In THT assembly, wired components are inserted through contact holes in the PCB and then connected to the PCB by a soldering process.
WHAT IS THT PCB ASSEMBLY?
THT (Through Hole Technology) – also called through-hole assembly – is another technology besides SMD assembly, where components are mounted on the top side of the board. Today, traditional THT assembly is largely replaced by SMD assembly.
However, there are still such components as flat cables (stranded wires), connectors, coils or ground capacitors where wired components cannot be omitted.
In contrast to SMD assembly, components are inserted into the PCB through contact holes and further processed by wave soldering (flood soldering), selective soldering, stirrup soldering or manual soldering, depending on the technical design. After through-hole assembly, these wired components are often still soldered by hand and tested manually.
Advantages of THT through-hole mounting
- When it comes to greater mechanical loads (e.g. for connectors or switches) or when larger currents flow, as in power electronics, wired components cannot always be replaced by SMD components.
- Assembly via THT creates a strong connection between the component and the PCB: After components are literally pushed through the PCB, they withstand environmental stresses more reliably.
- Ideal for larger components subjected to high power, high voltage and mechanical stress.
- THT components are easily interchangeable: optimal for prototyping and testing.
Disadvantages of THT assembly:
- In practice, however, quality problems are more common with THT manufacturing, because a lack of solder penetration (insufficient soldering temperature) or damaged components (overheating) are not uncommon with manual soldering processes. In addition, manual soldering cannot be repeated in a process-safe manner.
- Components with through holes are much larger and the component itself is located on the top side of the PCB.
- The solder joint is on the bottom side, so space is needed on both sides.
- THT also limits the available routing area on the inner layers because the holes must be drilled through all the PCB layers.
- We do not automate the THT assembly process. THT depends on operator skills, so it is less reliable and more expensive than SMD placement.
- Soldering process is less accurate and hardly reproducible.
THE PROCESS IN THT MANUFACTURING
The individual steps
At Stuhl REGELSYSTEME, THT assembly is carried out on state-of-the-art production lines by trained specialists. Here, both pure THT and mixed assemblies are possible. All equipment is of course up-to-date and regularly maintained. This ensures reliable and high-quality manufacturing.
APPLICATION AREAS OF THT ASSEMBLY
For individual electronics production
Although SMD manufacturing has largely replaced THT assembly, there are still some components for which THT assembly is still indispensable. Mechanically highly stressed assemblies such as switches, connectors and coils, for example, are attached to the PCB using THT. Usually, there are no other mounting options for such components.
SMT OR THT ASSEMBLY?
Unlike THT components, SMT components do not have long connecting wires, but connecting pads that are soldered to the pads of the PCB using solder paste.
If you are planning a project, you may be wondering whether SMT manufacturing or THT assembly is better. Although opinions in the industry are divided, in practice SMTs have many advantages over push-fit assembly, and as a result THT assemblies have often replaced them. SMT manufacturing does not require components to be soldered to the back of the PCB.
As a result, the board can be more densely populated with components and the backside serves as an additional surface for other components. Therefore, device manufacturing is more efficient and significantly cheaper. The device itself also becomes lighter because there are no connecting wires and smaller components can be used. The distance between the individual components is smaller and the conductor paths are shortened, which significantly improves the high-frequency characteristics.
However, the connections between THT components and the PCB are far stronger than SMT connections, making THT the ideal choice for components that are exposed to mechanical and environmental stresses or high heat. When prototyping, THT components can be easily modified – perfect for testing and hobbyists.
To combine the benefits of SMT and THT, some applications require mixed technology components. USB connectors are the most common. They must be mounted on densely populated PCBs with little free space, but at the same time need high mechanical stability, which only through-hole technology offers.
To mount such components, terminal pins in solder paste are used.
DEFINITION OF THR ASSEMBLY
In the early days of printed circuit board assembly, only THT was used. However, the use of SMT made it possible to dispense with contact holes. This made it possible to achieve a significantly higher component density on the PCB. In the course of this savings opportunity, an attempt was made to replace THT completely.
However, some components with high mechanical loads (e.g. plugs, switches) must still be securely connected to the board via through-hole connection pins. In the case of pure SMD components, there is a risk that the stressed solder joint will be damaged under high load. For this reason, PCBs are often mixed. The downside of this is that the process of THT assembly cannot be fully automated, resulting in higher costs.
Therefore, in the age of SMT assembly and soldering processes, it is advisable to make connectors, which are actually not suitable for this design, available for the reflow soldering process anyway. In this context, THR technology (Through Hole Reflow THR) was developed. Through-hole components are designed for automated placement and high thermal stress in reflow ovens. This can reduce the assembly costs of automated PCB assembly, since some process steps of the usual THT assembly can be omitted.
The terms PiP – Pin in Paste – and PIHIR – Pin in Hole Intrusive Reflow – are also used for THR technology.
What are THR components?
THR – Through Hole Reflow – is a combination of THT and SMT.
AH! THR components combine the best of two worlds: The through-hole technology of THT and the automatic assembly and reflow soldering of SMT components.
THE SOLDERING OF THE COMPONENTS
Suitable soldering methods in THT assembly
Wave soldering (also known as wave bath soldering) involves soldering THT components in a nitrogen environment over a solder wave on the underside of the PCB.
The preheated PCB moves over a wave of liquid solder generated by the pump and wets the component contacts with tin. For this purpose, the solder is melted in a solder pot and pressed into a wide slot nozzle by means of a tin pump. This creates a surge or wave. Depending on the application, moving and/or laminar waves are used. In a special form, several small hole nozzles are used instead of the wide slot nozzle.
Wave soldering is mainly used for wired components in THT (Through Hole Technology) and less frequently for pre-bonded SMD components. The advantage of wave soldering is the low thermal load on the PCB and components.
Selective soldering refers to the local soldering of individual joints of a component or assembly. In contrast, in mass soldering such as reflow soldering, wave soldering or dip soldering, all solder joints on one side of the workpiece are soldered at once.
Put simply, selective soldering replaces manual soldering in assembly production. It is an automated soldering process in 3D controlled via software. The X, Y and Z coordinates are defined on the board. The coordinates indicate where on the board the hot solder is deposited in order to solder a specific component. A conical nozzle approaches the PCB from below and sets targeted solder points. The selective soldering process is recorded as a video.
Most SMT circuit boards are populated on both sides. If additional THT components are placed so intricately that they cannot otherwise be soldered, a selective soldering process is used.
Manual soldering completes the soldering technology in cases where machine soldering is technologically not possible.
This is the case, for example, when a small number of pieces are involved. In this way, the initial automation costs can be saved. Manual soldering is also used on temperature-sensitive components such as batteries and cables. In addition, manual soldering supports machine-selective soldering when components have already been applied to the underside using the SMT process.