on the printed circuit assemblies can lead to the entire assembly being deemed defective. The defect in the connection and associated through vias must be either repaired or completely replaced else the defective printed circuit assemblies will have to be discarded. 
     There are a few known solutions for repairing of defective printed circuit assemblies. One solution being deep deletion of the defective through via. The deleted through via is then filled with an epoxy solution and then a new through via is drilled and the connections are soldered again. 
     The problem with such a solution is that additional wires need to be added to the printed circuit assembly. Also, since the printed circuit assembly has a plurality of interconnections; it becomes difficult to solder new interconnections in such a tight package of the plurality of interconnections. Such soldering may also lead to short circuits and some other additional defects in the printed circuit assemblies. 
     Also, in cases of Land Grid Array (LGA) applications, multiple interposers are used in a stacked structure to repair a defective LGA. The stacked structure may lead to overcrowding of interconnections in the LGA and may lead to short circuits within the LGA. 
     Therefore, in light of above discussion there is a need of a solution that facilitates easy and safe rework of defective printed circuit assemblies. 
     BRIEF SUMMARY 
     The present invention provides a flexible circuit useful for printed circuit assembly repair. The present invention overcomes the disadvantages in conventional systems for repair of printed circuit assemblies. The present invention entails better yield advantages. 
     Another object of the present invention is to provide a flexible circuit that enables improved reliability and reduced costs of printed circuit assembly rework. 
     Another object of the present invention is to provide a flexible circuit that provides insertion of additional function or design options during reworking of defects in printed circuit assemblies. 
     Another object of the invention is to provide additional wiring capability in localized high wiring density area of a printed circuit assembly, without adding additional wiring layers across entire printed circuit assembly board. 
     Another object of the present invention is to provide an improved probing technique using the flexible circuit of the present invention. 
     Yet another object of the present invention is to enable a flexible circuit to make interconnections in an LGA without adding an extra interposer. 
     In one embodiment of the invention, a rework device in form of a flexible circuit is provided in order to repair a printed circuit assembly, the printed circuit assembly including one or more through vias and interconnections. The flexible circuit is adapted to be threaded through a through via. The flexible circuit is a planar structure that includes a lead in portion and a tail portion. The flexible circuit also includes a first distended head portion proximal to the lead in portion and a second distended head portion proximal to the tail portion. Further, the second distended head portion is connected to the first distended head portion through a coverlay portion which may be of varying lengths, such that the length of the coverlay portion is substantially equal to a depth of the through via. 
     In an another embodiment of the invention, a rework device in form of a flexible circuit is provided in order to repair a printed circuit assembly, the printed circuit assembly including one or more through vias and interconnections. The flexible circuit is adapted to be threaded through a through via. The flexible circuit includes a lead in portion and a tail portion. The flexible circuit also includes a first distended head portion proximal to the lead in portion and a second distended head portion proximal to the tail portion. Further, the second distended head portion is connected to the first distended head portion through a coverlay portion, such that the coverlay portion includes a looped segment. In embodiments of the invention, the looped segment can be a coiled segment or a helical segment as per the different use the flexible circuit needs to be put to. 
     In yet another embodiment of the invention, a rework device in form of a flexible circuit is provided in order to repair a printed circuit assembly, the printed circuit assembly including one or more through vias and interconnections. The flexible circuit is adapted to be threaded through a through via. The flexible circuit is structured to include multiple branches. The multiple branches in the flexible circuit is configured to be threaded through a through via. The flexible circuit includes a plurality of lead in portions and a plurality of tail portions. Further, the flexible circuit also includes a plurality of first distended head portions corresponding to the plurality of lead in portions and a plurality of second distended head portions corresponding to the plurality of tail portions. The plurality of second distended head portions and the plurality of first distended head portions being through corresponding coverlay portions. 
     In yet another embodiment of the present invention, a debug probing device is provided. The debug probing device being configured to debug probing of a defective printed circuit assembly. The debug probing includes at least one leader thread connected to the debug probing device. Further, each of the at least one leader threads may have a connected rework device that can be threaded into through vias of a defective printed circuit assembly. 
     These and other exemplary embodiments, features and advantages of the present invention will be described or become apparent from the following detailed description of exemplary embodiments, which is to be read in connection with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
       The features of the present invention, which are believed to be novel, are set forth with particularity in the appended claims. The invention may best be understood by reference to the following description, taken in conjunction with the accompanying drawings. These drawings and the associated description are provided to illustrate some embodiments of the invention, and not to limit the scope of the invention. 
         FIG. 1  is a top view of a portion of an exemplary printed circuit assembly with two circular contact pads, in accordance with an embodiment of the present invention; 
         FIG. 2  is a side view of the printed circuit assembly of  FIG. 1  illustrating through vias and conductive layers, in accordance with an embodiment of the present invention; 
         FIG. 3  is a diagrammatic illustration of an exemplary flexible circuit, in accordance with an embodiment of the present invention; 
         FIG. 4  is a diagrammatic illustration of an exemplary flexible circuit having a plurality of branches, in accordance with an embodiment of the present invention; 
         FIG. 5  is a diagrammatic illustration of another exemplary flexible circuit having a plurality of branches, in accordance with another embodiment of the present invention; 
         FIG. 6  is a diagrammatic illustration of exemplary flexible circuits having different coverlay portions, in accordance with another embodiment of the present invention; 
         FIGS. 7 a , 7 b  and 7 c    are diagrammatic illustration of exemplary stages of repairing of a printed circuit assembly using an exemplary flexible circuit, in accordance with an embodiment of the present invention; 
         FIG. 8  is a diagrammatic illustration of an exemplary debug probing device used for probing a printed circuit assembly, in accordance with an embodiment of the present invention; and 
         FIG. 9  is a diagrammatic illustration of a stage of using a debug probing device for probing a printed circuit assembly, in accordance with another embodiment of the present invention. 
     
    
    
     Those with ordinary skill in the art will appreciate that the elements in the figures are illustrated for simplicity and clarity and are not necessarily drawn to scale. For example, the dimensions of some of the elements in the figures may be exaggerated, relative to other elements, in order to improve the understanding of the present invention. 
     There may be additional structures described in the foregoing application that are not depicted on one of the described drawings. In the event such a structure is described, but not depicted in a drawing, the absence of such a drawing should not be considered as an omission of such design from the specification. 
     DETAILED DESCRIPTION 
     Before describing the present invention in detail, it should be observed that the present invention utilizes a combination of method steps and apparatus components related to a rework device for repairing printed circuit assemblies. Accordingly the apparatus components and the method steps have been represented where appropriate by conventional symbols in the drawings, showing only specific details that are pertinent for an understanding of the present invention so as not to obscure the disclosure with details that will be readily apparent to those with ordinary skill in the art having the benefit of the description herein. 
     While the specification concludes with the claims defining the features of the invention that are regarded as novel, it is believed that the invention will be better understood from a consideration of the following description in conjunction with the drawings, in which like reference numerals are carried forward. 
     As required, detailed embodiments of the present invention are disclosed herein; however, it is to be understood that the disclosed embodiments are merely exemplary of the invention, which can be embodied in various forms. Therefore, specific structural and functional details disclosed herein are not to be interpreted as limiting, but merely as a basis for the claims and as a representative basis for teaching one skilled in the art to variously employ the present invention in virtually any appropriately detailed structure. Further, the terms and phrases used herein are not intended to be limiting but rather to provide an understandable description of the invention. 
     The terms “a” or “an”, as used herein, are defined as one or more than one. The term “another”, as used herein, is defined as at least a second or more. The terms “including” and/or “having” as used herein, are defined as comprising (i.e. open transition). The term “coupled” or “operatively coupled” as used herein, is defined as connected, although not necessarily directly, and not necessarily mechanically. 
     The present invention, according to preferred embodiments, provides rework device in form of a flexible circuit for repair or rework of a printed circuit assembly including of one or more through vias, such that the flexible circuit is configured to be threaded through a through via of the printed circuit assembly. 
     Referring to  FIG. 1 , there is shown a top view of a portion of an exemplary printed circuit assembly  100 , including but not limited to a prism to prism flex board, having two contact pads  102  and  104 . The two contact pads  102  and  104  are electrically coupled to two through via  106  and  108  respectively. A “through via” is a plated hole that connects one or more conductive pathways on one surface of a circuit board, for example the printed circuit assembly  100 , to conductive pathways on opposite surfaces of the circuit board, for example the printed circuit assembly  100 . 
     Moving on, in  FIG. 2 , there is illustrated a side view of the printed circuit assembly  100 . Each of the through vias  106  and  108  is shown to connect an upper surface  202  of the printed circuit assembly  100  and travel down to a bottom surface  204  of the printed circuit assembly  100 . Further, each of the through vias  106  and  108  are illustrated as a hole that is drilled or formed in the printed circuit assembly  100 . The through vias  106  and  108  provide electrical communication between different elements placed on the printed circuit assembly. 
     Referring to  FIG. 3 , there is illustrated an exemplary rework device in the form of a flexible circuit  300 , that can be used in rework or repair of a printed circuit assembly, for example the printed circuit assembly  100 . The flexible circuit  300  has a planar structure and includes a lead in portion  302 , a tail portion  304 , a first distended head portion  306 , a second distended head portion  308 , a coverlay portion  310  and an alignment portion  312 . In an embodiment, the lead in portion  302  and the tail portion  304  can be a detachable portion. 
     Coverlay portion  310  is a solid, tack-free film comprising one or more layers, each of which is formed from one or more flexible dielectric adhesive polymers. As used herein, “polymer” includes homopolymers, co-polymers, terpolymers, etc. Preferably, coverlay  310  comprises a single layer formed from one dielectric adhesive polymer or from a mixture of dielectric adhesive polymers. 
     Further it should be appreciated that in the figure, though the flexible circuit is shown to include only one lead in portion  302 , one tail portion  304 , one first distended head portion  306 , one second distended head portion  308 , one coverlay portion  310  and one alignment portion  312 , other embodiments of the present invention may include more than one such elements, without deviating from the scope of the present invention. 
     Through the lead in portion  302 , a leader thread can enter and guide the flexible circuit  300  to be threaded through a through via. The flexible circuit  300  enters a through via from the lead in portion  302 . The first distended head portion  306  is proximal to the lead in portion  302  and the second distended head portion  308  is proximal to the tail portion  304 . While reworking, the first distended head portion  306  emerges from a bottom surface of a through via through which the flexible circuit  300  has been threaded. The second distended head portion  308  remains on the opposite surface of the through via when the flexible circuit  300  has been threaded. The first distended head portion  306  and the second distended head portion  308  are so structured that they substantially conform to the two holes on the two surfaces of the through via. In an embodiment, the first distended head portion  306  and the second distended head portion  308  may be laminated with, but not limited to, a pressure sensitive adhesive, an epoxy, a thermoset or any other adhesive to respective surfaces of the printed circuit assembly  100 . 
     Moving on, the first distended head portion  306  and the second distended head portion  308  of the flexible circuit  300  are connected using the coverlay portion  310 . The arrangement of the first distended head portion  306 , the second distended head portion  308  and the coverlay portion  310  is such that after reworking there is no perceptible protrusion in the printed circuit assembly. This is ensured by keeping the length of the coverlay portion  310  substantially equal to that of a depth of the through via. This reworking process will be explained in more detail in association with  FIGS. 7 a , 7 b    and  7   c.    
     The flexible circuit  300  also includes the alignment portion  312  that is configured to properly align the flexible circuit  300  on the printed circuit assembly during repair or rework process. 
     In an embodiment, the breadth of the coverlay portion  310  may be at least 0.03 inches, preferably they may vary from 0.04 inches to 0.08 inches. However, it should be appreciated that in other embodiments of the invention, other dimensions as suitable for reworking of the printed circuit assembly can also be used without deviating from the scope of the invention. 
     Similarly, dimensions of the first distended head portion  306  and the second distended head portion  308  may be at least 0.01 inches, preferably they may range from 0.02 inches to 0.03 inches. In embodiments, the first distended head portion  306  and the second distended head portion  308  may be in the form of a spherical or circular bulbous structure having a diameter in the mentioned range. However, it should be appreciated that in other embodiments of the invention, other dimensions as suitable for reworking of the printed circuit assembly can also be used without deviating from the scope of the invention. 
     In other embodiments, the flexible circuit  300  may be substantially completely or at least partially covered with a suitable kapton or some similar purpose material layer running from the lead in portion  302  up to the tail portion  304 . In an embodiment, the kapton layer may be made up of, but not limited to a nickel based material, a ferrous based material and a gold based material. In an embodiment, the kapton layer may be a solder leveling or any other metal based platings. 
     Moving on, in  FIG. 4  there is depicted a flexible circuit  400  according to another embodiment of the present invention. The flexible circuit  400  is shown to include two separate branches and each branch has separate elements of a flexible circuit. For example, the flexible circuit  400  contains two lead in portions like  402   a  and  402   b.  Further it should be appreciated that in the figure, though the flexible circuit is shown to include only two branches, other embodiments of the present invention may include more than two such branches, without deviating from the scope of the present invention. 
     The flexible circuit  400  is shown to include two tail portions  404   a  and  404   b,  two first distended head portions  408   a  and  408   b  corresponding to the two lead in portions  402   a  and  402   b,  two second distended head portions  410   a  and  410   b  corresponding to the two tail portions  404   a  and  404   b  with help of corresponding coverlay portions  406   a  and  406   b.    
     It should be appreciated that individual elements of the flexible circuit  400  are similar in structure, features and functionality to the ones defined in conjunction with the flexible circuit  300 . 
     Moving on,  FIG. 5  depicts a multiple branched flexible circuit  500  in accordance with another exemplary embodiment of the present invention, The flexible circuit  500  can form a differential pair when the flexible circuit  500  is threaded through a through via. However, it should be appreciated that in other embodiments of the invention other similar electrical circuits can also be formed without deviating from the scope of the invention. 
     The flexible circuit  500  is shown to include two lead in portions  502   a  and  502   b  through which the flexible circuit  500  is threaded through a through via. The flexible circuit  500  also includes two tail portions  504   a  and  504   b.  The flexible circuit further includes corresponding two first distended head portions  506   a  and  506   b  and two second distended head portions  508   a  and  508   b,  such that they are connected through corresponding coverlay portions  510   a  and  510   b.    
     Also illustrated is a fold line  512 , along which the flexible circuit  500  may be folded to be converted into a differential pair, while being threaded through the through via. However, it should be appreciated that in other embodiments of the invention other similar arrangement like discrete electrical components which might be one of, but not limited to, an active component, an embedded resistance, an embedded capacitance, and a fuse can also be formed using the flexible circuit  500  without deviating from the scope of the invention. 
     Again, it should be appreciated that in the figure, though the flexible circuit is shown to include only two branches, other embodiments of the present invention may include more than two such branches, without deviating from the scope of the present invention. Also, it should be appreciated that individual elements of the flexible circuit  500  are similar in structure, features and functionality to the ones defined in conjunction with the flexible circuit  400  and the flexible circuit  300 . 
     Referring to  FIG. 6 , there are depicted partial portions of three flexible circuits  600   a ,  600   b  and  600   c,  in accordance with another exemplary embodiment of the present invention. The flexible circuits  600   a,    600   b  and  600   c  include first distended head portions  602   a,    602   b  and  602   c  respectively connected to second distended head portions  604   a,    604   b  and  604   c  respectively through coverlay portions  606   a,    606   b  and  606   c.    
     In embodiments of the present invention, the coverlay portion  606   a  of the flexible circuit  600   a  may be in form of a looped segment that is looped so as to form a coil as depicted by the coverlay portion  606   b  of the flexible circuit  600   b  or a helix as depicted by the coverlay portion  606   c  of the flexible circuit  600   c  so as to form a small inductive portion within the flexible circuit  600   b  and the flexible circuit  600   c.  In an embodiment, the looped portion can be any one of a coil shape, a helical shape and a spiral shape. Further in an embodiment, the at least one looped segment is positioned within the at least one through via. 
     The coiled coverlay portion  606   b  or the helix coverlay portion  606   c  are completely housed in the through via through which the flexible circuit  600   b  or  600   c  will be threaded. 
     In an embodiment, the flexible circuits  600   b  or  600   c  may be covered with a ferrous like material in order to increase the potential value of the inductance. 
     Again, it should be appreciated that individual elements of the flexible circuits  600   a ,  600   b  and  600   c  are similar in structure, features and functionality to the ones defined in conjunction with the flexible circuit  500 , the flexible circuit  400  and the flexible circuit  300 . 
     Moving on there is depicted in  FIGS. 7 a , 7 b  and 7 c    different stages of threading a flexible circuit  700  through a defective through via  702  of a printed circuit assembly containing a first contact pad  704  and a second contact pad  706 . A pin hole connector  708  as shown in the figure helps in interconnection of electrical components in a printed circuit assembly. The pin hole connector  708  runs through the defective through via  702  from the first contact pad  704  where electrical components are present, to the second contact pad  706 . The pin hole connector  708  is shown to be soldered at  710 . 
     The flexible circuit  700  is threaded through the defective through via  702  in  FIG. 7 b   , the flexible circuit  700  is entered from the first contact pad  704 , through the through via  702 , and emerges from the second contact pad  706 . The flexible circuit  700  is then soldered at the point  710  in order to repair the defective through via  702 . 
     Moving on, in  FIG. 8  there is depicted an exemplary debug probing device  800 , in accordance with an embodiment of the present invention. The debug probing device  800  can be used for probing a printed circuit assembly without damaging interconnection of electrical components present on the printed circuit assembly. Also, such a debug probing device eliminates additional wiring which may be required to be done in order to connect any conventional debug probing device to the printed circuit assembly  100 . 
     The debug probing device  800  includes a bulk portion  802  and a plurality of leader threads like  804   a,    804   b,    804   c,    804   d  and  804   e  of different shapes and different sizes. The plurality of leader threads  804   a,    804   b,    804   c,    804   d  and  804   e  may be connected to the flexible circuit  300  as depicted in  FIG. 3   
     Some shapes of the leader threads  804   a,    804   b,    804   c,    804   d  and  804   e  may include, but are not limited to, s-shaped, zigzag shaped, snake shaped etc. These different leader threads have different advantages and can help cater to different types of through vias. For example, in an embodiment illustrated by the leader thread  804   e  and the bulk portion  802 , the leader thread  804   e  is shown to include a plurality of openings  806 . The plurality of openings  806  are provided so as to allow clearance for through vias, surface features or keep out zones. 
     Moving on there is shown in  FIG. 9 , an exemplary method of using the debug probing device  800  on a printed circuit assembly  900 . For example, in the depicted method, the leader thread  802   a  is inserted through a through via  902  in order to probe on a surface of the printed circuit assembly  900 . If any defect is found in the through via  902  a flexible circuit connected to the leader thread  802   a  can be threaded in order to repair the defective through via  902 . 
     While the invention has been disclosed in connection with the preferred embodiments shown and described in detail, various modifications and improvements thereon will become readily apparent to those skilled in the art. Accordingly, the spirit and scope of the present invention is not to be limited by the foregoing examples, but is to be understood in the broadest sense allowable by law. 
     All documents referenced herein are hereby incorporated by reference.