PCB Design Stackup Considerations

Going from a design to a finished board is a complex process. Even with the sophistication of modern simulation tools, actual and expected performance can differ sharply. It’s essential not only for a board’s contents (i.e., components and circuitry) but also for its spatial orientation and material properties. While designers could safely omit the contributions of the board’s effects in the past, the speed of modern electronics has obliterated that assumption. When preparing to transition from design documents to the board design and layout, taking a few moments to weigh PCB design stackup considerations can improve manufacturability outcomes and performance.

Primary PCB Design Stackup Considerations
Layer count	The density of the design will influence design rules and preferred via structures for the stackup.
Controlled impedance	To promote signal integrity and performance, designers must specify trace characteristics (width, thickness, spacing, etc.) within a field solver.
Thickness	The physical thickness of the board is essential for interfacing with specific connectors (e.g., card-edge connectors).
Materials	Certain materials possess properties that make them more suitable for certain applications (high heat, vibration, mechanical flexing, etc.)

Building The Board Layer by Layer

As in all things engineering, form follows function. Applications differ depending on the board’s needs – a high-speed board requires material characteristics different from those of a high-power board. Electrically, the stackup uses alternating layers of dielectric (insulators) and conductors; physically, the board materials use copper-clad laminates (single or dual-layer cores) and prepreg to build the necessary board thickness. The board thickness depends on the enclosure’s physical constraints and the circuitry’s density, as a high-density interconnect (HDI) board needs additional layers for signal breakout.

How can designers begin to build the stackup? The primary consideration is the design documents – e.g., the schematic or materials sent from the engineer(s). For any high-speed design, start with these critical items:

• Material – Different materials have different properties. While electrical performance is often the driving force, it is rarely the only aspect. Specialty materials like PTFE are better for high-speed performance than traditional FR4 materials, which exhibit enhanced loss due to their inhomogeneity and anisotropy. Differences in mechanical properties (heat tolerance, peel strength, etc.) may require changes to manufacturing processes.

• Signal integrity – Within a plane, the further a signal travels from its source, the more susceptible it becomes to loss, distortion, and other forms of degradation. Similarly, the distance a signal travels between planes can lead to greater electromagnetic interference (EMI) due to an increasing current loop size. The distribution of reference (ground and power) planes relative to signal planes will reduce the signal return path, greatly improving electromagnetic compatibility (EMC).

• Density/thickness – Ideally, designers use the fewest layers necessary to route the board while adhering to best practices for EMC. However, additional financial considerations or system integration constraints may force designers to get creative. Microvia structures, via-in-pad, and other fabrication processes can help bridge the gap between dense layouts and restrictive layer counts.

The Connections Driving PCB Design Stackup Considerations

For multilayer boards, interconnections between conductive layers will also optimize the stackup layer count and layout compactness. Vias are the primary mechanism for connection: after board lamination fuses the layers of the board, manufacturers drill out the via holes and use an electroplating process to plate the holes for conductivity. However, vias come in many different flavors and design profiles. Larger and sparsely-populated boards can utilize standard vias for the layout, but high-density interconnect (HDI) boards will need to leverage smaller vias (known as microvias) using alternate drilling processes (namely, laser drilling).

Microvias can vastly shrink down the drill diameter but come with a catch: they can only drill so far into the board (typically 1~2 layers) without compromising the structural integrity of the via barrel according to the aspect ratio. However, microvias require an additional manufacturing stage known as sequential lamination that performs multiple laminate-then-drill cycles to build the necessary interlayer connectivity.

Your Contract Manufacturer Weighs All Stackup Options

Like any other constraint, PCB design stackup considerations require a holistic approach to board layout. As the earliest step in fabrication, the stackup materials, vias, and impedance structures will provide the roadmap for the layout and design documentation for the project. Designers want guaranteed manufacturability, and VSE is here to help. We’re a team of engineers committed to building electronics for our customers, including a full design review before manufacturing that ensures the best yield and performance at the lowest cost. We’ve been partnering with our valued manufacturing partners for over forty years to realize life-changing and life-saving designs.