In general, besides using conventional discrete passive elements, a printed circuit board can also use a thick film or a thin film process to develop the resistors required. In the thick film process, the resistors of the printed circuit board are made of carbon paste printed on the printed circuit board. Then the resistances of the resistors are fine-tuned by the laser trimming. In the thin film process, on the other hand, a nickel-plated copper foil and the epoxy resin of the printed circuit board is pressed together during the fabricating process of the printed circuit board. The nickel-plated side of the copper foil faces toward the printed circuit board and the non-plated side of the copper foil faces outward. Then, in a subsequent photolithography process, an acid etching solution is first used to etch both the copper and nickel layers, and then an alkaline etching solution is used to etch away the copper layer. A number of nickel blocks with the required dimensions are thereby formed. Laser is then used to trim each of the nickel blocks to achieve the precise resistance required.
In addition, currently, there is an electroless deposition technology that can replace the foregoing thin film method for building the resistor blocks to form thin film resistors.
In conventional thick film resistor fabricating methods, using high curing temperature carbon paste for the resistors is rather simple, mature, and less costly. However, because the laminate of the printed circuit board is susceptible to high temperature, low curing temperature carbon paste is usually used. The macromolecular polymer contained in the low curing temperature carbon paste will remain in the formed resistors even after the curing and solidification processes of the resistors. The hydrophilic property of the macromolecular polymer is the major factor causing the resistances of the resistors to vary along with the environmental change. Therefore, resistors having constant and precise resistances are difficult to achieve. On the other hand, the conventional thin film methods use the same temperatures and solutions as the conventional printed circuit board fabrication methods. The fabricated embedded resistors also have better stability and accuracy than those made by thick film methods. However, because the nickel-plated copper foil is difficult to manufacture, there are only limited supply sources and therefore the price is high. Although there are methods using the electroless deposition technology, the fabricated thin film resistors have inadequate adherence due to certain process factors. The application of these methods for mass production is thereby limited. Accordingly, the present invention is aimed at overcoming problems and disadvantages of conventional methods for fabricating thin film resistors of printed circuit boards.