Method for forming a thick-film resistor and thick-film resistor formed thereby

• US 6,229,098 B1
• Filed: 07/21/2000
• Issued: 05/08/2001
• Est. Priority Date: 06/05/1998
• Status: Expired due to Fees

• Current Assignee: Motorola Solutions, Inc.
• Original Assignee: Motorola, Inc. (Motorola Solutions, Inc.)
• Inventors: Dunn, Gregory J., Scheifers, Steven M.
• Primary Examiner(s): Gaffin, Jeffrey
• Assistant Examiner(s): NORRIS, JEREMY C
• Application Number:
  US09/621,148
• Time in Patent Office: 291 Days
• Field of Search: 174/255, 174/256, 174/260, 174/261, 174/262, 361/738, 361/761, 361/762, 361/763, 361/764, 361/765, 361/782, 361/766, 338/252, 338/253, 338/311, 338/322, 338/323, 338/324, 338/325, 338/326, 338/327, 338/328, 338/329, 338/330, 338/331, 338/332
• US Class Current: 174/260
• CPC Class Codes: H01C 17/003 using lithography, e.g. photolithography lithographic compositions and processing in general G03F
• CPC Class Codes: H01C 17/065 by thick film techniques, e.g. serigraphy
• CPC Class Codes: H05K 1/095 for polymer thick films, i.e. having a permanent organic polymeric binder
• CPC Class Codes: H05K 1/167 incorporating printed resistors
• CPC Class Codes: H05K 2201/09509 Blind vias, i.e. vias having one side closed
• CPC Class Codes: H05K 2203/0568 Resist used for applying paste, ink or powder
• CPC Class Codes: H05K 3/0023 by exposure and development of a photosensitive insulating layer
• CPC Class Codes: H05K 3/4644 by building the multilayer layer by layer, i.e. build-up multilayer circuits making via holes in the insulating layers H05K3/0011; special circuit boards as base or core whereon the multilayer is built H05K3/4602

Abstract

A process for forming a thick-film resistor whose dimensions can be accurately obtained, thereby yielding a precise resistance value. The method includes providing on a substrate a photoimageable layer that preferably forms a permanent dielectric layer of a multilayer structure. An opening is photodefined in the surface of the photoimageable layer, and then overfilled with an electrically-resistive material to form a resistive mass having an excess portion that lies on the surface of the photoimageable layer surrounding the opening. Following curing which causes the surface of the resistive material to become recessed below the surface of the photoimageable layer, the excess portion of the resistive mass is removed, preferably by abrading or a similar operation, such that the lateral dimensions of the resistive mass are determined by the lateral dimensions of the opening in the photoimageable layer. Thereafter, subsequent processing is performed to include the photoimageable layer as a permanent photoimageable layer of a circuit board, with the resistive mass and appropriate terminations forming a resistor in the permanent photoimageable layer.

Claims

1. A printed circuit board comprising:

a substrate;

a photoimageable layer on the substrate, the photoimageable layer having an opening in a surface of the photoimageable layer, the opening being characterized by a first opening dimension, a second opening dimension transverse to the first opening dimension, and a depth equal to a thickness of the photoimageable layer, the photoimageable layer being a permanent dielectric layer of the printed circuit board; and

a resistive mass within the opening, the resistive mass comprising carbon particles in a polymeric matrix, the resistive mass having first resistor dimension and a second resistor dimension that are approximately equal to the first opening dimension and the second opening dimension, respectively;

the resistive mass having a resistive surface recessed below the surface of the photoimageable layer and a resistor thickness less than the thickness of the photoimageable layer, the resistive mass lying in a plane containing the photoimageable layer, the resistive mass contacting at least two electrical terminals disposed in a plane outside the plane containing the resistive mass and the photoimageable layer.

2. A printed circuit board as recited in claim 1, wherein the resistive mass contacts the substrate.

3. A printed circuit board as recited in claim 1, wherein the electrical terminals are disposed above the resistive mass and the photoimageable layer.

4. A printed circuit board as recited in claim 1, further comprising:

a second photoimageable layer overlying the photoimageable layer and the resistive mass; and

two openings in the second photoimageable layer, the electrical terminals disposed in the two openings.

5. A printed circuit board as recited in claim 1, wherein the electrical terminals are disposed beneath the resistive mass.

6. A printed circuit board as recited in claim 5, wherein the electrical terminals lie on the substrate and the resistive mass overlies and contacts the electrical terminals.