Electricity: measuring and testing – Plural – automatically sequential tests
Reexamination Certificate
1999-06-03
2001-05-22
Metjahic, Safet (Department: 2858)
Electricity: measuring and testing
Plural, automatically sequential tests
C324S754120, C324S1540PB, C324S537000, C250S492220
Reexamination Certificate
active
06236196
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to testing of an electric workpiece including at least one electrical circuit provided on a substrate, printed circuit board, multi-chip module, and the like, and, particularly, to a system and method employing a thermal stimulus to a circuit for efficiently and accurately locating a defect, e.g., a short circuit or an incipient open circuit, which can be practiced in the absence of a priori knowledge of circuit geometry, and which uses electrical access only at circuit terminals.
2. Discussion of the Prior Art
It is often required to fabricate an electric workpiece including one or more electrical circuits, composed of suitable electrically conducting material conduits to carry either power or signal to and from various locations in electrical circuit structures.
These circuits may be either in the same plane, or may be in various planes stacked one over the other. Suitable electrical insulator materials separate these planes, in addition to separating the electrical conduits within a network. Electrical connections among conducting circuits in various planes are made through “vias” which perpendicularly connect plural planes. Some examples of an electric workpiece are: a printed circuit board, an electronic substrate, a multi-chip module, a flexible printed circuit sheet, etc.
During fabrication of an electric workpiece, it is possible that unwanted short circuit connections may inadvertently result, such as a short circuit or an incipient open circuit. These circuit defects may have a wide range of electrical resistance. Often, these defects must be located precisely for evaluation, analysis and possible repair. Incipient defects are unstable structures which may become either a short circuit or an open circuit condition at some time in the future. Incipient open circuits caused by a narrow neck in a circuit line, or chemical contamination. These defects may be detected by harmonic response analysis means disclosed by DiStefano, et al. in U.S. Pat. Nos. 4,496,900 and 4,868,506. Latent open defect detection is also described by Halperin, DiStefano and Chiang in “Latent Open Defect Detection Using Phase-Sensitive Nonlinearity Detection Technique”, IEEE Transactions on Components, Packaging and Manufacturing, Part B, Vol. 18, No. 2, May 1995, pp. 358. As described in this reference, a circuit is driven with a modulated current, and a circuit defect is detected with electric non-linearity. For example, in a circuit driven with current at two frequencies Fdrive1 and Fdrive2, the circuit defect generated heterodyne modulation at a frequency:
Fdefect=Fdrive
1+
Fdrive
2
In another example, there is a large current at Fdrive2=0 and the defect generated a homodyne modulation:
Fdefect=
2 *
Fdrive
1
There are several prior techniques to locate a short circuit defect including (1) human visual inspection or robotic pattern recognition. Other techniques apply a sufficient alternating current (AC) through two circuits connected by an inadvertent short, thus, generating a local magnetic field at the point where the short circuit occurs. This short circuit may be located by a technique (2) which requires scanning with a small non-contacting loop and measuring the induced Electromagnetic Field (emf). In another prior art technique (3), a thin film of a material having a suitably high magnetic susceptibility is applied to the workpiece. Adjacent to the short circuit, the magnetic field induces optical birefringence in the magnetic material. Under optical inspection with polarized light, the location of the defect may be determined. In yet another prior art technique (4), an AC current is applied to a small loop of wire. When the loop is near the short, the AC magnetic field induces a current to flow between two shorted circuits, thus indicating the proximity to the location of the defect.
The problem with method (1) is fatigue and unreliability, if human inspection is used; unreliable pattern recognition due to topography and transparent insulating films if robotic inspection is used.
Methods (2) (3) (4) are essentially spot scanning methods and require a long search time to locate a defect which might be anywhere in a search region. Inconveniently, long search times occur in several important cases: for short circuits between power planes, for defects involving one or more wires whose path geometry is effectively not known, or for complex printed circuit boards. To locate a defect in three dimensions, such as in a printed circuit with many layers, becomes even more difficult.
By contrast, if a defect is somewhere along a known path, which is topologically one-dimensional, and if it is practical to search narrowly along this path, then it is much less difficult to locate the defect. However, this favorable situation often does not occur for shorts, and does not occur for a workpiece without a detailed circuit map.
A further problem with methods (2), (3) and (4) is loss of reliability when the shorting resistance is greater than 1 kOhms.
A further problem with methods (2), (3) and (4) is false positives which indicate the presence of a circuit shorting defect where there is none. These false positives originate from certain circuit features such as input and output connections, inter-plane connections and depend on circuit design.
A recurring problem for electronic printed circuit board (PCB) and multi-chip module (MCM) substrate fabrication is the inability to locate efficiently and accurately short defects or incipient open defects. The ability to effectively locate defects in complex network of electrical conductors has important consequences; once a defect is located and carefully analyzed, the resulting information often helps to improve manufacturing technique and manufacturing yield.
To locate a defect is very different than to detect the same defect. These defects often are simple to detect, using just an ohmmeter. However, a short circuit defect between two power planes may occur almost anywhere on the workpiece region where the two power planes come near each other, so such a defect is often very challenging to locate. Alternatives such as automated optical inspection (AOI) may have some level of effectiveness. Unfortunately, however, AOI often is very time consuming, and often misses some defects. Therefore, other techniques are often necessary.
A first prior art technique for locating a short circuit is a voltage gradient technique, such as described in the reference “Quick Inspection of Power Plane Short Fault on Multilayer Substrates,” IEEE Transactions Comp. Packaging and Manufacturing Technology, USA, Vol. 18, No.3, September 1995, pp. 466-470 by Fang Lin Chao et al. This technique requires mechanical probe contact with the top surface metallurgy (TSM) and may be neither practical nor desirable. Non-contact methods of probing the surface are much more desirable.
A second technique utilizes magnetic induction, such as shown and described in co-pending U.S. patent application Ser. No. 08/807,076 [Atty. Docket # FI996163] entitled “METHOD AND APPARATUS FOR DETECTING SHORTS IN A MULTI-LAYER ELECTRONIC PACKAGE,” commonly-owned by the assignee of the present invention. In particular, Faraday induction techniques which typically use an AC pickup coil, are described in commonly-owned, co-pending U.S. patent application Ser. No. 09/116,396 [Atty. Docket # FI998018] entitled “METHOD FOR DETECTING POWER PLANE-TO-POWER PLANE SHORTS AND I/O NET-TO-POWER PLANE SHORTS IN MODULES AND PRINTED CIRCUIT BOARDS.” Faraday induction is often used effectively for low resistance shorts. However, Faraday induction is much less effective for high resistance shorts, particularly in the presence of significant capacitive coupling between power planes.
A third technique involves covering the printed circuit board surface with iron garnet, or another magneto-optical material, and inspecting this material with polarized light. A local magnetic field changes th
Guidotti Daniel
Halperin Arnold
Scaman Michael E.
Zingher Arthur R.
Capella, Esq. Steven
Hamdan Wasseem H.
International Business Machines - Corporation
Metjahic Safet
Scully Scott Murphy & Presser
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