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Even conventional epoxy glass laminate has some capacitance value, and the oldest formed embedded capacitors were carefully controlled thin conventional glass/epoxy laminate layers. Both epoxy resin and E-glass have dielectric constants of about 4, not a particularly large value. However, by eliminating the glass and adding compounds such as barium-titanate to the epoxy or polyimide resin, dielectric constants can be improved to the 10 to 20 range. If really high capacitance values are needed, ceramic or inorganic compositions must be used. Newer technologies demonstrate dielectric constants in the 100 to 2,000 range, capable of forming large value capacitors inside the circuit board. These larger capacitors are useful for decoupling, one of the largest uses of capacitors. Thickness is the divisor in the capacitance formula. The smaller the dielectric thickness, the higher the device capacitance. Early formed embedded distributed capacitors simply used the readily available 100 micron (4 mil) epoxy/glass innerlayer construction, common in multilayer circuit boards. Then, thinner woven glass allowed 50 micron glass/epoxy layers, doubling the device capacitance. Finally, polymer only or barium-titanate-filled polymer demonstrated thicknesses down to 8 microns, improving capacitance 6 to 20 times the value of the original 100 micron epoxy glass. With the thinner materials comes a higher price tag for precision laminate manufacture and special fabrication/handling techniques. The cost/benefit analysis is a trade-off between these premium materials and the smaller-size/higher-capacitance values of the filled polymers.
Inductors Inductors use the magnetic effects of electrons traveling in wires to influence the travel speed of other electrical signals. Typically inductors are a coil configuration. No special material technology has been demonstrated for inductors. These spirals of copper circuitry are typically formed similarly to the copper-conductive traces of innerlayers. Again, design software must determine the line width, length of the coil, and the number of turns of copper to size the proper inductor.
MATERIALS
Resistor Materials Resistors, discrete or formed, are actually very poor electrical conductors when compared to copper. Their function is to restrict the flow of electricity in a circuit. Common compositions of resistors include metal oxides, carbon particles, or small conductive particles separated widely by an organic polymer. Conventionally, the value of a resistor is rated in ohms ( ), with higher values restricting more current. Also, resistors are sized by tolerance in percentage, allowable maximum current, and parameters that deal with the electrical frequency of operation. Material suppliers have designated the range of resistance values that are optimum for their material. The designer determines the length and width that is allowable in the design, and then determines which material to specify for manufacture so that the final resistor value is achieved in the allocated circuit board space. Formed embedded resistors of similar values are designed on one specific layer so that the maximum economic value is achieved for that layer. Materials are also available that can combine mixed values of resistance on one layer. Polymer thick films (PTF) are applied by successive screen print/cure cycles so that all the desired resistance values can be made on one layer. Also, some sheet capacitance manufacturers laminate a layer of resistor foil as one electrode in their construction. Therefore, a fabricator can mix resistance and capacitance on one sheet type raw material.
EMBEDDED COMPONENTS
Resistor Fabrication Details Several different raw materials and processing systems are used to fabricate embedded resistors. The most commonly used are listed in this section. 21.5.2.1 Photoprint. Photoprint involves a supplier coating a thin, resistive layer on a copper-foil sheet and selling this to either a laminate supplier or directly to the board fabrication shop. The fabricator uses two imaging and etching steps: the first to image the copper conductors on the layer, and the second to size individual resistors by using a second, different etchant solution, specifically for the resistor composition. Typically, alkaline etchants are used in step one, and acidic etchants are used in step two. 21.5.2.2 Screen or Stencil Print. Screen or stencil print is the operation of adding specific resistive paste compositions directly to the etched innerlayer. Polymer compositions (PTF) are cured directly at normal board exposure temperatures (150 200 C). Ceramic compositions cure at much higher temperatures (900 C), so the sequence of manufacturing operations is changed. Copper foil first has the uncured ceramic paste screened. Then firing of the paste occurs in a nitrogen blanketed oven. The foil, with cured ceramic resistors, is laminated to B-stage epoxy, and the epoxy layer cured to C stage with resistors pressed into the resin. Finally, the innerlayer copper traces are etched, exposing the embedded ceramic resistors. 21.5.2.3 Plating. Plating is an additive process where the copper layer is first etched.Then the layer is catalyzed for electroless plating, and photoresist is applied. Resistor locations are imaged into the resist and the pattern developed. Exposed catalyst in the proper areas then initiates plating of the resistors. The resist is stripped, background catalyst is removed, and the resistors are in the proper locations. 21.5.2.4 Inkjet Printing. Inkjet printing is the additive process of applying appropriate resistor materials (usually PTF-type compositions) directly to etched innerlayers. Resistors are applied one at a time, in contrast to the mass screening of conventional PTF. It is also possible to print several resistor value compositions before a final single cure step. 21.5.2.5 Photoimageable Discrete. In a photoimageable discrete process, resistive compositions have been dispersed in traditional solder mask like materials, which are applied to the surface of an etched layer. Imaging and development leave the desired geometry resistor after the development step. 21.5.2.6 Resistor Trimming. Resistor trimming is a necessary step. All of the resistors given here have been shown trimmable to very precise values. This is normally done with a modified laser machine, adapted from trimming ceramic surface mount discrete resistors. Either the machine uses a probe card pattern to measure the values of a number of resistors at one time or it uses a flying probe technology from electrical test to measure and trim a single resistors. The flying probe system is used in hybrid circuit manufacture, and the probe card system is standard for arrays of resistors from discrete component suppliers. An interpretation of the laser trim process is shown in Fig. 21.4.
Capacitor Fabrication Normally, embedded formed polymer capacitors are made from purchased sheet materials. It is difficult for the board fabricator to apply a liquid dielectric formulation precisely to laminate, but one photolithographic capacitor formation technique has been developed. Unneeded dielectric is removed with a developer solution similar to liquid photoimageable solder mask.
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