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, especially wrapped within an industry’s nomenclature (or perhaps jargon)—for example, PCB vs. PCBA. 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 incredibly close and can be somewhat chaotic. Let’s take a closer look below.
PCB vs. PCBA: What’s the Difference?
|Fabrication produces a printed circuit board (PCB)—a bare board or a printed wiring board (PWB).||Assembly creates a PCB assembly (PCBA), a printed circuit assembly (PCA), or a circuit card assembly (CCA). In some contexts, a fully assembled board is also a PCB.|
PCB vs. PCBA: An Origin Story
The origins of the “printed” date back to the early days of PCB manufacturing, where mylar was the medium to recreate a photomask image of the circuit before photolithographically transferring it 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 must support manufacturing ease and performance.
After placement and routing are complete, the layout designer will output all relevant manufacturing files. These files then transfer to CAM, where necessary adaptations are made, such as scaling the artwork and performing panelization (arranging 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:
1. Patterning: As described, the artwork output as design files must be transferred to a laminate with an etch-resistant ink before an acid etch application. Direct subtractive methods like milling and laser etching are also an option, but these methods scale poorly and generally only see use in prototyping.
2. Etching: Copper features are expressed by removing excess copper or electroplating treated laminate.
a. Subtractive etching uses acid to eat away at the areas of the copper uncovered by the etch resist, but careful process control is necessary. As the acid eats away at the surface copper, it exposes copper beneath the etch resist. This acid removes the copper by attacking the newly-formed sides underneath the etch resist, typically forming the trapezoidal trace shape. However, an extended time in contact with the acid can undercut the traces, lead to impedances above calculated values, and open in more extreme cases.
b. Additive etching is a complex and expensive process, although 3D printing may eventually yield a viable path forward with this methodology.
c. Semi-additive is a hybrid of the additive and subtractive etch where a reverse image applies to a thin laminate. Copper plates to the board before image removal and the etching process begins. This sequence can improve copper features’ fine control while reducing the undercut.
3. 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.
4. Drilling: Connections spanning the board’s partial or full width require drilling. This process 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.
5. Plating: To form the interconnectivity between layers in the drilled holes, boards undergo a plating process. Following this, a final plating prevents corrosion on exposed copper features.
6. Solder mask and silk: A solder mask is applied and developed to the surface areas of the board that require solder. Finally, a silkscreen layer provides reference designators, pin-one indicators, polarity markers, part numbers, and other essential information that needs to be available 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 PCB vs. PCBA
Assembly is a less involved process than 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 that the 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 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 when mounting components on only one board side. 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 requires a pick-and-place machine. Auto-feeder reels of SMT components and high-speed heads accurately place the components for dense assemblies. Like soldering, certain components may require hand placement due to their size or incompatibility with the equipment. With assembly complete, the board undergoes final testing, confirming the assembly process and the combined fabrication-assembly of PCB products.
Some of these tests include:
- Optical inspection and power-off testing evaluate the board’s passive features, like the quality of the solder bond or impedance values measured by a meter.
- In-circuit and functional testing check the board’s performance when powered and ensure the board operates as intended.
Your Contract Manufacturer Can Clarify PCB Production
PCB vs. PCBA can be a confusing distinction, but proper context can reveal what stage of production is under consideration or even if a difference needs to be made in that case at all. 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.