What is THT Assembly?

1. Overview of PCB Assembly and Component Types

A component that is mounted on a printed circuit board (PCB) is part of the board assembly. The finished board with all parts is called a printed circuit board assembly, or PCBA. There are two main kinds of components. SMD parts are the most common today. THT parts are the traditional kind.

2. Definition of THT

THT stands for Through-Hole Technology. The name says it: these parts are mounted through holes in the PCB. The holes let the part leads pass from one side of the board to the other. The holes are usually made conductive by plating. The plated metal is often zinc, copper, or silver. This metal creates an electrical and mechanical bridge between the two sides of the board. The part leads can be soldered so the joint is strong and does not corrode.

3. THT Assembly Methods

Some parts can be soldered into holes by hand. Still, the usual way is to use an automatic insertion machine. The machine pushes the part leads through the hole and clinches the leads like small rivets. The result looks neat and consistent.

4. Key Feature of THT Parts

One feature of many through-hole parts is that they are fragile. They can be sensitive to heat. If a THT part gets too hot during soldering, the connection can fail. A failure may happen because nearby traces or solder joints lose contact. If that happens, the board may stop working or the part may not be fixed tight enough to hold.

5. Advantages of THT Parts

Even though SMD parts have replaced THT in many uses, through-hole parts still have advantages. Here are the main benefits:

 

  • Strong mechanical hold. When a part must resist a lot of mechanical force, through-hole mounting is stronger. Examples include terminal blocks and heavy connectors.
  • Good for high current. If the board must carry large currents, through-hole parts often handle heat and current better.
  • Mixed use is common. Most complex PCBs today use both SMD and THT parts together.
  • Robust mechanical performance. Through-hole parts give excellent grip to the board.
  • Works well for high power. For high-power parts, THT can be more reliable.
  • Lower first-time automation cost. For small runs, the initial cost to set up through-hole assembly can be lower than a full SMD line.
  • Better for large parts. If the component is large, through-hole mounting is often the better choice.
  • Better when high mounting strength is needed. For parts that must stay firmly fixed under stress, through-hole is preferred.

6. Differences Between SMT and Through-Hole Technology

Terms like SMD, THD, THT, SMT, THM, SOIC, QFN can confuse new electronic hobbyists. But the ideas are simpler than they look.

6.1 Basic Term Distinction

  • SM means surface mount. TH means through-hole. These two terms point to two ways to mount components on a PCB.
  • A letter after SM or TH can stand for different words. For example, T, D, M, C, or A can mean technology, device, mount, component, or assembly. People use them loosely.
  • Example: SMD parts are made for SMT processes. A board that uses both SMD and through-hole parts is called a mixed-technology board.

6.2 Historical Background & Structural Differences

  • At first, all parts were through-hole. Through-hole parts have metal leads. The leads go through plated holes in the board. Then the leads are soldered to pads on the opposite side. Drilling and plated through holes take up valuable space on the PCB surface. This is more visible on multi-layer boards. Drilling uses up space through all layers.
  • As space became more limited, surface-mount technology grew. SMD parts helped make smaller and more portable devices. A surface-mount part may or may not have leads. The main point is that these parts are soldered on the same side where the part body sits. So both sides of the board can be used for parts. No plated through-hole is needed for each component.

6.3 Space-Saving Design: Vias

To connect traces in inner layers, designers can use vias. A via is like a plated through hole but much smaller. Vias can be sized to link only certain layers. They take less space than a full plated through hole. Using vias saves PCB area. This helps SMD parts fit in the same footprint more easily.

6.4 Component Size & Assembly Requirements

  • SMD parts can be much smaller than through-hole parts. Leads can be removed and the contact area can be the end of the part body. This allows very tiny parts. Many components now come in SMD packages. Resistors, capacitors, inductors, and LEDs all come as SMDs. Some SMD parts are as small as a grain of sand, like the 0201 or 0603 package sizes.
  • Smaller packages save space. They let designers pack more function into a small board. But small size can hurt reliability. Soldering tiny parts needs high accuracy. SMD assembly lines use advanced machines. These machines include pick-and-place robots, reflow ovens, and custom stencils. The machines cost a lot. SMD assembly also needs careful process control.

6.5 Defect Risk & Durability

  • Because of those needs, SMD assembly lines can show many defects if not run well. The defects may be hard to fix by hand. The finished board can be delicate. Makers must handle it carefully.
  • By contrast, boards with through-hole parts often have larger solder joints that cover a wider area. These large joints make the board tougher. This toughness is useful for military and industrial uses. It helps the board withstand strong shocks and vibration.

6.6 Cost Comparison

  • Cost is another important factor. SMD components often cost less than through-hole parts. But SMT assembly can be more expensive to set up. It needs big, precise machines.
  • Through-hole assembly can be done with a soldering iron and some manual tools for small jobs. Still, hand-soldering many small SMD parts can be annoying and slow.
  • For mass production, SMT is faster and cheaper per unit. For small runs or prototypes, keeping some THT assembly for hand work can make sense.

6.7 Modern Application Status

SMT’s main benefit is high component density and smaller product volume. The drive for smaller and tighter devices pushed package sizes to the limit. In this modern age, THT can seem out of date. Still, predictions of its death were premature. Through-hole technology and components still have value. For many uses, both through-hole and surface-mount assembly services are available and affordable.

7. How to Choose the Right PCB Assembly Process

Picking the right PCB assembly process is important. The choice affects manufacturing efficiency, cost, and the final product’s performance.

 

Two main methods are used for PCB assembly: surface-mount technology and through-hole assembly. Surface-mount is the most used method. Through-hole is less common now. Still, through-hole remains popular in some industries.

 

Your choice depends on many factors. Below is a short guide to help you choose.

7.1 PCB Assembly: Surface-Mount Technology (SMT)

Surface-mount technology is the most common PCB assembly method. It is used in many electronics. Examples include USB flash drives, smartphones, medical devices, and portable navigation systems.

 

SMT advantages for selection:

 

  • SMT lets you make products smaller. If board space is tight, SMT is the best choice. This is true for designs with many resistors, diodes, and small passive parts.
  • SMT allows high automation. This leads to faster board assembly. High automation makes large volume production cost-effective.
  • SMT is flexible and customizable. If you have special needs, SMT can match them well. It can support custom PCB designs.
  • SMT allows parts to be placed on both sides of the PCB. Double-sided assembly lets you make more complex circuits without increasing board area.

7.2 PCB Assembly: Through-Hole Manufacturing (THT)

Even though through-hole use is falling, it is still common.

 

THT advantages for selection:

 

  • Through-hole parts fit large components. Examples include transformers, power semiconductors, and electrolytic capacitors. Through-hole gives a stronger mechanical bond between the part and the board.
  • Because of this, through-hole assembly offers higher durability and reliability. This extra strength makes the method a good choice for aerospace and military sectors.
  • If your product must resist high mechanical or environmental stress in use, through-hole is often the better choice.
  • If the product must run at high speed and with high reliability under stress, through-hole may be suitable.
  • If the product must work at very high or very low temperatures, through-hole’s strength and durability may be better.
  • If the product must work under high pressure and keep performance, through-hole might be right.

7.3 Hybrid Boards and Mixed Technology

Because devices keep getting smaller and more complex, many applications need both assembly types. This combination is called mixed technology.

 

A mixed board uses SMD parts where miniaturization and density matter. It uses THT for parts that need stronger mechanical support or current handling. Many modern products use both on the same board.

7.4 Key Factors to Consider When Deciding

This guide gives a quick view to help you choose the right assembly method. Still, other factors can make the decision more complex.

 

You should consider:

 

  • The size and shape of your parts.
  • The current they must carry.
  • The mechanical stress the product will face.
  • Temperature and environmental limits.
  • Production volume and cost targets.
  • Repair and rework needs.
  • Time to market and prototyping needs.

 

For small prototypes or repairs, manual through-hole soldering may be fine. For mass production, SMT usually gives the best cost per unit and the smallest form factor.

8. Common Practical Tips

  • Use THT for heavy connectors and large power parts.
  • Use SMT for dense circuits, RF parts, and tiny passives.
  • Use vias to connect layers and save board space.
  • If a part is available only in through-hole, plan a mixed board or use an adapter.
  • For prototypes, choose the method that lets you test quickly. For large runs, focus on assembly cost and yield.
  • When reliability is critical, lean to through-hole parts for mechanical strength. For size and weight limits, use SMT.

Summary

Both through-hole and surface-mount technologies have strengths. Choose the one that matches your design goals and production needs. If your product needs both strength and compactness, plan a mixed-technology board.

 

Through-hole PCB assembly and SMT assembly services are widely available. Prices are reasonable. If you need help selecting the correct process, list your product needs and limits. Then compare the options by cost, reliability, and manufacturability.
 
 
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