Fundamentally, single-sided and double-sided printed circuit boards serve the same purpose. Both serve as catalysts for electrical connections between components, however there are some key differences that set them apart from each other. More specifically, they differ in the costs associated with production and development processes, as well as ampacity differences.
Single and double-sided printed circuit boards share the same material make-up: FR-4, which is a form of fiberglass mixed with epoxy. In modern manufacturing, this is usually layered with copper for conductivity and then coated in solder mask for a professional finish. Sometimes, in a process called silk screening, industrial printers print markings and labels on the board as well.
Modern manufacturing techniques have not been around forever though. In the past, engineers and hobbyists would hand-etch PCBs in a concoction of chemicals by photo-exposing a design onto the board. Over the years, several methods were used, however most of them involved the same limitation: it was impossible to etch multiple layers on a single board. This led to board layouts that were larger than necessary, and generally needed quite a bit of optimization work.
In the 1960s, this wasn’t such an issue. Large through hole components were standard, and large circuit boards were far from uncommon. Large, rectangular chips with wide pins lining the edges, known as DIP packages, allowed traces to be routed between pins. Through hole discrete components like resistors and capacitors were large and forgiving with traces. Components were placed near each-other, and short traces were easy to accomplish, allowing for a single side layout to be easily attainable.
Emergence of Surface Mount Components
However, the 1980s brought fundamental changes to the circuit board scene. Surface mount components became the choice component among Electrical Engineers. Quad flat packages were becoming more and more common. Surface mount discrete components made it impossible to route traces under components. Smaller and smaller circuit boards became necessary. Consequentially, single sided PCBs were becoming increasingly more difficult to work with. This gave significant rise to double sided manufacturing techniques. No longer was it necessary to work within the confines of a single piece of copper to carry current to components, and plated holes drilled between layers, called vias, made it possible to switch sides of the board with ease.
Engineers made the decision to move to primarily double-sided boards mostly out of convenience. When a design constraint mandated that a part be in a specific part of the layout, engineers could react much easier with a double-sided layout than a single layer design.
Double-sided boards also allowed for the creation of large ground and power planes. If we examine a circuit board closely, we may see that instead of individual traces, we see what appears to be cutouts of a larger piece of copper. This gives countless benefits in areas such as electromagnetic interference, heat dissipation, manufacturability of the circuit board itself, and most importantly, fewer traces.
Even though there are clear advantages to double-sided layouts, single-sided designs still appear in several commercially available products and new designs today. A key driver to this is cost. In general, single-sided circuit boards are slightly cheaper to purchase, and sometimes come with a shorter lead time. For simple designs, this is a clear advantage. In cases of mass production of simple products, when fractions of cents matter the most, single-sided boards can spell the difference between success and failure. Also, circuits requiring high amounts of amperage through a trace are best suited by single-sided layouts for safety and ampacity reasons.
Let’s look at an application example. Let’s say that we were producing a simple circuit involving an LED and an old-school timing IC that will be put inside a custom 3D printed enclosure. In this project, we do not care about the placement of components and our component costs are minimal, so we would like the PCB to be as low-cost as possible to meet a very low final price point. Which design works best in a scenario like this? Clearly a single-sided board. Since single-sided boards are generally cheaper, and our circuit is so simple, it is a no-brainer to choose the simpler solution.
On the other hand, let’s assume we are trying to make a board for a slightly more complex circuit. We would like to design an interface board to plug into our desktop computer’s USB port. Since a board like this would most likely involve several resistors, capacitors and ICs, and component placement would be very specific. Thus, in this case, the clear winner would be a double-sided design. Not only would we be able to jump traces and route under physical components, but this practice could drive the price down. Our design would be significantly more compact, and components could be packed much closer together, therefore driving the overall cost of production even lower. Since more boards fit on a single panel to be assembled simultaneously, our manufacturing facility would charge us less, and since our enclosures would involve less material, our associated purchasing or manufacturing cost would decrease. Our shipping cost would be lower because of the smaller dimensions. The list goes on and on.
Essentially, single- and double-sided printed circuit boards serve the same purpose. They are, however, used differently in the modern era. Although both are still in use today, we are seeing fewer and fewer uses for single-sided boards. Even though it will be sad to say goodbye to beloved single-sided boards in the future, the polished exterior and conveniences of double-sided PCBs are a more than suitable replacement.