In terms of modern technology, the world is growing at a very fast rate, and its influence can easily come into play in our daily life. The way we live has changed dramatically and this technological advance has led to many advanced devices that we didn’t even think of 10 years ago. The core of these devices is electrical engineering, and the core is printed circuit board (PCB).
A PCB is usually green and is a rigid body with various electronic components on it. These components are welded to the PCB in a process called “PCB assembly” or PCBA. The PCB consists of a substrate made of fiberglass, copper layers that make up the trace, holes that make up the component, and layers that can be inner and outer. At RayPCB, we can provide up to 1-36 layers for multi-layer PROTOTYPES and 1-10 layers for multiple batches of PCB for volume production. For single-sided and double-sided PCBS, an outer layer exists but no inner layer.
The substrate and components are insulated with solder film and held together with epoxy resin.The welding mask can be green, blue or red, as is common in PCB colors. The welding mask will allow the component to avoid short-circuiting to the track or other components.
Copper traces are used to transfer electronic signals from one point to another on a PCB. These signals can be high-speed digital signals or discrete analog signals. These wires can be made thick in order to provide power/power for component power supply.
In most PCBS that provide high voltage or current, there is a separate grounding plane. Components on the top layer are connected to the internal GND plane or internal signal layer via “Vias”.
Components are assembled on the PCB to enable the PCB to operate as designed. The most important thing is PCB function. Even if the tiny SMT resistors are not placed correctly, or even if small tracks are cut from the PCB, the PCB may not work. Therefore, it is important to assemble components in a proper way. The PCB when assembling components is called PCBA or assembly PCB.
Depending on the specifications described by the customer or user, the function of the PCB may be complex or simple. PCB size also varies according to requirements.
The PCB assembly process has both automatic and manual processes, which we will discuss.
PCB layer and design
As mentioned above, there are multiple signal layers between the outer layers. Now we will discuss the types of outer layers and functions.
Understand PCB board assembly process and feel the green charm of PCBD
1 – Substrate: This is a rigid plate made of FR-4 material on which the components are “filled” or welded. This provides rigidity for the PCB.
2- Copper layer: Thin copper foil is applied to the top and bottom of the PCB to make the top and bottom copper trace.
3- Welding mask: It is applied to the top and bottom layers of the PCB. This is used to create non-conducting areas of the PCB and insulate the copper traces from each other to protect against short circuits. The welding mask also avoids welding unwanted parts and ensures that solder enters the area for welding, such as holes and pads. These holes connect the THT component to the PCB while the PAD is used to hold the SMT component.
4- Screen: The white labels we see on PCBS for component codes, such as R1, C1 or some description on PCBS or company logos, are all made of screen layers. The screen layer provides important information about the PCB.
There are 3 types of PCBS according to the substrate classification
1- Rigid PCB:
PCBs are most of the PCB devices we see in various types of PCBs. These are hard, rigid and sturdy PCBS, with different thicknesses. The main material is fiberglass or simple “FR4”. FR4 stands for “flame retarder-4”. The self-extinguishing characteristics of the FR-4 make it useful for the use of many hard-core industrial electronic devices. The FR-4 has thin layers of copper foil on both sides, also known as copper-clad laminates. Fr-4 copper clad laminates are mainly used in power amplifiers, switching mode power supplies, servo motor drivers, etc. On the other hand, another rigid PCB substrate commonly used in household appliances and IT products is called paper phenolic PCB. They are light, low density, cheap and easy to punch. Calculators, keyboards and mice are some of its applications.
2- Flexible PCB:
Made from substrate materials such as Kapton, flexible PCBS can withstand very high temperatures while being as thick as 0.005 inches. It can be easily bent and used in connectors for wearable electronics, LCD monitors or laptops, keyboards and cameras, etc.
3-metal core PCB:
In addition, another PCB substrate can be used like aluminum, which is very efficient for cooling.These types of PCBS can be used for applications that require thermal components such as high power leds, laser diodes, etc.
Installation technology type:
SMT: SMT stands for “surface mount technology”. SMT components are very small in size and come in various packages such as 0402,0603 1608 for resistors and capacitors. Similarly, for integrated circuit ics, we have SOIC, TSSOP, QFP and BGA.
SMT assembly is very difficult for human hands and can be a time processing process, so it is primarily done by automated pickup and placement robots.
THT: THT stands for through-hole technology. Components with leads and wires, such as resistors, capacitors, inductors, PDIP ics, transformers, transistors, IGBT, MOSFET, etc.
The components must be inserted on one side of the PCB on one component and pulled by the leg on the other side, cut the leg and welded. THT assembly is usually done by hand welding and is relatively easy.
Assembly process prerequisites:
Prior to the actual PCB fabrication and PCB assembly process, the manufacturer checks the PCB for any defects or errors in the PCB that could cause the failure. This process is called the Manufacturing design (DFM) process. Manufacturers must perform these basic DFM steps to ensure a flawless PCB.
1- Component layout considerations: Through-holes must be checked for components with polarity. Like electrolytic capacitors must be checked polarity, diode anode and cathode polarity check, SMT tantalum capacitor polarity check. IC notch/head direction must be checked.
The element requiring the heat sink should have enough space to accommodate other elements so that the heat sink does not touch.
2-Hole and through-hole spacing:
The spacing between holes and between holes and traces should be checked. Pad and through hole shall not overlap.
3- Brazing pad, thickness, line width shall be taken into account.
By performing DFM inspections, manufacturers can easily reduce manufacturing costs by reducing the number of scrap panels. This will help in fast steering by avoiding DFM level failures. At RayPCB, we provide DFM and DFT inspection in circuit assembly and prototyping. At RayPCB, we use state-of-the-art OEM equipment to provide PCB OEM services, wave soldering, PCB card testing and SMT assembly.
PCB Assembly (PCBA) step-by-step process:
Step 1: Apply solder paste using template
First, we apply solder paste to the area of the PCB that fits the component. This is done by applying solder paste to the stainless steel template. The template and PCB are held together by a mechanical fixture, and the solder paste is applied evenly to all openings in the board through an applicator. Apply solder paste evenly with applicator. Therefore, appropriate solder paste must be used in the applicator. When the applicator is removed, the paste will remain in the desired area of the PCB. Gray solder paste 96.5% made of tin, containing 3% silver and 0.5% copper, lead free. After heating in Step 3, the solder paste will melt and form a strong bond.
Step 2: Automatic placement of components:
The second step of PCBA is to automatically place the SMT components on the PCB. This is done by using a pick and place robot. At the design level, the designer creates a file and provides it to the automated robot. This file has the pre-programmed X, Y coordinates of each component used in the PCB and identifies the location of all components. Using this information, the robot only needs to place the SMD device accurately on the board. The pick and place robot will pick up components from its vacuum fixture and place them accurately on the solder paste.
Prior to the advent of robotic pickup and placement machines, technicians would pick up components using tweezers and place them on the PCB by carefully looking at the location and avoiding any shaking hands. This results in high levels of fatigue and poor vision for technicians, and leads to a slow PCB assembly process for SMT parts. So the potential for error is high.
As technology matures, automated robots that pick up and place components reduce the workload of technicians, enabling fast and accurate component placement. These robots can work 24/7 without fatigue.
Step 3: Reflow welding
The third step after setting up the elements and applying the solder paste is reflux welding. Reflow welding is the process of placing the PCB on a conveyor belt with components. The conveyor then moves the PCB and components into a large oven, which produces a temperature of 250 o C. The temperature is sufficient to melt the solder. The melted solder then holds the component to the PCB and forms the joint. After high temperature treatment, the PCB enters the cooler. These coolers then solidify the solder joints in a controlled manner. This will establish a permanent connection between the SMT component and the PCB. In the case of a double-sided PCB, as described above, the PCB side with fewer or smaller components will be treated first from steps 1 to 3, and then to the other side.
Understand PCB board assembly process and feel the green charm of PCBD
Step 4: Quality inspection and inspection
After reflow soldering, it is possible that components are misaligned due to some incorrect movement in the PCB tray, which may result in short or open circuit connections. These defects need to be identified, and this identification process is called inspection. Inspections can be manual and automated.
A. Manual check:
Because the PCB has small SMT components, visual inspection of the board for any misalignment or malfunction can cause technician fatigue and eye strain. Therefore, this method is not feasible for advance SMT boards due to inaccurate results. However, this method is feasible for plates with THT components and lower component densities.
B. Optical detection:
This method is feasible for large quantities of PCBS. The method uses automated machines with high power and high resolution cameras mounted at various angles to view the solder joints from all directions. Depending on the quality of the solder joint, the light will reflect off the solder joint at different angles. This automatic optical inspection (AOI) machine is very fast and can process large quantities of PCBS in a very short time.
CX – ray inspection:
The X-ray machine allows technicians to scan the PCB to see internal defects. This is not a common inspection method and is only used for complex and advanced PCBS. If not used properly, these inspection methods may result in rework or PCB obsoletion. Inspections need to be conducted regularly to avoid delays, labor and material costs.
Step 5: THT component fixation and welding
Through-hole components are common on many PCB boards. These components are also called plated through holes (PTH). The leads of these components will pass through holes in the PCB. These holes are connected to other holes and through holes by copper traces. When these THT elements are inserted and welded into these holes, they are electrically connected to other holes on the same PCB as the designed circuit. These PCBS may contain some THT components and many SMD components, so the welding method described above is not suitable for THT components in the case of SMT components such as reflow welding. So the two main types of THT components that are welded or assembled are
A. Manual welding:
Manual welding methods are common and often require more time than an automated setup for SMT. A technician is typically assigned to insert one component at a time and pass the board to other technicians inserting another component on the same board. Therefore, the circuit board will be moved around the assembly line to get the PTH component to fill on it. This makes the process lengthy, and many PCB design and manufacturing companies avoid using PTH components in their circuit designs. But the PTH component remains the favorite and most commonly used component by most circuit designers.
B. Wave soldering:
The automated version of manual welding is wave welding. In this method, once the PTH element is placed on the PCB, the PCB is placed on a conveyor belt and moved to a dedicated oven. Here, waves of molten solder splash into the substrate of the PCB where the component leads are present. This will weld all pins immediately. However, this method only works with single-sided PCBS and not double-sided PCBS, as melted solder on one side of the PCB can damage components on the other. After this, move the PCB for final inspection.
Step 6: Final inspection and functional testing
PCB is now ready for testing and inspection. This is a functional test in which electrical signals and power are given to the PCB at the specified pins and the output is checked at the specified test point or output connector. This test requires common laboratory instruments such as oscilloscopes, digital multimeters, and function generators
This test is used to check the functional and electrical characteristics of the PCB and validate the current, voltage, analog and digital signal and circuit designs described in the PCB requirements
If any of the PCB’s parameters show unacceptable results, the PCB will be discarded or scrapped according to standard company procedures. The testing phase is important because it determines the success or failure of the entire PCBA process.
Step 7: Final cleaning, finishing and shipping:
Now that the PCB has been tested in all aspects and declared normal, it is time to clean up unwanted residual flux, finger grime and oil. Stainless steel based high pressure cleaning tools using deionized water are sufficient to clean all types of dirt. Deionized water does not damage the PCB circuit. After washing, dry the PCB with compressed air. The final PCB is now ready to be packed and shipped.