Denis Lachapelle, P. Eng.
Anne-Marie Coutu, Tech.
Manufacturing Companies | PCB Cost Drivers | CAD Tools | Special Applications | Conclusion
The primary functions of a printed circuit board (PCB) are to support electronic components and facilitate their interconnection. PCBs, also known as printed wired boards (PWBs), serve as the foundation for a wide range of devices, from smartphones and dishwashers to large-scale weather simulation computers. Integrated circuits, which power these devices, are typically mounted on PCBs.
This paper aims to explore various aspects of the PCB ecosystem, including design, materials, categories, and usage. Below, two examples of PCBs are provided for reference.
Several major companies dominate the manufacturing of PCB laminate, prepreg, and associated chemicals, including DuPont, Rogers Corporation, Isola Group, Ventec, and Iteq Corporation. These companies produce a wide range of materials necessary for manufacturing printed circuit boards, which serve as the foundation for installing and interconnecting electronic components.
The manufacturing process begins with finished PCB companies receiving inputs such as copper laminates, prepregs, and various chemicals. They etch copper on both sides of the laminate to create traces and copper areas. For multilayer PCBs, they repeat this process and laminate the layers with prepreg in between. Additional steps, including drilling, plating, and alignment, follow, culminating in a fully formed PCB. While the explanation provided is simplified, it captures the fundamental principle of PCB manufacturing. Additional steps, such as drilling, plating, aligning, and printing, are integral parts of the process but have been omitted for brevity.
PCBs come in various layer configurations to accommodate different circuit complexities and requirements. Single-layer PCBs are suitable for straightforward circuits, while two-layer PCBs offer increased reliability and are used for moderately complex designs. For power electronic boards, which require robustness and efficient power distribution, four and six-layer configurations are often employed. High-density and high-speed circuits, known as high-density-interconnect printed circuit boards (HDI PCBs), typically utilize eight, ten, or more layers to accommodate intricate designs and ensure signal integrity.
After PCB fabrication, the next crucial step is soldering the components onto the board, a task typically performed by board stuffing or EMS companies. These companies receive the bare PCB along with all the electronic components to be mounted on it. Their assembly lines consist of several key sections:
While this explanation simplifies the process, it encapsulates the core principle of component soldering onto PCBs.
After assembly, PCBs undergo thorough inspection and testing to ensure their functionality and reliability. The methods employed for inspection and testing vary depending on the product’s complexity and application. For instance, while simple commercial circuits may require swift testing to control costs, safety-critical medical or aerospace circuits demand meticulous scrutiny to ensure utmost reliability. In space applications, where part replacement is nearly impossible, reliability takes precedence over cost concerns. Thus, inspection and testing pose significant challenges, balancing the need for rigorous scrutiny with practical considerations such as testing time and cost constraints.
Inspection and testing of PCBs involve a range of methods tailored to different needs. In some cases, specialized workers conduct manual inspections, meticulously examining each board for defects. Alternatively, camera inspection systems are employed for swift and precise examination of PCBs.
Testing methods vary as well. Flying probes are used to measure the board’s components and trace connections, providing detailed insights into its functionality. Another approach involves using a bed of nails, which connects multiple nodes of the circuit to specialized test equipment. This setup enables the execution of comprehensive test procedures to validate the board’s performance. Additionally, manual testing procedures, carried out by technicians, involve executing test procedures, taking measurements, and validating results through hands-on inspection.
At the end, the assembled boards are fully functional and ready to be integrated in the final product.
Whilst non-exhaustive, this section lists various types of PCBs.
The most common PCBs are typically constructed from FR4 epoxy glass laminate, which consists of a glass fabric filled with epoxy and laminated with copper on both sides. This material is available in thicknesses ranging from 2 to 200 mil. Additionally, prepreg, which is also composed of glass fabric filled with epoxy, is commonly used in PCB construction. Prepreg comes in thicknesses ranging from 3 to 8 mil, and multiple layers of prepreg are often incorporated into PCB designs for added strength and insulation.
Metal Plate PCBs feature a laminate that incorporates a metal plate, as illustrated in Figure 1. A dielectric layer is laminated onto the metal plate, onto which a copper foil is subsequently laminated. This material composition is specifically employed in power applications where heat generation necessitates efficient heat dissipation through the board. The inclusion of the metal plate offers exceptional thermal conductivity, making it an ideal choice for such applications. Additionally, in many instances, the metal plate is affixed to a heat sink to further enhance heat dissipation capabilities.
High Frequency PCBs are specifically designed for high-speed and high-frequency signal transmission applications. These PCB laminates, along with their corresponding prepregs, are engineered to minimize signal loss and enhance signal transmission speeds at high frequencies. Unlike standard PCBs, which typically have a dielectric constant (Dk) ranging from 3.5 to 4.1, high frequency laminates boast a lower Dk of around 3.2. Additionally, they exhibit a significantly lower Dissipation factor (Df) of approximately 0.004 compared to the 0.016 commonly found in standard FR4 PCBs.
For specialized applications such as radar, RF power amplifiers, and antennas, even more advanced PCB laminates are available, featuring higher Dk values of up to 10 and remarkably low Df values, as low as 0.002.
Flexible PCBs are designed to be bendable, allowing them to conform to different shapes and fit into tight enclosures. They are particularly useful for saving space and eliminating the need for connectors, and they are often employed in applications involving moving parts, such as printers and robotic joints. Typically, Flexible PCBs are manufactured using polyimide, although there are alternative materials available for applications requiring higher performance or lower cost.
Hybrid PCBs, also referred to as rigid-flex PCBs, combine flexible PCBs with rigid PCBs to create versatile circuitry solutions. In rigid-flex PCBs, rigid sections are typically attached to the enclosure walls, while flexible sections are used to link and transmit signals between these rigid sections. This allows the PCB to conform to complex shapes and fit into tight enclosures. Additionally, in some applications, components such as passives, integrated circuits, and connectors are installed directly onto the flexible PCB sections.
Heavy Copper PCBs are a specialized type of printed circuit board designed to handle high current levels within PCB traces. Unlike standard PCBs with copper thickness typically around 0.5 ounces per square foot (approximately 17 micrometers thick), Heavy Copper PCBs are engineered with significantly thicker copper layers. These thicker copper layers help reduce resistivity and dissipate heat more effectively, making them suitable for applications requiring high current-carrying capacity.
Manufacturers achieve heavier copper layers by utilizing laminates with increased copper content or by employing electrodeposition methods to enhance copper thickness. Some manufacturers offer Heavy Copper PCBs with copper weights exceeding 10 ounces per square foot, providing enhanced current-carrying capabilities for demanding applications.
High Tg PCBs, or high glass transition temperature PCBs, are designed to withstand extreme temperatures, typically exhibiting a glass transition temperature (Tg) exceeding 180°C. This is in contrast to the more common PCB types, which typically have Tg values ranging from 130°C to 150°C. High Tg PCBs find application in environments with extreme temperatures, either due to the operating conditions or the heat generated by certain components such as microcontrollers (MCUs) or field-programmable gate arrays (FPGAs).
Teflon based PCB are used in some very specific applications such as RF power amplifier, radar circuit, and when very high operating temperature is required. The cost of this type of board is very high and they are difficult to manufacture; you should have very good reasons to select them.
The following table lists a number of factors affecting PCB manufacturing cost.
Today, printed circuit boards are designed using computer-aided design (CAD) tools, which are software applications featuring graphical user interfaces and powerful algorithms to assist PCB designers. Before creating the PCB layout, a schematic is typically created using the same CAD suite used for PCB design. Additionally, circuit simulators are often employed to test specific circuit sections prior to PCB fabrication.
There are numerous software options available for schematic and PCB layout design, including Altium Designer, Siemens Xpedition and PADS, Eagle Autodesk, Cadence Allegro and Orcad, NI Ultiboard, KiCad, CircuitMaker, Zuken CR-8000, and many others. Some of these tools are open-source, while others are proprietary. Before selecting a CAD tool, it’s essential to understand your specific needs in terms of the number of boards per year, board complexity, and desired integrity level. Prices for these tools range from almost free to tens of thousands of dollars.
While some PCBs may be simple with few components and clearly understood circuits, more complex designs may benefit from circuit simulation. Simulation allows designers to ensure that circuit sections work as expected and extract critical parameters such as frequency response, peak power, and voltage or current levels. For simpler boards, circuit simulation may be skipped, but for designs with greater complexity, simulation can be advantageous in validating performance and functionality.
PCBs handling high-frequency digital signals such as USB, Ethernet, PCI, DDR, etc., require rigorous validation of signal integrity. Parameters such as voltage overshoot, undershoot, skew, crosstalk, and propagation delay must be carefully monitored. It’s crucial to control trace length matching, impedance, and spacing to ensure the functionality and reliability of the PCB.
PCBs containing large CPUs and FPGAs often draw substantial current, sometimes in the tens of amperes. Additionally, simultaneous bus switching can result in rapid variations in current, on the order of 1A/ns, leading to significant fluctuations in the supply voltage. To validate the power distribution of these boards, performing power integrity analysis is essential. This involves measuring the impedance of the power distribution network at various locations on the board.
PCBs containing large CPUs and FPGAs often draw substantial current, sometimes in the tens of amperes. Additionally, simultaneous bus switching can result in rapid variations in current, on the order of 1A/ns, leading to significant fluctuations in the supply voltage. To validate the power distribution of these boards, performing power integrity analysis is essential. This involves measuring the impedance of the power distribution network at various locations on the board.
As previously explained, printed circuit boards primarily support electronic components and facilitate their interconnection. However, PCBs can also serve as integral components in various applications. For instance, patch antennas utilize multiple radiating PCB copper sections arranged to create directional radiation patterns. Proximity sensors leverage metal sections on a PCB, with changes in capacitance between these sections indicating the proximity of objects with high permittivity. In RF design, PCB traces can function as passive components such as discrete inductors and capacitors. Additionally, in high-speed designs, the coupling between ground and power planes serves as VCC decoupling capacitance.
Printed circuit boards serve as commodities for some and complex, niche products for others, depending on factors such as technology level, operating environment, and safety and reliability requirements. Simple one- or two-layer PCBs, found in products like musical greeting cards, car remote starters, and garage door openers, contrast with more complex PCBs featuring four, six, or even more layers, used in power electronics, computer systems, aerospace applications, and beyond. Some manufacturers even fabricate PCBs with 20 to 30 copper layers.
The spectrum of complexity in PCB design and fabrication is vast, necessitating powerful design tools and a complex supply chain to meet diverse needs and requirement.
