Embedded components in printed circuit boards: a processing technology review

It is evident that embedded passive components (EPCs) allow packaging substrate miniaturization and have the potential to reduce costs. Moreover, they exhibit superior electrical behaviour with respect to the minimization of parasitic effects. However, as for most emerging technologies, there is no well-established process or method for EPCs that lead to the desired result, but many have been and are still being investigated. This article attempts to review the state of the art of resistor and capacitor EPCs, including an assessment of the pros and cons of the various technologies pursued. In the review, it is found that compared to discrete surface mount devices, EPCs provide (in order of current importance): space reduction of 30% or more, better HF signal integrity and potential cost reduction. Embedded resistors in thin-film technology are, in general, restricted to small resistance values up to a few kOmega. Embedded resistors in ceramic thick-film technology require a high temperature curing process and much care during lamination, but they can be combined with embedded capacitors and exhibit high stability. Whereas embedded resistors in polymer thick-film technology require a low curing temperature and can be combined with capacitors, they exhibit poorer electrical properties and stability. Moreover, tolerances of embedded resistors after manufacturing are exceeded by 15%, independent of the manufacturing technology, which means that laser trimming is required. Embedded capacitors are based mostly on barium-titanite with a dielectric constant of only approximately 20, which limits the capacitance density to a few nF/in(2). Ferroelectric material with a dielectric constant up to 2000 for embedded capacitors has been investigated but not yet established. Besides the traditional screen print technology for the deposition of pastes and inks, the ink-jet technology becomes more promising. Much higher flexibility with respect to material composition, layer thickness and even active components like LEDs have already been produced by ink-jetting.