The world is developing very rapidly in terms of modern technology, and it is easy to affect our daily lives. Our lifestyles have changed dramatically. This technological advancement has brought many advanced devices to the world that were unimaginable 10 years ago. At the heart of these devices is electronic engineering, and at the heart of that is the Printed Circuit Board (PCB).

　　A PCB, usually green in color, is a rigid body with various electronic components mounted on it. These components are soldered onto the PCB in a process called "PCB Assembly" or PCBA. A PCB consists of a substrate made of fiberglass, components, copper layers that form the traces, holes for mounting components, and layers that can be inner and outer layers. We can offer up to 1-36 layers for multilayer PCB prototypes and up to 1-10 layers for mass production multilayer PCBs. For single-sided and double-sided PCBs, there are outer layers but no inner layers.

　　The PCB substrate and components are insulated with a solder mask and held together with epoxy resin. The solder mask can be green, blue, or red, just like the colors commonly seen in PCB colors. The solder mask prevents components from short-circuiting with tracks or other components.

　　Copper traces are used to transfer electronic signals from one point on the PCB to another. These signals can be high-speed digital signals or discrete analog signals. These traces can be made thick to carry power/electricity to supply power to the components.

　　In most PCBs used to provide high voltage or high current, there is a separate ground plane. The electronic components on the top layer are connected to the internal GND plane or internal signal layers via "vias".

　　Components are assembled on the PCB to enable the PCB to function as designed. The functionality of the PCB is important. Even a tiny SMT resistor placed incorrectly, or even if a small track is cut by the PCB manufacturer, the PCB may not work. Therefore, it is very important to assemble the components in the correct way. A PCB after assembling components is called PCBA or assembled PCB.

　　The functionality of a PCB can be complex or simple, depending on the specifications described by the customer or user. The size of the PCB also varies depending on the requirements.

　　There are automated and manual processes in the PCB assembly process, which we will discuss.

　 　PCB Layers and Design

　　1- Substrate: A rigid board made of FR-4 material, on which components are "populated" or soldered. This provides rigidity to the PCB.

　　2- Copper Layers: Thin copper foil is applied to the top and bottom of the PCB to create top and bottom copper traces.

　　3- Solder Mask: This is a layer applied to the top and bottom of the PCB. It is used to create non-conductive areas of the PCB and isolate copper traces from each other to prevent short circuits. The solder mask also prevents solder from going onto unwanted parts and ensures that the solder goes into the areas meant for soldering, such as holes and pads. Holes will connect THT components on the PCB, while pads are used to hold SMT components.

　　4- Silkscreen: The white labels that we see in PCB manufacturing that identify components, such as R1, C1, or some description or company logo on the PCB, are made by the silkscreen layer. This silkscreen layer provides important information about the PCB.

　 　PCBs can be categorized into three types based on the substrate material.

　　1- How Circuit Boards are Made:

　　PCBs are common in various devices around us. These PCBs come in hard, rigid, and solid types with varying thicknesses. The main material is fiberglass or simply called "FR4". FR4 stands for "Flame Retardant-4". The self-extinguishing nature of FR-4 makes it very suitable for use in many hardcore industrial electronic devices. Both sides of FR-4 are pressed with a thin layer of copper foil, also known as copper clad laminate. The main applications of FR-4 copper clad laminate are power amplifiers, switching power supplies, servo motor drivers, etc. On the other hand, another rigid PCB circuit prototype substrate commonly used in home appliances and IT products is called paper phenolic PCB. They are lightweight, low density, inexpensive, and easy to punch. Calculators, keyboards, and mice are some of its applications.

　　2- Flexible PCBs:

　　Flexible PCBs are made from substrate materials such as Kapton, which can withstand high temperatures and are only 0.005 inches thick. They can be easily bent and are used in wearable electronics, connectors for LCD displays or laptops, connectors for keyboards and cameras, etc.

　　3- Metal Core PCBs:

　　Alternatively, another PCB substrate material such as aluminum can be used, which is very good at dissipating heat effectively. These types of PCBs can be used in applications that require heat-sensitive components, such as high-power LEDs, laser diodes, ultra-thin LCD PCBs, etc.

　　Types of Mounting Technologies:

　　SMT: SMT stands for "Surface Mount Technology". SMT components are very small in size and come in various packages, such as 0402, 0603, 1608 packages for resistors and capacitors. Similarly, for integrated circuits ICs, we have SOIC, TSSOP, QFP, and BGA.

　　SMT component assembly is very difficult and time-consuming for humans, so it is mainly done by automated pick-and-place robots.

　　THT: THT stands for "Through Hole Technology". For example, components with leads and wires, such as resistors, capacitors, inductors, PDIP ICs, transformers, transistors, IGBTs, MOSFETs.

　　The components must be inserted from one side of the PCB, then the pins are pulled out from the other side, then the pins are cut and soldered. THT component assembly is usually done by hand soldering and is relatively easy.

　 　Assembly Process Prerequisites:

　　Before going into the actual PCB manufacturing and PCB assembly process, the manufacturer checks for defects or errors in the PCB that could cause failures. This process is called the Design for Manufacturing (DFM) process. The manufacturer must perform these basic DFM steps to ensure a flawless PCB.

　　1- Component Placement Considerations: Through-hole components with polarity must be checked. For example, electrolytic capacitor polarity check, diode anode and cathode polarity check, SMT tantalum capacitor polarity check. IC notch/head orientation must be checked.

　　Components that require heat sinks should have enough space for other components so that the heat sink is not obstructed.

　　2- Hole and Via Spacing:

　　The spacing between holes and the spacing between holes and lines should be checked. Pads and vias must not overlap.

　　3- Copper pads, thickness, and trace width should be considered.

　　After performing a DFM check, manufacturers can easily reduce manufacturing costs by reducing the number of scrapped circuit boards. This will help to speed up turnaround by avoiding DFM-level failures.