Electricity: measuring and testing – Fault detecting in electric circuits and of electric components – Using radiant energy
Reexamination Certificate
2001-11-08
2003-09-16
Patidar, Jay (Department: 2858)
Electricity: measuring and testing
Fault detecting in electric circuits and of electric components
Using radiant energy
C324S754120
Reexamination Certificate
active
06621274
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a system for testing the networks of printed circuit boards without any physical contact therewith, and more particularly to a system and method of using an electron beam to test for shorts between and opens in conductor traces of a printed circuit board.
BACKGROUND OF THE INVENTION
It is desirable in the manufacture of printed circuit boards to test for defects at an early stage of fabrication in order to minimize the costs of repairing such defects and to maximize the yield of operable devices. The traditional approach to testing such circuitry has been to use mechanical devices, such as pins or other mechanical probes, to contact points on the circuitry to be tested and to run a current therethrough. However, the small size and high density of modern printed circuit boards makes the use of such mechanical testing devices unfeasible. The networks of modern printed circuit boards are becoming so small and so compactly placed on the boards that using a mechanical probe is difficult, if not impossible. Moreover, as the networks become smaller, the risks associated with damaging the networks using mechanical probes increases. For these reasons, systems using electron beams to provide contactless testing have been developed. However, although such systems exist, these known systems suffer from a number of disadvantages.
U.S. Pat. No. 4,843,330 to Golladay et al. discloses an electron beam contactless testing system which includes a conductive grid
48
placed above the surface of the specimen
36
to be tested. The
48
grid is negatively biased while the networks to be tested are charged in order to repel secondary electrons back toward the specimen and enhance charging of the conductive material. The bias is removed, or a positive bias is applied, during scanning of the specimen by a beam
12
so that secondary electrons can be collected by a detector. The testing system also includes a table
32
selectively movable in the X-Y directions perpendicular to the axis of beam
12
to position specimen
36
within the beam deflection field.
U.S. Pat. No. 5,602,489 to El-Kareh et al. discloses a method for testing the interconnect networks of a multichip module for opens and shorts. An electron beam
570
lands on a pad of an interconnect network located on a substrate
500
. The electron beam
570
is used to interrogate the pad. An extract grid
550
located above the substrate is maintained at a positive potential. While the electron beam
570
interrogates the pad, the pad emits secondary electrons until such a point that the pad reaches a positive potential near that of the positive potential of the extract grid
550
. The extract grid is then switched to a negative potential. The pad, still being interrogated by the electron beam
570
, then collects secondary electrons until such a point that the pad reaches a negative potential near that of the negative potential of the extract grid
550
. The test time, the length of time it takes for the pad to change from the positive potential to the negative potential, is measured and compared to a reference value. From this comparison it can be determined whether the interconnect network is defect-free, open, or shorted. The increase or decrease in emitted secondary electrons are evaluated by an electron detector
560
, and the substrate
500
is moved relative to the electron beam
570
by an x-y stage
540
.
U.S. Pat. No. 4,169,244 to Plows discloses a system for testing electronic networks. The system includes an electron gun
2
for production of an electron probe
1
, a scanning control for probe
1
, and a holder
16
capable of supporting a specimen
25
such that the probe
1
can impinge substantially normally on the specimen
25
. Deflection coils
6
are used to direct electron probe
1
to varying locations on the specimen
25
.
All of these prior art patents disclose devices which use a focused electron beam, such as that generated by a scanning electron microscope. Such a beam is capable of charging/reading a small area, for example an area of 1 inch by 1 inch. When a larger board, or a panel of small boards, is to be tested it is therefore necessary to move the board (as does the X-Y table of Golladay et al. and the x-y stage of El-Kareh et al.). Moving the board, however, can greatly increase the time necessary for testing, particularly when numerous points on the board must be tested.
Moreover, all prior are references disclose systems which can perform only top-to-top testing of board networks. If the network passes through the board, the network cannot be charged at a point on one side of the board and then tested at a point on the other side of the board. This is a serious disadvantage, as many modern circuit boards have networks which pass therethrough.
Furthermore, the systems disclosed in all prior art patents require that secondary electrons emitted by the networks being tested be collected by a discrete electron detector. Such systems are prone to error, however, as emitted electrons may not necessarily be deflected toward the detector, and may therefore not be detected.
A further disadvantage with respect to El-Kareh et al. is that the test time, that is, the length of time it takes for the pad to change from the positive potential to the negative potential, must be measured and compared to a reference value for every pad tested. Depending on the materials used and the size of the pads, this test time may be great, thereby greatly increasing the time necessary for testing a board.
What is desired, therefore, is a system for testing printed circuit boards which tests the networks of printed circuit boards without any physical contact therewith, which quickly tests numerous points on the boards, which can test large boards or panels of small boards without requiring movement of the boards, which tests networks passing through the boards, and which reliably detects the presence of emitted secondary electrons.
SUMMARY OF THE INVENTION
Accordingly, it is an object of the present invention to provide a system for testing printed circuit boards which employs electron beams to test the networks of printed circuit boards without any physical contact therewith.
Another object of the present invention is to provide a system for testing printed circuit boards which quickly tests numerous points on the boards.
A further object of the present invention is to provide a system for testing printed circuit boards which employs an electron beam such that large boards and panels of small boards can be tested without requiring movement of the boards.
Still another object of the present invention is to provide a system for testing printed circuit boards which tests networks passing through the boards.
Still a further object of the present invention is to provide a system for testing printed circuit boards which tests both sides of printed circuit boards using an electron beam on each side.
Yet a further object of the present invention is to provide a system for testing printed circuit boards which reliably detects the presence of emitted secondary electrons.
These and other objects of the present invention are achieved by provision of a system for testing for opens in and shorts between conductor traces on a surface of a printed circuit board. The system includes an electron gun assembly for generating an electron beam and an electron optics assembly for directing the electron beam to the conductor traces on the surface of the printed circuit board, which conductor traces are at a reference potential. A grid located proximate to and substantially parallel with the surface of the printed circuit board is placed at a first potential before the electron beam is directed to a first point on a conductor trace to charge the conductor trace to a first potential. The grid is then placed at a second potential, the second potential being between the reference voltage and the first potential, before the electron beam is directed to a second point on the conductor trace to cause emission
Image Graphics Incorporated
Nguyen Vincent Q.
Patidar Jay
St. Onge Steward Johnston & Reens LLC
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