I was a little obsessed with making what I deemed the “perfect” snacks when I was younger; to me, nothing was worse than a naked tortilla chip in my nachos or unbuttered popcorn at the bottom of the theater bag. I’ve learned to reel it in since, but I used to incorporate a layering process to ensure each bite was the same as the last. Similarly, I now apply these obsessive tendencies toward signal integrity during stackup design with PCB laminates that establish a consistent relative permittivity and match the design intent of the project.
PCB Laminates by Type
|FR (Flame Retardant) series||Fiberglass Cloth (G)
|Composite Epoxy Material (CEM) series
||PTFE (AKA Teflon) series||Other|
|Aluminum* (metal core boards for enchanced cooling of high power devices) Polymide* (flex and rigid-flux circuits)|
*: Indicates a common-choice laminate.
Using PCB Laminates to Build Boards
PCB laminates are the building blocks used to construct a stackup, providing various physical, electrical, and material properties of the final board. While there are exceptions, most common rigid PCB materials comprise a fiberglass weave set in an epoxy resin: the weave provides mechanical structure to the board, while the epoxy melts and flows during the lamination process, where high temperature and pressure fuse the various laminates into a single, unified product. A copper-clad laminate (CCL) is an extension of the base fiberglass/epoxy structure but with a layer of copper foil on one or both sides of the laminate, where the foil layers provide the etchable signal and plane layers during manufacturing.
At the start of a board layout, designers must utilize complex field solver software to determine some of the baseline features of the board according to the impedance targets of any single-ended or double-ended traces. Essentially, the designer begins with a few constraints that provide the framework for the board and designs the structure around these inputs. At the same time, designers will want to consider the materials on hand or available through vendors – a stackup built from expensive or exotic materials complicates the layout at the earliest possible point in the design. The designer will then select from different types of laminates to create the stackup:
- A PCB core is a double-sided CCL that adds two layers to the board stackup. Every stackup contains at least one PCB core but may contain many more; the maximum number N of PCB cores for a board of layer count L is L = 2N, where N is a positive, nonzero integer.
- Designers can use single-sided CCLs to add layers when the layer count of the stackup is not a power of 2 (e.g., 6, 10, 14, etc.) for the top and bottom layers of the design.
- Prepreg laminate materials without copper foil fit between the cores and above/below the copper side of any single-sided CCLs. The prepreg provides the necessary electrical insulation between otherwise adjacent interior copper layers while encapsulating the exterior copper layers when the materials fuse during lamination.
Laminate Parameters Meriting Consideration
The selection of laminates for a board will also depend on the electrical, mechanical, and thermal properties they impart. Designers can choose the material that most closely matches the DFM needs of their project, with an eye toward some of the most critical parameters:
- Dielectric constant (Dk) – Circuit performance depends on the dielectric constant the signal experiences over the length of its traversal. Also known as the relative permittivity (εr), the Dk value indicates the material’s polarizability by an electrical field. Generally, a low-Dk material is preferable, especially for high-speed designs, as the lower polarizability prevents signal degradation. However, certain applications may use high-Dk materials to induce a greater coupling between copper layers (effectively acting as a capacitor).
- Loss tangent (tan(δ)) – Alternatively known as the dissipation factor (Df), the metric indicates how dissipative the material is with the stored energy. It’s crucial to understand that energy within the PCB is stored within the electric fields between the conductive layers (again, harkening back to the PCB as a capacitor). Lower-loss tangent materials lose less of this stored energy as heat, representing a greater efficiency while experiencing less heat-related wear and aging.
- Coefficient of thermal expansion (CTE) – The CTE describes the expansion of materials during heating (or cooling) that is not a result of phase changes. Metals are generally low-CTE materials, while non-metals exhibit higher CTEs. The goal during pre-fabrication is to choose conductive and nonconductive materials with CTEs that are close – this will help prevent strain during expansion/contraction that can result in mechanical stress on the board features. A CTE mismatch is most noticeable at the vias, where barrel cracking or pad lifting may occur.
- Glass transition temperature (Tg) – The glass transition point is not a melting point but describes where materials change from strictly rigid to malleable. During processing and operation, a substrate that reaches its glass transition temperature will likely experience failure due to the change in underlying mechanical properties. In general, inexpensive materials are likely to have lower Tg values, making them less suitable for high-temperature applications.
Stackup Design Isn’t Immaterial – Talk To Your Contract Manufacturer
The selection of PCB laminates will depend on the material properties and financial considerations of the device – the design will dictate if a higher-quality or alternate-performance laminate is necessary. Otherwise, FR-4 is the typical standard for rigid boards due to its general aptitude for manufacturing processes and typical operating conditions. Optimizing the stackup design ensures a PCBA production that progresses smoothly without costly or time-consuming material reorders due to design incompatibility. Here at VSE, we’re a team of engineers committed to building electronics for our customers, layer by layer. Alongside our manufacturing partners, we’re committed to delivering the highest quality of life-saving and life-changing devices across numerous industries.