Cooking for me is more improv than planned—I love checking out the supermarket and building a mish-mash between what’s on sale and clearance. There’s a certain artistry to picking through a somewhat random assortment of ingredients and coming out with a satisfying dish on the other side. However, I adopt a much more intense posture when I’m cooking to impress anyone other than myself (as I’m far too easily amused). I want to ensure ingredients are chosen carefully and recipes are followed closely. I need my cooking to be well-received by others who may be far more discerning than myself.
Much in the same way, material selection for PCBs needs to be carefully weighed by end-use performance. Some of these are intrinsic properties of the materials, though materials engineering has made significant strides in designing elements of materials to maximize specific characteristics. When it comes to the best PCB materials, it is of utmost importance to first consider the function of the underpinning physics driving the final product’s function.
Pre-preg: Determining Best PCB Materials for High-Speed Designs
By weight and volume, pre-preg will be the most ubiquitous material throughout a finished design and is arguably the most important material decision of the PCB. Prepreg is commonly a lattice of glass fibers filled with epoxy-based resin, though entirely different materials exist, and performance differences can be engineered. However, dielectric media must be discussed before covering this aspect of the prepreg.
A dielectric is an insulating material that is susceptible to polarization when under the effect of an electric field. When an external field is applied to dielectric media, the material’s polarization occurs whereby the nonpolar material aligns and undergoes strain with the negative end of the field attracting the nuclei and the positive end of the field, thus attracting the electron cloud. This arrangement results in an antiparallel alignment of the induced electric field, and as it is a vector quantity, the net electric field experienced by the material is reduced. This is important because the dielectric material can store the same amount of charge at a reduced voltage compared to a non-dielectric material, increasing the capacitance and, therefore, total energy content.
The measure of the effectiveness of a dielectric against that of a vacuum is known as the dielectric constant (Dk), or relative permittivity. In addition to greater charge storage capacity, dielectric values greater than unity also represent a decrease in the speed of light through the media. This scenario can have a pronounced effect on signals on the board. Prepreg is neither homogeneous nor isotropic—signals traveling across the board will experience different average Dk values depending on the proportion of travel time they spend over the weave and resin.
In high-speed designs or critical traces of sufficient length (clock signals, differential pairs, etc.), this Dk disparity can become significant. Even traveling along the warp or the weft of the weave can incur noticeably different Dk values. Solutions can include fabricators tilting the image at an angle (to promote an average uniformity of Dk values for all signals) or designers routing at an angle. However, both methods increase the difficulty of the design.
Prepregs such as 1035s and 1078s have a solution—a flattened weave that reduces the gaps between fibers. Increasing the proportional area of the weave serves to average the Dk value across the material, alleviating design and fabrication concerns while offering a solution for high-speed designs unaddressed by more common 106 and 1080 pre-pregs. Flattened weaves typically possess a higher price point than more standard lattice weave prepregs, so cost may be a factor in mid-large production runs.
The Importance of Copper Weight and Additional Material Considerations
Generalizing a bit from prepreg, copper-clad laminate would be the next logical material choice to weigh. Copper-clad laminate is just prepreg sandwiched between layers of copper foil that will be etched to form the planes, pours, fills, and traces in the final design. The outer foil can be materials other than copper, but copper is prevalent due to its excellent characteristics and affordability.
Copper foil is ordered by weight and rolled to a uniform thickness to cover a one sq. ft. area. One oz. copper thickness is the most common choice for design for a few reasons. First, the thickness of the copper foil will determine the spacing between same-layer copper features, with thinner foils allowing for smaller spacing. This setup is advantageous for very signal-dense PCB designs to make the most of routing layers. Additionally, the feasibility of trace width design will depend on the thickness of the copper layers.
Etch compensation and copper pull-back can be used to modify the layout design so the trace widths will be within tolerance after etching, but thicker foil layers mean more extreme usage of these techniques. At some point, the fabricator will be coming into conflict with the intent of the layout designer. Thicker foil layers find use in power applications where the copper planes must be able to electrically and thermally withstand high voltage and current.
While more a result of processing than a material decision, the amount of copper etched away accounts can account for several fabrication concerns. The coefficient of thermal expansion (CTE) describes how a material expands volumetrically with an increase in temperature. Notably, board layers that have been “over-etched” will expand at a rate more similar to the prepreg material than that of the copper foil. This nonuniform expansion may result in board warpage or more catastrophic failure modes. One solution to combat this would be to add a ground pour on the over-etched layer.
Material Choices Made Easy With VSE
In truth, only a small facet of material selection and their reasoning have been considered. Material needs will depend largely on the job: production quantities, design features, electrical/thermal challenges, and more. A contract manager can simplify your search if you’re looking to cut through some of the confusion as to which materials will best suit your board needs. VSE’s motto is that we build electronics for customers by engineers—we’ll work with you and our valued partners to source the best PCB materials for your design to ease the production process while supplying you with a high-quality, optimized board.