Design and processing of ceramic resistors and capacitors fired onto copper foil and embedded into FR4 circuit boards are presented and discussed. Evolution of design guidelines and processing for embedded ceramics is presented and discussed. Design tolerances are slightly larger for embedded ceramics because of the extra firing at high temperatures experienced by the copper foil on which the resistors and capacitors are printed. Scaling of the resistor and capacitor terminations must be done during print-and - etch operations. Ceramic bodies are weak in tension and very strong in compression, so the most important processing precaution is to minimize tension experienced by the resistors during high temperature firing and lamination. Somewhat reduced lamination pressures is helpful. Limiting the size of resistors and capacitors is also helpful for capacitors. Size of resistors is usually not a problem except for very small resistors below 10 mils in size Designs for these applications include specially formulated pastes with thermal expansion behavior higher than is optimum for ceramic resistors and capacitors on alumina substrates. In addition, protection of the ceramic components from the stress of the lamination steps is desirable for the resistors. This is provided by encapsulating the resistors in a filled encapsulant that reduces the expansion of the epoxy resin, and scatters the laser energy applied during resistor trimming, so that the PWB itself is not harmed while being laser trimmed. Performance after processing appears excellent by usual environmental tests; and board flexure encountered in bend tests does not appear to be a problem. To date, five mil resistors appear too small to be screen printed with good CV and reliability. Ten mil and larger resistors are adequately stable.
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