Electricity: measuring and testing – Fault detecting in electric circuits and of electric components – Of individual circuit component or element
Reexamination Certificate
2001-02-27
2003-10-07
Le, N. (Department: 2858)
Electricity: measuring and testing
Fault detecting in electric circuits and of electric components
Of individual circuit component or element
C324S538000, C324S750010
Reexamination Certificate
active
06630832
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to equipment and methods for testing the electrical integrity of electrical circuits, and more particularly to equipment and methods for the non-contact electrical testing of printed circuit boards (“PCBs”), chip carriers and similar electrical circuits having conductors of various configurations.
BACKGROUND OF THE INVENTION
Electrical circuits, such as PCBs and chip carriers, are generally tested after manufacture to determine whether or not all of the conductors and other electrically conductive elements in the circuit are in their designated positions and to ensure that they are not unintentionally cut, shorted or otherwise have an undesired continuity or lack thereof. The conductors of electrical circuits are normally interconnected to define nets.
Conventional methods and apparatus for electrically testing electrical circuits typically employ some kind of physical contact with the nets. For example, in moving probe apparatus, a pair of probes may be physically moved by an X-Y mechanism into and out of contact with terminals of various nets. Because nets are tested sequentially by moving the probes from net to net, moving probe testing is a relatively slow method for electrically testing complicated electrical circuits.
Another method for electrically testing electrical circuits employs a so-called “bed-of-nails” testing fixture. A bed-of-nails fixture typically includes a large number of pins, which are positioned so that when a circuit to be tested is pressed thereagainst, the pins come into electrical contact with pads at the terminal ends of each net to establish electrical contact therewith. The conductivity of each net is subsequently measured. Although an electrical circuit can be tested much faster on an existing bed-of-nails fixture than by using a moving probe, bed-of-nails testing requires a dedicated fixture to be constructed for each electrical circuit configuration. As a result, bed-of-nails testing is, overall, a time consuming and costly solution.
Electrical testing methods which rely on physical contact with an electrical circuit to be tested, such as the moving probe and bed-of-nails methods described above, suffer from at least two additional fundamental disadvantages: First, as the size of pads at the terminal ends of conductors on electrical circuits decreases and their density increases, it becomes increasingly difficult to obtain adequate electrical contact therewith. Second, physical contact between conductor pads and the probes or pins may damage the pads.
To overcome these difficulties, a number of non-contact electrical testing methods have been proposed. One non-contact printed circuit board testing method is described in U.S. Pat. No. 5,218,294, issued to Soiferman. The patent describes stimulating a PCB under test with an AC signal through power and ground lines or layers, or in a non-contact manner by employing a near-field active antenna. The resulting stimulation generates an electromagnetic field which characterizes the PCB under test. The electromagnetic field proximate to the PCB under test is measured in a non-contact manner and compared to the electromagnetic field of a known faultless circuit board to determine whether the PCB under test is defective.
U.S. Pat. No. 5,517,110, also issued to Soiferman, describes non-contact stimulation of a PCB by a pair of stimulators located adjacent to the PCB on one side thereof. A resulting electromagnetic field is detected using a sensor array located between the stimulators on the same side of the PCB.
U.S. Pat. No. 5,424,633, issued to Soiferman describes a spiral loop antenna useful in the electrical testing of PCBs, as well as electrical testing in which an electromagnetic field is applied to a first side of a PCB under test by a non contact stimulator and an array of non-contact sensors on an opposite side of the PCB is operative to measure an electromagnetic field that is characteristic of the PCB when stimulation is applied. This system is able to electrically test nets that have terminal points on opposite sides of a PCB and relatively thin PCBs that do not have internal metal layers.
SUMMARY OF THE INVENTION
The present invention seeks to provide improved methods and apparatus for non-contact electrical testing of electrical circuits such as PCBs. For the purpose of the description and claims which follow, an electrical circuit being tested is referred to as a “board under test” or “BUT”.
One aspect of a preferred embodiment of the present invention provides for the non-contact electrical testing of BUTs, such as PCBs, that have nets which begin and terminate on the same side thereof, and that have other nets which begin and terminate on opposite sides thereof.
Another aspect of a preferred embodiment of the present invention provides for the non-contact electrical testing of BUTs, such as PCBs, that have internal metal layers and conductors that cross through or between the metal layers.
In accordance with a preferred embodiment of the invention, non-contact electrical testing of BUTs, such as PCBs, that have nets which begin and terminate on the same side as well as nets which begin and terminate on opposite sides is performed generally simultaneously. One side of a BUT is stimulated with an AC electric field at a first frequency and the other side of the BUT is stimulated with an AC electric field at a second frequency. Potentials induced by the different frequency stimulation in conductors on the BUT are measured and separated according to frequency.
It is readily appreciated that by applying stimulation to both sides of the BUT that results in separable potentials that are identified with stimulation applied to one side or the other of BUT, the electric continuity in different types of conductors on a BUT can be tested simultaneously.
In accordance with a still further aspect of the present invention, a pattern of potentials on a BUT is analyzed and compared to a pattern characteristic of an electrical circuit known to be not defective.
There is thus provided in accordance with a preferred embodiment of the present invention an apparatus for electrical testing of an electrical circuit having first and second side surfaces and including a plurality of conductors, the apparatus including at least one stimulation electrode disposed adjacent at least one of the first and second side surfaces of the electrical circuit and being operative to apply thereto a stimulation electromagnetic field in a non-contact manner, at least one sensing electrode disposed adjacent at least one of the first and second side surfaces of the electrical circuit and being operative to sense a resulting electromagnetic field produced by application of the stimulation electromagnetic field at various locations thereon in a non-contact manner, wherein at least one of the at least one stimulation electrode and the at least one sensing electrode includes at least two electrodes at least one of which is disposed adjacent each of the first and second side surfaces of the electrical circuit.
Further in accordance with a preferred embodiment of the present invention the at least one stimulation electrode includes at least first and second stimulation electrodes disposed adjacent respective ones of the first and second side surfaces of the electrical circuit.
Still further in accordance with a preferred embodiment of the present invention the at least one sensing electrode includes at least first and second sensing electrodes disposed adjacent respective ones of the first and second side surfaces of the electrical circuit.
Further in accordance with a preferred embodiment of the present invention there is provided at least one stimulation signal generator providing at least one stimulation signal to the at least one stimulation electrode.
Additionally in accordance with a preferred embodiment of the present invention the at least one stimulation signal generator provides stimulation signals to a plurality of stimulation electrodes in a manner such that signals induced
Golan Hanan
Harzanu Benyamin
Weber Raviv
Bromberg & Sunstein LLP
Dole Timothy J.
Orbotech Limited
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