What is PCB Assembly? What are the processes for printed circuit board assembly (PCBA)? How do they work?
What is PCB Assembly? What are the processes for printed circuit board assembly (PCBA)? How do they work?
In this section, we answer all these questions and we compare pcb assembly to other manufacturing technologies to help you find the best solution for your application.
A circuit board prior to assembly of electronic components is known as PCB. Once electronic components are soldered, the board is called Printed Circuit Assembly (PCA) or Printed Circuit Board Assembly (PCBA) or PCB Assembly. Different Manual and Automatic PCB Assembly Tools are used in this process.
It has to be noted that assembly of a circuit board is different from PCB Manufacturing Process. Manufacturing printed circuit boards involve several processes including PCB designing and creating PCB prototype. Once a PCB is ready, Active and Passive Electronic Components need to be soldered onto it before it can be used in any electronic equipment or gadget. This assembly of electronic components depends on Type of Printed Circuit Board, type of electronic components and purpose of the circuit board.
PCB Assembly Process Flow Chart (PCBA Process)
The PCBA process always starts out with the most basic unit of the PCB: the base, which consists of several layers, and each one plays a significant role in the functionality of the final PCB. These alternating layers include:
These materials and components remain largely the same across all PCBs, with the exception of the substrate. The substrate material of a PCB changes according to the specific qualities — such as cost and bendability — each designer is looking for in their finished product.
The three primary PCB types include:
PCB Types | PCBCart
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, PCBCart 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.
A few preparatory steps have to happen before real PCBA process even begins. This helps PCB manufacturers assess the functionality of a PCB design, and primarily includes a DFM check.
Most companies specializing in PCB assembly need the design file of the PCB to start out, along with any other design notes and specific requirements. This is so the PCB assembly company can check the PCB file for any issues that may affect the PCB's functionality or manufacturability. This is a design for manufacturability check, or DFM check, for short.
The DFM check looks at all the design specifications of a PCB. Specifically, this check looks for any missing, redundant or potentially problematic features. Any of these issues may severely and negatively influence the functionality of the final project. For example, one common PCB design flaw is leaving too little spacing between PCB components. This can result in shorts and other malfunctions.
By identifying potential problems before manufacturing begins, DFM checks can cut manufacturing costs and eliminate unforeseen expenses. This is because these checks cut down on the number of scrapped boards. As part of our commitment to quality at a low cost, DFM checks come standard with every PCBCart project order. PCBCart provides FREE DFM and DFA check with, however, priceless values because Valor DFM/DFA check PCBCart depends on is an automatic system contributing to high speed and accuracies.
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.
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.
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.
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:
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.
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:
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.
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.
Suffice it to say, PCB assembly process can be a filthy one. Soldering paste leaves behind some amount of flux, while human handling can transfer oils and dirt from fingers and clothing to PCB surface. Once all is done, the results can look a little dingy, which is both an aesthetic and a practical issue.
After months of remaining on a PCB, flux residue starts to smell and feel sticky. It also becomes somewhat acidic, which can damage solder joints over time. Additionally, customer satisfaction tends to suffer when shipments of new PCBs are covered in residue and fingerprints. For these reasons, washing the product after finishing all the soldering steps is important.
A stainless-steel, high-pressure washing apparatus using deionized water is the best tool for removing residue from PCBs. Washing PCBs in deionized water poses no threat to the device. This is because it's the ions in regular water that do damage to a circuit, not the water itself. Deionized water, therefore, is harmless to PCBs as they undergo a wash cycle.
After washing, a quick drying cycle with compressed air leaves the finished PCBs ready for packaging and shipment.
As a traditional PCB assembly method, thru-hole mounting process is accomplished through collaboration of manual procedure and automatic procedure.
Compared with thru-hole mounting process, surface mounting process stands out in terms of manufacturing efficiency because it features a totally automatic mounting PCB assembly process from solder paste printing, pick and place and reflow soldering.
With the development of modern science and technology, electronic products are becoming increasingly complex, driving complicated, integrated and smaller size PCB boards. It is almost impossible for PCBAs containing only one type of component taking part in.
Most boards carry Thru-hole components and SMD components, which requires collaboration of thru-hole technology and surface mount technology. Nevertheless, soldering is a complicated process that tends to be affected by too many elements. Thus, it becomes extraordinarily significant to better arrange the sequence of thru-hole tech and surface mount technology.
PCBA with application of mixed technologies should be carried out in the following situations:
Based on the comparison between mixed assembly procedures introduced above, it can be concluded that hand soldering works well for PCB assembly that requires with many components on both sides among which SMD components are more than THT components. Therefore, confronted with the situation when a small number of THT components are needed, it is wave soldering that is suggested.
PCB assembly has to go through such a complicated and technical process that numerous elements have to be taken into careful consideration and a little modification may cause huge change on cost and product quality. Descriptions concerning PCB assembly process in this article just center on typical PCBA procedures and technologies. Practical manufacturing process is largely determined and influenced by design files and specific requirement of customers. As a result, how to evaluate a reliable PCB assembler becomes a crucial question customers have to think it over prior to their PCBA order.