IL96288A - Probe and apparatus including same for testing printed circuit boards and other like articles - Google Patents

Description

D ' DDT 1 a D ?>yn ni m οαί> ΊΠΙΝ ί?ίη:>η ipnm α·»ΊΠΚ .'nil D'C'IDI PROBE AND APPARATUS INCLUDING SAME FOR TESTING PRINTED CIRCUIT BOARDS AND OTHER LIKE ARTICLES PROBE AND APPARATUS INCLUDING SAME FOR TESTING PRINTED CIRCUIT BOARDS AND OTHER LIKE ARTICLES The present . invention relates to a probe, and also to apparatus including such a probe, for testing single-layer or multi-layer PCBs (printed circuit boards), MCMs (multi-chip modules), and other articles having electrically-conductive nets (e.g., electrically-conductive pathways) on a surface thereof. The invention is particularly useful in apparatus for electrically testing for discontinuities in nets, and/or for shorts between nets, of PCBs, hybrid circuits, MCMs and other like articles, and therefore the invention is described below particularly with respect to such testing apparatus.

Electrical testing apparatus of the foregoing type commonly includes a plurality of probe elements for contact with pads (test or functional points) of the nets for measuring the resistance of the net between selected pads to indicate a discontinuity in the net, or for measuring capacitance between selected nets to indicate a short between nets.

One known type of testing apparatus for this purpose is based on "flying probes". Such apparatus includes a line of spring-biased pins that are pressed into contact with the PCB, MCM, or other article to be tested, as the pins are moved with respect to the article, or vice versa, to create contact with each net to be tested in its turn. This type of apparatus, however, is very slow and therefore not economical for testing most types of PCBs and similar articles being produced at high volume.

A more popular apparatus for testing PCBs and similar articles in high volume production is based on the use of a "bed of nails" probe fixture. Such a fixture includes a large number of spring-biased pins arranged in the form of a matrix or grid pressed into contact with the tested article. However, such a probe fixture generally must be custom-made for each type of article to be tested, and its expense increases drastically with an increase in resolution because of the mechanical connections required between the pins and the pads. As a practical matter, the probe resolution using such a fixture is limited to about 20 mils, and the test accuracy decreases with increased resolution. This is not sufficient for many applications at the present time, and will become even less sufficient in the future with the increasing density of nets in PCBs and SMT (surface mounting technology) strips, and very dense MC s . Moreover, the testing time in existing machines using a "bed of nails" probe fixture is relatively long, and will become longer as the pad density increases, because of the special arrangements required for providing appropriate interfaces between the probe fixture and the output circuitry.

Another disadvantage in using both the "bed of nails" and the "flying" probes is that the spring-biased pins in such probes are liable to damage the surface of the PCB or other article to be tested, in particular, thin film products such as MCMs.

It has also been proposed to use testing apparatus which includes selectively illuminated photoconductive gates. For example, US Patent 4,709,141 discloses such apparatus for testing an array of infrared detectors which are individually addressed. Since the purpose of the claimed apparatus is to test separate infrared detectors, one element is switched at a time. Therefore, such apparatus would not be usable for testing a plurality of pads simultaneously, or for testing the conductivity of two circuit elements on a straight line.

An object of the present invention is to provide a probe for use in electrically testing PCBs (particularly SMT boards), MCMs, and other similar articles having advantages in the above respects. More particularly, an object of the present invention is to provide a probe for use in testing such articles which permits a much higher probe resolution and a faster test time, which is less liable to cause damage to the tested articles, and which does not require expensive custom-made fixtures for the tested articles.

Another object of the invention is to provide apparatus utilizing the novel probe for testing PCBs and other similar articles.

According to the present invention, there is provided a probe for use in testing printed circuit boards multi-chip modules, and other articles having electrically-conductive nets on a surface thereof, the probe having a plurality of probe elements for contact with pads of the nets for measuring an electrical characteristic between selected pads to provide an indication of faults, characterized in that: the probe includes a light-transmissive member carrying the plurality of probe elements on one surface thereof; and in that each of the probe elements includes an inner layer of a light-transmissive electrically-conductive material in a form of a plurality of spaced parallel strips, an outer layer of an electrically-conductive material for contact with the pads of the article to be tested, and an intermediate layer of a photoconductive material forming a photoconductive gate whose electrical conductivity substantially increases when illuminated by light, the outer layer being in the form of a plurality of discrete deposits forming contacts for each of the parallel strips but normally insulated therefrom by the off-condition of the photoconductive gates . 4a - According to further features in preferred embodiments of the invention described below, the strips are arranged in a manner extending along one orthogonal axis of the light-transmissive member and spaced from each other along the other orthogonal axis .

According to still further features in the described preferred embodiments, the light-transmissive member further includes electrical output means, comprising at least one, and preferably two, additional strips of a light-transmissive electrically-conductive ma^ rial extending perpendicularly to and intersecting the plurality of parallel strips and insulated therefrom by further photoconductive gates. It is contemplated, however, that the mentioned output means could be in the form of other types of gates, e.g., transistor gates, and/or that the gates could be externally of the probe and connected thereto by electrical wires or other forms of conductive pathways .

According to another described embodiment, the inner layer of the light-transmissive, electrically-conductive material, and the intermediate layer of photoconductive material, are each in the form of a continuous layer, and the outer layer of electrically-conductive material is in the form of a plurality of discrete deposits forming contacts for the inner electrically-conductive layer but normally insulated therefrom by the photoconductive layer.

According to still further described preferred embodiments, the light-transmissive member is a flexible membrane deformable to provide good electrical contact between the contacts of the probe elements and the pads of the tested article. In one described embodiment, the light-transmissive membrane is a wall of an inflatable chamber which, when inflated, presses the contacts of the probe into firm engagement with the pads of the tested article. In a second described embodiment, the probe includes means for applying suction between the light-transmissive membrane and the article to be tested; and in a still further described embodiment, the light-transmissive membrane is of a resilient material deformable under mechanical pressure to enable its contacts to be firmly pressed into engagement with the pads of the tested article .

The invention also provides apparatus for testing PCBs and other similar articles comprising a table for receiving the article to be tested, a probe as described above for application to the article to be tested with the contacts of the probe elements in firm engagement with the pads on the surface of the article, and illuminating means for illuminating selected locations of the light-transmissive member for selectively actuating the photoconductive gates.

Preferably, the illuminating means comprises one or more lasers for producing a plurality of laser beams, and deflectors for deflecting the laser beams to the selected locations of the light-transmissive membrane.

As will be apparent from the description below, probes and testing apparatus constructed in accordance with the foregoing features permit a much higher probe resolution, e.g., in the order of 4 mils, as compared to the 20 mils probe resolution permitted by the previously known constructions. Moreover, the testing may be effected faster, is less liable to damage the tested article, and does not require special arrangements (e.g., interface cards) as compared to the testing apparatus presently being used.

The invention also provides a method of testing printed circuit boards, multi-chip modules, or other like articles, comprising: applying a probe as described above to selected pads of the article; illuminating selected locations of the light-transmissive photoconductive gates of the probe; and measuring an electrical characteristic between selected pads to thereby provide an indication of faults in the article. Examples of the electrical characteristic measured include resistance, capacitance, charging currents, and discharging currents.

Further features and advantages of the invention will be apparent from the description below.

The invention is herein described, by way of example only, with reference to the accompanying drawings, wherein: Fig. 1 schematically illustrates one form of testing apparatus constructed in accordance with the present invention; Fig. 2 is a plan view illustrating the construction of the probe used in the testing apparatus of Fig. 1 ; Fig. 3 illustrates the construction of one of the probe elements in the probe of Fig. 2; Fig. 4 illustrates the construction of one of the probe elements in the probe of Fig. 2 at its intersection with the two output strips formed on the probe; Fig. 5 diagrammatically illustrates the manner of using the probe of Figs. 2-4 for testing both for an interruption in a net and a short between nets; Fig. 6 illustrates a modification in the apparatus of Fig. 1; and Fig. 7 illustates a modification in the construction of the probe particularly for measuring charging and discharging currents .

The apparatus illustrated in Fig. 1 is intended for testing PCBs, SMT boards, MCMs and like articles having electrically-conductive nets on a surface which are to be tested both for interruptions in any particular net, and for shorts between nets. The illustrated apparatus includes a table 2 for receiving the article, designated PCB, to be tested, and a probe, generally designated 4, having a membrane 6 carrying a plurality of probe elements to be pressed into firm contact with the pads of the nets in the tested article. In the embodiment illustated in Fig. 1, membrane 6 is one wall of an inflatable chamber 8, which chamber is inflated by pressurized gas supplied from a tank 10 under the control of a valve 12.

Membrane 6 is of a light-transmissive material, e.g., silicon rubber, capable of transmitting laser beams produced from a laser 14 under the control of an optical system generally designated 16. Optical system 16 splits the beam. from laser 14 into four beams 14a-14d. These beams are separately controlled by separate beam-splitting scanning mirrors, schematically designated as 16a-16d, to selectively illuminate any one of four spots simultaneously on the light-transmissive membrane 6.

The construction of the light-transmissive membrane 6, and particularly its plurality of probe elements each designated 18, is illustrated in Figs. 2-4.

Thus, as shown particularly in Fig. 2, the surface of membrane 6 to face the PCB under test is formed with a plurality of parallel strips 20 of light-transmissive electrically-conductive material extending along one orthogonal axis (horizontally, Fig. 2 ) and spaced from each other along the other orthogonal axis (vertically, Fig. 2). Strips 20 carry the plurality of probe elements 18 which are brought into direct contact with the PCB under test. Light-transmissive membrane 6 further carries two additional strips 22, 24, also of light-transmissive electrically-conductive material extending perpendicularly to and intersecting all the strips 20. Strips 22, 24 are normally insulated from strips 20 as will be described below, and serve as input/output conductors for the probe elements 18 carried by strips 20.

The probe elements 18 carried by the light-transmissive electrically-conductive strips 20 are discrete elements consisting of a plurality of layers, shown in Fig. 3, which may be deposited in any suitable manner, e.g., via a mask or screen, as follows: a first layer 30 of a photoconductive material whose electrical conductivity substantially increases when illuminated by light (e.g., laser beams 14a-14d); a layer of tungsten 32 thereover; a layer of nickel 34 thereover; and a layer of gold 36.

All the light-transmissive electrically-conductive layers 20, 22, and 24, may be of indium oxide (I^O^) gold, etc. The photoconductive layer 30 is preferably of a doped amorphous silicon and acts as a gate in response to being illuminated by a laser beam and effects an electrical connection between the metal layers 32-36 and the light-transmissive electrically-conductive strip 20. Since layer 30 is normally insulating, it can be applied to the strips 20 as a continuous layer rather than in discrete spots. The tungsten layer 32 and nickel layer 34 provide long-life contacts with the PCB under test. The gold layer 36, which may be applied by evaporation, provides good electrical contact with the PCB.

Before strips 20 carrying the probe elements 18 are applied to the membrane 6, the two strips 22, 24, serving as the input/output conductors, are applied to the respective face of the membrane. After strips 22, 24 are applied, they are covered by discrete (or continuous) deposits of the photoconductive material as shown at 22a, 24a in Fig. 4.

Thus, the input/output strips 22, 24 are normally insulated from all the probe strips 20, but are connectible to the strips when a laser beam is directed to selected intersection points of strips 22, 24 with the probe strips 20.

Laser beams 14a, 14b illuminate the photoconductive layer 30 of selected probe elements 18 of the electrically-conductive strips 20, and thereby electrically connect the contact surface (layer 36) of the probe elements to the strips 20. Laser beams 14c, 14d illuminate selective intersection points between strips 22, 24 and 20, and thereby electrically connect the selected probe elements 18 via strips 22, 24, to an input/output circuit for measuring the appropriate electrical characteristics of the article to be tested at those probe elements .

Conductors 22, 24 are connected to an R, C measurement circuit 40, which in turn is connected to a processor 42 controlling the overall system. Processor 42 controls the optical scanning devices 16a-16d to direct the laser beams 14a-14d to selected points of the light-transmissive membrane 8. Processor 42 may also be used for controlling a zoom camera 44 for taking partial photographs of the PCB area under test, as will be described more particularly below.

The system illustrated in Figs. 1-4 operates as follows: After the article to be tested is placed on table 2 of the apparatus, chamber 8 of probe 4 is inflated by gas from tank 10 under the control of valve 12, to firmly press the light-transmissive membrane 4 into firm contact with the PCB. It will thus be seen that the discrete contact points (layers 36) of the probe elements 18 carried by the membrane 6 are pressed into firm contact with the PCB but are insulated from their respective electrically-conductive strips 20 because of the off-state of photoconductive gate 30 between each such probe element and its respective strip 20. The input/output strips 22, 24 are also normally insulated from strips 20 by the photoconductive gates 22a, 24a (Fig. 4).

By controlling the two laser beams 14a, 14b via their optical scanning devices 16a, 16b selected probe elements 18 may be made active by illuminating their respective photoconductive layers 30 to electrically connect their contact points to their respective electrically-conductive strips 20.

Similarly, by controlling the two laser beams 14c, 14d, via their optical scanning devices 16c, 16d, selected ones of the electrically-conductive strips 20 (and thereby of the selected active probe elements 18 ) may be connected by the photoconductive gates 22a, 24a, to the input/output strips 22, 24.

Thus, when continuity between two pads, which correspond to the same net, is to be tested, laser beams 14a, 14b simultaneously illuminate the light-transmissive membrane 6 at the location of the probe elements 18 for the respective pads. Fig. 2 illustrates the two locations illuminated by laser beams 14a, 14b as being those aligned with the photoconductive gates 30', 30' ' on electrically-conductive strips 20', 20' ', respectively. At the same time, gates 22a, 24a at the intersection points of strips 20', 20' ' with the two input/output electrically-conductive strips 22, 24 are illuminated so as to electrically connect the two strips 20', 20'', to strips 22 and 24, respectively.

At this instant, a testing voltage is applied by circuit 40 to the two input/output strips 22, 24, and the resistance is measured. The resulting measurement will provide an indication whether there is a discontinuity in the net between the two active probe elements 18; i.e., a low resistance will indicate no discontinuity, whereas a high resistance will indicate a discontinuity.

Fig. 5 illustrates the manner of testing both for discontinuity and for shorts. Shorts between different nets are tested mainly by capacitance (C) measurements via the input/output strips 22, 24. The comparison of the values of capacitance which correspond to the different nets, aided by the data stored in the processor 42 for the respective article to be tested, makes it possible to locate a short between two nets, as well as a discontinuity in a net.

The capacitance of each net is measured with respect to some reference point, e.g., that of pad P2 (Fig. 5). Thus, to detect a short between the nets of pads and P^, the photoconductive gates 30 of all three pads P1 , P2, P^ would be simultaneously illuminated by laser beams 14a, 14b, and measuring circuit 40 would measure the capacitance between pads P^ , ?2 an<^ pads Ρ3' I^ the capacitance between pads P^ and P2 is equal to C1 , namely the known capacitance between the nets of these two pads as stored in processor 42, this will indicate there is no short between these two nets. On the other hand, if the measured capacitance between the two pads is greater than that prestored in the processor 42 for the respective pair of nets, this will indicate there is a short between the two nets of the respective pads.

The inflation of chamber 8 in the apparatus of Fig. 1 presses the membrane 6 into firm contact with the article and thereby ensures good contact between its probe elements 18 and the pads.

Instead of using an inflatable chamber for this purpose, it is also possible to use a vacuum between the membrane and the unit under test. The flexibility of the membrane in both cases ensures good contact with the PCB even when there is some deformation in the board or non-uniformity in the thicknesses of its nets.

Fig. 6 illustrates a further possibility wherein neither pressurized gas nor suction is used. Instead, the flexible membrane, therein designated 106, is attached (e.g., mechanically or by glue) to a flat transparent panel 108. The article under test is pressed towards the membrane, and because of its resilient compressibility, it deforms sufficiently so that the electrical contacts of its probe elements firmly engage the pads of the article.

Preferably, the density of the light-transmissive electrically-conductive strips 20 should be such that two strips will always engage each pad. In some models, one strip will be sufficient. Preferably, the light-transmissive membrane 6 is of a transparent material, e.g., silicon rubber, Myler (Reg. TM) . etc.

Laser 14 is preferably a modulated CW (continuous wave) laser of about 10 mw, or an Ar gas laser. Less expensive diode semiconductor lasers could also be used.

Fig. 7 illustrates a probe construction useful for measuring charging and discharging currents between two nets on the article, one of which may be the ground plane. Thus, the probe illustrated in Fig. 7 includes a flexible membrane, therein designated 206, including an inner layer 208 of light-transmissive electrically-conductive material; an intermediate layer 210 of a photoconductive material; and an outer layer 212 of electrically-conductive material. In this embodiment, both the inner electrically-conductive layer 208 and the intermediate photoconductive layer 210 are continuous layers covering the complete surface of the membrane, whereas layer 212 is in the form of a plurality of discrete deposits each defining an electrical contact.

The probe illustrated in Fig. 7 is intended for measuring charging and discharging currents between selected pads 220 of a PCB or other like article and the ground plane 222 separated from the test pad by a dielectric layer 224. When the probe is used for this purpose, a pulse generator 230 is connected between a selected contact 212 of the probe, via a laser beam illuminating the membrane 206 at the location of the selected contact, and a charge meter 232 is used for measuring the charging current or discharging current between the selected pad 220 and the ground plane 222.

The two sides of the PC board may be tested simultaneously, and therefore two similar systems may be used on the two sides of the unit under test.

While the invention has been described with respect to several preferred embodiments, it will be appreciated that many other variations, modifications and applications of the invention may be made. 96288/2 - 18 -

Claims (26)

1. A probe for use in testing printed circuit boards, multi-chip modules, and other articles having electrically-conductive nets on a surface thereof, said probe having a plurality of probe elements for contact with pads of said nets for measuring an electrical characteristic between selected pads to provide an indication of faults, characterized in that: said probe includes a light-transmissive member carrying said plurality of probe elements on one surface thereof; and in that each of said probe elements includes an inner layer of a light-transmissive electrically-conductive material in a form of a plurality of spaced parallel strips, an outer layer of an electrically-conductive material for contact with the pads of the article to be tested, and an intermediate layer of a photoconductive material forming a photoconductive gate whose electrical conductivity substantially increases when illuminated by light, the outer layer being in the form of a plurality of discrete deposits forming contacts for each of said parallel strips but normally insulated therefrom by the off-condition of said photoconductive gates .

2. The probe according to Claim 1 , wherein said inner strips are arranged in a manner extending 96288/2 - 19 - along one orthogonal axis of the light-transmissive member and spaced from each other along the other orthogonal axis.

3. The probe according to Claim 2, wherein said light-transmissive member further includes electrical output means comprising at least one additional strip of a light-transmissive electrically-conductive material extending perpendicularly to and intersecting said plurality of parallel strips and insulated from each by a further photoconductive gate.

4. The probe according to Claim 3, wherein said electrical output means includes a second additional strip of light-transmissive electrically-conductive material extending parallel to and spaced from said first additional strip, said second additional strip also extending perpendicularly to and intersecting said plurality of parallel strips and insulated from each by a still further photoconductive gate .

5. The probe according to Claim 1 , wherein said inner layer of light-transmissive, electrically-conductive material, and said intermediate layer of photoconductive material, are each in the form of a continuous layer, and said outer layer of electrically-conductive material is in the form of a plurality of discrete deposits forming contacts for said inner - 20 - electrically-conductive layer but normally insulated therefrom by said photoconductive layer.

6. The probe according to any one of Claims 1-5, wherein said light-transmissive member is a flexible membrane deformable to provide good electrical contact between said contacts of the probe elements and the pads of the tested article.

7. The probe according to Claim 6, wherein said light-transmissive membrane is a wall of an inflatable chamber which, when inflated, presses the contacts of the probe element into firm engagement with said pads of the tested article.

8. The probe according to Claim 6, further including means for applying suction between said light-transmissive membrane and the tested article to press said contacts of the probe elements into firm engagement with said pads of the tested article.

9. The probe according to Claim 6, wherein said light-transmissive membrane is of a resilient material deformable under mechanical pressure to enable its contacts to be pressed into contact with the pads of the tested article.

10. Apparatus for testing printed circuit boards and other articles having electrically-conductive nets on a surface thereof, comprising: a table for receiving the article to be tested; - 21 - a probe according to any one of Claims 1-9 for application to the article to be tested with the contacts of the probe elements in firm engagement with the pads on the surface of the article; and illuminating means for illuminating selected locations of said light-transmissive member for selectively actuating said photoconductive gates.

11. The apparatus according to Claim 10 , wherein said illuminating means comprises laser means for producing at least one laser beam, and deflecting means for deflecting said laser beam to said selected locations of the light-transmissive member.

12. The apparatus according to Claim 11 , wherein said laser means produces a plurality of laser beams, and said deflecting means deflects said plurality of laser beams to said selected locations of the light-transmissive member.

13. The apparatus according to any one of Claims 10-12, further including measuring means for measuring an electrical characteristic between selected pads to indicate a fault in the article.

14. The apparatus according to Claim 13, wherein said measuring means includes an electrical circuit for. measuring resistance between selected pads to indicate a discontinuity in a net.

15. The apparatus according to Claim 13, wherein said measuring means includes an electrical - 22 - circuit for measuring capacitance between selected pads to indicate a short between selected nets.

16. The apparatus according to Claim 13 , wherein said measuring means includes an electrical circuit for measuring charge and discharge currents between selected pads.

17. A method of testing printed circuit boards, multi-chip modules, and other articles having electrically-conductive nets on a surface thereof, comprising: applying a probe according to any one of Claims 1-9 to selected pads of the article; illuminating selected locations of the light-transmissive member of the probe to activate selected photconducted gates thereof; and measuring an electrical characteristic between selected pads to thereby provide an indication of faults in the article.

18. The method according to Claim 17, wherein said selected locations of the light-transmissive member are illuminated by at least one laser beam.

19. The method according to Claim 18, wherein said selected locations of the light-transmissive member are illuminated by a plurality of laser beams.

20. The method according to any one of Claims 17-19, wherein said measured electrical characteristic is the resistance between pads, to thereby provide an 96288/2 - 23 - indication of a discontinuity in a selected net.

21. The method according to any one of Claims 17-19, wherein said measured electrical characteristic is the capacitance between selected pads, to thereby provide an indication of a short between selected nets .

22. The method according to any one of Claims 17-19, wherein said measured electrical characteristic is the charging and/or discharging current between one selected pad and another selected pad connected to a ground plane of the article.

23. The method according to any one of Claims 17-22, wherein said probe is applied to one face of the article to be tested.

24. The method according to any one of Claims 17-22, wherein a probe is applied to each of the opposite faces of the article to be tested.

25. A probe according to any one of Claims 1-9, for use in testing printed circuit boards, multi-chip modules, and other articles having electrically-conductive nets on a surface thereof, substantially as described with reference to and as illustrated in the accompanying drawings.

26. Apparatus according to any one of Claims 10-16, for testing printed circuit boards, multi-chip modules, and other articles having electrically-conductive nets on a surface thereof, substantially as 96288/2 - 24 - described with reference to and as illustrated in the accompanying drawings. The method according to any one of Claims 17-24, of testing printed circuit boards, multi-chip modules, and other articles having electrically-conductive nets on a surface thereof, substantially as described with reference to and as illustrated in the accompanying drawings . For the Applicant: Sarfford T. Colb P.O. Box 2273 Rehovot 76122 c:B6629