Building a PCB – or any electrical connection – requires a mixture of conductors and insulators to carry current in a controlled manner. Perhaps you’ve experienced a light cord or phone charger cable becoming exposed after extended flexure, which causes the insulating layer to degrade and reveal the metal wire innards. Circuit boards use the same basic relationship as a planar system but require alternating layers of dielectric material and conductor (usually copper) foil instead. The building-block material is known as a copper-clad laminate (informally, a PCB core), and manufacturers can influence aspects of the dielectric and conductor layers to maximize performance.
Copper Clad Laminate Variables
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Understanding the Components of a Copper-Clad Laminate
Copper-clad laminate is the essential building block of the PCB during the lamination process. The laminate consists of two discrete pieces: the copper foil, which forms the signal or plane layers, and the (commonly) glass-fiber weave and epoxy resin prepreg. These provide the board’s fundamental conductor/insulator relationship: the former carries the electrical signals, while the latter offers electrical isolation to prevent undue influence to or from each conductive layer. While all basic laminate adheres to this format, the implementation can vary wildly depending on the material needs of the board. The stackup design – the material layer-by-layer arrangement of the board – can use single- or double-sided copper-clad laminate (in combination with foil-less prepreg) to create a board structure that meets electrical and mechanical requirements.
Starting first with the glass fiber weave that forms the structure of the core’s prepreg, designers can specify the weave ply and tightness. Both characteristics affect the average dielectric constant encountered by signals’ transmission paths. As signals travel over the dielectric, they spend varying amounts of time over the fiberglass weave or the weave gaps (which contain only resin) – the difference in dielectric constant between the two materials can be significant enough on long enough signal pathways to impact synchronization. While it’s entirely possible the sum of the timing differences “average out” to a negligible amount, there’s also the chance of maximum difference where two time-sensitive lines experience the extremes of glass weave dielectric and epoxy resin dielectric.
Laminate manufacturers provide designers with various weave options to account for the weave effect (as it’s known). To be clear, line skew arising from the differences in the weave and resin relative dielectric can be trivial when the line transmission speeds are not appreciably fast. The most common/popular 106 and 1080 weave patterns are sufficient for the stated design intent in these cases and are economical to boot. Thicker weave dimensions and tighter-knit weaves will experience less random divergence (from a blind routing perspective) between the relative dielectric constants yet add to the bottom line of a fabrication – for smaller production runs (i.e., prototyping, limited NPIs, etc.), the total cost increase may be reasonable.
A consequence of a tighter weave is a reduction in resin %, as the “gaps” in the weave, which contained resin, exchange for additional glass fibers. This resin reduction has a marked change in how the board reacts during processing techniques, with the end product exhibiting different mechanical and material properties, especially when considering isotropic (direction-dependent) effects along the weave lines. In addition to the more extreme manufacturing outcomes, resin loss corresponds to a decrease in the electrical properties (relative dielectric constant, loss tangent), as the resin is more insulative than the weave.
Copper Foil Production Processes
Functionally, the prepreg only covers half of the laminate. Although seemingly less complex, copper foil manufacturing variations can drastically change its performance. What appears to be a flat planar surface without magnification, copper foil under magnification can show a variety of rough surfaces that influence its ability to grip the substrate and affect the impedance for high-frequency signals. Different processing techniques can cut down the copper teeth and offer distinct capabilities to the PCB:
- Electrodeposited – Manufacturers attach copper using an anode/cathode system connected to a copper solution and a rotating titanium drum. The electric field strength (DC power into the titanium drum) controls the deposition rate, with thicker copper requiring longer processing time. It possesses excellent tensile strength for applications with considerable mechanical requirements but has copper tooth formation.
- Rolled-annealed – A series of rollers mechanically reduce the thickness of the foil below recrystallization temperatures for strain-hardening. It significantly reduces surface imperfections, which makes it a fantastic choice for high-frequency boards.
- Reverse treated – A roughening step improves the adhesion between the foil and prepreg for better peel strength and, therefore, better resistance against mechanical delamination. By far, the most common foil choice for “standard” PCB applications due to its improved etch precision and miniaturization alongside lower-profile copper teeth.
- Double treated – Both sides of the foil undergo roughening to improve adhesion, allowing manufacturers to bypass a black oxide step during fabrication.
When It Comes to Laminates, Your Contract Manufacturer Has You Covered
Copper-clad laminates come in various styles that suit any circuit board; for the most part, designers can lean on standard selections that are broadly applicable. However, high-end performance can live and die on the laminate choice, and the most ingenious designs will be fighting uphill from the get-go in terms of optimization. At VSE, we’re a team of engineers committed to helping our customers build their designs while maintaining exemplary quality and reliability. For over forty years, we’ve worked alongside our manufacturing partners to bring life-changing and life-saving devices to market across several industries.
