Radiation imagery chemistry: process – composition – or product th – Imaging affecting physical property of radiation sensitive... – Making electrical device
Reexamination Certificate
1999-06-30
2001-08-28
Chu, John S. (Department: 1752)
Radiation imagery chemistry: process, composition, or product th
Imaging affecting physical property of radiation sensitive...
Making electrical device
C430S315000
Reexamination Certificate
active
06280907
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates in general to a process for forming resistors on a printed circuit board. More particularly, it relates to a process by which polymer thick film resistors and metal thin film resistors can be successively produced on the same layer of a printed circuit substrate.
2. Description of the Related Arts
The density of components and conductor circuits placed on printed circuit boards has been increasing in response to the need to accommodate increasingly complex electronic circuitry in smaller spaces. In addition to accommodating a greater number of conductor paths than heretofore, many printed circuit boards being manufactured today have a greater number of discrete components mounted thereon. Frequently, electronic circuits contained on printed circuit boards require a very large number of resistors to perform various functions, for example, terminating circuits. Heretofore, in most instances such terminations were required to be made by mounting resistors on the surface of a circuit board and connecting the resistors to conductors through holes which passed from the board's surface through one or more conductor planes to the conductor paths to which the resistors are to be connected- Resistors used to accomplish such terminating function could be either conventional carbon composition resistors or resistors screened onto the circuit board's top surface using polymer thick film (PTF) resistive pastes. In the event that surface mounting of the resistors is not suitable, e.g. where resistive terminations are required in a board having many layers and limited surface area, the circuit designer could use a material sold under the name “Ohmega-Ply” by Ohmega Technologies, Inc. The Ohmega-Ply material is composed of a layer of resistive material sandwiched between upper copper conductive layers and lower core material (PR-4 or Polyimide). The Ohmega-Ply material may be used to make resistor-conductor networks using a photolithographic process. The Ohmega-Ply material may be etched using conventional etching processes to produce circuit layers having integral discrete resistors. However, one problem with Ohmega-Ply material which makes it undesirable for use in multilayer printed circuit boards is that it is difficult to produce resistors for a layer whose values substantially differ from each other within the layer, e.g., vary by more than one decade.
U.S. Pat. No. 4,021,277 describes a method of forming a thin resistor film which comprises: depositing by cosputtering a metastable alloy film on a substrate, annealing said resistor film, and determining the geometry of The film by a normal photo-etch process. U.S. Pat. No. 4,368,252 describes a method for manufacturing a printed circuit board with resistance elements, in which a resistor pattern film and a conductor pattern film are formed on the two surfaces of a high conductive material layer such as a copper foil., respectively, and an insulating support is combined with the resistor pattern film on the high conductive material layer. Methods for forming metallic thin film resistors arc described in U.S. Pat. Nos. 4,540,463, 5,039,570, and 4,865,573. One major drawback with these methods is the incapability of producing resistors of high resistivity. Another method reading to the general subject matter is described in U.S. Pat. No. 4,870,746. The method includes the steps of manufacturing individual layers of the multilayer board with electrical circuits thereon and subsequently screening on one or more values of resistors using a polymer thick film resistive ink. The individual layers are thereafter bonded together into a multilayer board. The forgoing method has the disadvantage that the geometry and thickness of the resistors is difficult to control, resulting in variations in the resistivities.
SUMMARY OF THE INVENTION
An object of the invention is to provide a process for forming polymer thick film resistors on a printed circuit substrate, by which polymer thick film (PTF) resistor pastes having one or more resistivities may be placed on the same layer of the printed circuit board, and the geometry and thickness of the resistors can be precisely controlled.
Another object of the invention is to provide a process for forming polymer thick film resistors and metal thin film resistors on a printed circuit substrate, in which these two kinds of resistors can be successively formed on a printed circuit board.
The foregoing objects are achieved by applying a photoresist layer onto a substrate and forming openings in the photoresist layer to expose the preselected regions for resistors, such that PTF pastes having different resistivities may be filled into these openings by screen printing or dispensing, and resistive metallic films can also be deposited onto the substrate through these openings. In accordance with the present invention, since the geometry and thickness of the resistors can be precisely controlled, the resistance variations caused by dimensional instability can be minimized.
According to an aspect of the invention, the process for forming polymer thick film resistors on a printed circuit substrate comprises the steps of: (a) forming on a substrate a circuit pattern including electrodes which are reserved for resistors; (b) forming a photoresist layer covering the substrate and the circuit pattern; (c) forming one or more openings in the photoresist layer to expose the preselected regions for resistors; (d) filling the openings with polymer thick film resistor pastes having one or more resistivities; (e) curing the resistor pastes; and (f) removing the remaining photoresist layer to leave one or more polymer thick film resistors on the substrate.
According to another aspect of the invention, the process for forming polymer thick film resistors and metal thin film resistors on a printed circuit substrate comprises the steps of: (a) forming on a substrate a circuit pattern including electrodes which are reserved for resistors; (b) forming a photoresist layer covering the substrate and the circuit pattern; (c) forming a plurality of openings in the photoresist layer to expose the preselected regions for resistors; (d) filling at least one of the openings with polymer thick film resistor pastes having one or more resistivities; (e) partially curing the resistor pastes; (f) depositing a resistive metallic film onto the substrate through at least one other openings; (g) completely curing said resistor pastes; and (h) removing the remaining photoresist layer to leave at least one polymer thick film resistor and at least one metal thin film resistor oil the substrate.
Other objects, features, and advantages of the present invention will become apparent from the following detailed description which makes reference to the accompanying drawings.
REFERENCES:
patent: 4963389 (1990-10-01), Takada et al.
patent: 5624782 (1997-04-01), Hayakawa et al.
Chung Chia-Tin
Lin Bin-Yuan
Su Te-Yeu
Wang Hsin-Herng
Wang Wun-Ku
Chu John S.
Industrial Technology Research Institute
Sughrue Mion Zinn Macpeak & Seas, PLLC
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