Surface Mount vs Through-Hole: Pros & Cons

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Surface Mount Technology (SMT) and Through-Hole Technology (THT) are the foundational methods for assembling components on printed circuit boards (PCBs). SMT has gained popularity due to its reliability and cost-effectiveness, whereas THT continues to be relevant for specific applications owing to its unique strengths.

Pros and Cons Side by Side

Here are the major advantages and disadvantages of Surface Mount and Through-Hole Technology.

Pros

• SMT supports smaller, lighter, and faster devices due to space-saving attributes.
• Cost efficiencies in SMT lead to lower unit prices.
• Higher manufacturing capacity is achieved with Surface Mount technology.
• Automation simplifies the assembly process in SMT.
• Through-Hole Technology offers excellent environmental resistance.
• THT forms stronger mechanical bonds between components.

Cons

• Implementing SMT necessitates advanced design, production skills, and technology.
• Conducting visual inspections with SMT can be challenging.
• THT requires multiple holes to be drilled on the board.
• Production cycles for THT tend to be longer compared to SMT.

Types of PCB Assemblers

Diverse methods exist for assembling printed circuit boards, each suited for different board complexities and specific construction needs. The main types include:

• Surface Mount Assembly: It begins with loading the components into the pick-and-place machine's feeders, followed by programming the desired functionalities.
• Plated Through Hole Technology: This involves drilling holes through the board, often followed by in-circuit testing.
• Electro-Mechanical Assembly: Involves using various electromechanical equipment to assemble electronics onto PCBs.

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What Is Surface Mount Technology (SMT)?

Surface Mount Technology involves directly mounting electrical components onto the surface of a printed circuit board. Since its rise in popularity in the 1980s, SMT has become integral to the production of nearly all electronics.

With SMT, no holes need to be drilled; components are soldered directly onto the board, allowing for more components to fit on both sides of the board. This results in denser, more powerful, and compact PCBs.

Advantages & Disadvantages of SMT

Advantages of SMT:

• Improved manufacturing efficiency
• Greater design flexibility
• Enhanced performance of electronic components
• More compact designs for electronic devices
• Electromagnetic Compatibility (EMC) compliant designs
• Automatic correction of placement
• Reduced overall cost
• Consistent performance under vibrational conditions
• Minimized waste
• Faster lead times

Disadvantages of SMT:

• Issues arise under extreme environmental conditions
• Poor resistance to thermal stress
• Not suitable for small circuit testing
• Challenging to inspect
• Components are more prone to damage
• Less power handling capacity
• More expensive in small batch production

Components on SMT-assembled PCBs perform well under vibrational conditions. The elimination of the need for drilled holes significantly reduces costs, waste, and lead times. However, SMT can face issues under extreme mechanical, environmental, or thermal stress. Combining SMT with THT can exploit the advantages of both technologies.

What is Through Hole Technology (THT)?

Through-Hole Technology involves inserting component leads into drilled holes on a bare PCB. Although SMT emerged as the more efficient and cost-effective method in the 1980s, THT remains valuable for its specific advantages, particularly in durability.

Annular rings, which provide strong mechanical connections, make THT highly suitable for applications requiring durability.

Differences Between Axial and Radial Leads

Axial and radial leads are two types of THT components. Axial leads have wires attached on both ends of the component, while radial leads have both wires on one side. Radial leads are preferred for densely packed boards due to their space-saving design.

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Advantages & Disadvantages of THT

THT offers superior mechanical connections, making it ideal for components subjected to mechanical stress, such as connectors and transformers. Its wide placement of holes makes manual soldering easier and allows for easy interchangeability of components, perfect for prototypes and testing.

THT components are ideal for durable products that require strong interlayer connections. These connections ensure THT can withstand more environmental pressures than SMT components. Hence, THT is common in military and aerospace products exposed to extreme conditions.

However, THT requires drilling, which is time-consuming and expensive. It also limits the configuration area on multi-layered boards. Additionally, THT’s soldering methods are less efficient compared to SMT’s reflow ovens, requiring soldering on both sides of the board.

Cost Difference

SMT eliminates the need for manual drilling, resulting in lower costs for smaller PCBs. Automated pick-and-place systems and reflow ovens streamline the assembly process, offering cost savings over THT, which relies on manual soldering for high-density components.

THT is more expensive in small batch production due to the need for manual configuration and soldering. Consequently, SMT’s higher level of automation makes it more cost-effective, particularly for bulk productions.

Differences in Assembly

SMT assembly is largely automated, which significantly shortens assembly times compared to THT, which requires configuring auto-insertion machinery for various components. SMT thus allows for faster processing and shipping.

However, THT is necessary for specific designs that require robust or large components.

Conclusion – What Is the Best Method for You?

Choosing the right assembly method for your board depends on your specific requirements. While over 90% of today’s PCBs use SMT for its efficiency and affordability, THT remains essential for applications requiring unique mechanical, electrical, and temperature properties.

While not a definitive answer, every recent (last 20 years) military and space product I've worked on has has been built using SMT (Surface Mount Technology). There were some exceptions for individual components, but that was more because the part was only available in the through-hole package, and not for any reliability advantage.

Sometimes, particularly for larger packages, we have applied an adhesive under the package or an epoxy to the corners. But that was only done when the structural analysis showed it was needed.

None of these applications were for what I would call extreme acceleration environments, such as a cannon-launched shell (a smart munition). But they do have to survive launch and stage separation pyrotechnical shock, and many thousands of temperature swings.

Finally, many components are just not available in through hole packages. For example, show me a 1000+ pin FPGA in a through hole package. In those cases ways are found to make them work in the given environment.

Leaded parts are always preferred where large number of temp excursions are involved. The largest leaded part I used was an Actel (now Micro Semi) FPGA in either the 256 lead or 352 pin Ceramic Quad Flat Pack (CQFP) package, I forget which (was over 15 years ago). Even though it was SMT, because of the fine lead pitch, it could not be solder automatically and required a trained operator to hand solder it down.

Everything (COTS) I've seem above that IO count has been been in some sort of BGA or CGA package, which requires careful attention to board material (so as to minimize CTE mismatch between the package and the board) and staking and/or underfill of the package to the board.

You can get high IO count leaded packages, but they are usually custom designed and built MCMs (Multi Chip Modules).

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