The PCB assembly process occurs after PCB manufacturing, where the board structure is completely formed according to client requirements.The steps involved in PCB assembly range from stencil setup and solder paste printing to SMD part insertion, bond creation in the curing oven, and final PCB functionality testing.
The printing of solder paste occurs after the preparation of the stencil. At this point, extremely precise filling is required in the small apertures that serve as mounting points for electrical components. Otherwise, significant problems like bridging might happen. Frequently, the bridges are so little that they are invisible to the naked eye. Because of this, they are scarcely observable without the application of cutting-edge inspection tools. Even yet, they are the most frequent cause of short circuits or even component burning during the PCB assembly process.
The placing of electronic components using a pick and place machine is a crucial step in the PCB manufacturing process. The small conductive lines on the board must line up with the conductive components of the electrical components. As a result, the electronic gadgets need to be positioned and placed on the board with the greatest accuracy. To position the surface mounting devices, we employ the most accurate three fiducial points optical systems.
The solid and stable connection between SMDs and board is finally made in the curing oven at the conclusion of the PCB assembly line. Our ultimate goals, in addition to producing the highest-quality PCBs, are the PCBs’ lifespan and resilience, which are made possible via paste soldering.
Printed Circuit Boards Assembly (PCBA) Process
Step 1: Apply solder paste to the circuit board
The portions of the printed circuit board assembly where the components will fit are first covered with solder paste. On the stainless steel stencil, solder paste is used to accomplish this. A mechanical fixture holds the stencil and PCB together while the solder paste is applied uniformly by the applicator to all board openings.
The applicator evenly distributes the solder paste. Therefore, the appropriate amount of solder paste must be utilized in the applicator. The paste will stay in the desired PCB locations when the applicator is taken away. The solder paste is a grey hue and is made of 96.5 percent of tin, 3 percent silver, and 0.5 percent copper. It is also lead-free.
Step 2: Pick and place the machine
During this part of the assembly process, the board with the added solder paste is then passed into the pick and place process. Here a machine loaded with reels of components picks the components from the reels or other dispensers and places them onto the correct position on the board.
The components placed onto the board are held in place by the tension of the solder paste. This is sufficient to keep them in place provided that the board is not jolted.
In some assembly processes, the pick and place machines add small dots of glue to secure the components to the board. However this is normally done only if the board is to be wave soldered. The disadvantage of the process is that any repair is made far more difficult by the presence of the glue, although some glues are designed to degrade during the soldering process.
The position and component information required to programme the pick and place machine is derived from the printed circuit board design information. This enables the pick and place programming to be considerably simplified.
Step 3: Let the solder paste solidify
Once the solder paste and surface mount components are all in place, they need to remain there. This means the solder paste needs to solidify, adhering components to the board. PCB assembly accomplishes this through a process called “reflow”.
After the pick and place process concludes, the PCB board is transferred to a conveyor belt. This conveyor belt moves through a large reflow oven, which is somewhat like a commercial pizza oven. This oven consists of a series of heaters which gradually heat the board to temperatures around 250 degrees Celsius, or 480 degrees Fahrenheit. This is hot enough to melt the solder in the solder paste.
Once the solder melts, the PCB continues to move through the oven. It passes through a series of cooler heaters, which allows the melted solder to cool and solidify in a controlled manner. This creates a permanent solder joint to connect the SMDs to the PCB.
Many PCBAs require special consideration during reflow, especially for two-sided PCB Assembly. Two-sided PCB assembly need stenciling and reflowing each side separately. First, the side with fewer and smaller parts is stenciled, placed and reflowed, followed by the other side.
Step 4: Inspect the PCB assembly
Once the surface mount components are soldered in place after the reflow process, which doesn’t stand for completion of PCBA and the assembled board needs to be tested for functionality. Often, movement during the reflow process will result in poor connection quality or a complete lack of a connection. Shorts are also a common side effect of this movement, as misplaced components can sometimes connect portions of the circuit that should not connect.
Checking for these errors and misalignments can involve one of several different inspection methods. The most common inspection methods include:
• Manual Checks: Despite upcoming development trend of automated and smart manufacturing, manual checks are still relied on in PCB assembly process. For smaller batches, an in-person visual inspection by a designer is an effective method to ensure the quality of a PCB after the reflow process. However, this method becomes increasingly impractical and inaccurate as the number of inspected boards increases. Looking at such small components for more than an hour can lead to optical fatigue, resulting in less accurate inspections.
• Automatic Optical Inspection: Automatic optical inspection is a more appropriate inspection method for larger batches of PCBAs. An automatic optical inspection machine, also known as an AOI machine, uses a series of high-powered cameras to “see” PCBs. These cameras are arranged at different angles to view solder connections. Different quality solder connections reflect light in different ways, allowing the AOI to recognize a lower-quality solder. The AOI does this at a very high speed, allowing it to process a high quantity of PCBs in a relatively short time.
• X-ray Inspection: Yet another method of inspection involves x-rays. This is a less common inspection method — it’s used most often for more complex or layered PCBs. The X-ray allows a viewer to see through layers and visualize lower layers to identify any potentially hidden problems.
The fate of a malfunctioning board depends on PCBA company’s standards, they will be sent back to be cleared and reworked, or scrapped.
Whether an inspection finds one of these mistakes or not, the next step of the process is to test the part to make sure it does what it’s supposed to do. This involves testing the PCB connections for quality. Boards requiring programming or calibration require even more steps to test proper functionality.
Such inspections can occur regularly after the reflow process to identify any potential problems. These regular checks can ensure that errors are found and fixed as soon as possible, which helps both the manufacturer and the designer save time, labor and materials.
Step 5: Insert the plated through-hole component
The through-hole components are commonly found on many PCB boards. These components are also known as Plated through Hole (PTH). These components have leads that will pass through the hole in the PCB. These holes connect to other holes and vias by means of copper traces. When these THT components are inserted and soldered in these holes, then they are electrically connected to other hole in the same PCB as the circuit designed. These PCBs may contain some THT components and many SMD components so the soldering method as discussed above in case of SMT components like reflow soldering will not work on THT components. So the two main types of THT components soldering or prototype pcb assembly are
a. Manual Soldering :
The manual soldering method is the common and typically takes more time than compared to automated setup for SMT. Usually one technician is designated to insert one component at a time and the board is passed on to other technician who inserts another component on the same board. So the board will move all around the assembly line to get the PTH components stuffed upon it. This makes the process lengthy and so many PCB design and manufacturing companies avoid using PTH components in their circuit design. But still the PTH components are the most favorite and common components for most of the circuit designers.
b. Wave Soldering :
The automated version of manual soldering is wave soldering. In this method, once the PTH components are placed on the PCB, the PCB is put on the conveyer belt and is moved to specialized oven. Here a wave of molten solder is splashed on the PCB bottom layer where the components leads are present. This will solder all the pins at once. However this method is only for single sided PCBs and not for double sided because this molten solder while soldering one side of PCB can damage components on other side. After this, the pcb fabrication and assembly is moved for final inspection.
Step 6: Complete a final inspection
The PCB is now prepared for inspection and testing. This is a functioning test in which the PCB is supplied with electrical signals and power at the designated pins, and output is examined at the designated test points or output connectors. Common laboratory tools including an oscilloscope, a DMM, and a function generator are needed for this test.
This test examines the PCB’s electrical properties, functionality, and compliance with PCB and circuit design specifications for current, voltage, analog, and digital signals.
According to the company’s standard operating procedures, the PCB is rejected or trashed if any of its criteria yield undesirable results. The testing phase is crucial since it determines whether the entire process will succeed or fail.
It is now time to clean the undesired leftover flux, finger dirt, and oils stains after the PCB has been examined and pronounced to be in good condition overall. All forms of dirt can be removed with a high pressure washer made of stainless steel and deionized water. The PCB circuit won’t be harmed by the deionized water. After washing, compressed air is used to dry the PCB. The finished PCB is now prepared for packaging and shipping.
In short, PCB assembly is the process of bringing everything together into one place and creating something functional out of multiple components. It’s taking that newly manufactured board, adding components and resistors, and then making sure it does what it is intended to do.