Printed circuit boards, or PCBs, are at the heart of nearly all of today’s electronics, and they support and electrically connect the electronic components that are needed to make an electronic device function.
Manufacturing a printed circuit board is an involved process with many steps that vary from project to project. First, you need to design the board according to the needs of your project. Then, you need to manufacture the PCB according to the design by following a complex process that deposits copper onto the board and combines its various layers.
Once you have built the PCB, your work is far from done. You still need to assemble the board by affixing electronic components to it. Once you do, you have a completed printed circuit board assembly or PCBA. Sometimes, this assembled board is simply called a PCB. There are two leading methods for assembling a board — surface mount technology vs. through-hole technology.
Overview of the PCB Assembly Process
Once a PCB assembler has a bare board and a design to work with, they need to complete several steps to prepare for the actual assembly, the process by which they attach components to the bare board. The design that the client provides the assembler will tell the assembly professional where to place the components and which method, through-hole vs. surface mount, to use to do so. They may also use design notes and other information regarding specific requirements to guide themselves through the assembly process.
Before beginning assembly, many assemblers will check design files for any potential problems that could impact the manufacturability and functionality of the PCB. This process is called a design-for-manufacturability check or DFM. During this check, the assembler will look for missing or redundant features, problems with the spacing of components and other issues. Performing this check before beginning assembly is essential because the earlier you catch problems, the less they’ll increase costs and the time it takes to complete a project. It also prevents potential faults in the final product.
Once these checks are complete, the actual assembly process can begin. Exactly what this process looks like depends on surface mount vs. through-hole mounting technology is being used. In the upcoming sections, we’ll look more in detail at the steps of these two assembly methods. The assembly could involve just one of these methods or a combination of both.
Following the completion of assembly, the PCBA will undergo more checks. These quality control inspections aim to uncover any mistakes or problems within the PCBA. The thoroughness of the tests depends on the desires of the client regarding completion time, the details of the project and other factors. At EMSG, we visually inspect all of the PCBAs we produce under magnification. We can also optically inspect PCBAs using automated optical inspection (AOI) equipment, which can detect component-level and lead-level defects. The assembler may also provide in-circuit testing, which checks the functionality of an assembly.
These kinds of quality control inspections are important because components that fail after assembly can result in increased expenses and time-to-market. Defects that make it through into a final product can also cause financial losses, damaged reputation and even safety hazards. Quality control checks can prevent many of these undesirable outcomes.
After the inspection phase, PCBAs typically undergo a series of cleaning processes to remove dirt, oil and leftover flux residue. The assembly company may use deionized water applied with a high-pressure washing tool. Deionized water is used because the ions in water can cause damage to electronic circuits.
What Is Through-Hole Technology & The Thru-Hole Mounting Process?
One of the two most common methods for attaching electronic components to PCBs is the thru-hole, or through-hole, process. This technique is older than SMT, and for many years, it was the standard technology used for PCB assembly. When surface-mount technology began gaining popularity in the 1980s, many people thought it would make thru-hole PCB assembly obsolete. However, thru-hole technology has several advantages that have caused it to remain the preferred option for some applications.
The thru-hole manufacturing process, as its name suggests, involves drilling holes into the PCB. The board house drills these holes where the components will go according to the client’s design. Once the holes have been drilled, the leads are placed through them. It’s crucial that the leads are placed in the holes consistently and properly. The assembler needs to ensure that they have the correct polarity and orientation when manufacturing a through hole circuit board.
Next, the assembler must inspect the components and make any necessary adjustments.
Finally, the assembler will solder the leads securely in place, perhaps using a wave soldering in which the PCB passes slowly over liquid solder at a high temperature.
Originally, workers completed all of these steps manually. Today, automated insertion mount machines and other equipment can assist with many parts of the process. There are two main types of thru-hole components — axial lead and radial lead. Axial lead components have leads on both ends. They come out of the part in a straight line. Radial lead components have both leads on one side. The leads run through the entire board, so they can connect the various layers of the PCB.
Surface mounting technology, originally called planar mounting, began to become popular in the 1980s, and today, it’s used for the majority of electronic components. Surface mounted components, as their name suggests, are mounted to the surface of the PCB rather than inserted through holes in the board as with thru-hole assembly. In the surface mount assembly process, the assembler solders the components onto pads on the board.
Originally, much of the surface mounting process was done manually, but today, it’s heavily automated thanks to advances in modern technology. In the first step of the SMT technology process, the assembler applies a solder paste to the parts of the board where components will be mounted. A template called a stencil or solder screen helps to ensure that the paste is applied in the correct locations.
After completing solder paste printing, the board typically undergoes inspection to check for defects in the solder paste. If the inspections uncover any issues, the assembler will either rework them or remove the solder paste and apply it again. These steps are important because the quality of the solder paste printing correlates with the quality of the soldering that occurs later in the assembly process.
Next in the SMT technology process, the assembly professional will place the components on the board according to the client’s design. Originally, workers did this work manually with a pair of tweezers. Today, the process is largely automated and uses sophisticated pick-and-place machines. Then, the components will be soldered to pads on the board to secure them in place. This phase typically employs the technique of reflow soldering in which the board is sent into a furnace where high temperatures cause the solder paste to liquefy.
Because surface mounted components don’t use holes to connect the different layers of a board, they need a different method for doing so. That method is the use of vias, which are small holes that connect the various layers of the board. Unlike with thru-hole technology, however, no leads are directly attached to these holes. There are several different types of vias including through vias, blind vias and buried vias.
You need to use specific surface mount components for the surface mount technology processes. These components, called surface mount devices, function the same as those used for thru-hole technology but have different designs which is what makes surface mount assembly unique.
Learn more below about the specific surface mount technology advantages and disadvantages.
Thru-Hole Mounting Advantages
Surface mount technology has some advantages over thru-hole technology, namely cost, that contribute to its increasing popularity. However, thru-hole mounted components have some distinct features that help them remain the technology of choice for some industries and applications.
The most commonly cited example of thru-hole mounting process benefits is the fact that thru-hole mounted components offer enhanced reliability. Because the leads go all the way through the board in addition to being soldered in place, the bonds are stronger. As a result, the products they’re used in are more resistant to environmental stresses such shock and impacts.
Some types of components are also not yet available in a surface-mount variation, making thru-hole a necessity in some cases.
Thru-Hole Mounting Disadvantages
There are also some downsides to using the thru-hole process. Thru-hole components are larger than surface mount devices and take up more space, which increases costs because the boards need to be larger. This difference makes thru-hole technology unsuitable for high-density boards.
Surface Mounting Technology Advantages
Using surface mount technology is often more cost-effective than using thru-hole technology, which is the primary reason it’s so popular in electronics manufacturing today. As more surface mount devices are developed, the technology can be used for more types of products.
Part of the reason that the surface mount process can reduce costs is that surface mount devices are smaller and take up less space than thru-hole components. One of the big advantages of surface mount technology is that the method can be as much as one-third of the size and one-tenth of the weight of boards assembled using the thru-hole technique.
Another one of the advantages of surface mount technology is that it allows for higher connection densities, meaning they can have more electrical connections than other types of PCBs. You can also mount components on either side of the board directly opposite each other, allowing you to utilize more of the board space.
It’s also typically faster and easier to assemble surface mount PCBs than thru-hole PCBs. One of the biggest surface mount technology advantages is more of the process can be automated, which further reduces costs.
Surface Mounting Technology Disadvantages
Components that are mounted to the surface of a PCB are not as securely attached as leads that go all the way through the board. As a result, one of the disadvantages of surface mount technology is that components are not ideal for boards that will be exposed to high environmental stress or for conditions in which reliability is especially critical.
Thru-hole technology is ideal for products that require high reliability even under high levels of environmental and mechanical stress as well as high voltage, power and heat conditions. Large units are also more likely to use thru-hole technology because reducing costs through smaller components is less of a concern. Additionally, the thru-hole technique is useful for testing and prototyping because you can relatively easily adjust and replace leads manually, enabling you to try various layouts.
Transformers, semiconductors, electrolytic capacitors and plug connectors are likely to use thru-hole technology. The military, aerospace and industrial equipment sectors often use it because of its reliability and resistance to harsh conditions. Light-emitting diodes, or LED lights, used in outdoor applications such as billboards and displays in stadiums may also use thru-hole technology because of its ability to withstand outdoor conditions.
Today, the majority of electronic hardware employs surface mount technology. As the process gets increasingly cost-effective as compared to thru-hole technology, it’s becoming even more dominant in the electronics world. The trend toward making smaller electronic devices is also making surface mount components more common.
Smartphones, tablets, laptops, Internet of Things (IoT) devices and other electronic products all typically utilize the surface mounting process. It may be used in the manufacture of telecom and communications equipment, medical devices, traffic and transportation components, lighting, industrial and commercial hardware and more.
Which Process is Right for My Project
How do you know which process is right for attaching your components to your PCB? To determine the answer, you’ll need to consider:
- The type of project, whether prototyping or manufacturing
- For what purpose and under what conditions the unit will be used
- The type of product the PCB is for
- Which sector will use the product
- Cost concerns
- Reliability needs
- Size requirements
- Connection density
- Speed-to-market requirements
After considering all of these factors, you may decide to go with thru-hole technology, surface mount devices or a combination of both options depending on your precise needs. PCBs that use a mix of both techniques may have:
- A mix of both technologies on one side of the board
- Thru-hole technology on one side and surface mount on the other
- Thru-hole technology on one side and surface mount technology on both sides
- Other setups, which may be possible but may also be less cost-effective depending on the details of the project
EMSG can help you to determine which method, through hole vs. surface mount, is best for your needs. We can provide a thorough consultation, and we have the experience and knowledge required to help you make an informed decision. We also offer world-class assembly services for small-batch orders of 50,000 components or less as well as comprehensive testing, inspection and quality control services. We’re registered with the U.S. Food and Drug Administration (FDA), are ISO 9001:2015 certified and have high-quality equipment and facilities.
To learn more about our services, visit our surface mount technology and thru-hole assembly pages as well as the other informative pages on our website. You can also get in touch with us by filling out a contact form or calling us at 717-764-0002.