Patent Application
20060129955
The present invention relates generally to printed circuit board design and testing, and more particularly to techniques for the printed circuit board development cycle through the use of bead probe technologies and through automation of test pad location during PCB design and fixture probe location during fixture design.
Electronic products typically contain at least one printed circuit board (PCB). A PCB generally includes a plurality of electronic components that are electrically connected together by electrical paths called “nets” that are formed of various combinations of conductive traces, vias, wires, and solder, to form an operational circuit that performs a given function. A conventional PCB development cycle is illustrated by the Gantt chart shown in
Where possible, given the current state of the art of available development tools in the market, many steps involved in PCB design, test development and fixture development are automated, either fully or at least partially. For example, large automated industrial CAD development systems have been developed that allow floorplanning, schematic capture, trace routing, design verification, and even test generation. These automated features have significantly improved the time-to-market of an integrated circuit assembly. However, several steps in the PCB development cycle are still performed mainly by manual intervention and iterative effort. For example, as illustrated in
To this end, during the PCB design stage, a CAD system is used to design and lay out the PCB, including schematic capture and component and trace layout of the PCB under development. After design and layout of the circuit are complete, conventionally the circuit designer manually adds test pads for board testing (such as in-circuit test (ICT)), the goal being to provide probing access on all nets, or at least on all nets of interest, on the PCB. Conventional test pads are implemented as extensions of traces on the exposed trace layers in that they lie in the same plane as one of the exposed trace layers and are formed integrally with trades on the exposed trace layer using the same trace material. Test pads are typically round, square, or some other planar geometric shape and have a surface area large enough to accommodate a probe head so that when the PCB is probed at the test pad, the probe does not contact other traces or components on the PCB. Furthermore, the size of a test pad is also determined by pointing errors in probe placement that may cause lateral offsets. Thus a test pad may be somewhat larger than the probe head itself, to assure a good probability of hitting it. Because conventional test pads occupy area on the trace layer, the addition of test pads to the PCB design often require rerouting of nets of the PCB design. The addition of test pads to the design using conventional techniques also carries risks in adversely affecting surrounding circuitry or changing the transmission line characteristics of the traces over which high-speed signals are communicated. Accordingly, whenever a test pad is added to the PCB design using conventional techniques, the effects of the design change must be either calculated or simulated in order to ensure that the location of the test pad and its associated changes to the design do not adversely affect circuit operation. Because the test pad insertion step requires mainly manual effort and is iterative in nature, this step, as illustrated in
Referring again to the PCB development cycle of
The test fixture 220 includes a probe protection plate 240, standard spring probes 232 whose tips exactly correspond to test pads 204 on the bottom of the PCB under test 202, a top push-down gate 250 which opens and closes by way of a hinge 252, spacers called board pushers 258 mounted in the bottom of the push down gate 250 which limit the deflection of the PCB 202 under vacuum loading, a probe mounting plate 230 in which the spring probes 232 are installed, personality pins 226 which are wired to the spring probes 232, and an alignment plate 222 which aligns the wirewrap tails of the personality pins 226 into a regularly spaced pattern so that they line up with tester interface pins 214 mounted in the tester 210. As known in the art, a spring probe is a standard device, commonly used by the test community, which conducts electrical signals and contains a compression spring and plunger that move relative to the barrel and/or socket when actuated. A solid probe also conducts electrical signals but has no additional parts which move relative to each other during actuation.
During test, the PCB under test 202 is pulled down by vacuum or other known mechanical means so that the test pads 204 on the bottom of the PCB under test 202 contact the tips of the spring probes 232. The sockets of the standard spring probes 232 are wired to personality pins 226, and the alignment plate 222 funnels the long, flexible personality pin tips into a regularly spaced pattern. The tips of personality pins 226 contact the tester interface pins 214 mounted in the tester 210. Once electrical contact between the PCB under test 202 and the tester 210 is established, in-circuit or functional testing may commence.
Referring back to
Again referring to
It is clear from the above description of the PCB development process that it would be desirable to automate the time consuming manual and iterative steps of the PCB development cycle, namely test pad insertion during the PCB design stage of a PCB and fixture probe insertion during the corresponding test fixture design/build stage. It would further be desirable to automate the determination of the “best” set of fixture probe locations (or “preferred locations”) for PCB designs that implement multiple test pads on many of the nets to allow for alternate probing locations on these nets. It would further be desirable to utilize bead probe technology to implement multiple test pads along nets that can physically accommodate additional bead probes in order to provide a set of alternate probing locations for optimization of probe locations to reduce board stress while at the same time allowing some flexibility in the probing locations to accommodate fixture components such as board pushers. It would also be desirable to automate the designation of preferred and alternate probing locations for those nets that support alternate probing locations.
The present invention is a technique for improving the time-to-market of a PCB assembly through automation of PCB test pad insertion and fixture probe insertion. During the PCB design stage, the invention utilizes bead probe technology and a test pad location algorithm to automatically position and insert test pads in a PCB design. During the fixture design stage, the invention utilizes a fixture probe location algorithm to automatically position and insert fixture probes in a corresponding test fixture design. The test pad location algorithm and fixture probe location algorithm each utilizes a common probe location algorithm that ensures that fixture probe locations correspond to test pad locations on the PCB under test.
According to a preferred embodiment of the invention, a probe location algorithm automatically determines preferred and alternate locations of bead probes along nets of the PCB design. Bead probes are automatically inserted in the PCB design at all of the preferred and alternate bead probe locations. The same probe location algorithm is then used during fixture design/build to automatically determine the preferred probing locations from the PCB design.
A more complete appreciation of this invention, and many of the attendant advantages thereof, will be readily apparent as the same becomes better understood by reference to the following detailed description when considered in conjunction with the accompanying drawings in which like reference symbols indicate the same or similar components, wherein:
For purposes of clarity, the following terms are defined:
“net”—a signal transmission line which passes signals between two or more endpoints over an electrically conductive path; may be implemented as one or more of any of, including combinations of, the following: a trace, a via, a wire, a component lead, a solder ball, a wire bond, or any other electrically conductive element electrically connected between the two or more endpoints and through which the signal passes;
“test pad”—a point on a net accessible for probing, typically characterized by a surface area large enough to accommodate a probe head;
“fixture probe”—an electrically conductive element of a tester fixture which operates as a passive transmission line at least between a first end and a second end of the electrically conductive element, the first end configured to electrically contact a test pad of a circuit and the second end configured to electrically contact another electrical component of interest, such as a tester interface pin, a test pad of a wireless fixture PCB, a node of a tester measurement circuit, etc.).
Turning now to the invention, time-to-market of a PCB based product is reduced by automating what were previously manual steps in both the PCB design stage and the fixture build stage of the PCB development cycle. In particular, as shown in
Meanwhile, during the test and fixture development stages, the locations of the probes in the fixture must be determined in order to assign suitable tester resources to probing the various nets. Accordingly, during the test development stage, a novel automated fixture probe selection step (step 310) is performed to find the preferred, and where practical, alternate fixture probe locations. In the preferred embodiment, the probe location algorithm used to determine the preferred and alternate bead probe locations during the PCB development stage is also used to identify the preferred and alternate fixture probe locations for the fixture based on the released PCB design with test pads. Once the preferred and alternate fixture probe locations are known, an automated board pusher layout algorithm is preferably performed (step 312) to determine locations of board pushers in the top gate of the fixture in order to minimize board flex during test. In the preferred embodiment, points of maximum deflection on the PCB based on the fixture probe layout are automatically determined and board pushers are added to the fixture design at those points. Once the preferred fixture probe locations are known, tester resources can be assigned (step 314), and fixture build files containing drill data (where to drill, size of hole, depth of hole, which fixture layers, etc), insertion data (where and what to insert—e.g., probes, retainers, screws, board pushers, etc.), and wire data (map of personality pins to fixture probes to be wired) are generated for build (step 316). The fixture build design files may then be released for build (step 318) and the fixture can then be built (step 319).
According to a preferred embodiment, the invention includes a technique for implicitly encoding the preferred probing locations in the PCB design itself during PCB design, and for implicitly decoding the preferred probing locations during fixture design, in order to protect disclosure of the preferred probing locations only to authorized fixture builders. According to a preferred method 400, as illustrated in
In the preferred embodiment, the automatic bead probe insertion software 500 includes a potential bead probe location processor 520 that processes each net 522 to be probed on a PCB and generates a list of potential bead probe locations 526 that includes a number of possible locations on that net that meet the qualifying bead probe criteria 524. Because bead probes may be added anywhere along a net, there may be many locations along every trace that meet the qualifying bead probe criteria 524; accordingly, for practical purposes and to limit the algorithm processing time, the number of locations in the list of potential bead probe locations 526 may be artificially limited (i.e., fewer possible locations may be included in the list 526 than actually meet the qualifying bead probe criteria 524).
The automatic bead probe insertion software 500 utilizes a layout-independent test pad location algorithm 510 which automates the positioning and addition of test pads to a PCB design without altering the layout of the PCB design. Likewise, the automatic fixture probe insertion software 550 utilizes a fixture probe location algorithm 560 which automates the positioning of fixture probes in a corresponding fixture built for testing the PCB. In the preferred embodiment, the layout-independent test pad location algorithm 510 and the fixture probe location algorithm 560 each utilize a common probe location algorithm 570 for determining preferred and alternate probing locations given a set of alternate test pad locations or fixture probe locations, respectively.
In the preferred embodiment, the test pad location algorithm 510 exploits a novel test access structure, referred to herein as a “bead probe”, in combination with the probe selection algorithm 570 to determine locations of test pads on the PCB. Bead probes are described in U.S. patent application Ser. No. 10/670,649, to Parker et al., entitled “Printed Circuit Board Test Access Point Structures and Method for Making the Same”, filed Sep. 24, 2003 and assigned to the assignee of interest herein, the entire disclosure of which is incorporated herein by reference for all that it teaches. Research has shown that bead probes may be fabricated along existing nets of the PCB design without impacting the PCB layout or high-speed electrical characteristics of the net, as reported in “A New Probing Technique for High-Speed/High Density Printed Circuit Boards”, International Test Conference, October 2004, the entire document of which is incorporated herein by reference for all that it teaches. Since bead probes may be added to PCB designs without requiring net re-routing, in principle, as previously described, any net having at least a portion of the net implemented in an exposed layer of the PCB (i.e., is implemented on an outer PCB layer that is accessible for probing) can have a bead probe placed anywhere along the exposed portion of the net provided that the fixture probe probing the bead probe will not strike a nearby component on the board or interfere with a nearby fixture probe. The number of potential probing locations on a given net can therefore be quite high, allowing a high degree of flexibility in choosing the locations to probe. The question then becomes how does the potential bead probe location processor 520 choose the best location for each net to be probed in terms of achieving optimal probing conditions across the entire board or at least a region of the board.
Minimizing board flex provides an answer. Ideally, the test pads (locations at which probing force is applied) would be evenly distributed across the PCB in order to minimize the board flex of the PCB. However, given typical component package form factors and surrounding trace patterns resulting therefrom, it may not be possible to evenly distribute the test pads across the board. For example, a ball grid array (BGA) component typically results in concentrations of nets within and near the vicinity of the BGA component's floorplan coordinates. Therefore, the concentration of these nets may be such as not to allow even distribution of probing force in terms not only of the entire board itself, but also even within the local vicinity of the BGA component.
As stated earlier, the test pad location algorithm 510 utilizes a probe location algorithm 570 to choose the locations of the bead probes along the exposed portions of nets of the PCB given a list of potential bead probe locations. The probe location algorithm attempts to choose an optimal location from among a list 526 of potential bead probe locations, the optimal location being designated as the “preferred” bead probe location. Because a bead probe can be fabricated at essentially any location along an exposed portion of a net without impacting the electrical characteristics of the net, as discussed previously, there is a high degree of flexibility in choosing the actual location(s) of the test pad(s) along each net. However, because the number of potential bead probe locations may be quite high, as also discussed previously, for practical purposes (e.g., reducing processing time) the list of potential bead probe locations may be artificially limited based on parameters such as minimum distance between bead probes, number of bead probes per predefined net length, etc. The probe location algorithm 570 preferably selects from the list of potential bead probe locations at least a “preferred” bead probe location, and preferably also one or more “alternate” bead probe locations.
Since bead probes have minimal impact on the size and electrical characteristics of the PCB, it is preferable to implement bead probes not only in the preferred probing location along the net, but also in one or more alternate probing locations on many of the nets in order to allow for post-fabrication design changes (typically referred to as Engineering Change Orders (ECOs)) or to avoid interference with other fixture components. In the preferred embodiment, all bead probes, including those designated as preferred probing locations and those designated as alternate probing locations, are automatically added to the PCB design without test pads 530 (i.e., PCB CAD files) to generate a PCB design with test pads 540, and are implemented during PCB fabrication regardless of which probing location (preferred or alternate) ends up being actually probed during test.
During test, only one bead probe per net (either the preferred bead probe or one of the alternate bead probes) is probed. Therefore, if alternate probing locations are implemented on a net, the fixture designer must select only one probing location from among the preferred and alternate probing locations and implement only one fixture probe for probing the corresponding bead probe at the selected probing location. Since the preferred locations are calculated as the “best” probing locations in terms of board flex of the PCB under test, ideally the fixture design will implement fixture probes at fixture positions that correspond to all (or as many as possible) preferred probing locations. However, where preferred probing locations interfere with certain fixture components such as spacers, retainers, other fixture probes, the fixture design may implement fixture probes at fixture positions that correspond to several of the alternate probing locations.
In principle, of course, because the preferred probing location is the location that will minimize board flex, it is desirable that the fixture designer select the preferred probing location for every net. This may be addressed in one of two ways: (1) the PCB designer can directly communicate to the fixture builder which of the bead probes are preferred probing locations; or (2) the fixture builder can utilize the same probe location algorithm used by the PCB designer in determining the preferred probing locations on the board to determine the preferred fixture probe locations.
In this second method, the fixture designer/builder is provided with the PCB design with test pads inserted and the probe location algorithm 570. By running the same probe location algorithm 570 that was used by the PCB designer in determining the preferred and alternate probing locations of the bead probes on the PCB, the fixture builder can then identify the preferred probing locations on each net. Preferably, during fixture build, fixture probes are then inserted in the fixture at locations corresponding to the preferred probing locations on the PCB to be tested. At alternate probing locations, holes are preferably pre-drilled in the fixture; however, no actual probe is inserted in the pre-drilled holes. If, after fixture build, the PCB is subjected to an ECO that would require probing in one of the alternate probing locations rather than a preferred probing location, or if the other components in the fixture interfere with the preferred probing location within the fixture, the fixture probe that probes that preferred probing location may be removed from the fixture, and a fixture probe may be inserted in the fixture in the pre-drilled holes corresponding to the selected alternate probing location for the affected net(s).
The automation of the test pad location and implementation process during the PCB design stage and the corresponding automation of the fixture probe location process during the fixture design and build allow a streamlined PCB development process that greatly reduces the time-to-market of a PCB based product.
After creating the pool of possible bead probe locations 526 for each net 522, the bead probe insertion software 600 iteratively selects each net to be probed (step 602), preferably beginning with the nets with the fewest possible potential bead probe locations, and applies a probe location algorithm (e.g., probe location algorithm 570) that selects a preferred bead probe location from among the pool of potential bead probe locations associated with the net being processed (step 603). In the preferred embodiment, the probe location algorithm attempts to balance bead probes on both the top and bottom of the PCB to minimize board flex and minimize supplemental board pushers that will preferably be added to the fixture to optimally minimize board flex.
The bead probe insertion software then preferably also selects one or more alternate bead probe locations on each net, if possible, to allow for future ECOs (step 604). The selection is preferably performed by removing the preferred bead probe location from consideration by the probe location algorithm and re-running the probe location algorithm.
The bead probe insertion software 500 then inserts bead probes into the PCB design CAD files at the selected preferred and alternate (if such exist) bead probe locations 528 associated with the net (step 605). If more nets remain in the PCB design to be processed, (determined in step 606), the algorithm iterates steps 602 through 606 until all nets have been processed.
Once the bead probes have been added to the PCB CAD files, the CAD data may then be released to the test developer. The PCB CAD data contains information for each component on the PCB, including but not limited to the x- and y- coordinates of the endpoints of each trace, thickness of each trace, layer of each trace, locations and z-dimension thickness of each bead probe to implemented on the trace, and x, y- and z-dimension vertices and rotational information of nearby components to define the outlines or boundaries of components on the board. Unlike the prior art PCB development cycle, however, the PCB CAD data now include ICT probing locations that (1) will not affect the board's electrical performance, (2) did not alter the trace layout of the PCB; (3) likely provides 100% probing access plus alternates for flexibility, (4) took little manual effort from the PCB designer, (5) include at least one location on each net that, if properly chosen, will optimize board flex, and (6) were determined quickly, accurately, and automatically, reducing the time-to-market.
The test developer then translates the PCB CAD data into test data using test development software. During test development, fixture files are developed from the CAD data and sent to the fixture designer for fixture design/build. Included in the fixture files are the bead probe locations, both preferred and any existing alternate locations. While the preferred fixture probe locations may be designated in the fixture files as discussed above, in the preferred embodiment, the fixture designer/builder will run the second software module of the invention, namely the fixture probe insertion software 550 (
The fixture probe insertion software then preferably also selects one or more alternate fixture probe locations on each net, if possible, to allow for future ECOs (step 704). The selection is preferably performed by removing the preferred probing location from consideration by the probe location algorithm and re-running the probe location algorithm. Since in the preferred embodiment the probe location algorithm is the same as that used to determine the locations of the bead probes in the PCB design, the fixture probe insertion software will select alternate fixture probe locations that will coincide with the alternate bead probe locations when the PCB is mounted in the fixture.
The fixture probe insertion software then adds a fixture probe to the fixture build file at the selected preferred fixture probe location associated with the net, and, if alternate fixture probe locations exist for the net, adds holes to the fixture build file at the alternate fixture probe locations to allow for future insertion of actual probes in the alternate fixture probe locations in case the location of the physical fixture probe must be changed due to future ECO or other fixture design problems (step 705). The fixture probe insertion software iterates through all nets to be probed.
Once all nets to be probed have been processed (determined in step 706), the fixture probe insertion software adds board pushers to the fixture design (step 707). In the preferred embodiment, the step for adding board pushers to the fixture design is implemented using the support location algorithm described in U.S. patent application Ser. No. 10/215,108, supra, which finds the locations of board pushers that result in minimized board flex. Board pushers are then added to fixture build files from which the physical fixture will be built (step 708).
During fixture build, holes are drilled in the fixture plates and physical fixture probes are inserted in the fixture at the selected preferred fixture probe locations indicated in the fixture build file, and holes associated with alternate fixture probe locations are either fully or partially (i.e., may optimally be punched out) drilled in the fixture plates to allow insertion of a physical fixture probe therein if later needed. Board pushers are also added to the fixture where indicated in the fixture build file.
In summary, the automated PCB and fixture probe insertion technique of the invention automates previously manual steps of the PCB development process to significantly improve the PCB development cycle and reduce time-to-market of PCB based products. Referring to the Gantt chart shown in
The techniques described herein provide many advantages to the PCB development process. First, the layout independent property of bead probes provides additional opportunities for fast, flexible and cost-effective probing.
It may be desirable to place some bead probes manually in order to address other concerns such as signal coupling between probes that may disturb measurement quality. These measurements can be made at ICT, during the functional part of ICT (for example, on a dual-stage fixture), or in a functional test. The manual probe placement step would be imposed before automatic probe location selection is performed on the remaining nets.
Another advantage included in the techniques described herein is that the automatic selection process can include alternate placements to enable easier Engineering Change Orders (ECO) for a board. (ECOs occur when a board designer needs to revise a board's design.) For example, a preferred probe location plus a few alternates can be chosen from the list of potential locations on a given net. The ICT fixture would have a fixture probe at the preferred location but only partially drilled holes at the alternates to reduce vacuum leakage in the fixture. If an ECO were implemented that eliminated the preferred location, the fixture would have one of the partially drilled alternates fitted with a fixture probe. This is a simple modification to the fixture. Further, if the original location was kept, the fixture could be used for both versions of the PCB, minimizing revision control issues in production.
Finally, since the probe selection process occurs at the PCB layout step, ECOs can be further streamlined by providing a list of previously selected probe locations to the automated selection process. In this “ECO mode”, the automated selection process would try to use existing locations first, before assign new ones, thus reducing costly changes to an existing test fixture.
Those of skill in the art will appreciate that the methods described and illustrated herein may be implemented in software, firmware or hardware, or any suitable combination thereof. Preferably, the method and apparatus are implemented in software, for purposes of low cost and flexibility, which run on computer hardware. Thus, those of skill in the art will appreciate that the method and apparatus of the invention may be implemented by a computer or microprocessor process in which instructions are executed, the instructions being stored for execution on a computer-readable medium and being executed by any suitable instruction processor. Alternate embodiments are contemplated, however, and are within the spirit and scope of the invention.
Although this preferred embodiment of the present invention has been disclosed for illustrative purposes, those skilled in the art will appreciate that various modifications, additions and substitutions are possible, without departing from the scope and spirit of the invention as disclosed in the accompanying claims. It is also possible that other benefits or uses of the currently disclosed invention will become apparent over time.
