Abstract:
A wiring board for mounting an electrical device, which has an array of connectors thereon arranged in a grid pattern, wherein the connectors have at least two levels of criticality of connection to the substrate. The substrate has a plurality of mounting structure or features arranged in the same grid pattern to connect with the array of connectors on the electrical device. The mounting structures or features are divided into a plurality of at least two groups, with each group corresponding to a level of criticality of the connectors on the device. Each group of mounting structures has a discernible feature differing from each other group, to thereby permit different levels of inspection criteria for each group. The invention also contemplates a method of forming a substrate having the features for connecting the connectors on the device that have different levels of criticality

Description:
FIELD OF THE INVENTION 
     This invention relates generally to printed wiring board and, more particularly, to a technique of forming printed wiring boards and the resulting board to provide built-in inspection aids based on the criticality of the various device joining structures on the printed wiring board. 
     BACKGROUND INFORMATION 
     Present day printed circuit boards have many types of discrete components attached or mounted to them. Some components are surface mounted to metal features, usually formed with a photolithographic process using various techniques, such as solder, wire bond, mechanical compression, as well as other techniques, while others are inserted into holes previously drilled and metal plated. In most cases, there are a multitude of connection points between the component and the printed circuit board on some fixed and repetitive grid patterns. A typical grid may be a matrix of pads, such as a ball grid array, as well as others, with centerlines on the order of 0.050 inch to 0.100 inch apart. Much testing and inspection of these features at many different stages is performed to insure that they meet a predetermined defect criteria for items, such as metal voiding, missing metallization, size, contamination as well as may other criteria. The past practice has treated all the features in the grid with the same degree of criticality during the board inspection process, and defective parts are either scrapped or repaired according to quality specifications which are of the highest quality. Connections on the I/C chips are typically provided for all the attachment structures in the grid. 
     However, all of the connections on the chips and, hence, all of the corresponding features or attachment structures in a specific grid array may not be actually electrically functional or may have a redundant function to others on the same chip or printed circuit board. In fact, in many grid arrays of component mounting features, there are more non-critical features than critical features. Generally speaking, a non-critical feature is one where the pad is not used for electrical function or is used for a redundant function for other features on the pad. It should be noted that while many of the connections may be less critical from an electrical or circuit point of view, nevertheless in the design of the circuit board, the features are included for several reasons. First, they may be necessary for providing structural integrity to the chip. Second, the features may be necessary for future use, such as engineering changes. Thus, they are present. 
     Which mounting features or mounting structures are critical and which are not critical is not readily obvious to the equipment or operator inspecting the board. Therefore, defects in non-critical features frequently result in the board being scrapped or repaired unnecessarily. In high volume manufacturing environments, processing many part numbers with different types of component mounting scenarios and varying levels of human expertise, mapping of each individual part number&#39;s functional features is not practically accomplished. Therefore, there is a need to identify which features or mounting structures are critical and which are not during the inspection phase, so that the boards need not be rejected because of defects in non-critical features or mounting structures. 
     SUMMARY OF THE INVENTION 
     According to the present invention, a wiring board for mounting an electrical device, which has an array of connectors thereon arranged in a grid pattern, is provided, wherein the connectors have at least two levels of criticality of connection to the substrate. The substrate has a plurality of mounting structures or features arranged in the same grid pattern to connect with the array of connectors on the electrical device. The mounting structures or features are divided into a plurality of at least two groups, with each group corresponding to a level of criticality of the connectors on the device. Each group of mounting structures has a discernible feature differing from each other group, to thereby permit different levels of inspection criteria for each group. The invention also contemplates a method of forming a substrate having the features for connecting the connectors on the device that have different levels of criticality. 
    
    
     DESCRIPTION OF THE FIGURES 
     FIG. 1 a  is a perspective view of a portion of a substrate having mounting structures or features thereon and a portion of an I/C chip for mounting on the substrate according to the prior art; 
     FIG. 1 b  is a view similar to FIG. 1 a  showing mounting features according to one embodiment of the present invention; 
     FIG. 1 c  is a view similar to FIG. 1 a  showing mounting features according to another embodiment of this invention; 
     FIG. 2 a  is a perspective view of a portion of a substrate having mounting structures which include pads connected to vias and a portion of an I/C chip for mounting on the substrate according to the prior art; 
     FIG. 2 b  is a view similar to FIG. 2 a  showing mounting structures with pads connected to vias according to the present invention; and 
     FIG. 3 is a side elevational view of a portion of a chip carrier and a portion of an I/C chip showing various connection features on the chip and chip carrier. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring now to the drawings and, for the present, to FIG. 1 a , a perspective view of a portion of a printed circuit wiring board and a portion of an I/C chip are shown somewhat diagrammatically. Printed Circuit wiring board includes a substrate  10  on which are formed connection pads  12 . Although connection pads have taken various shapes for various functional reasons, a typical connection pad  12  is generally circular in shape and the connection pads  12  typically are arranged in a grid array to mount an electrical device, such as an I/C chip  14 . (While the invention is described in conjunction with an I/C chip, it is to be understood that it is equally useful in connecting other electrical devices to a substrate.) I/C chip  14  has a plurality of connectors  16  and  18  which, in one embodiment, take the form of solder bumps which can be solder reflowed to join the I/C chip  14  to the connection pads  12 . (Only a few connectors  16 ,  18  and connection pads  12  are shown for illustration. However, many more connectors  16 ,  18  and connection pads  12  are typically present on a chip and substrate.) 
     Connectors  16  and  18  on the I/C chip  14  are generally identical in shape, although some of them are critical connectors, which connectors are designated as connectors  16 , and some are non-critical connectors, which connectors are designated as connectors  18 . Generally speaking, the critical connectors are those which perform or are involved in an electrical function or carry an electrical signal as a primary connector. Non-critical connectors which are designated as  18  generally are connectors on the chip which serve no electrical function or which serve only a redundant function to some of the connectors  16 . Thus, while it is critical that the connectors  16  make good, solid electrical contact with the contact pads  12  on the substrate  10 , such is not the case of connectors  18  which, if not formed to a good, solid electrical contact, is not critical and the chip  14  will still function. 
     If the connection pads  12  on the substrate  10  are all similarly sized and shaped, then, unless one has a visual map or other mapping device of the connectors  16  and  18  on the chip  14 , the inspector will inspect each of the connection pads  12  to the same criteria, which has to be the highest criteria needed for the connectors  16 . This is a difficult task and, generally, is beyond the skill of the people making the inspection. Therefore, it is conventional practice to reject and scrap or return for rework substrates  10  which have defects in any of the connection pads  12  whether or not they are connected to critical connectors  16  or non-critical connectors  18  on the I/C chip  14 . 
     According to the invention, one embodiment which is shown in FIG. 1 b  has connector pads that are arranged in the same grid pattern as in FIG. 1 a , i.e. the same grid array to mate with the connectors  16  and  18  of the I/C chip  14 . However, in this embodiment of the present invention, connector pads  20  are formed of one shape, i.e. a circle, and connector pads  22  are formed of another shape, i.e. a hexagon (although these shapes are arbitrary), the connector pads  20  being positioned to mate with the connectors  16  on the I/C chip  14  and connector pads  22  being positioned to mate with the connectors  18  on the I/C chip  14 . Thus, during an inspection process, if the inspector should note that there are defects with respect to the connector pads  22 , such as, but not limited to, metal voiding, missing metal, improper size, contamination and many other defects, these can be ignored since these connector pads  22  are non-critical. However, if the same type of defect is noticed in a connector pad  20  which is deemed critical, then the printed wiring board would be returned either for rework or scrapped. Hence, a great number of boards, which would otherwise be returned for rework or scrapped, can pass inspection since the defects noted therein are on the non-critical connector pads  22 . Thus, both critical and non-critical pads are provided as needed for other purposes, but the inspection criteria are different. 
     FIG. 1 c  shows another embodiment of the present invention where connector pads  24  and  26  are provided, these connector pads  24  being critical connector pads to mate with the connectors  16  on the I/C chip  14 , and the connector pads  26  being non-critical connector pads being positioned to mate with the connectors  18  on the I/C chip  14 . In this embodiment, rather than being of a different external shape, the connector pads  24  are completely filled-in circles whereas the connector pads  26  are generally annular in shape or, expressed another way, have a centered void which will identify connector pads which are for the non-critical connectors  18 . 
     FIG. 2 a  shows another type of connection or mounting structure for an I/C chip according to the prior art. In this case, connector pads  27  are connected to vias  28  through circuit lines  29 . As in the embodiment of FIG. 1 a , all of the connector pads  27  and circuit lines  29  are the same whether the pads  27  are to be connected to critical connectors  16  or non-critical connectors  18  on the I/C chip  14 . 
     FIG. 2 b  shows how this type of mounting device can be formed to show critical and non-critical connections. In this embodiment, the connector pads  27  are all the same size and shape, i.e. circular, but other shapes could be used, and the vias  28  are all the same size and shape. However, there are different size circuit lines  30  and  32  leading to the vias  26 . The circuit lines  30  are for connection to the pads which connect to connectors  16  of the I/C chip  14  and circuit lines  32  are for connection to the pads connected to connectors  18  of the I/C chip  14 . In this case, the circuit lines  32  are wider than the circuit lines  30 , thus affording an inspector an indication of which connector pads are for critical connectors and which are for non-critical connectors and, hence, allowing any defects to pass in the connector pads for connection to the non-critical connectors  18  of the I/C chip  14 . 
     It should be understood that these are but a few of the techniques which can be used to provide the necessary differentiation between mounting structures which are to be connected to critical connectors  16  of the I/C chip  14  to non-critical connectors  18  of the I/C chip  14 . The particular differentiation of features is not critical other than that the mounting structures to be mounted to the connectors  16  of the I/C chip must be differentiated in such a manner that they are discernable from those which are to be mounted to the connectors  18  of the I/C chip  14 . 
     The invention has been described with respect to manual visual differentiation by human inspectors. However, other alternatives exist, such as automated optical inspection (AOI), where it is possible to program the tool to recognize and differentiate the shape variation between pads and treat the defect criteria differently between the two. For example, an algorithm can be written whereby defect criteria on a polygon-shaped pad would be treated differently by the AOL tool than on a round pad, etc. 
     The invention has been described in connection with mounting an I/C chip to a substrate. This can be either an I/C chip mounted directly to a printed wiring board or it can be an I/C chip mounted to a chip carrier  34 , as shown in FIG.  3 . Moreover, the chip carrier  34  shown in FIG. 3 can itself be the electrical device and the mounting of connectors  36  and  38  on the chip carrier can be mounted in the same way as connectors  16  and  18  on the chip  14  are mounted to a substrate  10  of a printed wiring board. Thus, it is possible that a chip carrier and a printed wiring board can have mounting structures which differentiate between critical and non-critical connections to an electrical device. 
     It should be noted that a particular method of forming the mounting structure on the substrate  10  is not critical and can be accomplished by any well known teachings used in the art, such as by photolithography using either positive or negative photoresist and using either additive plating or full panel plating and subtract etching, or any other technique which will provide the desired differentiation between the mounting structures on the substrate. Moreover, the mounting structures are not limited to those that can receive solder connections, but can be for wire bond, compression bonding, pin-in-hole and other type of connections. 
     It is also to be understood that the invention has been described with two levels of criticality; however, three or more levels of criticality can be accommodated by having a different discernible shape of mounting structure for each level of criticality. 
     While the discussion is centered around inspection of mounting features on a substrate, this invention is not limited to features which necessarily require mounting or connection. For example, there are instances of through holes and vias which have various levels of criticality which may not have any connection or component mounted to the through hole or via. There can be associated with that through hole or via a distinguishing mark or identification to enable an inspection or selected manufacturing operation to be performed associated with that through hole or via. A substrate does not necessarily have to be limited to a circuit board. Substrates could encompass various other types, such as semiconductor chips, flexible circuitized carriers, ceramic carriers, microwave carries, and so on. Features can be in various forms as well, such as transistors and components, microwave transmission and reception features, geometric shapes and trenches, and so on. 
     Accordingly, the preferred embodiments of the present invention have been described. With the foregoing description in mind, however, it is understood that this description is made only by way of example, that the invention is not limited to the particular embodiments described herein, and that various rearrangements, modifications, and substitutions may be implemented without departing from the true spirit of the invention as hereinafter claimed.