Before starting any project, it’s necessary to determine which materials will compose the final product. Whether cooking, painting, or building electronics, it’s crucial to pick suitable materials that express the necessary characteristics of the finished piece.
PCB material selection is no different: form follows function. Designers need to include the material grade, choice, and quality into design rules to most efficiently realize a finished board. Material selection distinguishes itself as more than a subset of circuit design. It must incorporate the non-idealized performance of the circuit, components, and design materials into the specifications. A diverse team of engineers, designers, and fabricators must lend their respective expertise to furnish the board with the desired capabilities.
Getting Down With the (Copper) Thickness
Generally, the two most important material choices will be the selection of copper (or rarely other metals) foil thickness as well as the substrate. Of the two, the copper foil thickness is the more straightforward. Depending on the particular characteristics of the finished board, fabricators will order the appropriate copper foil, with its thickness denoted in weight per area.
Copper foil thickness is directly responsible for many of the final board parameters:
- Unsurprisingly, the thickness of the copper features. Thicker copper has the advantage of improved thermal- and current-sinking capabilities, which sees it find use in high-current/voltage power designs that necessitate the added thickness.
- Copper-copper clearance has a direct relationship with copper thickness. That is to say, the thicker the copper, the larger the clearance values must be. This relationship is because copper is etched away in a subtractive process in acid baths, but etch resist is only applied to the surface of the copper. As the acid eats away at the uncovered copper, it begins to eat away at the sides of the copper – this is why traces have a trapezoidal shape as the acid can etch away more of the sides at the top of the trace (where it spends more time in contact) than at the bottom. To ensure the integrity of the copper structures, copper-copper clearance must rise commensurate with the foil thickness.
- Impedance structure calculations will also depend on the thickness of the trace or traces. As thickness decreases, impedance falls, and vice versa (much like the width of a copper structure). While trace thickness is a universal input for even rudimentary impedance calculators, modeling equations can become less accurate as the thickness approaches an appreciable fraction of the substrate thickness (unless this condition is accounted for).
What Goes into PCB Material Selection?
Aside from economic and procurement considerations, these parameters will form the determining factor in PCB material selection. To understand how board materials can withstand the demanding operating conditions of a PCB environment, take a moment to learn some of the different aspects that designers and fabricators have to weigh:
- Dielectric constant. Compared to a vacuum, the relative permittivity represents a material’s ability to store more charge and, therefore, electrical energy. Higher dielectric constants can store larger electric field values before breakdown.
- Loss tangent. The loss tangent represents the inefficiency of the material as it converts electrical energy to heat. Purely resistive lossless surfaces are much like frictionless surfaces in that they are both idealized. Thus, all substrates experience some amount of heat dissipation from the stored electric field.
- Coefficient of Thermal Expansion (CTE). The CTE determines the degree to which materials grow in dimension, area, and volume at certain thermal thresholds. Fabricators must account for the CTE of structures like plated through-holes in the bare board and to the substrate to avoid warpage and other unsatisfactory manufacturing defects. To that end, designers may employ a copper flooding technique on layers with minimal features to ensure a consistent expansion in all directions.
- Glass Transition temperature (Tg). Represents the point where an amorphous material (typically epoxy resin) transforms from its brittle state to a much more ductile form. Though it is not a phase change, the material can be easily molded with a lamination press.
- Fire retardant. Boards are susceptible to high temperatures and electrical discharges that, without mitigation, could lead to health and safety hazards due to the combustion of the substrate. FR materials inhibit the growth and propagation of the flame so that it may be contained before growing to a size that would threaten life or equipment. Its need is so common in boards that an entire family of substrate materials is designated FR.
Substrate Choice Opens Up a World of Design Unto Itself
As the bulk material of the board, the substrate will directly impact a significant amount of material properties of the board. With the wide range of intrinsic features the substrate imparts and the various functions of boards, it should be no surprise that various materials are available to designers and fabricators. While materials can quickly descend into an almost limitless catalog of choices, including thickness, weave, and more granular attributes, essential attributes shared among the family of materials are listed below:
- FR is the most popular material for PCB substrate, specifically FR-4. Depending on the disposability of the board, the substrate construction could differ from resin-impregnated paper to glass-fiber bounded by epoxy resin, with additional configurations available. One of the main distinguishing characteristics of FR types is the temperature rating. Still, designers may seek different types and grades for improved arcing resistance, dielectric capacity, desiccation, and more. FR-4 is a jack-of-all-trades, performing well in numerous environments and designs.
- Composite epoxy material (CEM) consists of a glass-weave surface with epoxy resin, making it functionally similar to FR-4. Different types of CEM utilize various core materials and may or may not possess flame retardant capabilities.
- Metal cores are generally seen in power boards for additional thermal dissipation of power circuitry. Like a typical core, it will usually be sandwiched between two prepreg layers for the lamination process.
- Flex materials are instrumental in providing the rigidity and flexibility required of specific designs, primarily in wearable electronics.
An Expert Contract Manufacturer Simplifies Material Selection
Boards are complex fabrication processes, and PCB material selection can compound this issue. Design and the floor need open communication to collaborate for the best performance, and the choice of foil, substrate, or other processes comprise a symbiotic relationship that helps draw out the best board performance. If you’re still facing issues centered around material selection or any additional parts of the PCB design and manufacturing process, VSE is happy to help. At VSE, our engineers are passionate about building electronics for customers spanning many industries. We’ll work with our valued partners to ensure every aspect of your design is evaluated and optimized for maximum performance.