This column will focus on land pattern development criteria, and methodology for accommodating low and moderately high-profile passive components within a multilayer circuit board.
As noted in Part 3 of this series, a broad range of discrete passive component elements are candidates for embedding, but the decision to embed these component elements within the multilayer circuit structure must be made early in the design process. While many of these components are easy candidates for integrating into the substrate, others may not be suitable, or they are difficult to rationalize because they involve more complex process methodology.
Basic material sets selected for this embedded component application defines a 200-μm FR-4 epoxy-glass core base with 18-μm copper foil bonded to both sides. In this example, the buildup material includes pre-impregnated layers of glass cloth with an uncured epoxy resin furnished pre-dried, but not hardened, and layers of 18-μm copper foil. There are more than a dozen thickness options available for prepreg materials, but for this process description, a 200-μm material will be referenced. The prepreg layer and copper foils are sequentially bonded together using a combination of pressure and heat. The process begins with imaging and chemically etching the circuit pattern on the copper foil. The lamination, imaging, and etching processes continue until the layer structure is complete.
When embedding components in the multilayer structure, several factors must be considered:
- Circuit layer selected for attachment
- Component size (length, width, and height)
- Terminal area and metalization
- Location and orientation
- Method for termination
While thin passive components may not require any preparation before lamination, taller components will likely need developing a cavity-like pocket in the dielectric layer to ensure that the overall flatness of the finished multilayer PCB can be maintained. Regarding the attachment method for components, tin-alloy plated terminals will be most compatible with a tin-based solder alloy or a conductive polymer material that is either deposited or stencil printed onto the mounting site.
To read this entire column, which appeared in the October 2018 issue of Design007 Magazine, click here.