Design for Embedded Components in PCB Structures
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.
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.
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.