Electronics are an integral part of our daily lives. Everything from our smart phones to our cars includes electronic components. At the heart of these electronics is the printed circuit board, also known as a PCB.
There are two types of mounting technologies are prevailing in the modern PCBA industry:
Surface Mount Technology: Sensitive components, some very small, such as resistors or diodes are placed automatically onto the surface of board. This is called SMD assembly, for surface mount device. Surface mount technology can be applied on small size components and integrated circuits (ICs). For example, Viasion is capable of mounting package with min. size 01005, which is even smaller than the size of a pencil point.
Thru-Hole Technology: works well on components with leads or wires that have to be mounted on board by plugging them through holes on board. The extra lead part has to be soldered on the other side of the board. This technology is applied on PCB assemblies containing large components such as capacitors, coils to be assembled.
Due to distinctions between THT and SMT, they have to go through different assembly processes as well. The following article will discuss other material and design considerations beyond the base of the PCB as they apply to the PCB assembly process in relation to THT, SMT and mixed technologies.
Actual PCBA process steps:
Step 1: Solder Paste Stenciling
The first step of PCB assembly is applying a solder paste to the board. This process is like screen-printing a shirt, except instead of a mask, a thin, stainless-steel stencil is placed over the PCB. This allows assemblers to apply solder paste only to certain parts of the would-be PCB. These parts are where components will sit in the finished PCB.
The solder paste itself is a greyish substance consisting of tiny balls of metal, also known as solder. The composition of these tiny metal balls is 96.5% tin, 3% silver and 0.5% copper. The solder paste mixes solder with a flux, which is a chemical designed help the solder melt and bond to a surface. Solder paste appears as a grey paste and must be applied to the board at exactly the right places and in precisely the right amounts.
In a professional PCBA line, a mechanical fixture holds the PCB and solder stencil in place. An applicator then places solder paste on the intended areas in precise amounts. The machine then spreads the paste across the stencil, applying it evenly to every open area. After removing the stencil, the solder paste remains in the intended locations.
Step 2: Pick and Place
After applying the solder paste to the PCB board, the PCBA process moves on to the pick and place machine, a robotic device places surface mount components, or SMDs, on a prepared PCB. SMDs account for most non-connector components on PCBs today. These SMDs are then soldered on to the surface of the board in the next step of PCBA process.
Traditionally, this was a manual process done with a pair of tweezers, in which assemblers had to pick and place components by hand. These days, thankfully, this step is an automated process among PCB manufacturers. This shift occurred largely because machines tend to be more accurate and more consistent than humans. While humans can work quickly, fatigue and eyestrain tends to set in after a few hours working with such small components. Machines work around the clock without such fatigue.
The device starts the pick and place process by picking up a PCB board with a vacuum grip and moving it to the pick and place station. The robot then orients the PCB at the station and begins applying the SMTs to the PCB surface. These components are placed on top of the soldering paste in preprogrammed locations.
Step 3: Reflow Soldering
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: Inspection and Quality Control
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: Through-Hole Component Insertion
Depending on the type of board under PCBA, the board may include a variety of components beyond the usual SMDs. These include plated through-hole components, or PTH components.
A plated through-hole is a hole in the PCB that's plated all the way through the board. PCB components use these holes to pass a signal from one side of the board to the other. In this case, soldering paste won't do any good, as the paste will run straight through the hole without a chance to adhere.
Instead of soldering paste, PTH components require a more specialized kind of soldering method in later PCB assembly process:
• Manual Soldering: Manual through-hole insertion is a straightforward process. Typically, one person at a single station will be tasked with inserting one component into a designated PTH. Once they're finished, the board is transferred to the next station, where another person is working on inserting a different component. The cycle continues for each PTH that needs to be outfitted. This can be a lengthy process, depending on how many PTH components need to be inserted during one cycle of PCBA. Most companies specifically try to avoid designing with PTH components for this very purpose, but PTH components are still common among PCB designs.
• Wave Soldering: Wave soldering is the automated version of manual soldering, but involves a very different process. Once the PTH component is put in place, the board is put on yet another conveyor belt. This time, the conveyor belt runs through a specialized oven where a wave of molten solder washes over the bottom of the board. This solders all of the pins on the bottom of the board at once. This kind of soldering is nearly impossible for double-sided PCBs, as soldering the entire PCB side would render any delicate electronic components useless.
After this soldering process is finished, the PCB can move on to the final inspection, or it can run through the previous steps if the PCB needs additional parts added or another side assembled.
Step 6: Final Inspection and Functional Test
After the soldering step of the PCBA process is finished, a final inspection will test the PCB for its functionality. This inspection is known as a "functional test". The test puts the PCB through its paces, simulating the normal circumstances in which the PCB will operate. Power and simulated signals run through the PCB in this test while testers monitor the PCB's electrical characteristics.
If any of these characteristics, including voltage, current or signal output, show unacceptable fluctuation or hit peaks outside of a predetermined range, the PCB fails the test. The failed PCB can then be recycled or scrapped, depending on the company's standards.
Testing is the final and most important step in PCB assembly process, as it determines the success or failure of the process. This testing is also the reason why regular testing and inspection throughout the assembly process is so important.