GB2312519A

GB2312519A - Multiple probe printed circuit board test arrangement

Info

Publication number: GB2312519A
Application number: GB9705959A
Authority: GB
Inventors: George Guozhen Zhong
Original assignee: Individual
Current assignee: Individual
Priority date: 1996-04-23
Filing date: 1997-03-21
Publication date: 1997-10-29
Also published as: GB9705959D0

Abstract

A method and apparatus for testing a printed circuit board 5 comprises two excitation plates 3, 4 in which one excitation plate is arranged either side of the board under test. The excitation plates 3, 4 inject a high frequency signal into the conductive members of the board 5 by means of capacitive coupling. One of the excitation plates 3 includes perforations through which an array of solenoid-actuated probes 31 may obtain access to sample signals from the said conductive members. The excitation plates 3, 4 are formed by conductive plates with a dielectric coating. Translation tables operating in the x-y plane are used to scan the test probes over the board 5. A translation table operating in the z direction may be used to adjust the gap between the excitation plate 3 and the board 5. A ground plate 14, iron tube frame 32 and brass tube frame 15 may be used to provide screening for the probes 22. The solenoid-actuated probes may be activated individually or as a group. The test arrangement may be used in an automatic test system with automatic loading and unloading of the boards to be tested.

Description

AUTOMATIC MULTI-PROBE PWB TESTER This invention relates to a new structure of an automatic test equipment (tester) for detecting shorts and opens of bare printed wiring boards (PWBs or PCBs for printed circuit boards) and its associated test process. The board under test (BUT) can be single-sided boards, double-sided boards, or multi-layer boards; and it can be through-hole-mounted type or surfacemounted type.

Detecting shorts and opens of bare PWBs is an important part of total quality assurance operation for PWB productions. Testers available on the market for detecting shorts and opens of PWBs include two basic types: contacting type and noncontacting type. A traditional (flying-pin) contacting tester usually has two probes. To detect opens of a single conductor (or trace) on a PWB, the two probes have to contact every terminals of the conductor pair by pair and measure the continuity. To detect shorts of conductors is not a straight forward task: first, all of the adjacent conductors have to be found out; then the two probes have to test the terminals of every adjacent conductors pair by pair. It is obvious that this test process is very time consuming and inefficient. Another type of contacting testers use multiple probes in a bed of nails test fixture, which have no drawback of the two probe testers, but need considerably long time to build the fixture and have a limitation of testing fine pitch PWBs because the probes of small test center are fragile and have to be replaced often in production environments. On the other hand, noncontacting testers use visual-band imaging and x-ray imaging techniques for detecting shorts and opens. However, the testers using visual-band imaging can only be used for inspection of single layer boards. The testers using x-ray imaging can assist human visual inspection of multilayer boards, but the test process is not fully automatic. Other limitations of the noncontacting testers include their much higher cost than that of the contacting testers.

I have found that the disadvantages and limitations of the traditional contacting and noncontacting testers can be overcome by a contacting tester with multiple solenoid-actuated probes and two excitation plates. The two excitation plates are set on both sides of a BUT to excite the conductors of the BUT through a capacitance coupling. The solenoid-actuated probes are assembled on a top excitation plate in a two-dimensional array. The top excitation plate is perforated to allow the solenoid-actuated probes to contact the BUT through the small holes to sample signals. The signals sampled from the terminals of the BUT are used for determining the coupling capacitance of the conductors of the BUT with the excitation plates and for composing the electric signature of the BUT. Shorts and opens of the BUT are detected by comparing the electric signature of the BUT with the electric signature of a known good PWB. Since the solenoid-actuated probes can be operated simultaneously or independently to each other, and each of them needs only to sample signals from the terminals in a small portion of the BUT to obtain the electric signature of the BUT, the invented tester has a much faster test speed than the traditional two-probe contacting tester while maintaining a capability of testing not only doublesided PWBs but also multi-layer PWBs (through hole mounted or surface mounted). And the cost of the invented tester is much lower than that of the non-contacting tester (vision system) which needs a scanning mechanism with much higher precision and resolution, a much more powerful (high speed and huge memory) and expensive computer system, and a more sophisticated software to process the huge vision test data.

In drawings which illustrate an embodiment of the invention, Figure 1 is an assembly of the embodiment, Figure 2 is a right side view of this embodiment, Figure 3 shows a detailed partial view of a probe assembly case and relevant components, Figure 4 is a section view of the line A A of Figure 3, Figure 5 shows two conductors connected by a short, and Figure 6 shows a conductor divided by an open. Figures 7 and 8 show another embodiment of the invention.

As is seen from Figure 1 and Figure 2, the tester comprises a probe assembly case 1 which is held by a brace 9 mounted on a z table 10 (single axis table) which is in turn mounted on a frame 8, a top excitation plate 3 which is mounted on the bottom of the probe assembly case 1, a bottom excitation plate 4 fastened on the top of a fixture (or vacuum fixture) 6 which is mounted on the top of astable 11 which is in turn mounted on the top of anxtable 12. Thextable 12 and the frame 8 are fixed on a base 7. A BUT 5 is loaded on the top of the bottom excitation plate 4 and under the top excitation plate 3. The top excitation plate 3 and the bottom excitation plate 4 are made of conductive material with dielectric coating 27. The solenoid-actuated probes are assembled inside the probe assembly case 1. The x, y, and z tables have individually attached displacement transducers and are driven by stepping motors. The stepping motors and the electronic circuits in the probe assembly case are connected to a microcomputer control system (not shown in the Figure) to carry out the feedback motion control of the x, y, and z tables, data acquisition, and analysis process.

Figure 3 shows a detailed partial view of the probe assembly case 1 and relevant components. The probe assembly case 1 has a case wall 29 and a cover plate 2. The solenoidactuated probes 31 are assembled between the top excitation plate 3 and a ground plate 14 in a two-dimensional array as shown in Figures 3 and 4. The top excitation plate 3 is perforated to allow probes 22 to contact the BUT 5 through the small holes when solenoids 24 are powered.

(The holes should be as small as possible to reduce the missing area of excitation, but larger enough to avoid the contact between the probe and the excitation plate.) The probe 22 is attached by an insulation ring 17 and an iron ring 18. The iron ring 18 provides driving force to the probe when the solenoid 24 is powered; the insulation ring 17 provides insulation between the probe 22 and the iron ring 18. The probe 22 is made of conductive material or plastic material with single plated conductor or with double symmetric plated conductors from the tip to the tail of the probe 22. The former can be used for a preamplifier with single-ended input and the latter for a preamplifier with double-ended input. An iron tube frame 32 is used for housing the solenoid 24 and enhancing magnetic flux density of the solenoid 24. A brass or aluminum tube 15 is used for assembling the solenoid 24. The air chambers are formed inside the brass tube frame 15 between the iron tube frame 32 and a cap 16 so that the air in the chambers against the insulation ring 17 and the iron ring 18 damps the fast reciprocating motion of the probe 22 so as to prevent its oscillation and stabilize its contact against the BUT when it is actuated. A plastic bearing 21 is used for guiding the probe 22 in reciprocating motion. A spring 19 is used for raising the probe 22 when the solenoid 24 is de-energized. A sensor board 13 is mounted on the top ofthe ground plate by bolt 25, nut 30, plastic tubes 28, and insulation washer 26 and used for signal pre-amplification, data acquisition, and control of the solenoid-actuated probes. The ground plate 14 screens the sensor board 13 from the top excitation plate 3 to avoid the electromagnetic interference. The iron tube frames 32 and the brass tube frames 15 assembled by the ground plate 14 also screen the probes 22 from the top excitation plate 3. A push-on connector 20 is used for wiring from the probe 22 to a preamplifier on the sensor board 13.

The test process is illustrated as follows: First, the BUT 5 is loaded on the bottom excitation plate 4 and positioned by positioning pins 23. Second, the x table 12 moves the BUT 5 until the BUT 5 is entirely covered by the top excitation plate 3. Then the z table 10 drives the probe assembly case 1 and automatically adjusts the air gap between the top excitation plate 3 and the BUT 5. After the desired air gap is obtained, a high frequency alternate current power source is switched to the excitation plates 3 and 4 which excite the conductors of the BUT 5 through the capacitance coupling. The x-y tables 11 and 12 then start to move the BUT 5 in a step-by-step and row-by-row sequence following a grid pattern1 of the BUT 5. The step displacement of the BUT 5 matches the unit length of the grid pattern, and the row length is equal to the distance which separates the probes 22 from each other in the two-dimensional array.

When the BUT 5 steps to the end of a row, the x table 12 moves the BUT 5 one step to the next row, then the y table 11 continues to move the BUT 5 step by step in the reverse direction through the next row. Since all the terminals are located at the intersections of the grid pattern as recommended by the standards2, each terminal will have a chance to align with a probe 22. When one or several terminals on the BUT 5 align with the probes 22, the corresponding solenoids 24 are powered, the probes 22 make contact with the terminals, and the signals from the terminals are sampled by the microcomputer-based data acquisition system. To realize this scanning and sampling process, the x-y coordinates of every terminals of the BUT 5 are stored in the computer memory so that the microcomputer knows at any instance which terminals are aligned with the probes 22 and, therefore, actuates the solenoids 24 and samples the signals from the terminals accordingly. After the signals are sampled, the solenoids 24 are de-energized and the probes 22 are raised by the springs 19 to release the contact with the BUT 5. This process continues until the signals from all of the terminals on the BUT 5 are sampled. Then the fixture 6 withdraws from the test area under the top excitation plate 3 to unload the BUT 5. For double-sided PWBs, the BUT 5 must be turned over to test the other side after the first side is tested to sample signals from all of the terminals on both sides of the BUT 5. But if a terminal, such as a plated through hole, can be tested from either side of the BUT 5, only one side of the terminal needs to be tested. It is noted that during this test process, each of the probes 22 needs only to sample signals from the terminals which are located in a small square area (one square inch) cornered by the A modular grid system to identify all holes, test points, lands, and overall board dimensions with modular units of length of 0.1, 0.05, 0.025, or other multiples of 0.005 inch in that order of preference.

2 MIL-STD-275E, MILSTD-2118, and IPC-D-300G.

adjacent probes. A terminal is defined as an end part of a conductor, that is, a node (such as a land, a pad, a plated hole, or a test point) with only one conductor entry. For example, a node is not a terminal if it has two conductor entries, even one from each side of the BUT.

The method of the fault detection is based on the following principle: Each conductor of the BUT has an inherent coupling capacitance with the excitation plates. Theoretically, the magnitude of signals sampled from a terminal of the BUT is proportional to the coupling capacitance between the terminal- connected conductor and the excitation plates. The coupling capacitance is in turn proportional to the effective coupling area between the conductor and the excitation plates. If the conductor has shorts or opens, the effective coupling area of the conductor changes so that the magnitude of the signals at the terminals of the conductor changes.

Therefore, the magnitude of the signal sampled from a terminal of the conductor can be used for determining the coupling capacitance of the conductor and the coupling capacitance of the conductor can be used as the electric signature of the conductor for the fault detection. All of the electric signatures of the conductors of the BUT compose the electric signature of the BUT. This electric signature is compared with the electric signature of a known good PWB and the opens or shorts of the BUT can thus be detected by the comparison.

It is emphasized that the test process for detecting shorts and opens needs to sample signals from every terminals to compose the electric signature of the BUT because shorts or opens of a conductor have significant effects on at least one terminal of the conductor but not on every terminals of the conductor. Figure 5 and 6 show two simple cases where a short and an open change the effective coupling area of the conductors. Figure 5 shows two conductors connected by a short; Figure 6 shows a conductor divided by an open. In these two cases, the short and the open only have significant effects on the terminals pointed by arrows. In fact, it can be concluded that the maximum variation in the effective coupling area of the conductors is greater than half of the effective coupling area of their original conductors. The similar conclusions can also be drawn from cases of double-sided or multi-layer PWBs and cases of conductors with branches.

Therefore, a criterion which is implemented in the test process for detecting shorts and opens can be stated as follows: a short or an open may exist in a conductor if the electric signature of the conductor is different from the electric signature of a known good PWB and the difference in the coupling capacitance of the conductor is greater than 50%. This is a conservative fault detecting criterion assuming that a short and an open do not concur near a terminal of a conductor, but the percentage value of the criterion can easily be modified to suit a particular application. Since this fault detecting method is based on the significant difference between the BUT and the known good PWB, it is not susceptible to the electric noise influence.

Since increasing the effective coupling area will increase the variation in the effective coupling area caused by shorts or opens, and since a large variation in the effective coupling area will result in a large variation in the magnitude of signals at the terminals during a test, it is therefore desired to increase the effective coupling area between the conductors and the excitation plates so as to increase the sensitivity of the tester to detect the faults. This illustrates the advantage of the invented tester using two excitation plates on both sides of the BUT to increase the effective coupling area.

In the embodiment shown in Figure 4 there are a total of 576 (24x24) solenoid-actuated probes assembled on the top excitation plate and the probes are separated by one inch from each other in a two-dimensional array; but other number of probes and separating dimensions may be used if desired.

In the embodiment shown in Figures 1 and 2, the probe assembly case 1 is hanged by the brace 9 which is driven by the z table 10 mounted on the frame 8 for adjusting air gap between the top excitation plate 3 and the BUT 5 but, if desired, the probe assembly case 1 can also be hinged with the frame 8 and be lifted up and lowered down by an automatic mechanism for loading and unloading BUT 5, and the air gap between the top excitation plate 3 and the BUT 5 can be set manually for each type of PWBs.

Although the presented invention is mainly described as a tester for the purpose of testing bare PWBs, it may readily be applicable to PWBs with surface-mounted electronic components.

Although the presented invention is mainly described as a contacting tester, the solenoidactuated probes can be replaced by capacitance-type pickups to construct a noncontacting tester.

This modification needs a different fault detecting criterion and results in different performance characteristics of the tester, but the basic structure of the tester is unchanged.

Although the present invention has been fully described by way of example with reference to the accompanying drawings, it is to be noted that various changes and modifications will be apparent to those skilled in the art. Therefore, unless otherwise such changes and modifications depart from the scope of the present invention, they should be constructed as being included therein.

Claims

CLAIMS The embodiment of the invention in which an exclusive property or privilege is claimed is defined as follows:

1. A tester for automatically detecting shorts and opens of bare PWBs or PWBs with surface-mounted electronic components, comprising a top excitation plate, a bottom excitation plate, multiple solenoid-actuated probes, a probe assembly case, a sensor board, a ground plate, a fixture, and x, y, and z tables, as well as an associated microcomputer-based automatic test process.

2. A tester as defined in claim 1, in which the top excitation plate and the bottom excitation plate are mounted on both sides of a BUT, cover the entire BUT, and excite conductors of the BUT through a capacitance coupling during test.

3. A tester as defined in claim 1 and 2, in which the bottom excitation plate is mounted on the fixture on the top ofthe x-y tables so that the x-y tables can drive the BUT to realize scanning and sampling process.

4. A tester as defined in claim 1 and 2, in which the top excitation plate is mounted on the bottom of the probe assembly case which is held and driven by the z table to adjust the air gap between the top excitation plate and the BUT.

5. A tester as defined in claim 1 and 2, in which the top excitation plate is perforated to allow the solenoid-actuated probes, which are assembled on the top of the top excitation plate, to contact the terminals on the BUT through the small holes to sample signals.

6. A tester as defined in claim 1, in which the solenoid-actuated probes are assembled between the top excitation plate and the ground plate in a two-dimensional array and can be operated independently to each other to sample signals from terminals on the BUT so that each of the solenoid-actuated probes needs only to sample signals from the terminals in a small square area cornered by the adjacent probes.

7. A tester as defined in claim 1, in which the ground plate is mounted between the sensor board and the top excitation plate for assembling the solenoid-actuated probes and for screening the sensor board and the probes from the excitation plate to avoid electromagnetic interference.

8. A tester as defined in claim 1, in which each of the solenoid-actuated probes comprises a brass tube frame, an iron tube frame, a solenoid, a probe attached by an insulation ring and an iron ring for actuating the probe when the solenoid is powered, a plastic bearing for guiding the probe in reciprocating motion, and a spring for raising the probe when the solenoid is deenergized.

9. A tester as defined in claim 1 and 8, in which the air chambers are formed in the tube frames so that the air in the chambers against the insulation ring and the iron ring damps the fast reciprocating motion of the probe, prevents its oscillation, and stabilizes its contact against the BUT when it is actuated.

10. A tester as defined in claim 1, in which the automatic test process is achieved by using multiple solenoid-actuated probes, and each of the probes only samples signals from terminals in a small portion, cornered by four adjacent probes, of the BUT in the row by row scanning manner. The automatic test process includes two basic steps: first, the electric signature of the BUT is composed by the coupling capacitance of the conductors with the excitation plates, which is determined by the magnitudes of signals which are sampled from every terminals of the BUT; second, the electric signature of the BUT is compared with the electric signature of a known good PWB and the shorts and opens of the BUT are detected by using a fault detecting criterion which states that a short or an open may exist in a conductor of the BUT if the electric signature of the conductor is different from the electric signature of the known good PWB and the difference in the coupling capacitance of the conductor is greater than 50%.

Amendments to the claims have been filed as follows The embodiment of the invention in which an exclusive property or privilege is claimed is defined as follows: I. A tester for automatically detecting shorts and opens of bare PWBs or PWBs with surface-mounted electronic components, comprising a top excitation plate, a bottom excitation plate, multiple solenoid-actuated probes, a probe assembly case, a sensor board, a ground plate, a fixture, and x, y, and z tables, as well as an associated microcomputer-based automatic test process.

8. A tester as defined in claim 1, in which each of the solenoid-actuated probes comprises a brass tube frame, an iron tube frame, a solenoid, a probe attached by an insulation ring and an iron ring for actuating the probe when the solenoid is powered, a plastic bearing for guiding the probe in reciprocating motion, and a spring for raising the probe when the solenoid is deenergized

10. A tester as defined in claim 1, in which the automatic test process is achieved by using multiple solenoid-actuated probes and each of the probes only samples signals from terminals in a small portion, cornered by four adjacent probes, of the BUT in the row by row scanning manner. The automatic test process includes two basic steps: first, the electric signature of the BUT is composed by the coupling capacitance of the conductors with the excitation plates, which is determined by the magnitudes of signals which are sampled from every terminals of the BUT; second, the electric signature of the BUT is compared with the electric signature of a known good PWB and the shorts and opens of the BUT are detected by using a fault detecting criterion which states that a short or an open may exist in a conductor of the BUT if the electric signature of the conductor is different from the electric signature of the known good PWB and the difference in the coupling capacitance of the conductor is greater than 50%.

11. An apparatus (tester) for automatically testing a printed wiring board (pWB) including electrically conductive paths, terminals, parts and surfaces whose electrical and physical continuity and conformance to a known standard is to be verified, comprising: (a) two dielectrically coated excitation plates placed on both sides of a BUT for injecting high frequency signals into conductors of the BUT through a capacitive coupling between the conductors of the BUT and the two excitation plates, one of which, being perforated and being placed on the top of the BUT, called top excitation plate, the other, being placed on the bottom of the BUT, called bottom excitation plate; (b) an integrated solenoid-actuated probe assembly including a large number (at least four) of integrated solenoid-actuated probes which are mounted on the top excitation plate in a two-dimensional array, separated evenly from each other in the x and y directions in a plane parallel to the surface of the BUT, which can be independently actuated in the z direction to sample signals from terminals of the conductors of the BUT, but can not independently move in the x or y direction; (c) one or several sensor boards, being part of the integrated solenoid-actuated probe assembly, for signal amplification and data acquisition; (d) motorized x and y tables for driving the BUT along with the bottom excitation in a stepby-step and row-by-row sequence following the grid pattern of the BUT in the x and y directions so that the integrated solenoid-actuated probes mounted on the top excitation plate can sample signals from terminals, which are momentarily in alignment with the probes, of the conductors of the BUT to realize the test process of scanning, sampling, and data acquisition; (e) adjusting means for adjusting the air gap and maintaining a desired air gap between the top excitation and the BUT; (f) a fixture mounted on the top of the x and y tables; (g) means for loading/unloading the BUT to/from, and positioning the BUT on the bottom excitation plate; (h) a computer system for analyzing the sampled signals to detect defects in the BUT.

12. A tester as claimed in Claim 11 wherein each of the integrated solenoid-actuated probes comprises a brass tube frame, an iron tube frame, a solenoid, a probe attached by an insulation ring and an iron ring for actuating the probe when the solenoid is powered, a plastic bearing for guiding the probe in reciprocating motion, and a spring for raising the probe when the solenoid is de-energized.

13. A tester as claimed in Claim 11 and Claim 12 wherein each of the integrated solenoid-actuated probes has air chambers formed in the brass tube frame and the iron tube frame so that the air in the chambers against the insulation ring and the iron ring damps the fast reciprocating motion of the probe, to prevent its oscillation and to stabilize its contact against the BUT when it is actuated.

14. A tester as claimed in Claim 11 wherein the integrated solenoid-actuated probe assembly comprising: (a) a large number (at least four) of solenoid-actuated probes which are mounted on the top excitation plate in a two-dimensional array, separated evenly from each other in the x and y directions in a plane parallel to the surface of the BUT, which can be independently actuated in the z direction, to sample signals from terminals of the conductors of the BUT, but can not independently move in the x or y directions; (b) the perforated top excitation plate; (c) one or several sensor boards, placed above and in close proximity to the integrated solenoid-actuated probes for signal amplification and data acquisition; (d) a ground plate between the sensor boards and the integrated solenoid-actuated probes as a means to avoid electromagnetic interference; (e) means for fastening the top excitation plate, the solenoid-actuated probes, the ground plate, and the sensor boards together but electrically insulating them from each other.

(f) a probe assembly case for housing the integrated solenoid-actuated probe assembly and for mounting the probe assembly to the adjusting means.

15. A tester as claimed in Claim 11 and Claim 14 wherein the top excitation plate is perforated and is mounted on the bottom of the probe assembly case, wherein the perforated small holes on the top excitation plate are in alignment with the probes, one perforated small hole for each probe, so that the probes can be actuated to contact terminals of conductors of the BUT and sample signals from the terminals through the perforated small holes.

16. A tester as claimed in Claim 11 and Claim 14 wherein the top excitation plate is perforated and the size of the perforated holes is as small as possible to reduce the area of missing excitation but large enough to allow the probes to contact the BUT through the perforated holes without contact with the top excitation probe.

17. A tester as claimed in Claim 11 wherein the bottom excitation plate is mounted on the top of the fixture which is in turn mounted on the top of the motorized x and y tables, wherein the said excitation plate and the fixture have positioning means so that the BUT can be loaded on the top of the said excitation plate in a definite position.

18. An test method for automatically detecting shorts and opens of PWB(s) comprising steps of: (a) using the top excitation plate and the bottom excitation plate for injecting high frequency signals into conductors of the BUT through a capacitive coupling between the conductors of the BUT and the two excitation plates; (b) using a large number of the integrated solenoid-actuated probes to sample signals from every terminals of conductors of the BUT (each terminal having only one conductor entry); (c) each of the solenoid-actuated probes only samples signals from the terminals in a small rectangular area, cornered by adjacent solenoid-actuated probes, of the BUT; (d) using the motorized x and y tables for driving the BUT along with the bottom excitation plate in a step-by-step and row-by-row sequence following a grid pattern of the BUT in the x and y directions in a plane parallel to the surface of the BUT so that the integrated solenoid-actuated probes mounted on the top excitation plate can sample signals from terminals, which are momentarily in alignment with the probes, of the conductors of the BUT to realize the test process of scanning, sampling, and data acquisition; (e) using the magnitudes of sampled signals from all the terminals of conductors of the BUT, which are in principle proportional to the coupling capacitance between the conductors of the BUT, the top excitation plate, and the bottom excitation plate to compose the electric signature of the BUT; (f) comparing the electric signature of the BUT with the electric signature of a known good PWB and detecting short and opens of the BUT by using a fault detecting criterion which states that a short or an open may exist in a conductor of the BUT if the magnitude of the sampled signal from one of the terminals of the conductor is different from the magnitude of the known good PWB and the difference is greater than a preset threshold.

19. A test method for automatically detecting shorts and opens of PWB(s) as claimed in Claim 18 wherein the preset threshold would be 50%.

20. An apparatus (tester) for automatically testing a printed wiring board (PWB) including electrically conductive paths, terminals, parts and surfaces whose electrical and physical continuity and conformance to a known standard is to be verified, comprising: (a) two (or more than two) dielectrically coated excitation plates which are placed on both sides of a BUT for injecting high frequency signals into conductors of the BUT through a capacitive coupling between the conductors of the BUT and the two excitation plates, at lease one of which is perforated and called perforated excitation plate; (b) an integrated solenoid-actuated probe assembly including a large number of integrated solenoid-actuated probes which are mounted on the perforated excitation plate in a two-dimensional array, separated evenly from each other in the x and y directions in a plane parallel to the surface of the BUT, and which can be independently actuated in the z direction to sample signals from terminals of the conductors of the BUT through the perforated small holes in the perforated excitation plate; (c) driving means for generating the x and y relative displacements between the BUT and the integrated solenoid-actuated probe assembly to realize the test process of scanning and sampling; (d) positioning means for determining the x and y positions of the BUT relatively to the integrated solenoid-actuated probe assembly; (e) adjusting means for adjusting the air gap and maintaining a desired air gap between the perforated excitation plate and the BUT; (f) means for loading and unloading the BUT; (g) means for computerized analysis of the sampled signals to detect faults in the BUT.

21. A tester as claimed in Claim 20 wherein the integrated solenoid-actuated probe assembly comprising: (a) the perforated excitation plate; (b) a large number of solenoid-actuated probes which are mounted on the perforated excitation plate in a two-dimensional array, separated evenly from each other in the x and y directions in a plane parallel to the surface of the BUT, and which can be independently actuated in the z direction, to sample signals from terminals of the conductors of the BUT through the perforated small holes in the perforated excitation plate; (c) one or several sensor boards placed in close proximity to the integrated solenoid-actuated probes for signal amplification and data acquisition; (d) a ground plate between the sensor boards and the integrated solenoid-actuated probes as a means to avoid electromagnetic interference; (e) means for fastening the perforated excitation plate, the solenoid-actuated probes, the ground plate, and the sensor boards together but electrically insulating them from each other.

22. A tester as claimed in Claim 20 wherein each of the integrated solenoid-actuated probes comprises a brass tube frame, an iron tube frame, a solenoid, a probe attached by an insulation ring and an iron ring for actuating the probe when the solenoid is powered, a plastic bearing for guiding the probe in reciprocating motion, and a spring for raising the probe when the solenoid is de-energized; the air chambers are formed in the brass tube frame and the iron tube frame so that the air in the chambers against the insulation ring and the iron ring damps the fast reciprocating motion of the probe, to prevent its rebound and to stabilize its contact against the BUT when it is actuated.

23. A tester as claimed in Claim 20 wherein the perforated small holes in the perforated excitation plate are in alignment with the probes, one perforated small hole for each probe, and the size of the perforated small holes is large enough to allow the probes to contact the BUT and sample signals from the terminals through the perforated small holes without contact with the perforated excitation plate.

24. A tester as claimed in Claim 20 wherein the positioning means for determining the x and y positions of the BUT relatively to the integrated solenoid-actuated probe assembly are the positioning pins which are mounted on one of the excitation plates attached to the fixture.

25. A tester as claimed in Claim 20 wherein the driving means are the motorized x and y tables which drive the BUT along with one of the excitation plates attached to the fixture to generate the x and y relative displacements between the BUT and the integrated solenoid-actuated probe assembly to realize the test process of scanning and sampling.

26. An test method for automatically testing a printed wiring board (PWB) including electrically conductive paths, terminals, parts and surfaces whose electrical and physical continuity and conformance to a known standard is to be verified, comprising steps of: (a) using two (or more than two) excitation plates which are placed on both sides of the BUT for injecting high frequency signals into conductors of the BUT through a capacitive coupling between the conductors of the BUT and the two excitation plates, at least one of which is perforated and called perforated excitation plate; (b) using a large number of integrated solenoid-actuated probes which are mounted on the perforated excitation plate in a two-dimensional array, separated evenly from each other in the x and y directions in a plane parallel to the surface of the BUT, operated independently in the z direction, to sample signals from terminals of conductors of the BUT through the perforated small holes in the perforated excitation plate, each of the solenoid-actuated probes only sampling signals from the terminals in a small rectangular area, cornered by the adjacent solenoid-actuated probes, of the BUT; (c) generating the x and y relative displacements between the BUT and the integrated solenoid-actuated probes to realize the test process of scanning and sampling; (d) analyzing the sampled signals to detect faults of the BUT.

27. A test method as claimed in Claim 26 wherein the test process of scanning and sampling is realized by: (a) using the motorized x and y tables for driving the BUT in a step-by-step and row-by-row sequence following a grid pattern of the BUT in the x and y directions in a plane parallel to the surface of the BUT; (b) using the integrated solenoid-actuated probes which are mounted on the perforated excitation plate to sample signals from the terminals, which are momentarily in alignment with the probes, of the conductors of the BUT.

28. A test method as claimed in Claim 26 wherein the faults of the BUT are shorts and opens.

29 A test method as claimed in Claim 26 wherein analyzing the sampled signals to detect faults is realized by: (a) using the magnitudes of sampled signals from the terminals of conductors of the BUT, which are in principle proportional to the coupling capacitance between the conductors of the BUT and the two excitation plates to compose the electric signature of the BUT; (b) comparing the electric signature of the BUT with the electric signature of a known good PWB and detecting faults of the BUT by using a fault detecting criterion which states that a fault may exist in a conductor of the BUT if the magnitude of the sampled signal from one of the terminals of the conductor is different from the magnitude of the known good PWB and the difference is greater than a preset threshold.

30. An apparatus for automatically testing a printed wiring board (PWB) including electrically conductive paths, terminals, parts and surfaces whose electrical and physical continuity and conformance to a known standard is to be verified, comprising: (a) at least one integrated solenoid-actuated probe assembly including a large number of integrated solenoid-actuated probes which are mounted on a perforated plate in a twodimensional array, separated evenly from each other in the x and y directions in a plane parallel to the surface of the BUT, and which can be independently actuated in the z direction to sample signals from conductors of the BUT; (b) driving means for generating the x and y relative displacements between the BUT and the integrated solenoid-actuated probe assembly to realize the test process of scanning and sampling; (c) positioning means for determining the x and y positions of the BUT relatively to the integrated solenoid-actuated probe assembly; (d) adjusting means for adjusting the air gap and maintaining a desired air gap between the perforated plate and the BUT; (e) means for loading and unloading the BUT; (f) means for computerized analysis of the sampled signals to detect faults or defects in the BUT.

31. A tester as claimed in Claim 20 wherein each of the integrated solenoid-actuated probes comprises: (a) a solenoid with an iron tube frame for generating electro-magnetic field; (b) a probe which runs through the solenoid and is attached by an insulation ring and an iron ring for actuating the probe when the solenoid is powered; (c) means for guiding the linear motion of the probe and for insulating the probe from the iron tube frame; (d) means for raising the probe off the PWB when the solenoid is de-energized.

32. A tester as claimed in Claim 20 wherein each of the integrated solenoid-actuated probes comprises: (a) a solenoid with an iron tube frame for generating electro-magnetic field; (b) a probe which runs through the solenoid and is attached by an insulation ring and an iron ring for actuating the probe when the solenoid is powered; (c) the air chambers which are formed in the iron tube frame as damping means to prevent bounce of the probe and to stabilize its contact against the BUT when it is actuated; (d) means for guiding the linear motion of the probe and for insulating the probe from the iron tube frame; (e) means for raising the probe off the PWB when the solenoid is de-energized.

33. An test method for automatically testing a printed wiring board (PWB) including electrically conductive paths, terminals, parts and surfaces whose electrical and physical continuity and conformance to a known standard is to be verified, comprising steps of: (a) injecting signals into the BUT; (b) using at least one integrated solenoid-actuated probe assembly including a large number of integrated solenoid-actuated probes which are mounted on a perforated plate in a twodimensional array, separated evenly from each other in the x and y directions in a plane parallel to the surface of the BUT, and which can be independently actuated in the z direction to sample signals from conductors of the BUT, each of the solenoid-actuated probes only sampling signals from the conductors in a small portion of the BUT; (c) generating the x and y relative displacements between the BUT and the integrated solenoid-actuated probes to realize the test process of scanning and sampling; (d) analyzing the sampled signals to detect faults of the BUT.

33. An apparatus for automatically testing a printed wiring board including electrically conductive paths, terminals, parts and surfaces whose electrical and physical continuity and conformance to a known standard or specification is to be verified, comprising: (a) means for injecting signals into conductors of the BUT; (b) at least one integrated probe assembly including a large number of integrated solenoid-actuated probes, which are fixed in a two-dimensional array, separated evenly from each other in the x and y directions in a plane or planes parallel to the surface of the BUT, and which can be independently actuated in the z direction to sample signals from test points of the BUT; (c) driving means for generating the x and y relative displacements between the BUT and the integrated probe assembly to realize the test process of scanning and sampling; (d) positioning means for determining the x and y positions of the BUT relatively to the integrated probe assembly; (e) adjusting means for adjusting the air gap and maintaining a desired air gap between the integrated probe assembly and the BUT; (f) means for loading and unloading the BUT; (g) means for computerized analysis of the sampled signals to detect faults or defects in the BUT.

34. The tester as claimed in Claim 33 wherein each of the integrated solenoid-actuated probes comprises: (a) a solenoid with an iron tube frame for generating electro-magnetic field; (b) a probe which is attached by an insulation ring and an iron ring and runs through the center of the solenoid for actuating the probe when the solenoid is powered; (c) means for guiding the linear motion of the probe and for insulating the probe from the iron tube frame; (d) means for raising the probe off the BUT when the solenoid is de-energized

35. An test method for automatically testing a printed wiring board including electrically conductive paths, terminals, parts and surfaces whose electrical and physical continuity and conformance to a known standard or specification is to be verified, comprising steps of: (a) injecting signals into the BUT; (b) using at least one integrated probe assembly including a large number of integrated solenoid-actuated probes, which are fixed in a two-dimensional array, separated evenly from each other in the x and y directions in a plane or planes parallel to the surface of the BUT, which can be independently actuated in the z direction to sample signals from test points of the BUT, and each of which only samples signals from the test points in a small rectangular portion of the BUT; (c) generating the x and y relative displacements between the BUT and the integrated solenoid-actuated probes to realize the test process of scanning and sampling; (d) analyzing the sampled signals to detect faults or defects of the BUT.

36. The test method as claimed in Claim 35 wherein the test process of scanning and sampling is realized by: (a) using the motorized x and y tables for generating the x and y relative displacements between the BUT and the integrated probe assembly in a step-by-step and row-byrow sequence following the grid pattern of the BUT; (b) using the integrated solenoid-actuated probes for sampling signals from the test points which are momentarily in alignment with the probes.