In part I, the author reviews design steps and material choices.
Printed circuit boards are produced in different forms, e.g., rigid, flexible, rigid-flex, and high-density interconnect (HDI). The primary differences are in the materials used to fabricate them; these materials, by their properties, give PCBs their ability to flex or to remain rigid.
Regardless of the materials used, the primary steps in the PCB manufacturing process are generally the same. But before PCBs reach the manufacturing stage, the PCB designer must make some choices depending on the application:
These steps are important for the success and reliability of the final product. For example, if the application demands that a component move back and forth during operation, as does the head of a printer, a flexible circuit must be used. Most wearables are shrinking in size, and HDI technology is most suitable for the rigid-flex boards (PCBs) they use. At this stage, therefore, the designer selects the appropriate type and material for the PCB.
The complexity of the electrical circuit decides the number of layers the PCB will have. The density of the PCB increases as products trend toward miniaturization.
The option left to the designer is to design a multilayer PCB to contain the functionality within the PCB’s mechanical boundaries.
The designer must decide the stackup, or the design of consecutive layers, depending on the nature of the application. For example, if the application incorporates high-frequency circuits, the designer must define the impedance and minimize crosstalk. They may have to use power and ground layers alternately, with traces carrying signals in between, to achieve this. Concurrently, the designer must decide on the width, spacing and routing of traces, placing of vias and test pads, and more.
Once the design process is complete, the designer produces an output in the form of standard documentation, which helps the manufacturer fabricate the PCB. This documentation can follow various formats, including Gerber, IPC-2581 or ODB++.
Substrate materials. The most widely used material for rigid PCBs is FR-4, an epoxy/glass laminate. It’s reasonably priced, offers satisfactory stability under temperature variations, and does not break down easily. Other less expensive materials such as paper phenolic are available and are typically used for low-cost commercial products. At the other end of the price spectrum are such materials as Teflon or PTFE. These materials offer very low losses and a stable dielectric constant for high-performance, high-frequency designs. Flexible, rigid-flex and HDI PCBs typically use polyimide-based substrates, although Teflon and Kevlar are also used for high-frequency and high-performance applications.
Cladding materials. A PCB requires copper traces and planes, and these originate in the form of copper cladding on the substrate, i.e., a thin sheet of copper bonded to the substrate. The designer can specify the thickness of the copper cladding, as it comes in a few standard thicknesses. Selecting the right thickness of copper for a board is critical to reaching and maintaining desired performance levels for adhesion to the substrate, and to ensure good performance at high frequencies.
Commercially, manufacturers use two types of copper foils for PCBs: electrodeposited (ED) and rolled-annealed (RA). These two types of foils are characterized by different forming processes and undergo different treatments for improving and preserving adhesion to various substrate materials. Formation of ED copper, for example, follows electrical deposition of copper on a slowly rotating polished stainless-steel drum placed in a solution of copper sulfate. The copper foil is removed in a continuous roll, with the side against the drum providing the smoother finish. Conversely, ingots of solid copper successively passed through a rolling mill produce RA copper foils.
These two types of copper foils possess different qualities due to the nature of their forming. ED copper used on PCB substrates is well-suited for applications where mechanical stress may be a critical factor, while RA copper is suitable for applications involving the potential for thermal shock. When ED copper is subjected to thermal cycling, for example, PCBs may develop cracks in narrow conductors, hence the better suitability of RA.
The grain structure of PCB copper varies according to its manufacturing process. ED copper foils are offered in coarse, moderate and fine grain structures, textures that are reflected in the copper surface finish. Although a copper foil with a coarse structure also has a coarse surface finish, and enables a stronger bond between the copper foil and dielectric laminate, it typically exhibits a greater insertion-loss performance at higher frequencies. Additionally, skin effect at higher frequencies, which causes the effective resistance of the conductor to increase at higher frequencies where the skin depth is smaller, worsens the insertion-loss performance of the rough surface of copper traces on a PCB.
Next month: The multilayer fabrication process.