Electricity: conductors and insulators – Conduits – cables or conductors – Preformed panel circuit arrangement
Reexamination Certificate
2000-07-21
2001-05-08
Gaffin, Jeffrey (Department: 2841)
Electricity: conductors and insulators
Conduits, cables or conductors
Preformed panel circuit arrangement
C174S255000, C338S252000, C338S311000, C338S322000, C361S761000, C361S765000
Reexamination Certificate
active
06229098
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention generally relates to printed wire board circuits and their fabrication. More particularly, this invention relates to a method for forming a thick-film resistor to have precise dimensions determined by photolithography techniques, thereby avoiding the variability associated with conventional screen printed resistors.
Thick-film resistors are employed in hybrid electronic circuits to provide a wide range of resistor values. Such resistors are formed by printing, such as screen printing, a thick-film resistive paste or ink on a substrate, which may be a printed wiring board (PWB), flexible circuit, or a ceramic or silicon substrate. Thick-film inks used with ceramic printed wire boards are typically composed of a glass frit composition, an electrically-conductive material, various additives used to favorably affect the final electrical properties of the resistor, and an organic vehicle or polymer matrix material. Thick-film inks used in organic printed wire board construction are typically composed of an electrically-conductive material, various additives used to favorably affect the final electrical properties of the resistor, an organic binder and an organic vehicle. After printing, the thick-film ink is typically heated to dry the ink and convert it into a suitable film that adheres to the substrate. If a polymer thick-film ink is used, the heating step serves to remove the organic vehicle and to cure the polymer matrix material. Other thick-film inks must be sintered, or fired, during which the ink is heated to burn off the organic vehicle and fuse the remaining solid material.
The electrical resistance of a thick-film resistor is dependent on the precision with which the resistor is produced, the stability of the resistor material, and the stability of the resistor terminations. Control of the x, y and z dimensions (the width, electrical length and thickness, respectively, of the resistor) is particularly challenging in view of the techniques employed to print thick-film inks and the dimensional instability that may occur during subsequent processing. For rectangular screen-printed resistors, the x and z dimensions are determined by the screening process, and the y dimension is determined by the termination pattern. Conventional screen printing techniques generally employ a template with apertures bearing the positive image of the resistor to be created. The template, referred to as a screening mask, is placed above and in close proximity to the surface of the substrate on which the resistor is to be formed. The mask is then loaded with the resistive ink, and a squeegee blade is drawn across the surface of the mask to press the ink through the apertures and onto the surface of the substrate.
Compared to many other deposition processes, screen printing is a relatively crude process. As a result, screen printed thick-film resistors are typically limited to dimensions of larger than about one millimeter, with dimensional tolerances generally being larger than about ±10% at this lower limit. Consequently, screen printed thick-film resistors having adequate tolerances in the x and y dimensions are often larger than chip resistors. The thickness of a thick-film resistor can generally be controlled to tolerances of about 20% to 30% by screen printing, due in large part to variability in the x, y and z dimensions. While the z dimension (thickness) of a screen-printed thick-film resistor can be reasonably well controlled through precision in the screening operation, the control of x and y dimensions is fundamentally limited by the relatively coarse mesh of the screen and by ink flow after deposition. As a result, resistance tolerances of less than ±20% cannot be achieved with screen printed thick-film resistors without laser trimming, an operation that is usually cost prohibitive for complex circuits.
From the above, it can be seen that present practices involving the fabrication of thick-film resistors can necessitate a compromise between the precision of the resistance value and the size of the resistor. In other words, while smaller resistors are often preferred to yield a more compact circuit, an undesirable consequence is that resistance values are less predictable due to the dimensional variability of the resistors. Accordingly, a need exists for a method for producing a thick-film resistor in which resistance values and tolerances can be more accurately controlled than prior art screen printing techniques permit.
SUMMARY OF THE INVENTION
According to the present invention, a process is provided for forming a resistor whose x (width), y (electrical length) and z (thickness) dimensions can be accurately obtained, thereby yielding a resistor whose resistance value can be precisely obtained even for resistors having dimensions on the order of about 100 micrometers. The resistor can be formed with conventional thick-film resistor inks used to form screen printed thick-film resistors, although the precision of the final dimensions of resistors formed in accordance with this invention is better than that possible for prior-art screen printed thick-film resistors.
The method of this invention preferably entails the use of one or more photoimageable resins as masks that may subsequently form permanent dielectric layers of a multilayer structure. The method includes providing a dielectric layer on a substrate, and then defining an opening in the surface of the dielectric layer. In the preferred embodiment where the dielectric layer is formed with a photoimageable resin, the opening is photodefined to precisely achieve the dimensions for the intended thick-film resistor. The opening is then filled with an electrically-resistive material, preferably using a screen printing technique with an oversized aperture such that the electrically-resistive material forms a resistive mass having an excess portion that lies on the surface of the dielectric layer surrounding the opening. The excess portion of the resistive mass is then selectively removed such that the resistive mass has lateral dimensions defined by the opening in the dielectric layer. Thereafter, subsequent processing is performed such that the dielectric layer forms a permanent layer of a circuit board, with the resistive mass and appropriate terminations forming a resistor in the permanent dielectric layer.
In the preferred embodiment, the portion of the resistive mass lying on the surface of the dielectric layer is removed by abrading, such as grinding or polishing. For this purpose, the resistive mass is heated to cure the electrically-resistive material prior to the abrading operation, such that shrinkage of the resistive mass occurs and the surface of the resistive mass becomes recessed below the surface of the dielectric layer. During the abrading operation, the excess portion of the resistive mass remaining on the surface of the dielectric layer is removed without substantially altering the recessed surface of the resistive mass and with minimal removal of the dielectric layer.
As is conventional, the terminations of the thick-film resistor of this invention determine the electrical length of the resistor. The terminations may be in the form of electrical contacts lying either above or below the resistor. To form terminations that overlie the resistor, the method of this invention further includes the step of forming a second dielectric layer on the dielectric layer and over the resistive mass after the excess portion of the resistive mass has been removed. Openings are formed in this second dielectric layer to expose at least two regions of the resistive mass, and an electrically-conductive material is then deposited in the openings to form electrical contacts that serve as terminations for the resistor. Alternatively, terminations that lie below the resistor may be formed in or on the substrate underlying the dielectric layer in which the resistor is formed.
From the above, those skilled in the art will appreciate that the method described above is conducive t
Dunn Gregory J.
Scheifers Steven M.
Fekete Douglas D.
Gaffin Jeffrey
Motorola Inc.
Norris Jeremy
LandOfFree
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