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Design007-Sept2018

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38 DESIGN007 MAGAZINE I SEPTEMBER 2018 pliers with experience in build-up of circuits can embed both passive and active components. Although a majority of companies furnish- ing high-volume embedded component circuit boards are in Europe and Asia, there are number of proficient North American suppliers as well. Providing formed resistors and capacitors within the circuit structure remains the most economical process with the least level of risk; however, value range and tolerance control is somewhat limited. Placing discrete component elements furnishes a greater value range and superior tolerances. Embedded Component Process Planning The process for embedding discrete passive component elements within the multilayer PCB structure is a sequential process requir- ing specialized SMT process skills and assem- bly systems typically outside the realm of the average circuit board supply chain—a process capability more common to OEMs and board- level assembly service providers. To prepare for efficient high-volume production, the PCB fabricator will need to establish in-house com- ponent placements and attachment capabili- ties and reliable component sources or develop partners for the procurement of components that are suitable for embedding. The PCB sup- plier must also determine which systems and methodologies will be required for applying termination materials (conductive polymer or solder) that can ensure precise component placements, curing polymers or reflowing sol- ders, and specialized systems required for elec- trical testing during the progressive stages of the fabrication process. When choosing discrete resistor, capacitor, and inductor components for embedding, both device outline and thickness must be consid- ered. While some passive components are very thin and require no special preparation, others may have profile dimensions requiring encroachment into contiguous circuit layers. Discrete Resistors Several companies are now able to furnish thin, small outline resistors and capacitors that prove to be ideal for embedded component applications. The outline of currently avail- able components can be as small as 0.4 mm x 0.2 mm (01005) and 0.6 mm x 0.3 mm (0201). Discrete thick-film resistors are offered in a 0.015-mm profile. Specifically developed for embedding, these devices are described as a flat thick-film resistor furnishing a value range of 1.0 Ω to ~10 MΩ with a power rating of 0.063 watts and working voltage maxing out at 50 volts. Although these devices have a relatively low power rating, the operating temperature range, resistor value, and tolerance (1% and 5%) specified for the smaller devices are the same as the larger resistor variations and available with copper terminals. While tin-based alloy terminals are acceptable for solder or conduc- tive polymer termination, copper terminals will be necessary when using plated microvia interface technology. For applications requir- ing resistor value tolerances less than 1%, an alternative nickel-chromium thin-film family is available, but the package dimensions are significantly greater than the thick-film resistor previously described. Discrete Capacitors Low-profile ceramic capacitors have been developed for height-restricted applications where device thickness is a key design con- straint—perfect for embedded or other appli- cations with thickness limitations. Ideal for decoupling and filtering applications, the com- Providing formed resistors and capacitors within the circuit structure remains the most economical process with the least level of risk; however, value range and tolerance control is somewhat limited.

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