Why Use Through-Hole Technology in PCB Design?

March 28, 2017 Alexsander Tamari

Close up of Through-Hole Components, very tiny protrusions

When it comes to technology, we never look back. Sometimes, it seems that the old technology just won't phase out. One example can be seen in through-hole technology; this class of legacy components seems to hang around to this day, even as new designs require ever smaller components.

But is it really that simple? Why use through-hole mounting technology in printed circuit boards (PCBs) when it seems surface mount technology (SMT) is a better choice? Like most design decisions, there are tradeoffs involved using each type of component. To start, let’s give a quick breakdown of through-hole mounting technology and surface mount technology as it pertains to the PCB design process.

 

Through-Hole Components

Through-hole components come in one of two types of packages: radial and axial. Axial through-hole components have electrical leads that run along the axis of symmetry of the component. Think about a basic resistor; the electrical leads run along the cylindrical axis of  the resistor. Many diodes, inductors, and capacitors are mounted in this way.

Meanwhile, radial components have electrical leads that protrude in parallel from the same surface of the component. Many large electrolytic are packaged in this way, allowing them to be mounted to a board while taking up a smaller amount of space.Illustration showing the component side of the PCB design

Side view of an axial through-hole component.

Through-hole components came about at a time when designers were more concerned with making electronic systems mechanically stable and were less concerned about aesthetics and signal integrity. There was less of a focus on reducing space taken up by components, and signal integrity problems were not a concern. Later, as power consumption, signal integrity, and board space requirements began to take center stage, designers needed to use components that provide the same electrical functionality in a smaller package. This is where surface-mount components come in.

Axial and radial through-hole mounting technology.

Radial (left) and axial (right) electrolytic capacitors.

 

Surface Mount Technology Components

If you take a look at any modern PCB design, you’ll likely see boards that are dominated by surface mount components. Newer designs still use through-hole components, but these components tend to be used more often in power electronics and other devices that generate a lot of heat. Surface mount technology is the most commonly used component package technology today. These types of components do not use pins for electrical leads. Instead, the leads appear as small pads of metal on the same side of the component. The primary purpose of these pads is to allow soldering directly onto the surface of a PCB during assembly.

Components for surface mounting on PCB board

Surface Mount Devices

The use of pads in surface mount technology compared to through hole technology provides certain advantages, which will be discussed below. In addition, the smaller pad size and overall component size causes these components to have less prominent parasitics. This allows them to be operated at higher speeds/frequencies before you start to notice signal integrity problems.

 

Through-hole vs. Surface Mount Technology Costs

Surface mount components tend to be smaller than an equivalent through-hole component. However, this does not necessarily mean the cost of a surface mount component is always cheaper simply because less raw materials are used in manufacturing these components. Surface mount components themselves might cost a similar price as an equivalent through-hole component. However, once automated assembly costs per component are considered, the total cost per surface mount component tends to be cheaper than a through-hole component with the same component values, power/voltage ratings, and tolerances.

This difference arises because placing through-hole components requires drilling holes in your PCB, which incurs tooling costs. In contrast, drilling is not required with surface mount components, which accounts for the cost difference. All this begs the question: if surface mount components are smaller, faster, and cheaper, then why use through-hole technology at all? The answer depends on the use case for your PCB design. Yes, through-hole PCB technology is old, big, and expensive, but there are some advantages.

 

PCB Through Hole Technology Pros and Cons

PRO

CON

Easier prototyping

Higher board cost due to drilling

Strong physical connections                           

Takes up more board real-estate

Heat tolerance

PCB assembly process is more involved        

Power handling capability

Slower speeds

 

Surface Mount Technology: Advantages and Disadvantages

PRO

CON

Small size → Denser boards

Weaker physical connections to the PCB

Reduced parasitics → reliable at higher speeds

Lower heat tolerance

Faster & cheaper assembly

Lower power handling capability

No drilling → Cheaper board fabrication        

DFM: tombstone, pop cornering, etc

When comparing the two PCB design technologies, it's easy to see why surface mount is the reigning champ. Surface mount components are smaller, cheaper, and can be run at higher speeds. This is especially important in upcoming mixed signal and analog-heavy applications like IoT devices, new wireless devices, and wearables. As network speeds increase and new devices run at higher data rates, surface mount components will continue to make an appearance as they generally cannot be substituted for through-hole components.

Technology leaders are driving towards a connected society, and size does matter when it comes to PCB design. In the drive for ubiquitous computing, IoT, or the “ambient intelligence” we all crave, the drive to make smaller and smaller components includes the board itself. Smaller components enable smaller boards, allowing us to build printed circuit boards in almost any form factor. Smaller sizes mean lower manufacturing costs. Less expensive components and boards provide cost savings to the end customer.

Through-hole mounting technology is great for prototyping and testing as you’re able to easily swap out components on a printed circuit board. Even before you design your board, you can breadboard your design with through-hole technology.

PCB board prototype or breadboard using through-hole

Breadboard with through-hole components

In addition to prototyping and testing, through-hole components have very strong physical bonds to the board as they are soldered from both the top and bottom of the board. This makes boards with through-hole mounting technology very durable, which is partly why they are used in military and aerospace. They also have a high temperature tolerance. You can find through-hole technology in all sorts of places. One example is on LED lights in billboards or stadiums. Through-hole LEDs are extremely bright and durable, allowing them to handle the outside elements.

LED billboards powered by PCBs using through-hole technology

Through-hole LEDs in outdoor signs

Also, if you look at industrial machines and equipment, you can find many boards that are built almost exclusively using through-hole components. Again, this is due to the harsh operating conditions, such as extremes of temperature or situations involving high power consumption. Through-hole technology may be old and seem outdated, but it has a purpose and can be used for its physical endurance and strength in today’s connected world.

Through-hole components close up

Through-hole components in a power supply

 

Curious to learn more? Visit our popular PCB design guidelines blog and learn from our experts. 

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Hole Tolerance Definitions

About the Author

Alexsander Tamari

Alexsander joined Altium as a Technical Marketing Engineer and brings years of engineering expertise to the team. His passion for electronics design combined with his practical business experience provides a unique perspective to the marketing team at Altium. Alexsander graduated from one of the top 20 universities in the world at UCSD where he earned a Bachelor’s degree in Electrical Engineering.

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