The Printed Circuit Board (PCB) is the heart of every electronic gadget, directing the functionality of the device. Creating this central “command centre” happens at a PCB manufacturing plant. It is a highly-sophisticated, multi-step process from concept to prototype to assembly to testing. Some manufacturers only offer PCB fabrication, just one part of the process required to turn an idea into a completed PCB product. Other manufacturers offer turnkey PCBA (Printed Circuit Board Assembly) which covers both board fabrication and the assembly of all components on the fabricated chip. This important distinction between the two processes is sometimes unclear, resulting in disappointment.
Let’s look at the difference between PCB assembly and PCB fabrication in more detail:
PCB Fabrication
PCB fabrication starts with the design of the circuit board followed by prototype production, followed by testing. When you’re shopping around for a manufacturer, take heed that this testing is for the board only—not for a completed, function circuit board.
At the fabrication stage, the PCB consists of a flat sheet of insulating material and a layer of copper foil laminated to the substrate that supports electric components that are connected to it later. The flat sheet consists of 4, 6, or 8 layers depending upon the number of signal layers.
Chemical etching creates pads for connections. Vias connect different layers of copper by dividing the copper into separate lines called circular traces or tracks. The tracks act as fixed wires and are kept apart from each other via insulation of air and board substrate material. Further, the surface of the PCB often also has a coating to protect the copper from corrosion and reduce the chances of solder shorts between tracks or any other electrical contact with stray bare wires. This coating is known as the solder resist.
Thus, the PCB, in its most basic form, consists of four layers that are heat-laminated together to create a single board. These four layers are:
- Substrate
The substrate is the layer that provides structure and shape to the PCB. It is what holds the components of the circuit board together. The substrate can be made of different kinds of materials such as fibreglass (which is the most commonly used) or more flexible materials like polymers.
- Copper
The copper layer is the most important—it is the conductor layer that performs the function of wires. The copper traces form pathways to carry electric charges from one component to another on the circuit board. The high conductivity of copper ensures that these traces transfer signals without losing electricity. The thickness of this layer is generally around 35 microns, but it varies depending upon the power requirements.
- Solder Mask
This layer insulates the copper tracks from the solder, metallic part, and other conductive items. It’s a protective layer that ensures only those specific areas of the board are exposed to the solder. The solder mask (solder resist) is usually green in colour, but any colour can be used for the solder mask. The material used is a liquid photopolymer and the technology used is called epoxy or epoxy-acrylate resin technology.
- Silkscreen
The silkscreen identifies components on a PCB for engineers and manufacturing teams. A non-conductive epoxy ink is used for the silkscreen in colours that vary from yellow to white to black. While the silkscreen does not contribute to board functionality, it flags components quickly and is, therefore, an important part of the PCB fabrication process.
Once these steps are complete, engineers make necessary modifications to the design or materials of the prototype. A green light here means the board read for assembly—it is a bare circuit board that is essentially a blank canvas! There are no electronic components—capacitors, inductors, transistors, and resistors—yet that make the gadget work. These components are soldered to the fabricated PCB at the PCBA or Printed Circuit Board Assembly stage.
That comes next.
PCB Assembly
Printed circuit board assembly (PCBA) is the complete process of transforming a bare printed circuit board into a functional unit by soldering components onto the board. Without components, the electronic device can’t accomplish pre-decided electronic functions. The assembly process starts after bare-board fabrication. First, a DFM (Design For Manufacturing) review ascertains that the PCB design fits manufacturing capability. This review cuts manufacturing costs and eliminates unforeseen delays due to design issues that would require re-engineering.
Next, a process called surface encapsulation assembles all the electronic components on the circuit board. Manufacturers use technologies like Through-hole, Ball-Grid Array, or Surface Mount Technology (SMT) to place and solder all the electronic components onto a PCB. With Through-hole technology, electronic components are attached to the PCB by threading them through pre-drilled holes that are surrounded by conductive pads. Using Surface Mount Technology (SMT), components are placed onto the PCB with automated pick-and-place machines, ensuring perfect alignment of the pins with the conducting pads. Glue or solder paste fastens the components onto the board.
A process known as reflow soldering melts the solder paste in a reflow oven to create solder joints. It then passes the board through coolers to solidify the paste once more and thereby hold all the components firmly in place.
A thorough quality check identifies and corrects any misalignments of components or errors in the layout.
The final process is box assembly, where the PCB—with its components soldered on—is finally encased with the outer covering to form the final product.
PCB Testing
The finished PCB goes through multiple tests:
- Power-off testing: an analog signature performed with the circuit board in switched-off mode.
- In switched-off condition, the finished board is inspected visually by design engineers or expert inspectors.
- In a switched-on condition, in-circuit testing checks working functionality of the PCB functionality.
- Automatic optical inspection (AOI) uses high-speed camera scans to inspect assembled PCBs.
- X-ray inspection uses high-resolution scans to detect flaws like opens, shorts,, or voids on large boards, BGAs, and micro-BGAs.
- Functional testing establishes if the PCB performs as per the original design.
Fabricating a PCB and assembling a PCB are two separate processes. Any faults or errors in these processes can cause the device to fail or malfunction. Error-proof fabrication and flawless assembly together is the key to a successful electronic device.
Both PCB fabrication and PCB assembly must be undertaken by reputed, experienced PCB manufacturers to ensure quality and reliability.
MIS electronics is a turnkey, one-stop PCB manufacturer. We provide PCB design, fabrication, and assembly (PCBA) services. One-stop PCB manufacturing is cost-effective as end-to-end fabrication and assembly facilities are available under one roof. It reduces the cost of transportation and labour; it reduces the chances of friction between different teams, leaves less room for errors, and aligns responsibility (and recourse) in one spot. Our experienced designers and engineers ensure that you get your electronics to the market fast and at an optimal cost. Get a no-obligation quote from us today.