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PCB Assembly

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Almost all industries use Printed Circuit Boards or PCBs. These small electronic devices use several components to come through and are responsible for a variety of applications in these sectors. But for this, it is important that all components in a PCB are assembled and affixed to the board in a manner that makes them useful and functional.

So, what is the procedure for assembling a PCB, and how do we do it correctly? In this article, we’ll discuss the PCB assembly in detail.

What is PCB Assembly?

Printed Circuit Board, short form PCB assembly is a process that includes embedding electronic components on PCB’s surface. This creates a functional electronic circuit or product.

For a better understanding of how PCBs and PCBAs differ, please take a look at our comprehensive comparison between PCB and PCBA.

Types of PCB Assembly

Thru-Hole Technology

All the components, such as capacitors, inductors, coils, and resisters, were inserted using holes. This gave it its name, thru-hole technology.

The technology works for one-sided, two-sided, and multi-sided but does not suit the new age of electronics. While SMT dominates the market when it comes to PCB assembly, a few applications, such as capacitors and substantial transformers, require thru-hole technology.

Surface Mount Technology

Simply put, surface mount technology, or SMT welds components to circuit boards. The results are far better than traditional assembly methods, and hence, it is used more now. All phones, computers, home appliances, etc., are built using this technology.

Mixed PCB Assembly Technology

It is impossible to assemble different components using only SMT and THT technology. This is because, as technology advances, electronic devices are expected to be sleek and multi-functional. The duo cannot be achieved using a single technology.

Therefore, a number of different PCB assembly methods are utilized, and soldering paste has no role in them. Although some components can be connected using SMT, many others cannot, so a combination of techniques is employed.

Dive deeper into Surface Mount Technology (SMT) Assembly and its advantages in our specialized guide.

The Basic Design of a PCB

Normally, a Printed Circuit Board comprises the following layers. Each layer has a different construction and a different function within a circuit.

ingle-layer-PCB

Substrate layer: To provide rigidity to the board, this layer is made using fiberglass. Most boards use this material, but flexible PCBs use flexible plastic, such as Kapton. Besides this, substrate layers of PCB use materials such as epoxies, but they are not as durable as FR4.

Copper Layer: Thin copper foil is laminated to the board; this forms the next layer in a PCB. It may have one or more copper foil layers, and their thickness depends on how much power the PCB needs to handle.

In a single-sided PCB, one side of the PCB has a layer of copper foil. The other side is for embedding electrical components. On the other hand, double-sided PCB features a conductive copper layer on both the sides of the board.

Soldermask Layer: After copper lamination, it is now time for the next layer, the soldermask layer. This layer provides insulation to the copper layers and keep it aloof from other components to prevent short. It is also responsible for lending the PCB its green color.

Furthermore, it must be stated that the solder mask layer is by far the most vital layer, especially in the manufacturing process. So, when some electrical components are to be soldered, this layer helps in the correct placement of the components.

Silkscreen Layer: The silkscreen layer is the last and final layer. It is mainly for the users to help them differentiate the many pins and learn about the functionality of each. Symbols, letters, or numbers are written on this white layer.

Learn more about the crucial role and intricacies of the Solder Mask Layer in PCBs in our dedicated article.

Components of PCB Assembly

Basically, there are two types of components: Passive components and active components. The duo vary due to their purpose.

Passive components

These are electronic components that have no impact on the signal and let it flow without modifying it. Moreover, they do not require any electronic power to operate.

Capacitors: A dielectric layer divides two conductive layers and this creates a capacitor. As we know, these electronic devices keep an electrical charge for a brief period before being supplied to another location in a circuit.

You can measure their capacity by determining their capacitance. Capacitors exist as different types based on their dielectric power. The density of the dielectric material defines the intensity of the electric charge.

Resistors: Most widely used components in a PCB, these are available in various shapes and sizes. The primary function of these in a circuit is to obstruct the flow of the current.

Among the various types of resistors, the axial' style is the most common, featuring leads on both ends and colored bands on the body. These signify resistance value, tolerance, and, at times, the temperature coefficient.

Inductors: In a PCB, inductors are one of the three passive linear components of a circuit, along with capacitors and resistors. An inductor is made up of an insulated wire wound into the form of a coil.

When electricity runs through the coil, it creates an electromagnetic field. But if the electric current changes, the coil resists the change. Therefore, inductors try to keep the current in the circuit steady.

Diodes: These electronic devices enable the flow of current in only one direction. Thus allowing it to travel from an anode to a cathode and vice versa. This can be attained by maintaining zero resistance at one end and infinite resistance at the other. An example of this is LED.

Transformers: As the voltage fluctuates, these devices transport power between circuits. They consist of a magnetic core with two or more two coils wound around it. The primary coil connected to the power supply is accompanied by the secondary coils.

Active Components

Transistors: These are the main building blocks of an electronic circuit. As semiconductors, they are used as amplifiers and electrical switches.

Although they can function independently, they are often found in large quantities, sometimes in billions, integrated into a single circuit.

ICs (Integrated circuits): Other names are microchip or chip. These are electronic devices that connect wires and other components within a circuit. They are constructed as a single unit on many tiny dielectric substrates or semiconductor wafers, and allow the creation of the most advanced smart devices.

To explore more about electronic components involved in PCB assembly, check out our detailed guide on Electronic Component.

Types of ICs

BGA (Ball Grid Array): This one is specifically designed for Integrated Circuits, making them suitable for mounting microprocessors. The package has more pins than any other package and its unique feature allows using the the entire bottom surface. This design, along with average trace length, adds to its high speed and performance.

SMD packages (Surface mount device): As technology evolved, it paved the way for new chip packages. One such development is surface mount technology (SMT), featuring several small components. These packages have minute pins that can be soldered onto the PCBs. A few examples of the same include Quad Flat Packages (QFP), Small Outline (SOP), etc.

Understand the complexities and design considerations for High Speed PCB in our comprehensive guide.

How to Select the Components for PCB Assembly

Functionality: Determine that the component selected meets all the requirements of the application. This will ensure that the PCB works as desired.

Power Consumption: Designers consider CAD libraries to find out how much power a computer can handle. Sometimes, these libraries do not provide the correct information. This is a serious problem and may lead to the component failing to start or not working at all.

Current and voltage rating: Oftentimes, the minimum and maximum current and voltage ratings of the PCB are ignored. Note that if you buy a component with a higher voltage than needed, it will make the circuit heavier. At the same time, if the components have lower ratings, they may melt, leading to the failure of the component. Buy one with optimum range.

Regulations: Adhere to the regulations. Failing to do so does not mean that your component will fail immediately. However, there are always chances of it being called back.

The operating environment: How do you plan to operate the PCB? Just remember that some parts need to work in specific temperatures and moisture levels. So whether it's meant for industry, business, or military, the use depends on the surroundings.

Purchasing components
Lack of components is one of the key issues faced during PCB assembly. While there are alternatives, they may not be readily available. This propels the manufacturers to buy low-grade components or fall into false marketing gimmicks. Therefore, find a reliable PCB assembly service that delivers turnkey services.

Placing the components
Besides buying, placement is another issue. To overcome this issue, the machines should be supplied with Gerber data. They should also focus on features such as panel handling strips, panel sizes, and fiducial marks, etc. These are some of the characteristics that reduce faults in placement and also help in constructing a quality product.

Be prepared for changes
No first design is the final design. You should be willing to consider new and different components as some modifications are made to the design. Also, before you pick up a component, consider things such as power component area density, dissipation, the cost of components, and their availableness.

Apply Sound Grounding Practices
Whatever design you decide upon, it must include a sufficient number of ground planes and bypass capacitors. This practice is important as it enables your PCB to achieve optimum susceptibility performance and electromagnetic compliance. Here's a suggestion: in the case of ICs, use decoupling capacitors.

Check spare gates
Connect the inputs of any spare gates to a signal. If there are any left out, connect them as well. Though this is not very common, you should ensure that all gates are connected to avoid issues such as preventing systems from working.

Pre-requisites for the PCB Assembly Process

Prior to commencing the PCB process, it is vital to assess the PCB for all the irregularities that may end up in the malfunction or failure. It is known by the name Design for Manufacturing (DFM) process.

Here are some basic steps to carry out this process:

Component Layout Consideration: Check components with polarity. Make sure electrolytic capacitors, diodes, and SMT tantalum capacitors are put in the right way. Also, double-check the notch or head direction of ICs.

Additionally, if a component needs a heat sink, make sure there's enough room around it so it doesn't touch other components.

Hole and Vias Spacing: Take a look at the space between the holes. Undertake a similar inspection for the distance between the trace and the hole. You should ensure that the via holes and the solder pads are not overlapping.

Consider thickness, copper pads, and Trace width: once the DFM check is over, you can minimize the overall manufacturing cost. With this approach, you can prevent the DFM issues.

PCB Assembly

Solder Paste Stenciling

Put solder paste on the part of the circuit board where you'll place the electronic parts. Use a stainless steel stencil for this. Hold the PCB and stencil in place with a tool, then spread the solder paste on the board. Spread it evenly with just a little solder.

When you take the tool away, the paste will stay on the board. Grey-colored solder paste is 96.5% tin, 0.5% copper and 3% silver. Also, it is free of lead.

For more insights into what solder paste is and how it's applied in PCB assembly, read our detailed article on Solder Paste.

Pick-and-Place

While the earlier step is carried out manually using tweezers, this step uses advanced automation. In this step, the SMT components are picked and placed on the PCB board.

Prior to the placement, an expert creates the design, which is then entered into the robot in the form of a CAD file. This file shows where each part is placed on the PCB by giving their pre-set X and Y coordinates.

With the help of the information provided, the robot places the SMD devices on the board. The specialized pick and place robots are designed to pick the parts with their vacuum parts and drop them on the solder paste.

Before the introduction of the robotic pick-and-place machines, technicians used tweezers to manually pick up components. This was not only tedious but also caused health issues such as weak eyesight in technicians. Instances of human error were also high.

Sometimes, it also led to reduced speed in PCB assembly. But no more with robots. These can work non-stop and also ensure there are no errors whatsoever.

Reflow Soldering

After you finish applying the solder paste and placing the component, you must also ensure that they stay in the right place. In other words, the solder paste should solidify fixing components to their pre-determined place. This process, reflow soldering, is a step in that direction.

In this process, the components are placed on the conveyor belt. The belt then moves the components onto a large oven, which comprises many heaters. These heating appliances heat the board to a temperature of 250 degrees Celsius. This melts the solder.

The melted solder affixes the components to the board, thus creating joints. Now, the PCB is cooled down. In the same oven, the circuit under construction moves through a series of coolers. This solidifies the solder in a controlled environment.

The components create a permanent joint on the board. However, a few boards need to be treated slightly differently. Two-sided PCBs, for instance, require stenciling and reflowing for both sides.

To elaborate, in two-sided PCBs, the side with smaller components is created first. It is stenciled, placed, and reflowed. Once set, the other side goes through the same treatment.

Quality Control and Inspection

Once you conclude the above three steps, it is essential to ensure that the circuit is ready to function accurately. Why you must do this because there can be a misalignment that can lead to a short circuit later on. Also, the movement in the reflow process can create poor connections or have no connections at all.

Therefore, it is suggested to check the PCB quality by using the inspection method. PCB inspection is one of the many methods that ascertain quality assurance, and this can be manual or automatic.

Manual Inspection: Although automatic methods are available, a manual check is still favored. It is suitable for smaller batches of PCBs. A visual inspection of the board is sufficient to find out the flaws, if any.

But, looking at such boards for an hour or longer causes optical fatigue and, hence, errors. Therefore, it is recommended only for THT components and those with lesser component density.

Optical Inspection: This method is appropriate for larger batches of PCBAs. A specialized machine is used to inspect the assemblies. It is called an AOI machine, and it has a series of cameras to check for flaws or errors.

These cameras come with high power and high resolution and are strategically placed at different angles. These angles make sure that no joint is missed.

When the light falls on the joints, it is reflected in different directions. This helps the AOI locate a joint that is of lower quality. The best part is that the machine works at a high speed, making it possible to inspect a large batch quickly.

X-ray inspection: Another method of Inspection is X-ray. This method is not applied in normal circumstances. It is for complicated and advanced PCBs. This helps to gaze at the lower layers and find out if there are any flaws there.

But one must use them cautiously as in case of wrong application, you may have to rework or stash the PCB in waste. This kind of evaluation should be carried out frequently to prevent delays or material costs.

Once the inspection is complete, move to the next step. Irrespective of whether you found any error or not, it is important to check whether the PCB component is functioning for what it is built for or not.

So, test the connections for their quality. If the boards require more programming or calibration, then this process will take slightly longer than intended. This is because it requires more steps to confirm the functionality.

Note: The inspection steps should be performed more frequently to diagnose and fix the issues as soon as they arise. This will not only save time but also help prevent many PCBs with serious issues, and from landing in waste.

Discover the importance and methodology behind Automated Optical Inspection (AOI) in our in-depth exploration.

Through-Hole Component Insertion

It must be noted that each PCB Assembly (PCBA) will have a different set of components if the board is different. This may include a range of components going beyond the typical surface-mounted devices (SMDs). For example, through-hole components or PTH components.

These plated-through holes or components are found on the PCB. They have wires that go through the holes in the circuit board, which then connect with other holes made up of copper.

When a manufacturer attaches these parts with wires in the holes and solders them, they get connected to other holes on the same board. This should be done exactly how it is depicted in the design. Soldering paste has no role to play in this board as it won't adhere to the hole.

Therefore, through-hole components need a specialized method, such as the soldering method.

Manual soldering: Using the manual soldering method is the most common method. However, the process takes slightly longer than the automated procedure. Nevertheless, here's how it is carried out.

One technician is allowed to insert one component. Then, the board moves to the next technician and so on in the assembly line. The time it takes to create one board with THT depends on the number of components that must be inserted in one cycle.

While this makes the procedure long and tedious, designers prefer it and employ it as a common practice. However, many companies avoid it in their designs as it consumes a lot of time.

Wave Soldering: It is an automatic process, wherein the components are placed on the PCB and then positioned on a conveyor plate.

The conveyor plate takes the components into an oven and they get splashed with solder. This happens on the bottom layer where the wires are present. The melted solder will join the pins to the board.

The downside of this method is that it is suitable for one-sided PCBs only and does not work on double-sided PCBs. Why so? Because wave soldering on the other side can render the delicate components of their functionality.

Once the soldering process finishes, one can move to the last and final stage. But if the PCB requires the addition of more parts or assembly of the other part, then go back to the first step.

Inspection and Testing

With all the steps covered, the PCB is now up and ready to be checked for efficacy and functionality. Hence, it is also known as a functionality test. Give the circuit board electricity at certain points to see if specific spots or connectors are working correctly.

Lab instruments such as an oscilloscope, function generator, and DMM were used to perform the test. Apart from verifying functionality and electrical features, this test also helps in establishing current, analog and digital signals, and voltage, as in the requirements of PCB design.

During inspection, if the PCBA does not meet the set standards, it is disposed of or recycled if possible. However, the latter depends on the company's standards. This step is extremely important as it helps establish PCBA's success rate.

Final Cleaning and Finishing

PCB assembly is a messy procedure. There is flux left behind by soldering paste in addition to oils and dirt that may have gotten transferred from human hands. While testing is an important procedure, the aftermath may not look as pleasing to the eye.

The flux tends to stay on the PCB for a long time until removed and may start smelling bad. You may never know, as it could turn acidic and thus harmful to the joints. Not only this but the shipment may be returned if the new pieces of PCBs look used and filthy with smelly residues and fingerprints.

Therefore, it is important to wash these PCBs before releasing them into the market. One can use deionized water in a stainless-steel, high-pressure washing apparatus to get rid of the residues. This process is safe and will not harm the board. The reason is that deionized water is free of ions that can cause harm, but this water has none. So there is nothing to worry about.

After rinsing the PCBA, dry it using compressed air. This prepares the board for shipment.

Best Practices for Successful PCB Assembly

For making PCB assembly results better, here are a few suggestions:

Component size: Always select the right size of the components for PCB designing. If you select a smaller size, problems may arise in the assembly, making it a lengthier procedure. 

Therefore, larger size components are recommended. Even if you are to disassemble or solder, removing bigger parts is far easier and quicker than the smaller ones. 

Component footprint: Before beginning the assembly process, think about how each component will fit on the PCB board. Once you have the blueprint in your mind and on paper (read the datasheet), design each component as illustrated on the sheet. 

Remember, if the footprint is not replicated correctly, some issues may arise, such as uneven heating, leading to one side of the PCB sticking in place of two. Passive electronic components such as the resistors and capacitors may also get affected. 

Space between components: When you do not leave any gap between the components, you are inviting trouble. Note that this can lead to failure. When you place two components closer together, it can create a host of problems, which may even prompt you to redesign or refabricate the circuit. 

So, make sure when you use an automated assembly method, the components are well placed. Keep them at a distance from the edge, the mechanical part, and the other components. 

There is yet another spacing issue. Insufficient space between components or incorrect rotation of components can also lead to issues in the wave soldering process. So, space your components accurately.   

Updated BOM: Always update the Bill of Materials (BOM). If there is an error in it, the manufacturers are required to identify and fix it before proceeding with PCB assembly. To keep things streamlined, update the BOM each time you modify the design.

Use of fiducials: They are indicators for PCB machines and are round in shape. They help the automated equipment to find out how the board is positioned and how the components would be placed. 

These are of two types: global fiducials and local fiducials. 

  • Global fiducials are positioned on the edge of printed circuit boards. As the pick and place machines are designed to identify the orientation, the correct placement will help them great deal.
  • Local fiducials are placed near the corners of square SMD components. A placement like this makes it much easier for the machines to find the component. In turn, it helps reduce errors in positioning during PCB assembly.

Conclusion

There are several ways to perform PCB assembly. Pick and choose the one that suits your components and requirements. This will ascertain that the product so manufactured complies with the set standards and works just as intended. 

So, if you are interested, get in touch with a reliable PCB manufacturer, especially he who has experience and expertise in both high- or low-volume PCB assembly service.

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