One of the best feelings when picking up a new hobby or task is the gradual wave of competency from experience. The more extended immersion one commits to practice, the sooner one can see past the surface level to some underlying guidelines and techniques. PCB design is no different, and specifically, a four-layer PCB stackup involves some practices that differ intensively from a more populated board. Fortunately, most four-layer PCB stackups exist on the less complicated side of the design spectrum. As in all things, form follows function, and the designer should be well aware of the limitations of a four-layer stackup and how best to maximize their space to excel during the design phase.
Four-Layer PCB Stackup Example
Can I Implement a Four-Layer PCB Stackup Design Strategy for My Board?
Four-layer stackups are a popular jumping-off point for simple designs. As a cheap build with high design manufacturability, the four-layer design still exists alongside more intensive and expansive boards in today’s design world. Deciding upon a four-layer stackup begins by analyzing the number of components for placement and the routing needed. For example, a relatively simple FPGA would instantly preclude a four-layer design due to the density of the routing and components. Consider the major factors that impact whether a design can achieve a proper layout in four layers without sacrificing performance:
- Board size – The components and rat’s nest size are a function of the board’s dimensions. Since the stackup has constraints, the board dimensions refer only to the in-plane area of the board surfaces. Board dimensions that provide ample space for layout and routing can easily overlook the need for extra layers.
- Power planes – Typically, two internal layers will designate one as power and one as ground. If the power nets are grouped well in placement or few, the power planes should easily handle the copper features. On the other hand, many power nets spread disparately across the board may inhibit a four-layer implementation, even if the total number of nodes per net is reasonable.
- Design density – Because the routing layers are likely to include only the top and bottom layers, denser designs (whether those are components, traces, or both) may not be achievable in a four-layer stackup. Components must be on an outer layer; although embedded component technology is possible, the cost and challenge of designing around the internal plane layers make it an extreme exception rather than a consistent design choice.
Board Thickness, Aspect Ratio, and Maximizing Design Space
A 4-layer stackup has an additional wrinkle in its design compared to a board with more populous inner layers: while it is not necessarily a thin board, it is a possibility. As layer count increases, so does the board thickness. Although thinner boards can use a thicker core to meet design requirements, such as providing a snug fit in an enclosure to dissuade vibrations in the field, there are advantages to working with a thinner board.
The aspect ratio, defined as the ratio of the board thickness to a drilled hole diameter, sets a maximum guideline to prevent manufacturing defects. A general rule of thumb is an aspect ratio < 10:1 (that is, a drilled hole circumference no less than a tenth of the height of the barrel) will prevent overplating in a drilled plated hole. Extreme instances can even result in plating that shuts near either side of the hole before a continuous barrel running the hole length can form. For vias, this could result in a defect hidden within the body of the PCB, which does not form connections on the inner layers.
Continuity errors are not the only potential issue: the coefficient of thermal expansion mismatch can result in catastrophic failure to the board during production or use in elevated temperatures. However, if space is at a premium on top or bottom layers, reducing the circumference of the drilled hole will have a knock-on effect of reducing the via pad circumference. While the individual space savings may seem small, the aggregate space is freed up by shrinking pads (while remaining above the minimum aspect ratio). Therefore, the designer can ascertain a “Goldilocks” band for the drilled hole circumference to ease the layout.
Cost Cognancy May Play a Role in Design
With all the talk of via design, asking a designer whether a particular connection needs a via would be remissive. While it would be rare to have a design with no signal communication between top- and bottom-layer components, careful placement can greatly reduce the need for transitions. Although signal integrity must always remain a priority, designers should also know when it’s possible to bend it to their advantage. Large production runs will save a not inconsequential amount of time and money by reducing the number of vias on a board, as each reduction in design could account for thousands or more during manufacturing.
The via-in-pad option is a final consideration for tight designs in a 4-layer PCB stackup. Via-in-pad may not be a designer’s first choice due to the added expense of the process, but a dense design in a small space may validate it. The extra cost comes due to the need to fill the via with a conductive material and cap it over so the solder cannot fill the hole during the appropriate processes.
However, capping is not necessarily mutually inclusive with a via-in-pad. The advantages of via-in-pad are immense: fan-outs become unnecessary for most devices while reducing connections running from top-to-bottom-layer components. Suppose via-in-pad is awaiting approval for the design. In that case, a more seasoned designer can often quickly determine if a via-in-pad is an appropriate solution based only on the board dimensions and the number and size of the land patterns.
Whether it’s Four Layers or Forty Layers, VSE Can Bring Your Design to Life
A four-layer PCB stackup involves some different calculus than what a standard design may encounter. Although almost any design these days has space at a premium, dense four-layer builds can push this maxim to the extreme. Whether building prototypes or consumer-ready devices, you want the assurance that your board manufacturing utilizes the best techniques and practices. That’s where VSE comes in: here, we design and build electronics for customers with a team of engineers to ensure a phenomenal final product ends up in your hands. Our production partners adhere to exacting standards (much like ours), so your design undergoes as much care and pride as ours.