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Embedding Components, Part 5: Alternative Termination Methodologies and Surface Plating Variations

Embedding discrete passive components in a multilayer PCB combines two process disciplines: 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 assembly 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 systems with a high level of positional accuracy. Interconnection can be accomplished using either deposited solder paste and reflow processing or applying a conductive polymer material. Due to the extremely small land pattern geometries required for mounting the miniature passive components, companies commonly rely on precision dispensing these materials.

Component Attachment Site Preparation

Land patterns for embedded passive components—previously explained in Part 4 of this series—furnish geometries that maximize adhesion and ensure electrical interface between the terminals and lands of the components.

Key attributes for solder or conductive polymer 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.

To read this entire column, which appeared in the November 2018 issue of Design007 Magazine, click here.