Denis Lachapelle, P. Eng.
Anne-Marie Coutu, Tech.
Introduction
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.
Manufacturing Companies
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:
- Solder Paste Applicator: This section applies solder paste precisely to the areas where components will be attached on the PCB.
- Pick-and-Place: Automated machines in this section accurately position the components onto the PCB according to the design specifications.
- Oven: The assembled PCBs are then passed through an oven, where the solder paste is melted, creating a permanent connection between the components and the board.
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.
Types of PCBs
Whilst non-exhaustive, this section lists various types of PCBs.
Most Common 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
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.
Figure 1, Metal Base Laminate
High Frequency PCBs
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
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.
Figure 2, Rigid-Flex, source Altium
Hybrid PCBs
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.
Figure 3, Two rigids attached by a flex.
Heavy Copper PCBs
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
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 PCBs
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.
PCB Cost Drivers
The following table lists a number of factors affecting PCB manufacturing cost.
| FACTORS | IMPACTS |
|---|---|
| Dimensions | cost increases mostly linearly with PCB area. |
| Shape Complexity | Complex shapes and features (V-groove, jump scoring, countersink holes, etc.) require more machining time and may reduce material utilization efficiency, increasing costs. |
| Number of Layers | Cost rises approximately linearly with the number of layers, with additional layers adding complexity and material costs. |
| Copper Thickness | Thicker copper layers increase material costs and may require specialized manufacturing processes, impacting overall cost. |
| Overall PCB Thickness | Meeting thickness requirements while controlling trace impedance can increase costs due to complex stack-ups and material choices. |
| Number of Drill Sizes | Fewer drill sizes reduce tool changes and machining time, potentially reducing costs. |
| Number of Holes | More holes increase drilling time and may lower yield, impacting manufacturing costs. |
| Complexity | The use of through-hole vias is the simplest option, but costs increase with the complexity of other via types such as blind vias, buried vias, via-in-pad, back-drilled vias, and filled vias. |
| Controlled Impedances | Controlled impedance with tight tolerance necessitates precise control over factors such as glass fabric thickness, dielectric constant, and trace width. Achieving these specifications may require the use of specialized materials with low dielectric constant and low loss factor. |
| Material | As discussed in section 3 there are many types of PCB with standard FR4 being the most common. However, some materials are specifically engineered for high-speed digital PCBs, RF boards, high operating temperatures, or demanding dimensional stability requirements. Opting for these specialized materials can substantially increase costs. |
| Material Utilization | As discussed in section 3 there are many types of PCB with standard FR4 being the most common. However, some materials are specifically engineered for high-speed digital PCBs, RF boards, high operating temperatures, or demanding dimensional stability requirements. Opting for these specialized materials can substantially increase costs. |
| Material Utilization | Standard panel dimensions, such as 18x24 and 24x36, are commonly used in PCB manufacturing. Maximizing board utilization within these panels is crucial for cost efficiency. For example, if your board occupies 90% of the panel area, you lose only 10% in material waste. However, if it occupies just 75%, the material waste increases to 25%, leading to higher costs. |
| Traces and Gap Width | When traces and the gaps between them are very thin, such as 0.004 inches or less, precise control over the etching process is essential. Over-etching can increase impedance or lead to trace breakage, while under-etching can result in reduced impedance or short circuits between traces. Additionally, maintaining a constant trace impedance along the entire length becomes more challenging with such thin traces, as controlling width tolerance becomes crucial. |
| Plating such as gold | In some cases, special plating is necessary for specific applications such as keypad contacts, edge connector fingers, and side plating. While gold plating offers high reliability, it comes with a significant cost. Alternatively, less expensive options like Electroless Nickel Immersion Gold (ENIG) and Nickel-Palladium-Gold (NiPdAu) exist, although they may offer lower reliability compared to gold plating. |
| Aspect Ratio | The aspect ratio in PCB manufacturing refers to the ratio of hole length to diameter. Maximum values vary among manufacturers, with some limiting it to 8:1 while others allow up to 12:1. High aspect ratios pose challenges during the copper plating process, as ensuring plating penetrates up to the center of the via becomes more difficult. Additionally, high aspect ratios can lead to poorer mechanical strength, signal integrity degradation, and challenges in thermal management, especially if the via supports high current. Therefore, careful consideration of aspect ratio is essential to ensure optimal performance and reliability in PCB designs. |
| Tolerances | Board dimensions, shape, and mounting point tolerances significantly impact PCB manufacturing costs. It's advisable to specify larger tolerances where possible to lower costs. It's important to inquire with the selected manufacturer to understand their tolerance thresholds, as these can vary between manufacturers and may affect pricing. By understanding these thresholds, you can make informed decisions to optimize costs while meeting your design requirements. |
| Rigid, Flex and Rigid-Flex | Rigid boards are typically the most cost-effective option, followed by flexible boards, with rigid-flex PCBs being the most expensive. However, it's essential to consider the total cost, as the use of flex or rigid-flex PCBs can offer benefits such as sparing connectors, saving space, and reducing workmanship. These advantages can lead to overall cost savings despite the higher initial cost of flexible or rigid-flex PCBs. |
| Surface Finish | After assembling the laminates and prepregs, applying copper plating, and solder mask, it's crucial to protect the remaining exposed copper areas from oxidation and enhance solderability. Several surface finish processes are available, including: · HASL or HAL (Hot Air Solder Leveling) · Electroless Nickel Immersion Gold (ENIG or ENi/IAu) · Electroless Nickel Electroless Palladium Immersion Gold (ENEPIG) · Immersion Silver Plating (IAg plating) · Organic Solderability Preservative (OSP) · Immersion Tin Plating (ISn) · Direct Immersion Gold (DIG) · Immersion Gold (ENEPIG) Among these, HASL lead-free and ENIG are the most common options. |
| Silkscreen | Silkscreen printing on PCBs offers a variety of color options, with the ability to choose multiple colors if needed. However, using more than one color can increase production costs |
| Package Type | Some package types, once soldered, require x-ray inspection due to the inability to visually inspect the solder joints. Examples include packages like Ball Grid Array (BGA), which feature very small pitches (distances between two balls) such as 0.5mm, 0.4mm, or even smaller. With such small pitches, the pad size can be as small as 0.3mm, leaving only 0.2mm for routing traces in between, including two gaps and a trace. This presents significant routing challenges and requires careful consideration during PCB layout. |
| Turn-around Time and Quantity | The quantity ordered significantly affects PCB costs, as non-recurring costs (NRE) are distributed across more units in larger orders. Additionally, turnaround time is a major cost driver, as expedited production may require overtime pay for staff and reorganization of production planning by the manufacturer. |
| Coating | Depending on their application, PCBs may require coating with acrylic to enhance resistance against humidity, moisture, and pollutants once assembled. |
CAD Tools
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.
Special Applications
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.
Conclusion
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
