Embedding Passive and Active Components: PCB Design and Fabrication

Vern Solberg
Solberg Technical Consulting
Saratoga, California USA

Embedding components within the PC board structure is not a new concept. Until recently, however, most embedded component PC board applications adapted only passive elements. The early component forming processes relied on resistive inks and films to enable embedding of resistor and capacitors elements. Although these forming methods remain viable, many companies are choosing to place very thin discrete passive components and semiconductor die elements within the PC board layering structure.

In addition to improving the products performance, companies have found that by reducing the component population on the PC board's surface, board level assembly is less complex and the PC board can be made smaller, The smaller substrate, even when more complex, often results in lower cost. Although size and cost reductions are significant attributes, the closer coupling of key elements can also contribute to improving functional performance.

This paper focuses on six basic embedded component structure designs described in IPC-7092. The process variations define the structure, depending on whether components are passive or active, placed and/or formed and if they are on one side of the PC board base-core or both. The formed and placed components may be located on any number of layers, however, formed components are generally assigned to dedicated layers.

The layering description actually becomes part of the type designation that is very similar in describing an eight layer (2-4-2) HDI board and the naming indicates whether the base-core represents a final assembly or is simply a mounting base onto which additional layers are sequentially added.

The IPC-7092 standard suggests that the first decision the developer must address is whether or not to consider embedding components. Depending on the application, cost, performance, or some other metric, the cost for adoption will likely influence this decision. Several materials properties must be considered as well; the availability of passive and active components intended for placement, and implementation of new processes. Although cost of materials and additional process steps can be an issue, 'formed' component processes are mature, economical and well within the process capability of most circuit board fabricators.

Manufacturers recognize, however, that values and tolerances of the formed components can shift significantly as the board materials age or when the product is exposed to severe assembly or environmental conditions. On the other hand, with the availability of smaller passive device outlines from a wide supply base, many companies, although reluctant to embed active semiconductor components, will see the practical advantage for embedding discrete passive elements. For resistor elements especially, the value range is broad and the tolerance will remain stable.

Embedding the semiconductor is where many companies may find a significant concern. Procurement of semiconductors in a wafer format and outsourcing metallization and thinning of the wafer is not always possible. Additionally, micro-via termination processes used in circuit board manufacturing requires component parts to have copper plated bond sites. The good news is, a growing number of foundries, are already preparing their wafers for the emerging copper wire-bond users.

The originating companies may elect to bring together the two primary suppliers; the circuit board fabrication specialist and the assembly service provider with both assembly and test capability. Some PC board fabrication companies may have already established both of these capabilities in-house but others will need to develop partnerships. These partnerships must be willing to adjust their portion of the generated revenue against the overall process yield. That includes the sharing of losses from fabrication process defects and damaged components.