I wrote a letter to a former teacher the other day, and given his impact on my career, I wanted to ensure I presented myself well. I used online tools to smooth over spelling and grammatical errors, but upon a reread, I realized I had used some imprecise (yet roughly synonymous) language. Most wouldn’t notice or care. But as a stickler for details, I couldn’t let it slide.
Language is tricky, though, especially wrapped within an industry’s nomenclature (or perhaps jargon). For example, PCBA vs. PCB. A few people may be knowledgeable of the latter and certainly fewer than that of the former. As one may surmise from two acronyms separated by a single letter, their definitions are extremely close and can be somewhat muddled. Let’s take a closer look below.
PCB vs. PCBA? What’s the Difference?
The PCB manufacturing process can be broken down into two overarching steps:
- Fabrication which produces a printed circuit board (PCB), which can also be referred to as a bare board or a printed wiring board (PWB).
- Assembly results in a PCB assembly (PCBA), a printed circuit assembly (PCA), or a circuit card assembly (CCA). In certain contexts, a fully-assembled board may also be called a PCB.
The PCBA vs. PCB terminology schema is the least ambiguous: the product of assembly is a PCB populated with its components, while fabrication provides the base of the PCB and all the connections therein.
PCB and PCBA: An Origin Story
The origins of the “printed” date back to the early days of PCB manufacturing, where a photomask image of the circuit was recreated on mylar and then photolithographically transferred onto a copper-clad laminate. While the spirit remains the same with modern PCB computer-aided design (CAD) and computer-aided manufacturing (CAM), significant improvements have been made to support manufacturing ease and performance.
After placement and routing are complete, the layout designer will output all relevant manufacturing files. These files then pass onto CAM, where necessary adaptations are made, such as scaling the artwork and performing panelization (the arrangement of PCBs on the same production panel to increase efficiency). At this point, pre-production processes have concluded, and the intricate, technical machining begins.
Fabrication Starts the PCB Manufacturing Process
As the foundation of the printed circuit board, fabrication acts as the first of the two major stages of manufacturing. Below is a list of the processes that comprise a standard fabrication job, but steps may be omitted or substituted depending on the exact requirements of the board:
- Patterning: As described, the artwork output as design files needs to be transferred to a laminate with an etch-resistant ink before an acid etch application. Direct subtractive methods like milling and laser etching can also be performed, but these methods scale poorly and generally only see use in prototyping.
- Etching: Copper features are expressed by removing excess copper or electroplating treated laminate.
- Subtractive etching uses acid to eat away at the areas of the copper uncovered by the etch resist, but care must be taken. As the acid eats away at the surface copper, it exposes copper beneath the etch resist. This copper can be removed by the acid attacking the newly-formed sides underneath the etch resist, which typically forms the trapezoidal trace shape. An extended time in contact with the acid, however, can undercut the traces and lead to impedances above calculated values and opens in more extreme cases.
- Additive etching is currently a complex and expensive process, although 3D printing may eventually yield a viable path forward with this methodology.
- Semi-additive is a hybrid of the additive and subtractive etch where a reverse image is applied to a thin laminate. The unmasked copper is plated up, the reverse image removed, and then the board is etched. This has the advantage of improving the fine control of copper features while simultaneously reducing the undercut.
- Lamination: The alternating layers of copper and prepreg are placed in a high-temperature press to fuse the materials and form the rough shape of the board.
- Drilling: Connections that span the partial or full width of the board need to be drilled out. This could be for through-hole packages or smaller layer-layer connections. Dense layouts may require laminating and then drilling between a certain number of layers before a final lamination; this increases space for layout by shrinking the size of the drilled hole at an additional cost.
- Plating: To form the interconnectivity between layers in the drilled holes, boards undergo a plating process. Following this, a final plating occurs that prevents corrosion on exposed copper features.
- Solder mask and silk: A solder mask is applied and developed to the surface areas of the board that do require solder. Finally, silkscreen is added to the board which provides reference designators, pin one indicators, polarity markers, part numbers, and other essential information that needs to be rendered at-a-glance.
Tests will then verify that the board has passed through fabrication successfully, and then it’s on to assembly.
Assembly: The Dividing Line Between PCBA vs. PCB
Assembly is not quite as involved a process as fabrication. However, the logistics of correctly placing thousands of components in the correct position and orientation at an acceptable production speed is nothing to sneeze at.
There are two divergent paths assembly should consider:
- Through-hole packaging. Surface mount technology (SMT) is usually smaller, endearing it toward densely populated boards. A board with only SMT components can utilize a reflow oven to handle all of its soldering applications, whereas through-hole devices require a pass through a wave solder machine. One advantage of through-hole components is the increased mechanical stability due to the greater bonding area – connectors that undergo constant mating cycles are prime candidates.
- Single- or double-sided assembly. Assemblies are simpler to perform when components are placed on only one side of the board. However, double-sided assemblies are extremely common, especially with layouts that require routing on a component-side layer or when the total number and area of the parts are extensive.
Before soldering, however, the components must be placed; for mass-volume PCB lots, this is accomplished with a pick-and-place machine. The machine is fed reels of SMT components which are accurately placed and verified using high-speed heads. Like soldering, certain components may require hand placement due to their size or incompatibility with the equipment.
With assembly complete, the board is submitted to final testing. This confirms not only the process of assembly but the combined fabrication-assembly of PCB products.
Some of these tests include:
- Optical inspection and power-off testing that evaluates the passive features of the board, like the quality of the solder bond or impedance values measured by a meter.
- In-circuit testing and functional testing check the board’s performance when powered and assure the board operates as intended.
Your Contract Manufacturer Can Clarify PCB Production
PCBA vs. PCB can be a confusing distinction, but proper context can reveal what stage of production is being discussed or even if a distinction needs to be made in that case at all. It is, without a doubt, a very technical field, and it can be difficult for those with limited experience in the industry to understand the sheer breadth of the topics. But difficult doesn’t mean impossible!
Here at VSE, we’re a team of engineers committed to building electronics for our customers. That motivation extends to educating our customers about the assembly process and assuaging any concerns about translating their design into a fully realized board. Paired with our professional manufacturing partners, we strive to deliver the best PCBAs in production quantities from NPIs up to mass production.
