PCB Stackups: A Brief History

The evolution of layer stackups, from simple constructions to sequential lamination.

Stackup, the buildup of PCBs, has grown in importance over the past few decades. In the early days PCBs functioned primarily as interconnect, and the need for stackup was minimal. On a single-sided PCB, the “stack” is simply one piece of clad material. Even the board thickness was rarely a consideration unless it had to plug into an edge connector.

With the advent of multilayer PCBs, however, the stack of materials is more critical and, as always, cost is often the underlying driver. Stackups can range from the simplest four-layer PCB to complex stackups requiring sequential lamination.

Let’s take the humble four-layer PCB. PCB designers not familiar with fabrication may be forgiven for thinking that the four-layer board consists of two double-sided rigid boards bonded together with an additional layer of unclad material. It may come as a surprise, then, that most four-layer boards are stacked as a central two-layer PCB with two sheets of copper foil laminated on the outside, with the bonding provided by a layer or two of glass cloth “pre-impregnated” with uncured resin (prepreg).

Foil and core builds. This most basic four-layer stackup (and simplest to build for the fabricator) is called a “foil build” (Figure 1), with a foil on the outer layer of the stack. This is the most common build for even higher-layer-count boards.

A common question is, When are two two-layer boards laminated together? Most often, this construction is used in microwave applications where expensive microwave materials are laminated together with a lower-cost internal bonding layer. This type of four-layer (or more) stackup is called a “core build” because it comprises core materials on the outer layers (Figure 2).

Outside of the microwave world – where designers working with ever-higher device integration need more layers for routing – some of those layers or layer groups (sometimes called cells of layers) may comprise high-speed materials. These may require different processing, but invariably, high-speed materials are higher in cost. So, for volume use, designers try to build stackups with a mix of appropriate materials for each layer type, resulting in even more complex stackups.

Sequential lamination. By taking the PCB through the production process multiple times, boards can be built with drilled holes that appear to go only through parts of the stack. The process for this type of complex stackup is called sequential lamination. It can offer benefits for signal integrity (shorter vias) and provide greater flexibility in interlayer interconnect than a traditional stackup, which uses holes drilled through the entire board.

HDI stackup. The icing on the cake for complex rigid multilayers is the HDI stackup, where additional layers are laminated with multiple passes through the production process and laser-drilled to provide even greater interconnect density and enhanced signal integrity. A typical HDI build with its three press cycles is shown in Figure 3. From a stackup perspective, consideration should be given to glass styles that benefit from laser drilling, such as flat or spread glass.

Complex? Yes. From its humble origins, the PCB has developed into a complex interconnect device containing components including fabricated inductors, capacitors and transmission lines. PCB design engineers have the task of ensuring that the layer stackup is designed in a cost-effective, reliable way that is appropriate for the application and the lowest cost and easiest to fabricate. Flex and flex-rigid PCBs bring in a whole new layer (excuse the pun) of complexity.End of article content

Martyn Gaudion is managing director of Polar Instruments (polarinstruments.com); mgdirect@polarinstruments.com.