Method and apparatus for a twisting fixture probe for probing test access point structures

Abstract

A twisting fixture probe for cleaning oxides, residues or other contaminants from the surface of a solder bead probe and probing a solder bead probe on a printed circuit board during in-circuit testing.

Description

BACKGROUND OF THE INVENTION

Printed circuit assemblies (PCA's) are typically tested after manufacture to verify the continuity of traces between pads and vias on the board and to verify that components loaded on the PCA perform within specifications. Such printed circuit assembly testing is generally performed with automated in-circuit testers or ICT's and requires complex tester resources. The tester hardware must generally be capable of probing conductive pads, vias and traces on the board under test.

In-circuit testers (ICT) have traditionally used “bed-of-nails” (BON) access to gain electrical connectivity to circuit wiring (traces, nets, pads) for control and observation capability needed for testing. The Agilent 3070 is one typical in-circuit tester and is available from Agilent Technologies, Inc. of Palo Alto, Calif. This necessitates having access targets within the layout of circuit nets that can be targets for ICT probes. Test access points are usually circular targets with 28 to 35 mil diameter that are connected to traces on the printed circuit board. In some cases these targets are deliberately added test pads, and in other cases the targets are “via” pads surrounding vias already provided in the printed circuit.

Lower diameter targets are increasingly difficult to hit reliably and repeatably, especially when a test fixture may contain several thousand such probes. It is always desirable to use larger diameter targets, but this is in fundamental conflict with the industry trend towards higher densities and smaller geometry devices.

Yet another industry trend is to use higher and higher speed logic families. One Megahertz (MHz) designs became ten MHz designs, then 100 MHz designs, and are now reaching the Gigahertz domain. The increases in logic speed necessitate industry attention to board layout rules for higher-speed interconnects. The goal of these rules is to create a controlled impedance pathway that minimizes noise, crosstalk and signal reflections. Printed circuit boards traces that carry high-speed signals tend to have critical layout requirements and require controlled characteristic impedances. When traditional test probe targets are added, this causes discontinuities in the controlled impedances and may damage signal fidelity.

FIG. 1A illustrates a classic pair of differential transmission signal traces 102a, 102b on a portion of a printed circuit board 100. As illustrated, the printed circuit board 100 is formed as a plurality of layers. In the illustrative embodiment, the printed circuit board 100 includes a ground plane 104 layered over a substrate 105, a dielectric 103 layered over the ground plane 104, traces 102a, 102b layered over the dielectric 103, a solder mask 106 layered exposed surfaces of the dielectric 103, and test access targets 115a and 115b. In such a layout, there are a number of critical parameters that affect the impedance of the signal path. These parameters include trace width 110, trace separation 111, trace thickness 112, and dielectric constants of the solder mask and board material. These parameters influence the inductance, capacitance, and resistance (skin effect and DC) of the traces, which combine to determine the transmission impedance. It is generally desirable to control this value across the entire run of each trace 102a, 102b.

In some higher speed designs it is also important to control the symmetry of the traces. However, routing signals on a crowded printed circuit board necessitates curves and bends in the path, which makes matching lengths and symmetries more difficult. In some cases, series components (such as series terminations or DC blocking capacitors) may be included in the path, and these have dimensions that differ from the layout parameters. Signals may also have to traverse connectors, which add to the difficulties.

Another trend is toward higher and higher density boards, which are also layout critical. When traditional test probe targets are added to a high-density board, the board layout is generally disturbed, in which adding test probe targets to one or more nodes necessitates moving several others out of the way. Such changes to high-density boards in many cases, may not be practical, as there may not be any room to move traces. If any of the signal traces also happen to be high-frequency signal traces, then the bends needed to re-route them may also have a negative performance impact as well as the negative effects of the conventional target itself.

Additional difficulties arise when testing is considered. Testing requires tester access to circuit traces at particular probe targets. Layout rules typically require test targets to be at least 50 mils apart and may require the diameter of the test point targets to greatly exceed the width of the traces. FIGS. 1A and 1B illustrate test targets 115a, 115b symmetrically positioned 50 mils apart on the differential signal traces 102a, 102b. As seen in FIGS. 1A and 1B, large test targets 115a is being probed by a typical fixture probe 116 with a sharp head 120, such as those available from Interconnect Devices, Inc., 5101 Richland Avenue, Kansas City, Kans. 66106. The sharp probes 116 help break through any oxide or residue build up on the test targets 115a and 115b.

The positioning of test targets 102a, 102b can be problematic. In many cases the need to keep a minimum separation between targets (typically 50 mils, minimum) is in direct conflict with controlled impedance layout rules. These conflicts lead to either a compromise in controlled impedance integrity and optimal circuit layout for performance and space, or a forced reduction in target placement with a resulting reduction in testability.

While high-speed printed circuit boards are one example of layout-critical circuits, another more general case is that of high-density boards. Adding conventional probe targets to a high-density board will most likely disturb the layout. For example, adding test points to just one or more nodes may require moving several other traces out of the way. In many cases, in a high-density circuit design, this may be impractical, if not impossible, as there may not be any room to move these traces in a crowded circuit layout. If any traces are also high-frequency signal traces, then the re-routing may have an additional negative performance impact as well as the negative effects to the optimal circuit layout itself.

As discussed more fully in U.S. patent application Ser. No. 10/670,649 entitled Printed Circuit Board Test Access Point Structures And Methods For Making The Same filed Sep. 24, 2003 by Kenneth P. Parker et al. and U.S. patent application entitled Method And Apparatus For Manufacturing And Probing Printed Circuit Board Test Access Point Structures filed Oct. 25, 2004 by Kenneth P. Parker and shown in FIGS. 2A-2C, a new paradigm has been developed to replace the old test paradigm where large probe targets are laid out on a printed circuit board and probed with sharp-pointed fixture probes mounted in a test fixture.

As shown in FIGS. 2A-2C small hemi-ellipsoidal solder beads or test access points 18a and 18b are added to traces 12a and 12b on a printed circuit board 10 without perturbing the board's layout or performance by taking advantage of the z-dimension. As illustrated, the printed circuit board 10 is formed as a plurality of layers. In the illustrative embodiment, the printed circuit board 10 includes a ground plane 14 layered over a substrate 15, a dielectric 13 layered over the ground plane 14, traces 12a, 12b layered over the dielectric 13, a solder mask 16 layered exposed surfaces of the traces 12a and 12b and dielectric 13, and test access points 18a and 18b.

As noted above, an important factor in probing is obtaining a good electrical contact between the fixture probe and the test target. In the old paradigm, this was typically handled by probing with a sharp probe tip 120. However, this method may be problematic with bead probes or test access point structures 18a and 18b, as the test access point structures may have very small dimensions in the x-y plane, in the order of 3-5 mils wide by 15-20 mils long or less. Current design for test guidelines for ICT (in circuit testers) require a minimum 30 mil diameter test pad or target by a chisel or spear tipped probe. The small dimensions of the test access point structures in the x-y plane in combination with the small dimensions of the chisel or spear tipped test fixture probe would very likely create test access accuracy and reliability problems. Simply, it may be hard to hit a small target with a sharp tipped probe with current industry test fixture standards. Also, while a chisel or spear tipped test fixture probe may be effective in disturbing surface contaminants on a 30 mil or greater test pad, assuming it could reliably hit a 3-5 mil test access point structure, it would very likely catastrophically damage the test access point structure. There is a need in the industry for a method to ensure good electrical contact between bead probes or test access point structures and fixture probes.

SUMMARY OF THE INVENTION

The present invention solves the problem of how to obtain a good electrical contact between a solder bead probe or a test access point structure and a fixture probe for printed circuit board testing.

In one embodiment a surface of a solder bead probe may be cleaned by a rotational motion of a twisting fixture probe as the twisting fixture probe is brought into compressive contact with the solder bead probe.

In an exemplary method for implementing the twisting fixture probe, a substantially flat tip is attached to a rotating member, which is brought into operational contact with connected to a rotating mechanism and a spring force.

In an exemplary method for cleaning and probing a solder bead probe on a printed circuit board, a twisting fixture probe is aligned with a solder bead probe and brought into compressive, twisting contact therewith. Testing may then be run with the fixture probe in contact with the solder bead probe.

BRIEF DESCRIPTION OF THE DRAWINGS

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:

FIG. 1A is a top view of a conventional printed circuit board with traces showing the x- and y-dimensions in the x-, y-, z-coordinate system;

FIG. 1B is a cross-sectional side view of the printed circuit board with differential signal traces showing the x- and z-dimensions in the x-, y-, z-coordinate system;

FIG. 2A is a top view of a printed circuit board showing the x- and y-dimensions of the differential signal traces in the x-, y-, z-coordinate system with a test access point structure;

FIG. 2B is a cross-sectional side view of the printed circuit board in FIG. 2A showing the z- and x-dimensions in the x-, y-, z-coordinate system with a test access point structure;

FIG. 2C is a cross-sectional side view of the printed circuit board in FIG. 2A showing the z- and y-dimensions in the x-, y-, z-coordinate system with a test access point structure;

FIG. 3 is a cross-sectional side view showing a portion of a printed circuit board with a test access point structure on a trace and a fixture probe contacting the test access point structure;

FIG. 4A is a cross-sectional side view showing a portion of a printed circuit board with a test access point structure on a trace and a twisting fixture probe contacting the test access point structure in accordance with the invention;

FIG. 4B is a top view of a test access point structure with a twisting fixture probe imposed there over in accordance with the invention;

FIG. 5 is an side perspective view of a test access point structure with a twisting fixture probe in accordance with the invention;

FIG. 6 is an operational flow chart illustrating a method for implementing a twisting fixture probe in accordance with the invention; and

FIG. 7 is an operational flow chart illustrating a method of testing a test access point structure on a trace of a printed circuit board with a twisting fixture probe in accordance with the invention.

DETAILED DESCRIPTION

Turning now in detail to the invention, on a trace defined in an x-, y-, z-coordinate system where the x-dimension represents the trace width, the y-dimension represents the trace length, and the z-dimension represents the trace thickness, it will be recognized by those skilled in the art that present techniques for test access point placement on a printed circuit board utilize only the x- and y-dimensions. The present invention takes a different approach by taking advantage of the z-dimension, that is, the trace thickness. In this regard, the test access point structure of the invention is a localized “high point” on a printed circuit board trace that does not significantly perturb the impedance of the trace and that can be targeted with a probe. Throughout this specification, test access point structure and bead probe structure are used interchangeably.

As discussed more thoroughly in the above referenced patent applications, FIG. 3 illustrates one potential method to ensure electrical contact between the fixture probe and the test access point structure. FIG. 3 illustrates a side cross-sectional view of a fixture probe contacting a test point access structure in accordance with the invention. As shown in FIG. 3, a printed circuit board 10 comprises a substrate 15, a ground plane 14, and at least one dielectric layer 13 with at least one trace 12 printed, deposited or otherwise attached thereon. A solder mask 16 with a hole formed over the trace 12 at a location where a test access point structure 18 is positioned over the exposed surfaces of the dielectric layer 13 and trace layer 12. Test access point structure 18 is conductively attached to trace 12 within the solder mask hole 17 at the test access point. The test access point structure 18 projects above the exposed surrounding surfaces of the solder mask 16 to form an exposed localized high point on the trace 12 that may be electrically contacted as a test target by a fixture probe 35 during testing of the printed circuit board 10.

As discussed above and shown in FIG. 3, as the fixture probe 35 is brought into initial compressive contact with test access point structure 18, test access point structure will be deformed and form a substantially flat upper surface 32, which will move any potential surface oxide, residues or contaminates and permit a good electrical contact between the fixture probe 35 and the test access point structure 18. The compressive force between the fixture probe and the test access point structure 18 may be from any known means, such as a spring loaded fixture probe 35 with a shaft 36, a spring force mechanism 37 and a substantially flat contact area 38.

For the purposes of discussion, assume the fixture probe has a flat surface that comes in contact with the test access point or bead 18. If the radius of curvature is tight enough, the test access point structure 18 made of solder is subject to deformation when a fixture probe contacts it with a predetermined amount of force. A typical fixture probe force is approximately 4-8 ounces. The yield strength of typical solders (both leaded and lead-free) is approximately 5000 psi. Thus, when a fixture probe compressively contacts a newly formed bead or test access point 18 for the first time, the test access point 18 will deform with a substantially flattened top, as shown in FIG. 3. The flat area 32 grows as the solder deforms, until the flat area 32 is great enough to support the fixture probe force. The process of deforming the bead or test access point 18 displaces any surface oxides, contaminants or residues and gives the fixture probe excellent electrical contact to the solder of the test access point 18.

By way of analogy, one can think of a potato as a bead probe and the potato skin as surface contaminates, residues or oxides. The potato is placed on a flat, hard surface. A second object, representing a fixture probe, having a flat, hard surface with a diameter at least as large as that of the potato is brought into compressive contact with the potato until the surface of the potato begins to deform and flatten. As this happens, the potato skin will be deformed and the flat surface of the second object, representing the fixture probe will come into contact with the inside of the potato, representing uncontaminated solder of the bead probe.

As an exemplary model, the yield strength of solder is approximately 5000 psi or 0.005 pounds per square mil, or 0.08 ounces per square mil. Thus, to support a typical 4-ounce flat probe, the flattened area 32 of the test access point 18 must be 4/0.08 or 50 square mils. Assume a 5 mil wide by 20 mil long bead 18 that is approximately 3 mils high. Assume that when the fixture probe first touches the bead 18, there is no flattened surface area. Then as the fixture probe pushes down on the solder, the area that is flattened 32 approximates an ellipse with a 1:4 width/length ratio. As this area increases, the solder yield begins to slow until there is a “footprint” of 50 square mils, or about ½ the total area of the bead itself. Once the surface area is large enough to support the fixture probe force, no further deformation occurs. Subsequent probing does not produce any further deformation.

Once probing has flattened a bead probe, the solder bead is subject to growing a new layer of oxide coating or other surface contaminants as time passes. Therefore, if the board is returned from the field for re-test or if the board had to pass through a repair and re-test cycle, there may be a new oxide or contaminant layer on the surface of the bead probe 18. Since the flattened bead is already capable of supporting the probe force, no new deformation should occur to displace this oxide or contaminant layer. While lead-free solder (containing a majority of tin) may grow a conductive tin-oxide layer, older leaded solder may grow a lead-oxide layer that is a poor conductor. Thus, this oxide layer or other surface contaminants may adversely affect re-testing.

This problem may be addressed by using a smooth faced probe that rotates or twists about its long axis (around the z axis). FIG. 4A illustrates a twisting fixture probe 40 in contact with a bead probe or test access point structure 18 on a printed circuit board 10 in accordance with the invention. As the twist probe 40 is depressed inside its rifled barrel, an internal flange or other know mechanism causes it to rotate a predetermined number of degrees about the z-axis. This causes the twisting fixture probe 40 to rotate as it contacts the bead probe 18, which causes a wiping action that polishes the bead probe surface 32 clean of any oxide, residue or contaminant buildup. The twisting fixture probe 40 may have a relatively flat or smooth faced tip 44, because at a microscopic scales this is actually a rough surface that grinds the oxide, residue or contaminants off the bead probe surface 32, restoring adequate electrical contact between the bead probe 18 and the twisting fixture probe 40.

FIG. 4B illustrates a top view of bead probe 18 with probe tip 44 superimposed thereon, illustrating the rotating action as fixture probe 40 is brought into compressive contact with bead probe 18. FIG. 4B is shown with some misalignment between the bead probe 18 and the fixture probe tip 44, illustrating that for a bead probe 18 of approximately 3-5 mils by approximately 15-20 mils and a fixture probe tip 44 with a diameter of approximately 23-35 mil.

FIG. 5 illustrates one exemplary embodiment for twisting fixture probe 40, having a cylindrical member 41 housing a shaft 42 attached to a relatively flat tip 44. Shaft 42 causes tip 44 to rotate about the z-axis when brought into compressive contact with bead probe 18 by a force, such as spring force 43. Shaft 42 may be a relatively square shaft that has been twisted about its long axis, like a twisted licorice stick. As the twisted square shaft 42 is compressed by the spring force 43, it may come into contact with a flange or indentation 45 in the wall of the cylindrical member 41, causing the shaft 42 and the tip 44 to rotate. There are other anticipate means for causing tip 44 to rotate a predetermined amount as the tip 44 is brought into compressive contact with the bead probe 18, effectively cleaning the surface 32 of the bead probe 18.

FIG. 6 is an operational flow chart 50 for a method of implementing a twisting fixture probe according to the invention, in which a relatively flat tip member 44 is attached to a rotating member 42, such as a twisted shaft at step 52. The rotating member 42 is brought into operational contact with a rotating mechanism, such as a flange or indentation 45 at step 54. The rotating member is brought into operational contact with a compressive force mechanism 43, such as a spring force at step 56. The rotating mechanism may be other known means, such as a substantially square hole and the end of cylindrical member 41 next to tip 44, such that as the compressive force 43 forces the twisted shaft member 42 through the substantially square hole, the twisted shaft member 42 will rotate, causing the tip 44 to rotate a predetermined amount of rotation.

FIG. 7 is an operation flow chart for a method 60 of probing a bead probe with a twisting fixture probe in accordance with the invention. In operation, a twisting fixture probe 40 in a test fixture is aligned with a test access point structure 18 on a trace 12 of a printed circuit board 10 at step 62. A substantially flat tip 44 of twisting fixture probe 40 is brought into contact with a top surface of a test access point structure 18 at step 64. Electrical contact is ensured as the top surface 32 of test access point structure 18 is cleaned of any oxides, residues or contaminants as the substantially flat tip 44 is rotated by applying a compressive force 43 to a rotating member 42 at step 66. After electrical contact is ensured, tests may be run requiring electrical contact between the test access point structure 18 and the fixture probe 40.

It will be appreciated from the above detailed description of the invention that the present invention uniquely solves the conflict problems faced by traditional techniques for test access point placement on printed circuit boards. 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. For example, the tip of the twisting fixture probe may be a shape other than circular, as shown in FIG. 4B. Also, bead probes may be implemented on either or both sides of a two-sided printed circuit board. It is also possible that other benefits or uses of the currently disclosed invention will become apparent over time.

Claims

1. A method for implementing a twisting fixture probe, said method comprising: attaching a substantially flat tip to a rotating member; attaching the rotating member in operational contact with a rotational mechanism; and attaching the rotating member into operational contact with a compressive force.

2. The method according to claim 1, wherein the substantially flat tip is a substantially circular, flat tip.

3. A method in accordance with claim 2, wherein the substantially circular, flat tip has a diameter of approximately 23-35 mil.

4. A method in accordance with claim 1, wherein the rotating member is a twisted square shaft.

5. A method in accordance with claim 4, wherein the rotational mechanism is a flange in a cylindrical housing surrounding the twisted square shaft.

6. A method in accordance with claim 5, wherein the compressive force is a spring force at a first end of the twisted square shaft opposite to a second end of the twisted square shaft to which the substantially circular, flat tip is attached.

7. A method for probing a solder bead probe on a printed circuit board, said method comprising: aligning a twisting fixture probe with a solder bead probe conductively attached to a trace on a printed circuit board; bringing a tip of the twisting fixture probe into contact with the solder bead probe; and rotating the tip of the twisting fixture probe as the tip is brought into compressive contact with the solder bead probe.

8. The method of claim 7, further comprising the step of running one or more tests with the twisting fixture probe in compressive contact with the solder bead probe.

9. The method of claim 7, wherein the tip of the twisting fixture probe is substantially flat.

10. The method of claim 9, wherein the tip of the twisting fixture probe is substantially flat at the surface that contacts the solder bead.

11. The method of claim 10, wherein the tip of the substantially flat twisting fixture probe is substantially circular.

12. The method of claim 11, wherein the diameter of substantially circular tip of the substantially flat twisting fixture probe is approximately 23-35 mil in diameter.

13. A fixture probe comprising: a substantially flat tip; a rotating member attached to the substantially flat tip; a rotating mechanism attached to the rotating member; and a compressive force mechanism.

14. The fixture probe of claim 13, wherein the substantially flat tip is substantially flat in the x-y plane of the x-, y-, z-coordinate system.

15. The fixture probe of claim 14, wherein the substantially flat tip is substantially circular with a diameter of approximately 23-35 mil.

16. The fixture probe of claim 13, wherein the compressive force mechanism is a spring force.

17. The fixture probe of claim 13, wherein the rotating member is a substantially square twisted shaft.

18. The fixture probe of claim 17, wherein the rotating mechanism is a flange in a housing around the substantially square twisted shaft.

19. The fixture probe of claim 17, wherein the rotating mechanism is an indentation in a housing around the substantially square twisted shaft.

20. A method for cleaning a solder bead probe on a printed circuit board, the method comprising: bringing a fixture probe with a substantially flat tip into contact with the bead probe; and twisting the flat tip of the fixture probe as the fixture probe is held in compressive contact with the solder bead probe.