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Design for Embedded Components in PCB Structures

Tue, 08/03/2004 - 5:11am

Even as PCBs become denser with additional components, traces and vias, pressure is still on to place more and more components, even after the PCB is saturated. Welcome to embedded components.


By Dean Wiltshire

Most electronic products start with the definition of the physical package. This is turned into mechanical diagrams with physical space designated for electronic circuitry. The physical package design is limited by its application. Cell phones are a perfect example. They must fit into the average size human hand, support a speaker near an ear, microphone, and must feature a telephone key pad which the average finger tips can access.


Figure 1. A PCB layer stack with embedded integral passive components.

These are minimum standard requirements that haven't changed since the first cell phones were delivered for consumers. Yet, every six months, consumers see increased capability from new cell phones with the same physical packaging requirements. There is an increase in active and passive electronic components, and signals between these components, without an increase in physical space designated for electronic circuitry, therefore the PCBs become denser with additional components, traces and vias. How is it possible to have more and more components after they are already saturated? Welcome to embedded components.

The Scoop

Integral components are fabricated into a PCB and allowed on internal (embedded) and surface layers. Embedded discrete components are assembled into the PCB, typically into an exposed cavity. There are several factors driving the trend to use embedded integral passive components over discrete passive components and embedded discrete active components over surface mounted discrete active components.


Figure 2. Meander integral resistor.

Advantage: Embedding

Increased functionality of active components has the number of passive components growing. The number of discrete passive components is, in many cases 70% to 80% of the total part count and continues to rise as passive-to-active ratios grow. While active components are being packaged into large pin BGAs the ideal surface placement space for discrete passive components becomes more difficult to obtain.

In addition, increased numbers of capacitors are needed as component speeds and digital content increase. Capacitors must be placed close to the IC pin to avoid unacceptable noise levels that cause timing errors. Eliminating discrete passive components from the surface layers and embedding them frees surface space and allows the passive components to be closer to active pins.

Embedded integral passive components allow for higher frequency (faster) PCBs. The linearity of signals through embedded integral passive components reduces inductance of "core to surface, return to core" signal paths. Along with lower inductance, embedded integral passive components can lower power system impedance and radiated emissions — improving the overall electrical performance of a PCB. Also, reliability of the PCB is improved through reducing the overall number of solder joints.

For leading-edge companies, there is also the need for embedded discrete active components. While additional surface space is made available for other active components, embedded discrete active components are not packaged, leading to smaller footprints for the active component. The driving factor to use embedded discrete active components is the reduction in the PCB size format while increasing the active functions.


Figure 3. An etched integral resistor (image courtesy of Omega Technologies, Inc).

Design Tools and Their Roles

This article will explain the different types of embedded components and what is required for a systems design tool to fully embrace embedded components.

There are three types of mainstream embedded integral passive components: resistors, capacitors and inductors.

Figure 1 shows a FR-4 PCB layer stackup with several embedded integral passive components. The following sections detail embedded components and speak to some of the design tool requirements for each type.

Embedded Integral Resistors

An embedded integral resistor resides on a single layer, which can be any physical layer on a board, even plane layers. A resistor has two copper pads with resistance material between the pads. The shape of the resistance material can be a simple rectangle, or a meander resistor shape as shown in Figure 2. In all cases, the resistance material overlaps the copper pads. A layer-specific trace/via/drill keepout and placement outline is required around the area of resistance material to prevent signal shorts/openings with the resistor.

Fabricating embedded integral resistors involves an etching or printing process. Figure 3 shows a single layer buildup and process to support etched resistors. Both etch and printing processes leave resistors with +8% to +16% tolerances. This may be acceptable for some signals but for this technology to become mainstream, tolerances must be lowered to that of passive resistor components, or +1%. In order to achieve +1% tolerance, today's integrated test and trim equipment addresses these issues. For example, ESI has developed a large stage system that accommodates panels up to 24 inches× 30 inches. This machine allows for probing and testing of multiple resistors with concurrent laser trimming. It also measures the resistance of each resistor and laser trims it to the correct value-the results are within the desired tolerance of +1%.


Figure 4. An embedded integral capacitor.

When the cost of laser trimming is understood, many steer away from embedded integral resistors and this should not be the case. What is often forgotten in most cases is that it is not the absolute tolerance of a resistor that is critical but the relation between several resistors in the same circuit. As all the embedded integral resistors are manufactured in the same process, their relative accuracy is much better than the 8 - 15% absolute tolerance that is typical for a technology.

Since embedded integral components are part of the fabrication processes and not part of the assembly process, BOM does not reflect the components as physical components. Fabricators prefer to work with spreadsheets in order to manage embedded integral passive components.

Embedded Integral Capacitors

Typically, capacitors represent the largest percentage of components on a PCB. Today, there are two techniques for fabricating embedded integral capacitors: through dielectrics and "thick-film" process (additive).

Great promise surrounds the ability of materials with high dielectric constants to enable embedded integral capacitors. There may be one to many "capacitor" dielectric layers on the board and a dielectric layer could support one to many capacitors. Since the signal actually flows through the dielectric, the pads of a capacitor (for trace connections) can be located on opposite sides of the dielectric layer(s) as shown in Figure 4. Discrete capacitors have parasitic inductance that limits frequency behavior, an advantage for embedded integral capacitors since they have almost no parasitic inductance.

Embedded integral capacitors can also be added through a thick-film screening process directly onto a layer. This technique is used in today's MCMs and Hybrids. The capacitor material overlaps the copper pads and, in this scenario, the pads are on the same layer. Similar to embedded integral resistors, layer-specific trace/via/drill, and placement outlines from the capacitor material (dielectric or screened) need to be established and respected throughout the design process.


Figure 5. An embedded integral inductor.

Embedded Integral Inductors

Inductors are another candidate for embedded integral components. Unlike embedded integral resistors or capacitors, they require no special materials or special fabrication processes. They are copper and use thin lines and spiral type geometry. A combination of line width, number of turns, shape and line-length define the inductance. Figure 5 shows copper spiral lines connected through a blind/buried pad.

Proximity to other signals can reduce the inductance of embedded integral inductors. Therefore, layer specific trace/via/drill and placement outlines should be larger than just the geometry footprint. Because of the fine lines and sizes of inductors, a high resolution is needed within output and photoplotting to insure correct creation of the outlines.

From a PCB design point of view, embedded integral inductors have one pad located on the outside of the shape and the other in the center. Routing to the center pad requires a different layer. Therefore, the center pad is blind, buried or through. Inductor geometries may span more than one signal layer, using a blind/buried pad for interconnect between layers. A template library of shapes and modeler to generate/verify a shape would be very useful when performing this task.

Embedded Discrete Active Components

Higher functionality densities are obtainable if discrete active components like Integrated Circuits (ICs) are not packaged; the dies are placed directly on or in PCBs. Wirebonds, cavities, die on board, die on die along with modified packaging of flip chip are all being requested to be brought into PCBs (see Figure 6).

Microvias support fanouts from wirebond layers and solder bumps, all of which may be to an internal layer. Figure 7 shows solder bumps on a PCB layer. A flip chip is placed into these solder bumps. Solder bumps are created through an assembly process of depositing the solder bumps onto pads. The flip chip is then placed with its connection points landing on the solder bumps. Then solder bumps are heat-treated, causing a solid interconnect bond between the flip chip and the pads.

With the specific embedded components and their requirements defined, we'll discuss the needs of system design tools to successfully incorporate embedded components.


Figure 6. A PCB cavity and embedded discrete active component with wirebonds to different signal layers.

Appropriate System Design Tools

One of the most important points to realize is that use of embedded integral passive components is a fabrication process and not an assembly processes. When considered in the design phase, this provides great design flexibility, allowing PCB Designers to meet increasing density challenges. Of course, this flexibility can only be realized if the tool fully supports embedded components. As we explore the impacted roles of a design team, we'll point out how the design tools complement the roles.

The first role is that of an electronic engineer capturing the logic to be designed into the PCB. This process does not change. The physical values and tolerances of components are expressed as they are for embedded components as they are for discrete components.

The role of component librarian changes with the addition of managing vendor specific material libraries and PCB layer stackups. These libraries can be provided by the EDA vendor or material vendor as part of a design kit.

Some EDA vendors claim support for embedded integral components, yet require the librarian to generate the embedded integral components — one physical shape per value, material, tolerance, layer, etc. The management of a library, which offers every potential shape/size of embedded integral passive components, become exponentially large when managed in a typical component library scheme. To alleviate this problem, the librarian might only offer one or two embedded integral passive components per value.


Figure 7. A bump-to-mount flip chip.

The unfortunate trade off is that the PCB designer now has a limited, fixed set of embedded integral components — artificially setting a limit on the overall design density. The best solution is that a librarian doesn't create any embedded integral passive components — the design tool synthesizes the physical shape, thus removing the library management nightmare and providing the greatest flexibility for the PCB designer.

The PCB designer needs to understand material layer stackup with the flexibility of potentially modifying the materials to optimize the design. To support, the design tool must offer material representation and stackup/material changes during the PCB design.

A tool that fully supports embedded integral passive components synthesizes them from vendor specific materials, electrical requirements and physical design parameters. From a design flow point of view, the component synthesis has to occur in the placement and interconnect phases of PCB design. This provides the PCB designer optimum control over the component size, XY and layer placement during critical aspects of the design.

A proper component synthesis provides designers with several physical options for each component reference designator. So, two instances of integral 10 KW resistors can be represented completely differently in a design to best fit the design densities in their physical layer location.

Interactive and automatic tools allow designers to synthesis the shapes and values individually or as selected groups to optimize design densities, values and fabrication rules. DRC, both online and offline, must reassure the PCB designer throughout the design process that embedded integral passive components are placed on the appropriate layer for fabrication and meet the appropriate value and tolerance.

Embedded discrete active components have different needs. Flip chips IC packaging is becoming popular because of its smaller physical size. They can be placed in an exposed cavity or embedded within the substrate layers. When compared to embedded integral passive components, they have a much greater thickness (Z-axis height) for both the physical component and the mounting. They are assembled onto the PCB with mounting and conductivity occurring through "solder bumps" or wirebonding.

If placed in a cavity, the active discrete component may be completely covered by additional PCB layers — truly embedding the component and potentially leaving surface layers for other components. If wirebonding is used, then the embedded discrete active component can sit in a cavity, but is exposed to one of the surface layers for wirebonding. The design tool must provide the PCB designer the flexibility to generate wirebonds from the die to the PCB and too specific layers on the PCB and then provide the PCB designer with the ability to move the wirebond pads to accommodate traces and vias, while not allowing the wirebonds to short to one another.

The final step in PCB design is post processing — generating outputs. This task is typically done by the role of the PCB designer. The design tool has to provide a boolean AND, OR and XOR filters which understand materials to determine what is included on plotter outputs. These are used to create the appropriate masks of materials and fabrication processes. Additional outputs must support laser trimmers, bare board testers, XY and layer locations, pad locations, wirebonds and NC Router. All of these output capabilities are needed per substrate layer.

Conclusion

Embedded components offer the opportunity for higher functionality, performance and density on PCBs, at lower overall product costs. However, these benefits can only be realized by employing system design tools, which understand and embrace the special needs of embedded components. Library management is different for embedded components; the library and design tools must accommodate special needs — not cause component management nightmares or false design constraints. Design tool must allow the PCB Designer to employ component flexibility at any point in the design so the appropriate design density can be realized.

With these tools in place, the next level of density within fixed physical packaging constraints can be realized.

About the Author

Dean Wiltshire is a Product Architect with Mentor Graphic's System Design Division. He has worked in the PCB industry since in 1982, where he started as a designer in the SciCards service bureau. Wiltshire has held various design, product marketing, application engineering, and engineering positions for EDA companies like SciCards, Cadnetix, Dazix, Intergraph Electronics, VeriBest, and Mentor Graphics. Along with a skill set as a Senior PCB Designer, Wiltshire earned a Master of Science in Computer Information Systems.

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