A Comprehensive Walkthrough on PCB Assembly Processes and Components

What is PCB Assembly?

Printed circuit board manufacturing and printed circuit board assembly are two separate but related processes. PCB manufacturing deals with fabricating the PCB. This includes all the necessary traces, vias, and holes. 

On the other hand, Printed Circuit Board Assembly (PCBA) involves mounting electronic components onto the manufactured PCB. Printed circuit board assembly and PCB manufacturing ultimately have different requirements and standards.

The primary steps of assembling a PCB include:

• Solder paste application
• Component placement
• Soldering (reflow)
• Inspection, testing, and quality control
• Cleaning
• Repair and rework
• Application of conformal coating 

What’s the Preparation for PCB Assembly? 

PCB Assembly Requirements

Before it can move on to the assembly phase, the manufactured printed circuit board must have a solder mask.

Manufacturers apply the solder mask over the PCB’s conductive layer to protect it from oxidation. In addition to this, it prevents solder bridges from forming between closely spaced pads and gives the printed circuit board its green color.

The main component and equipment requirements for the PCB assembly process are as follows:

• Electronic circuit board components
• Solder flux
• Soldering machinery and equipment
  • Reflow oven 
  • Wave soldering machine
• Pick-and-place machinery
• Inspection and testing equipment

Choosing The Right Installation Technology

There are three main installation technologies to choose from for your assembly:

• Surface Mount Technology (SMT): This is a method of mounting electrical components directly on the surface of the PCB. The electrical components installed in this way are referred to as surface-mount devices (SMD).
• (Plated) Through-hole technology (THT/PTH): This is a mounting scheme where the assembler attaches and connects components (through their leads) to PCBs using drilled holes. Once the lead is inserted and secured to the PCB, the assembler solders the component to the pads on the opposite side.
• Hybrid technology: An installation that combines both the SMT and THT approaches. 

Design For Manufacturing (DFM) Inspection

Design for Manufacturing (DFM) involves designing parts, products, or components for ease of manufacturing.

This allows manufacturers to produce the end product at a lower cost.

DFM inspection primarily focuses on ensuring that a printed circuit board design is functional.

The DFM inspector must create a review of the bill of materials, and identify parts not recommended for manufacturing, and components that require immediate replacement. 

DFM inspection has several benefits. It allows designers to understand a product’s reproducibility and repeatability.

It also guides on tolerances during the manufacturing process and whether the demands of the product are reasonable.

By identifying the issues before the assembly process, DFM allows you to rectify your mistakes before you send them for assembly.

The test also reveals any unnecessary, missing, and potentially problematic issues.

These include inaccurate spacing between components, footprint corroboration, component-to-component spacing, component-to-edge spacing, first pin indication, component polarity, cathode mark for the diode, and so on.

Some of the issues covered during DFM inspection include checking how tolerance issues will impact the gauging system and detecting matters concerning non-conformances.

PCB Assembly Process: Detailed Steps 

Solder Paste Application

Solder paste application is the first of the SMT assembly process. This is done using a solder paste printer, which applies solder to the PCB with the guidance of stencil apertures.

If you have seen how a t-shirt is printed using a silkscreen, then you have a pretty fair idea of how the process works.

The stencil is held over the board using a mechanical fixture. An applicator then takes over and distributes the solder carefully by moving it across the board.

Close-up Macro Shot Laser Cut Stainless Steel Stencil

Solder is only applied to the open spaces where the SMT components will be mounted. The process will have to be repeated on the other side of the board if you deal with double-sided PCBs.

The solder paste is made from powder metal solder and combined with flux which gives it adhesive properties.

It’s generally grayish and holds the different components in place by fusing them. The nature of the application and use of the PCB determines the composition of the solder paste.

Generally, it contains varying percentages of copper, tin, and silver. 

After the soldering process, engineers inspect the board to ensure the solder has been applied only over the intended spots.

They will also check the amount of solder over the pads to provide sufficient amounts that have been used to install the components.  

Pick and Place Process

SMDs don’t come with legs or leads like through-hole devices and are soldered on the PCB. They are the most common non-connector PCBA components.

In old times, engineers would pick and place each component using tweezers, which would take a long time and cause fatigue.

Thanks to technology, we have capable machines that ‘pick and place’ SMD on target pads.

The machines usually pick up the components using a vacuum and align the PCB.

Then, it precisely places the SMDs on the chosen coordinates of the PCB on the solder. You may have to carry out this step in multiple types based on the number of SMDs you plan to use.

For double-sided PCBs, you need to carry out the ‘pick and place’ process one side at a time.

The result is more accurate and consistent than human effort and also suitable for mass production. Also, machines don’t feel tired or need to sleep and can work around the clock.

Technology has made component placement quick and easy, and all you need to do is program the machines properly to pick and place SMDs.

Automatic Pick Place Machine Quickly Installs Components

Reflow Soldering

The SMDs are now on the pads, and the next step is to make sure they are adequately secured. That means you have to heat and solidify the solder in a process called reflow soldering.

The PCBs are carried on a conveyor belt through an industrial oven that heats the PCBs to temperatures of 480 degrees Fahrenheit. The solder in the solder paste melts as a result while the PCBs keep moving on the conveyor.

Next, the PCBs are treated with a series of more excellent heaters so that the molten solder can cool down and solidify properly. Now you have permanently attached the SMDs on the pads successfully!

Infrared Soldering Oven Production

A critical consideration about this step is that the SMDs are heated at much higher temperatures than manual and wave soldering.

But you have nothing to fear as the present-day SMDs are suited to withstand extreme reflow soldering temperatures.

Remember that you cannot use this technology for many through-hole components due to the heat profile involved. You have to attach them using wave or manual soldering.

You will also have to carry out stenciling and reflowing for each side of double-sided PCBs separately. Begin with the side that has smaller and fewer components, then take care of the other side.

Through-Hole Assembly

The first step of the through-hole assembly process requires the manufacturer to drill and create plated through holes for the component leads.

Once the assembler completes this step, they can then insert the components. 

Next, the assembler uses waver soldering to finalize the connections between the components and the board. This is typically done using wave soldering where the PCB is passed over a flow of molten solder. The solder attaches itself to the exposed metal areas of the PCB. 

Alternatively, assemblers can manually solder the components onto the PCB using a solder iron. This is intended for small productions or where high degrees of accuracy are required. Manual soldering can be lengthy. Hence, many companies try to avoid using it. 

Nevertheless, once the assembler completes the soldering process, they move the PCB to the next station for cleaning. This removes any residual solder flux and contaminating chemicals to prevent corrosion.

Inspection & Quality Control

X-ray inspection

Manual inspection is notorious for being unreliable. Consequently, most modern manufacturers and PCB assemblers use machine-controlled methods such as Automatic Optical Inspection (AOI). These techniques are far more suitable for large batches as they can process high quantities of PCBs quickly. 

Alternatively, assemblers can use X-ray inspection. While this method is uncommon in most assemblies, it’s popular in the inspection of complex and layered PCBs. 

X-ray inspection allows you to see through the layers of the PCB and visualize the lower layers to see hidden problems.

Component Package Types in PCB Assembly

There’s an SMT variant for every THT component. We refer to SMT components as Surface Mount Devices (SMDs). 

Surface Mount Components (SMDs)

Some of the most common SMDs include:

• Resistors: Allows circuit boards to alter the flow of current. Each resistor has a printed code on its encasement denoting its size. For instance, 0402 indicates that an SMD resistor has a package length of 0.04 inches and a width of 0.02 inches. 
• Capacitors: Their main purpose is to store electric energy temporarily. Circuit boards can use them to filter out electric noise, deal with voltage drops or provide bursts of energy to components. We categorize them according to their materials, the most common being ceramic, electrolytic, and tantalum.   
• Inductors: Store electric energy in a magnetic field to help maintain the flow of current in a circuit. 
• Diodes: Facilitate rectification.  
• Transistors: Facilitate switching. They can also amplify electrical signals. 
• Integrated Circuits (ICs): Embedded miniaturized circuits.  

Surface mount technology has revolutionized how the industry creates and mounts integrated circuits (ICs) in particular. The two packages most concerned with ICs are Ball Grid Arrays (BGAs) and Quad Flat No-Leads (QFN).

Ball Grid Array (BGA)

Ball grid arrays are derived from pin grid array (PGA) technologies. Whereas PGA packages use pins to conduct electrical signals from the circuit board, BGAs use tiny drops of solder.

The manufacturer places these solder balls evenly apart on the board in an array of symmetrical grids and gently heats them in a specialist oven. Surface tension ensures that the components are kept in position.

BGA packages have several advantages over other methods. As such, they have become a popular technique used in manufacturing integrated circuits and electronic devices. 

The BGA’s solder balls allow the package to provide a higher number of connections in a smaller area. In addition to this, the BGA’s design makes it easier for it to dissipate heat. BGA packages deliver superior electrical performance and are compact, lightweight, and cost-effective.  

Quad Flat No-Lead (QFN) Packages

Because of its brilliant electrical performance, cost-effectiveness, and miniature profile, the Quad Flat No-Lead (QFN) has become one of the most widespread SMT packages. The primary purpose of the QFN package is to facilitate electrical and physical connections between integrated circuits (ICs) and SMT-based PCB assemblies. 

Low Profile Qfn Case Electronics

Many consider QFNs to be a type of chip scale package (CSP). This is because they’re lead frame-based, feature exposed pads, and have a similar size and form factor to other CSPs.

QFNs can have one of two pin configuration types:

• Single-row: Features a single row of pins/leads underneath the package. These pins are located near the edges of each side of the QFN. Ideal for assemblies where the conservation of space is important. 
• Dual/Multi-row: Instead of a single row of pins on each side of the package, this configuration features multiple rows of pins. Due to a higher pin count, they can form denser and more reliable connections. 

Regardless of the configuration, by using QFN packages, your PCB assembly can stand to benefit from their compact size and lightweight. These attributes make them ideal for compact applications such as those required for Internet-of-Things (IoT) devices. 

Their short pins allow them to reduce inductance and noise. This, in turn, makes it easier for your PCBA to maintain pristine digital signal integrity. QFN packages tend to feature exposed thermal pads underneath their encasements, making thermal management easier.

Hybrid and Specialized Components

In addition to QFN and BGA, manufacturers can use hybrid and specialized packages like:

• System-in-package: A combination of multiple ICs and passive components integrated into a single hybrid package. Because of its ability to help reduce board size, you can find these packages in compact devices such as smartphones.
• Multi-Chip Modules (MCM): A hybrid package containing multiple ICs. Each IC has a separate function or purpose. MCM modules offer a reduction in interconnect relays and enhanced performance. We use them in high-level applications such as aerospace, advanced computing, and telecommunications. 
• Leadless Chip Carrier (LCC): Instead of conventional leads, these packages have solder bumps underneath their encasements. This enables them to form high-density connections. They offer excellent heat management and electrical performance.
• Chip-on-board (COB): Consists of an exposed die that manufacturers mount directly on the PCB and wire-bond. This package is space-saving, cost-effective, and has exceptional thermal performance. We use it in LED lighting and displays, as well as sensors.   
• Wafer-level Chip Scale Package (WLCSP): The manufacturer packages the die at the wafer level. This results in a package that is almost the same size as the die. It offers the smallest footprint of any package and excellent electric performance. 
• Flip-Chip Package: You can find this package type in graphics processing units and high-speed communication devices. Manufacturers create this package by flipping and bonding the die onto the substrate using a set of solder droplets (bumps). 

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What Are Testing and Quality Assurance in PCB Assembly?

It’s important to ensure that your PCBs work accordingly. Your PCB’s failure can come at a high cost. Testing helps manufacturers and electronics makers prevent failures and recalls, which can reduce costs, save time, and enhance reputation. The aim of testing PCBAs is to ensure quality and reliability. We can categorize testing methods as either common or advanced.

Common Testing Methods

The three most common types of testing methods are: 

• Visual Inspection: An operator or worker manually inspects the PCBA for any visible defects. Manufacturers typically use this method as a part of a multi-stage testing process. Operators check for flaws in the solder joints, damage to the PCB, and component misalignment.
• In-Circuit Testing (ICT): Allows manufacturers to isolate and test each component of the PCBA individually. ICTs can help determine if there are any defects in the connections, helping manufacturers find shorts and opens. ICT uses automated equipment, making it highly reliable, fast, and precise.
• Functional Circuit Testing (FCT): Manufacturers test how the board will operate in a real-world setting. They run practical simulations and place the board under stress to determine how well it will cope and function. It’s essentially the final guard against PCBA failures. 

Advanced Testing Techniques

• X-Ray Inspection (AXI): Utilizes X-rays to generate a transparent image revealing the topography and structure of the PCB. It can help identify any hidden defects that escape other tests. X-ray inspection is particularly useful in finding connection and solder-based faults in multi-layer PCBs.
• Boundary Scan/ Joint Test Action Group (JTAG) Testing: Verifies and checks the integrity of PCB and IC connections. The manufacturer inputs a boundary scan cell using the IC’s pins. The boundary scan cell can record the state of the pin or be used to drive signals through the IC. Boundary testing grants manufacturers and programmers access to the PCB/IC’s internal nodes. This allows them to observe and access the various signal points and paths in the device. 

What Are Cost Structure and Optimization in PCB Assembly?

Some factors that influence cost include: 

• Labor costs: Your workforce and the amount of money you pay them will affect the final costs of the PCB.
• Turnaround time: How fast you want your PCBs assembled and delivered will also impact their prices. The manufacturing firm may prioritize your orders first, which will increase costs. 
• Quantity: The cost of manufacturing and assembly is directly proportional to the number of required units. The larger the production, the greater the costs.
• Technology: Can either increase or decrease costs. Some equipment and machinery are more cost-efficient than others. The more complex and advanced the equipment is, the more likely it is to be expensive to operate. Nevertheless, the more advanced it is, the better the results it will likely produce. 
• Material selection: Cost also hinges on the type of materials and components your design requires. For instance, fire-resistant and high-performance materials tend to cost more. Moreover, it’s not just about the material or components; it’s about how much of them your board needs. 
• Insertion Technology: The scale of your production should help determine which component insertion technology is best for it. For instance, SMT is best for high-volume assemblies but not as cost-effective for small-scale ones. The inverse is true for THT. 

Strategies for Cost Reduction

If you want to reduce the costs of PCB assembly, there are several steps you must take. First, you must ensure you deliver complete Bills of Material to your manufacturer.

Secondly, ensure that you take advantage of your manufacturers’ sourcing services. Thirdly, don’t skimp on matters to do with inspection.

Fourth and last, ensure that you optimize your bare board schematics. 

What Are Common Problems in PCB Assembly and How to Solve Them?

Because of its complexity, a lot can go wrong during PCB assembly. Some issues include:

• Solder Joint Defects: One of the most common problems, especially in SMT productions. This includes solder bridging, cold solder joints, and solder bridging. Assemblies must ensure that they do not apply too much or too little solder. AOI can help assemblies detect solder bridging in the early stages. In addition to this, manufacturers must ensure that they apply enough heat during the reflow process and that the pads are wetted correctly. 
• Incorrect Component Placement: Sometimes, components can be placed on the wrong pads on the board. This can be due to human error or issues with the pick-and-place machine’s programming. To avoid this, manufacturers must instate comprehensive quality control checks. This includes peer-reviewed visual inspections and AOI. Manufacturers must also ensure that they’ve marked and labeled all components on the PCB. 
• Component Shifting: Components may unintentionally move during the reflow process. A suboptimal reflow profile is usually the culprit in such cases. To prevent this, manufacturers must use the correct reflow profile and ensure that the PCB is stable during the soldering process. They should also verify the placement accuracy using AOI. 
• Delamination: The PCB’s substrate may unravel due to exposure to heat and moisture. Manufacturers can prevent this by preheating/baking the PCB. This will help remove or limit the moisture. It’s also to carefully manage the reflow temperature and use high-quality materials that won’t easily fray during the soldering process.
• Electrostatic Discharge (ESD) Damage: This can occur during handling or assembly. Small sensitive components can be especially susceptible. Operators should use anti-static equipment like anti-static shoes, bracelets, gloves, and/or mats. Workstations should be grounded properly. 

Operator Wearing ESD Protection Bracelet

What Are Future Trends in PCB Assembly?

As we mentioned earlier, insertion technologies such as SMT aim to help automate and optimize the PCB assembly process.

While it has indeed made the process faster and reduced errors, it isn’t perfect.

Artificial Intelligence (AI) and the Industrial Internet of Things (IIoT) may allow us to take PCB assembly to the next level.

These technologies may allow supervisors to remotely view and manage multiple assemblies.

It may also reduce the human workforce, making the assembly cheaper and less dangerous.

AI can determine which are the best suppliers to source components from.

AI-powered X-ray inspection can potentially find flaws and defects without human intervention and supervision.

Through these advancements, manufacturers can create fully automatic human-free PCB assembly chains. 

These technologies may also enable the industry to develop more sustainable manufacturing and assembly techniques and practices.

For instance, the soldering and reflow processes take a huge toll on the environment.

This is because they use a tremendous amount of energy and generate lots of heat. AI can help us implement better temperature control and optimize the soldering process at large. 

Conclusion

PCB assembly is a complicated process with many moving parts. Executing it properly is crucial to ensuring the durability and functionality of your PCB.

The above guide provides you with a comprehensive dive into the world of PCB assembly. It covers the various stages of PCBA and how it differs from manufacturing.

It covers the various components and packages and how to deal with challenges that may present themselves.

By following this guide, you should be able to assess the quality of a PCB assembler.

As with most industries that deal with technology, PCB manufacturing and assembly are continuously evolving.

Thus, it’s important to ensure that you’re up to date with the latest industry advancements.

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