Electricity: measuring and testing – Impedance – admittance or other quantities representative of... – Lumped type parameters
Reexamination Certificate
2000-05-01
2001-11-27
Brown, Glenn W. (Department: 2858)
Electricity: measuring and testing
Impedance, admittance or other quantities representative of...
Lumped type parameters
C324S715000
Reexamination Certificate
active
06323661
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to methods and devices for measuring contact resistance between conductive surfaces in electrical devices and, more particularly, to measuring contact resistance between a metallized surface of a printed circuit and a contacting conductive surface, e.g., a metallized surface of a phased-array ultrasound transducer for use in a diagnostic medical imaging system.
BACKGROUND OF THE INVENTION
Most modern transducers for use in diagnostic medical ultrasound imaging are phased arrays, made up of single or multiple rows of electrically and mechanically independent transducer elements. Each transducer element is a layered structure comprising an acoustic absorber, a piezoelectric ceramic (hereinafter “piezoceramic”) layer, one or more acoustic matching layers, and a front wear plate or focusing lens. Typically, one or more flexible printed circuits (hereinafter “flex circuits”) are used to make electrical connections (signal and ground) from the piezoceramic layer to the signal processing electronics, or to a bundle of coaxial cables which ultimately connect to the signal processing electronics. One known method of connecting the flex circuit to the piezoceramic uses ohmic contacts; i.e., exposed metal pads on the flex circuit are laminated, using high pressure and a thin layer of non-conductive epoxy, to the electroded surface of the piezoceramic layer. If the flex circuit and piezoceramic surfaces are microscopically rough and the epoxy layer is sufficiently thin, then an electrical connection is achieved via a distribution of direct contacts between high points on the piezoceramic surface and high points on the flex circuit.
The quality of such an ohmic electrical connection is very sensitive to material and process parameters that can be difficult to control (such as surface roughness, flatness, and parallelism; epoxy viscosity; lamination pressure). The resistances of both good and bad contacts are small compared to other impedances in the circuit for a transducer element, so that it is next to impossible to non-destructively measure the quality of the electrical connection. Typical measurements of transducer performance, such as low-frequency capacitance measurements or high-frequency impulse response measurements, can detect open or severely degraded ohmic contacts but cannot discriminate between good contacts and weak contacts which may degrade and become unreliable over time. If contact problems are suspected, either the back or front layers of the transducer must be removed to obtain access to the flex circuit-to-ceramic bond area. The ceramic electrode and the flex circuit metallization are separately exposed and connected to the leads of a resistance meter. This method of analysis is both destructive and laborious.
SUMMARY OF THE INVENTION
In accordance with a preferred embodiment of the invention, electrical resistance of an ohmic contact between two conductive substrates, e.g., a flexible printed circuit and a metallized substrate made of piezoelectric ceramic material may be measured by employing two metal pads on the surface of the flex circuit, separated from each other but in close proximity thereto. Two electrical leads extend from each flex pad to locations on the flex circuit that are accessible for probing with a four-lead resistance meter (e.g., a milliohmmeter) and may be remote from the flex circuit-to-substrate ohmic connection. For laboratory measurement of contact resistance, many sets of such pads, of a variety of sizes, may be combined into a single flex circuit.
In accordance with a further aspect of the invention, for in-process monitoring of transducer manufacturing, a small number of contact resistance measurement pads may be designed into production flex circuits. If bad electrical contacts can be detected, then bad parts can be scrapped early in the production line. If process drift can be monitored in production, then lamination problems can be corrected before many transducers are affected. For in-process monitoring, the contact resistance measurement circuit must be arranged in a way which does not interfere with either the normal fabrication processes or the optimal design and operation of the transducer. Preferably, the contact resistance measurement circuit and method should add negligible cost to the transducer components and fabrication processes.
The invention is described in the context of, but is not limited in applicability to, the manufacturing of phased-array ultrasound transducers. Examples of other applications of the invention include fabrication of multi-layer printed circuit boards and assembly of flex circuit-to-device connections for flat-panel displays, cellular telephones, etc. Flex circuit-to-device assembly connections are typically made with an anisotropic conductive adhesive (epoxy containing a sparse distribution of conductive particles) rather than a very thin layer of non-conductive epoxy, but the need to characterize the electrical resistance of the contact between the flex circuit and the substrate (e.g., flat-panel displays) is the same.
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Sogoian George Charles
Wildes Douglas Glenn
Breedlove Jill M.
Brown Glenn W.
Cabou Christian
General Electric Company
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