Advantages of Embedding
Before starting the design, it is imperative to understand the advantages that embedding components brings, while at the same time considering the drawbacks of adding the fabrication steps leading to the embedding. In fact, there are potential effects on cost and production yield that the design team must factor in when considering embedding components within PCBs. Some of these advantages are:
- Reduction in size and cost
- Minimizing electrical path lengths
- Decreasing parasitic capacitance and inductance
- Reducing EMI effects
- Improving thermal management
For RUSH PCB UK LTD, innovation in PCB technology comes basically from reduction in size and cost. Embedding components within the PCB substrates help to reduce the size of the board assembly. For complex products, a PCB embedded with components can potentially reduce the manufacturing costs.
High-frequency circuits are highly susceptible to the parasitic effects of long electric path lengths during PCB design. Embedding components within the PCB helps in minimizing electrical path lengths, thereby reducing the parasitic effects to a large extent.
Such reduction in path lengths when connecting embedded passive components to the pins of an IC can decrease the parasitic capacitance and inductance, thereby reducing load fluctuations and noise within the system. For instance, it is possible to place embedded passive components directly underneath the pins of an IC. This not only reduces the via inductance, but also minimizes potential negative parasitic effects, and improves device performance. In fact, embedding components within the substrates of a board allows reduction of path lengths over surface mounting.
It is possible to integrate an electromagnetic Interference shield around an embedded component. For instance, simply adding PTH all around the component can reduce noise coupling from outside. In certain applications, this may even eliminate the need for any additional surface-mounted shield.
It is also possible to add heat-conducting structures to an embedded component for improving thermal management. For instance, embedding thermal micro-vias to be directly in contact with the embedded component can help it to dissipate the heat to a thermal plane on an external layer. Adding thermal micro-vias also reduces thermal resistance, as the amount of heat traveling through the PCB substrate reduces.
One of the major concerns when embedding components within a PCB is the long-term reliability of the design. Solder joints on embedded components formed and placed within the laminates of a PCB can be affected when the PCB undergoes soldering processes such as reflow during assembly of surface mount devices. Embedded components can be an additional problem after manufacturing, as they cannot be easily tested or replaced once they have failed.
What Components can be Embedded?
RUSH PCB UK LTD considers two main categories of components fit for embedding into PCB laminates—passive and active. They are used in different ways and for different applications. As a large majority of embedded components are of the passive category, embedded resistors and capacitors are the most popular.
However, an embedded passive component does not mean that a discrete resistor or capacitor is placed inside a cavity within the substrate of a board. Rather, it is the selection of a specific layer material to form the resistive or capacitive structure of an embedded passive.
Benefits such as listed above make embedded components an alternative to discrete surface-mount passive components. Applications such as series termination resistors benefit from this technology tremendously, a huge number of transmission lines terminate at dense memory devices and ball-grid array (BGA) ICs.
RUSH PCB UK LTD can embed a chip within a PCB, but steps for other manufacturers may vary. Typically, the fabricator has to create space for the body of the IC, and this takes the form of a cavity. Approaches to chip embedding technology may take the following approaches:
CIP or Chip in Polymer: this involves embedding thin chips when building up dielectric layers of the PCB, rather than integrating them within the core layers. The fabricator can use standard laminated substrate materials.
ECBU or Embedded Chip Buildup: this involves mounting chips on polyimide films and building up interconnect structures thereon.
EWLP or Embedded wafer-level package: this involves performing all technology steps at the wafer level. IO area available is limited to the footprint size of the chip, as this technology essentially requires fan-in.
IMB or Integrated module board: this involves aligning the components and placing them within a cavity and using controlled-depth routing to place the cavity within a core laminate. Filling the cavity with molding polymer ensures chemical, electrical, and mechanical compatibility to the substrate. Impregnation of isotropic solder in the polymer helps to form reliable solder joints while laminating the embedded part into the stack.
Component Design Considerations for Embedding
RUSH PCB UK LTD considers layout of components and their physical orientation as important factors when designing for embedded purposes. It is also necessary to select proper substrate materials and compatible components, as this reduces the chances of failure during PCB fabrication.
Selecting specific materials is the key to determine the electrical properties of embedded passives. For instance, an embedded resistor is simply a sheet of resistive film, its dimensions defining the value of the resistance. The resistance of such material is dependent on the resistivity of the material, its length, and its cross-sectional area. Resistive film materials vary in their resistivity, and this directly influences the final resistance value. Therefore, selection of the material is critical to the design and the manufacturing process.
Manufacturers make embedded capacitors by arranging properly dimensioned copper cladding to act as plates, and placing suitable dielectric material in between. Designers calculate capacitance based on the dielectric constant of the material, the permittivity of free space, distance between the plates, and the area of the plates. The final capacitance value increases with an increase in the dielectric constant of the chosen material, an increase in the area of the plane, and decreases with an increase in the plane-to-plane distance in the board layers. Manufacturers use special material for maintaining dielectric strength, with a thin but dimensionally stable dielectric layer for creating embedded capacitors for power supply decoupling.
For making other active components such as ICs, manufacturers and designers select materials that provide substrate durability and long-term reliability of components within cavities. CTE or coefficient of thermal expansion defines the manner in which the material will change during high-temperature events such as reflow soldering of surface-mount components. It is highly imperative for the designer to select substrate material and polymer with matched CTE for filling cavities to maintain the integrity of the board structure.
RUSH PCB UK LTD has two ways of aligning and placing embedded components in cavities—face-up and face-down, with face-down being the preferred process. For a face-down alignment, the cavity depth needs to match the package height, and therefore, the manufacturer can embed chips of different thicknesses on the same layer. This allows for good thickness control for the dielectric material, and accurate component placement during assembly.
Manufacturing Processes for Embedding Components
Individual manufacturers will vary their fabricating processes for embedding depending on the type of PCB and the available equipment at their disposal. Broadly, manufacturing process at RUSH PCB UK LTD for embedding components follow two methods—one, aligning component and placing them within cavities, and the other, molding components into the substrates, building up additional structures thereon.
Manufacturers use different manufacturing and configuration techniques to make cavities in PCBs. Advancement in technology has led to better and more efficient methods of developing cavities for embedding active components. The new methods offer additional benefits such as higher production yields and improved reliability.
Drilling cavities with lasers offers the highest positional accuracy and precision of all methods, as it is possible to control a laser beam precisely for achieving uniform depth and wear as it removes dielectric material. Using a longer wavelength prevents the laser from penetrating copper layers, thereby forming a distinct stop layer. After forming the cavity, the fabricator adds an anisotropic conductive adhesive before placing a component inside the cavity. Application of heat and certain amount of pressure helps to melt the solder particles in the adhesive material, thereby forming reliable solder bonds.
More conventional methods use milling for creating cavities, as milling is more cost-effective than lasers are. Although improved technology allows making miniature milling tools, there is a practical limit to using milling and routing for cavity creation. Even so, milling is more popular as compared to lasers.
Some manufacturers prefer using thin wafer packages, integrating them directly into dielectric layers during the buildup, rather than drilling or routing cavities into the core material. The fabricator begins by die-bonding the thin chip to the substrate, following it up with a layer of liquid epoxy or an application of a laminated RCC or resin-coated copper film as a dielectric. He/she then applies a heated press lamination process, optimizing it to embed the chip without void formation.
Any design with embedded components will require proper documentation for reducing manufacturing time and cost. As the process of embedding components combines component assembly, packaging, and PCB manufacturing into a single manufacturing process, necessary documentation requires layer stack diagrams, NC drill files, fabrication notes, pick-n-place files, and assembly notes for effective PCB fabrication.
Market demand is pushing for high-density, low-profile electronic devices. Manufacturers are complying to this demand with the technology for embedding passive and active components within the board substrate. RUSH PCB UK LTD has successfully broken through potential barriers of reliability concerns and risks to production yields and cost.