Safety First! : Surface Mount vs. Through Hole – When to Use and Where

The printed circuit board (PCB) invention ushered in a radical change in electrical and electronic technology. This success has led to the continued improvement of many devices and smaller and even miniature profiles for others. It has also resulted in entirely new classes of products across almost every industry. And it isn’t easy to imagine a powered device or appliance without a PCB.

 As the technology has matured, two distinct methods for producing PCBs emerged. The first is surface mount technology (SMT). Because PCB components became smaller over time, SMT became the dominant technology by volume. In SMT, components are mounted directly onto the bare board surface. Instead of wires, tabs and connectors are used with solder to complete the board circuitry.

Through-hole technology (THT), holes are drilled in the bare board, and wires are run through the holes to connect the components. One side of the board will have “trace holes” to mark the path of the wiring. Wires are soldered to the leads of each component for completing the circuitry.

Types of SMT

There are two types of components used in the manufacture of SMT PCBs, active and passive. Passive components do not add power gains to the circuit because their function doesn’t require energy. Examples of passive SMT components are resistors, whose function is to slow current flow; capacitors that store and release energy at a programmed rate and time; and inductors that manage and transmit voltage where needed.

Active components consist of transistors, integrated circuits, and others that are the workhorses of a PCB. Active components are responsible for completing the task, such as transistors to amplify or switch current or integrated circuits acting as microprocessors or timers.  

Types of THT

Like SMT, THT has two different types as well, radial and axial. Radial THT components have two leads protruding from the same side of the unit. The leads can be placed through holes in the board so that the radial component sits perpendicular to the board. This allows more components to be used on a board, thus preserving board space. It helps in the manufacture of boards requiring high component density.

Axial leads have a lead on each end of the component. They are lower profile and allow small distances to be traversed to reach another component on the board. They are unsupported, and the leads go through board holes as well.

Advantages and Disadvantages of SMT

Advantages of SMT include:

• Size – SMT components are much smaller than THT. Some of them are microminiature. This means that they cost less and means that PCBs can be produced at a higher density than THT.  
• Manufacturing – Because there are no holes to be drilled, SMT can be manufactured faster. High-speed automated component placement combined with no drilling means lower production costs. 
• Stability – Because components are glued or soldered to the board, they can withstand higher levels of shaking and vibrating compared to THT. This makes them more reliable.
• Circuit Speed – One of the most significant advantages of SMT PCBs is faster circuit speeds. In an era of ever-increasing device speed, this makes them an ideal choice for many applications.
• Flexibility – SMT components can be placed on both sides of the bare board. This increases the number of available connections and means devices require fewer PCBs.
• Lower Interference – Since leads are very short or nonexistent, the circuit viability is improved, and RF resistance is lower.

Disadvantages of SMT include:

• Capital Cost – While cheaper in component price and when produced in volume consistently, the capital expense for SMT equipment is high. The need for accurate placement, automated soldering, and other issues related to high-speed production makes initial investment costly.
• Test and Inspection – The density of the board and the miniaturization of components make testing and inspecting boards more difficult. This often makes visual inspection impossible. As the density of a board rises, so does its complexity.  The result is the need for higher skillsets and training when testing is required.
• Fragile – The advantage of size and weight also leads to the disadvantage of being more delicate compared to THT. Boards can be broken in shipment or handling.

Advantages and Disadvantages of THT

Advantages of THT include:

• Reliability– Because leads must pass through holes, THT is more robust compared to SMT. The through-hole placement of leads forms stronger mechanical bonds than SMT. They can withstand higher levels of stress and harsher conditions of heat, vibration, shock, and adverse environmental conditions.
• Repairability – Because components use leads, in some cases, they can be replaced.
• Heavy-Duty Use – In addition to the benefits of strength, THT PCBs can withstand high acceleration and collision. They are much less fragile than SMT components and board.
• Higher-Power – Because THT components are larger, PCBs built with THT can handle high-power applications compared to SMT.

Disadvantages of THT include:

• Manufacturing – THT boards are more challenging to manufacture. One issue is the drilling required. The ability to utilize only one side of the board also drives up manufacturing complexity and cost.
• Automation – Because THT components must be placed by hand and soldered manually, automation is more complicated. This increases overall cost compared to SMT.

When and where to use SMT and THT

The answer to whether to use SMT or THT boards depends on the application.  This may be application-related or industry-related and will depend on the use case for each.

THT

1. Military Hardware – Because it is subjected to high rates of shock, vibration, speed, and other factors, military applications often require THT. The strength of the leads connected through the board makes it a much-desired choice for situations where board failure could cause catastrophic results.
2. High Power Applications – THT boards are larger than SMTs. This means they can be used in higher power operations. They are often a component of transformers that have both added heat and high power.
3. Aerospace – Like the military application, aerospace requires a high degree of both reliability and stress performance due to issues such as pressurization variations, whether in the form of wind, rain, and lightning, and other factors, aerospace use of THT PCBs.
4. Prototyping and Testing – As they are often manually made anyway, THT boards are ideal for testing and prototyping. The skillset to repair and adjust on board is lower, and because components are more prominent, repair and change of function are easily accomplished. Once the prototype is complete and the product enters production, it may be replaced with an SMT board for mass production.

SMT

SMT is the method with the highest volume in use worldwide. Applications to consider for SMT use include:

1. High Density Requirements – Today’s appliances, devices, and even factories are feature-rich with functionality and connectivity. SMTs provide more bang for the buck by delivering much more density compared to THT. High density allows more operations with fewer boards.
2. Compact Design – as components for SMY boards can be microminiature, boards can be much smaller. In a product where form and function are both required, small or compact products can still take advantage of the capabilities of an SMT.
3. Light Weight Applications – Like the compact design, many applications require lighter weight for carrying, load limits, etc. The smaller components and boards mean that weight is significantly reduced compared to THT.
4. Higher Circuit Speed – Listed as one of the significant advantages of SMTs, applications that require a high circuit speed should use an SMT. Personal devices and connected devices whose functionality depends on the speed of operation will benefit from SMY boards.
5. High Volume – THT’s may be suited for bespoke or specialty applications requiring adverse conditions. But for mass-produced goods, SMTs are more cost-effective when produced in high quantities. They provide a high level of automation and quality through that automation. 

Mixed Assembly

There are applications where users need to take advantage of the benefits of both technologies. In these cases, PCBs are manufactured using mixed assembly where the board is part SMT and part THT. Here, the SMT components are welded to the bare board first. Then, drill holes are made for THT components and then added and wave soldered to complete the board.  

Mixed board technology is used in a wide range of applications, including:

• Industrial Controller Assemblies – This allows the circuit speed of SMT for motion control within large control systems as well as the more rugged construction needed for harsh manufacturing.
• Processing Equipment for the Video Industry – Speed and lack of RF interference are the benefits of the SMT component, while THT allows high power use for video processing and compression.
• Servers – Again, SMT circuit speed is beneficial. However, many servers are often used together or in a “farm,” creating heat and high-power usage that requires THT.
• CPUs – This could include both consumer CPUs as well as CPUs in advanced automation and control systems. The near solid-state construction of THT combined with SMT speed and low power consumption make this an excellent mixed assembly use.

PCBs have been part of an electronics revolution that has changed the world as we know it. Which method to use will always depend on end-use, application, and required features. However, there is a solution between the two or by using mixed assembly to address any need.