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Design007-Nov2018

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46 DESIGN007 MAGAZINE I NOVEMBER 2018 Embedding discrete passive components in a multilayer PCB combines two process dis- ciplines: PCB fabrication and SMT assembly. Locating a supplier with both capabilities may be a challenge; however, a growing number of PCB fabricators have installed SMT assem- bly capabilities, or they have partnered with a company already established to offer assembly services. Because they are furnished with a very thin profile, resistor and capacitor components with different values can be mounted directly onto land patterns on a subsurface layer of the printed circuit structure. However, handling and placing of these small components requires sys - tems with a high level of positional accuracy. Interconnection can be accomplished using either deposited solder paste and reflow pro- cessing or applying a conductive polymer mate- rial. Due to the extremely small land pattern geometries required for mounting the miniature passive components, companies commonly rely on precision dis- pensing these materials. Component Attachment Site Preparation Land patterns for embedded passive components—previ- ously explained in Part 4 of this series—furnish geometries that maximize adhesion and ensure electrical interface between the terminals and lands of the com- ponents. Key attributes for solder or conductive poly- mer process control include: • Uniform surface planarity • Minimal intermetallic propagation • Inhibited surface oxidation The surface finish on the passive component terminals and the land patterns provided on the substrate must enable a reliable electrical and mechanical interface. Solder Processing The alloy composition commonly furnished on discrete passive component terminals is tin (Ag) over nickel (Ni) base plating. AgNi-plated component terminals will be compatible with any one of the commercially available lead-free solder alloy compositions listed in Table 1. Solder paste is essentially a fine metal alloy powder suspended in a thick flux medium. Flux Embedding Components, Part 5: Alternative Termination Methodologies and Surface Plating Variations Designers Notebook by Vern Solberg, CONSULTANT Table 1: Lead-free solder alloy compositions. (Source: Indium Corporation)

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