Radiation imagery chemistry: process – composition – or product th – Imaging affecting physical property of radiation sensitive... – Including heating
Reexamination Certificate
1998-03-18
2001-05-01
Huff, Mark F. (Department: 1756)
Radiation imagery chemistry: process, composition, or product th
Imaging affecting physical property of radiation sensitive...
Including heating
C430S312000, C430S313000, C430S315000, C430S324000, C427S096400
Reexamination Certificate
active
06225035
ABSTRACT:
FIELD OF THE INVENTION
The present invention generally relates to the manufacturing of printed circuit boards (PCB) and printed circuit assemblies (PCA). More particularly, this invention relates to a method for forming a thick-film resistor on a PCB with photolithographic or photoimaging techniques.
BACKGROUND OF THE INVENTION
Discrete resistors that require mounting directly to the surface of a circuit board are widely used in the electronics industry. A disadvantage with discrete resistors is the additional weight, assembly complexity and reduced circuit density that are incurred with their use. While the size of discrete resistors has been considerably reduced, with chip resistors commercially available having dimensions of as little as about 20×40 mils (about 0.5×1 millimeter), further miniaturization can generally be cost prohibitive for boards requiring numerous resistors.
Thick-film resistors are available alternatives to the use of discrete resistors. However, thick-film resistors have limited applications because of the size and tolerance control of the printing process used to form such resistors, which generally entails screen printing a thick-film resistive paste or ink on a substrate, such as a printed circuit board (PCB), flexible circuit, or a ceramic or silicon substrate. Thick-film inks are typically composed of an electrically-resistive material dispersed in an organic vehicle or polymer matrix material. After printing, the thick-film ink is typically heated to convert the ink into a suitable film that adheres to the substrate. If a polymer thick-film ink is used, the heating step serves to dry and 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.
In addition to the size and tolerance limitations noted above, the use of a thick-film resistor is further complicated by the predictability and variability (or tolerance) of the electrical resistance, which are dependent on the precision with which the resistor is produced, the stability of the resistor material, and the stability of the resistor terminations. Conventional thick-film resistors are rectangular shaped, with “x,” “y” and “Z” dimensions corresponding to the electrical width (W), electrical length (L) and thickness, respectively, of the resistor. Control of the “x” and “y” dimensions of a thick-film resistor is particularly challenging in view of the techniques employed to print thick-film inks and the dimensional changes that may occur during subsequent processing. For rectangular screen-printed resistors, the x and z dimensions are determined by the resistor 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 imprecise process. Screen printed thick-film resistors having adequate tolerances in the x and y dimensions are often larger than chip resistors. Resistance value predictability is generally low, and precise tolerances typically cannot be maintained at aspect ratios (L/W) below 0.5 squares. As a result, one ink of a given resistivity, requiring one screening, cure and associated process steps, is required for each decade of resistance value needed in a circuit design, which often necessitates the use of three to four inks to complete one circuit. While resistance tolerances can be improved by laser trimming, such an operation is usually cost-prohibitive for complex circuits. As a result, screen printed thick-film resistors have found only limited application as a substitute for discrete resistors.
Accordingly, a need exists for a method for fabricating a thick-film resistor having more precise dimensional tolerances and a wider range of readily-producible resistances per single resistive ink than present thick-film resistors offer.
SUMMARY OF THE INVENTION
The present invention provides a process for forming a resistor whose dimensions can be accurately determined by photoimaging processes, thereby yielding a resistor whose size can be sufficiently small and whose resistance value can be sufficiently predicted to be a commercially viable alternative to discrete chip resistors. More particularly, resistors of this invention are formed of a photoimageable resistive thick-film material that enables the dimensions of the resistor to be determined directly by photodefinition instead of conventional screen printing. As a result, the dimensions of a resistor formed in accordance with this invention can be more precisely determined than possible for prior art screen printed thick-film resistors. Moreover, resistors of this invention can be formed and sized to maximize the circuit density on a circuit board while incurring minimal additional weight.
The method of this invention generally entails depositing a photoimageable electrically-resistive layer on a substrate. Suitable materials for the resistive layer preferably contain an electrically-conductive particulate filler in a curable and photoimageable polymeric matrix. Once deposited, the resistive layer is photoimaged and developed so as to remove a portion of the layer while the remaining portion remains to define a resistor on the substrate. Electrically-conductive terminations are provided that determine the electrical length of the resistor.
The terminations may be formed on the substrate prior to depositing the resistive layer, such as by electroplating, or after the resistive layer has been photoimaged and developed. If the latter approach is used, the material for the photoimageable electrically-resistive layer is further modified to render the resistive layer plateable. The terminations can then be formed by depositing a photoimageable layer on the resistor, photoimaging and developing the photoimageable layer so as to form openings that expose two regions of the resistor. The exposed regions of the resistor, associated with the developed photoimageable layer, is then plated with a conductive material to form terminations that overlie the resistor. In an alternative embodiment, a plateable photoimageable layer is deposited and photoimaged in the manner described above, after which a second photoimageable layer is deposited on the plateable photoimageable layer prior to developing the plateable photoimageable layer. The second photoimageable layer is then photoimaged and the photoimageable layers developed simultaneously to form openings that expose two regions of the resistor and two regions of the plateable photoimageable layer that are adjacent the exposed regions of the resistor. Thereafter, the conductive material can be plated on the exposed regions of the resistor and the plateable photoimageable layer to yield terminations and connectors to the resistor.
From the above, those skilled in the art will appreciate that forming a thick-film resistor from a photoimageable material in accordance with this invention enables the electrical width of the resistor to be precisely determined by photoimaging, while the electrical length of the resistor can be precisely determined by the process used to form the terminations. The result is a thick-film resistor whose dimensions can be controlled far better than possible by screen printing, and whose size can be less than that of discrete resistors.
REFERENCES:
patent: 4479890 (1984-10-01), Prabhu et al.
patent: 4610810 (1986-09-01), Hasegawa
patent: 4870746 (1989-10-01), Klaser
patent: 5162144 (1992-11-01), Brown et al.
patent: 5260170 (1993-11-01), Brown
patent: 5338567 (
Brown Vernon L.
Conway Lola
White George E.
Zhang Min-xian
Barreca Nicole
Fekete Douglas D.
Huff Mark F.
Motorola Inc.
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