Greetings and welcome to today’s lecture. It's our 7th tutorial in the PCB learning series. In our previous lectures, we have studied the two main types of PCB i.e. Single-sided and Double-sided PCB. Today is going to be a very interesting and interactive class about Through Hole Technology(THT), which is applied in the process of designing printed circuit boards.
A PCB board has a properly designed circuit on it and it's composed of connecting traces/paths and various electronic components. The electronic components are mounted on the board in two different ways i.e. Through-hole and Surface-mount. We will cover Surface-mount in our next lecture and today, we will discuss how to mount components on PCB boards using though-hole technology.
So, let's get started with Through-hole technology:
Introduction to Through-Hole Technology of PCB Design
- In Through-Hole Technology(THT), small holes are drilled in the PCB board to mount the components.
- The normal-sized components(unlike SMD components) have long copper leads/pins and are plugged into these drilled PCB holes and soldered from the other side. The extra leads are trimmed off.
- In through-hole technology, components are placed on one side of the board and soldered on the other side, where the copper foil is present.
- Through-hole technology is normally used in single-sided boards.
History of Through-hole Technology
- THT is the oldest among these PCB components' attachment methods.
- For quite a long time, it remained the standard method to use in the PCB assembly process.
- In the 1980s, when the surface mount technology process was introduced in the field of PCB assembly, designers thought that the THT method will become extinct but that is still not the case.
Why do we still need THT?
- The surface mount technology, being advanced had many advantages over the old Through-hole technology but because of low cost, THT is still operating.
- Some high-power components that required strong connections could not be fitted by the surface mount process, THT is the only option that could be used in such a process.
- For example, the connection of the terminal blocks, power jacks and other power connectors can not be done by any other method apart from the THT.
- Also, equipment working in places with high temperatures and vibrations can only have boards that are connected through the through-hole technology.
- Therefore, while others thought that, the entry of the surface mount devices was the end of the through-hole technology, their thought was disapproved over time, as of today, through-hole technology remains one of the important mounting processes in the manufacturing of the PCB.
How to place THT PCB Order?
There are many online PCB Fabrication Houses, that provide Through Hole PCB board services. One of them is JLCPCB Manufacturing House, which offers this facility of mounting Through Hole components as well as SMT components, a sister company of EasyEDA. JLCPCB is a top-notch PCB Fabrication House, provides low rates for PCB orders.
JLCPCB has an excellent support team, so you should discuss your PCB order with them first. They will completely guide you and will give you the final price and time to complete it. JLCPCB provides a perfect product as per your requirements, a personal experience.
- Drill holes on the PCB following the clients’ design.
- Holes have several specifications from location to diameter sizes.
- In the design folder submitted by the designer, there is a very specific file named drill file. This is the file that will give the location and sizes of the holes to be drilled on the boards accordingly.
- Select the components, that must have long leads and every component has its own specification. Components are mentioned in the BOM files, schematics and the GERBER files. If you go through the files, you will get the components to be used in the design, whether, THT or SMD components. THT components will be selected in this case.
- The next phase is to place the components on the board and ensure that their leads are on the holes as defined in the design. This process can be done manually by human hands or can be done by pick-and-place machines that have been designed specifically for this purpose.
- Ensuring that the components are in the positions described in the design to avoid any errors.
- Solder the terminals on the board accordingly. Soldering is one of the most important processes in THT PCB design.
- Soldering is done by use of the soldering gun which is of different varieties as shown below:
Initially, this process was done manually and was time-consuming, but today, due to AI technology, it has become quite simple and automated to design through-hole technology. An automated insertion machine helps in inserting the electrical elements into the drilled holes of the PCB before the soldering process starts.
Classifications of Through Hole Components
We have talked about the electrical components being placed on the PCB boards through the hole.s These components can be classified into two main types:
- Radial-leads components.
- Axial-leads components.
- In Radial-leads components, all the leads originate from one side of the component.
- The components can be mounted either vertically or horizontally.
- Radial-lead components include capacitors, op-amp, power regulators(LM317, LM7805) etc.
Vertical mounting of Radial THT components
- In this case, the component is placed perpendicular to the board while the base is placed parallel to the board. Soldering is done on one side of the terminal and the other side is filled with flax.
Horizontal mounting of Radial THT components
- The component body is fixed horizontally as you can see in the image above.
- The leads exhibit a very nice bend and spacing. This allows the proper filling of the solder on the component hence forming the strongest bond.
- The component is free from vibration effects as it is well-fixed on the board.
The Axial leads Components.
- In axial-leads components, the leads appear from opposite sides of the component package.
- Axial components are placed horizontally over the PCB board and are strongly attached to the sheet.
- Common examples are diodes, resistors etc.
Vertical axial mounting method.
- One side of the mounting lead is long and allows room for its bending to achieve horizontal fixing.
- It has a strong mechanical joint as it can allow soldering on both sides of the board.
THT Components types
This is mostly used in integrated circuit embedded systems. Those ICs that have terminals on one single side are said to be single-ended THT components. See the attached figure below.
Again, it is applied in ICs systems. Some IC for example the 8051 AT89C51 have pins that are arranged in two files each from its side of the length as shown below. The type of THT that will be used to fix them in the PCB is called double-ended THT mounting.
Pin grid arrays THTs
In this type of THT, the components have so many pins that are arranged throughout the component in a grid manner. See the example of such components below.
Applications of THT
- High voltage areas eg in the medical equipment. When high voltages are involved, there is a possibility of shorts and overheating of the components. Since this type of component mounting provides a strong joint, thus THT comes in handy.
- High mechanical stress devices eg military equipment. High stress, requires strong joints to avoid breakages. This can only be offered via the THT type of mounting.
- High power areas eg in the steam boilers. High power is accompanied by vibrations and magnetic forces. This type of mounting is good for such devices.
- High temperatures operating devices eg in the nuclear plants. High temperature means the possibility of high reliability. Hence, we can use THT as it can be easy to replace components in case of problems.
- Prototyping and testing of components since it is the most flexible method offering easy ways for replacement of components during the process.
Advantages of THT
- The use of the THT method will ensure that you have a very strong mechanical bond between your components and PCB board.
- THT-bound components have high resistance to wear and tear and this is made possible by the use of the large soldering deposits at the terminals.
- The THT components are very easy to swap/repair, thus, best for prototyping works.
- It is best suited for places where strong mechanical applications are needed such as aerospace and military equipment.
Disadvantages of the THT method of PCB components Mounting.
- Due to the drilling of the required holes, the production machines have to be very accurate.
- Drilling of the holes requires some additional time, hence THT method takes a longer time compared to other methods.
- This method limits the available routing paths for the multilayer PCBs because the drilled holes have to go through the given layers.
- The technology being used in the industrial mass production of THT is less reliable compared to other methods.
The through-hole technology PCB mounting is the best method to be used when manufacturing large PCB boards. It is best for mechanically strong component mounting and also the cheapest when it comes to doing testing and prototypes. It can't be extinct and it will never.
So, that was all for today. In the next lecture, we will discuss the second method of mounting components on PCB boards named Surface-mount technology. Till then, take care!!!